------ Connects with drawing on Page 35.
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........... our new eight input stereo/mono mixer.
Many of our readers require a multi-input stereo mixer with comprehensive facilities such as automatic muting, tone controls, headphone monitoring, signal metering and ability to handle a whole range ol signal sources. Here is our answer to your requirements.
Mixer Specifications
Eight input stereo mono mixer with automatic noise muting, level metering, bass and treble controls and stereo headphone socket. All input and output connections are made via 6.5mm jacks.
Mixer Section
Frequency response at nominal output: 30Hz to 70kHz between - 1 dB points; -3 dB, -10 dB at 10 khz; constant turnover Baxandall circuit.
3dB points at 10Hz and 150kHz. Tone controls: " , * 10dB at 100Hz.
Variable slope, Sensitivity (without preamplifiers): 40mV at 5k input impedance for rated output of 0.775V RMS. Input impedance may be Increased with a series resistor in the input circuit, with consequent reduction of sensitivity.
Maximum Output Signal: 5V RMS; with headphones connected the output signal clips at Just above the rated output of 0.775V RMS.
Output impedance: 4.7k for stereo mode; 2.35k for mono mode. Output signal levels are halved when both channel outputs are connected together for driving a mono amplifier.
Headphone socket: To suit, any 1W impedance dynamic phones of 8 ohms or more.
Maximum control interaction: Less than 0.5dB Signal-to- noise ratio: better than 60dB with respect to 0.775V RMS output.
Distortion: typically less than 0.04% at 1kHz at rated output.
Separation between channels: typically better than - 50dB Microphone Preamplifier- for 600 ohm microphones)
Sensitivity: 0.5mV at 100k input impedance Frequency response: 20Hz to 70kHz at - 1dB points Input overload: 10 mV at 1kHz.
Signal-to-noise ratio: - 42dB with respect to rated output
Distortion: masked, by residual noise but less than 0.5% Microphone Preamplifier (for high Impedance dynamic .. microphones):
Sensitivity: 2.5mV at 100k input impedance.
Frequency response: 25Hz to 70kHz at -1dB points.
Input overload: 95mV at 1 k.
Signal-to-noise ratio:- 50dB with respect to rated output with short circuit input.
Distortion: less than 0.1% Magnetic Cartridge Preamplifier:
Sensitivity: 3mV at 1kHz for rated output; 56 input' impedance.
Frequency response: within less than 1dB of RIAA response.
Input overload: 90mV at 1kHz.
Signal-to-noise ratio: - 60d(5 with respect to rated output with short circuit input.
Distortion: typically less than 0.1%
Ceramic Cartridge Preamplifier:
Sensitivity: 30mV at approximately 3 megohms input impedance.
Frequency response: 30Hz to 70kHz - ldB points.
Input overload: 1 volt at 1kHz.' Signal-to-noise ratio: - 56dB or better with typical cartridge connected.
Distortion: typically 0.15% at 1kHz at 30OmV input.
Guitar Preamplifier:
Sensitivity: 60mV at 100k input impedance.
Frequency response: 30Hz to 70kHz at – 1 dB points.
Input overload: 140mV at 1kHz.
Signal-to-noise ratio: - 5(dB with respect to rated output with short circuit input! Distortion: less than Or -1%
Mute:
Max gain reduction: 30dB.
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Fast attack, slow decay, on signal levels: deactivate circuit.
In the steady stream of correspondence which we receive from our readers we often have letters indicating a need for a multi-channel mixer.
So recently we decided to look at the many facilities required by our readers and see what could be incorporated into a new mixer, without making it Into a complex monster.
Attack and decay times dependent Adjustable threshold and switch to Right at the outset, let us state that few people seem to want the most basic of mixer stages which only have unity gain. Often, considerable amounts of gain are required.
For example, a common requirement in a simple mixer, as requested by our readers is the ability to accept inputs from a couple of low impedance microphones and maybe a few high level sources such as a radio or stereo recorder, mix them and then feed the output signal into the high level Inputs of a stereo cassette deck. This sort of request comes from amateur tape , recordists whose machines do not have mic/line mixing.
As an afterthought, these readers often request that the mixer have tone controls. Now these requirements are all quite reasonable but they do not add up to what could be called a "simple mixer".
Another group of readers who often write to use about mixers are those who are members of pop and jazz bands.
These days, the trend is to feed all the Inputs from microphones and amplified instruments such as guitars, keyboards and synthesizers, into one master mixer and then into one or more amplifiers having a power output of several hundred watts.
Here, not only must the mixer ca/pr for a wide range oi input and impedance levels but it must have more inputs; as many as twelve would not be excessive for some bands. As well, these people require output level monitoring so that they do not overdrive their expensive amplifiers and loudspeaker systems.
A third grouping of readers who require mixers are those who are members of amateur stage societies. These people require a stereo mixer to feed a stereo public address system. They often have as many as eight microphones spread across the stage with perhaps one or two off-stage. Again, they require output level monitoring.
Headphone monitoring of the output signal is often quoted as another desirable facility, by the three groups of potential users.
So all these readers really want a fairly elaborate mixer. And that is without mentioning specialized features such as pan pots, tone controls on each input channel and even octave equalizers. If we Incorporated all these features we would have produced a monster which only relatively few readers would be game to build.
What we have produced is a new Playmaster mixer which will cater for a total of eight inputs, (four per channel on a stereo basis), has tone controls, output signal monitoring and stereo- headphone socket for monitoring.
With the two output channels bridged together it can be used as a mono mixer with eight inputs.
One of the problems with complex mixers, is that they tend to be noisy when all inputs are running with high gain.
The situation can be particularly bad where several low impedance microphones are used. Because they have such a low signal output voltage to begin with, typically several hundred microvolts, they inevitably give a poor signal-to-noise ratio when plugged directly into a preamplifier.
Professional equipment used in broadcasting and recording studios gets around this problem by using low impedance microphones with balanced lines and step-up transformers.
The balanced lines eliminate hum and other mains-induced noise while the step-up transformers improve the Inherent signal-to-noise ratio of the system by increasing the signal level to the preamplifier.
Unfortunately, microphone transformers are expensive, so we really could not incorporate them into the Play-master mixer, however, there is another way of improving the signal/noise ratio.
In a typical situation where a mixer is used, microphone channels with a poor signal/noise ratio will be most noticeable during quiet sections of the program and less noticeable during loud sections. During the quiet sections chances .are that some or all of the noise microphone channels in question are not actually being used.
However , in typical use by amateurs, it may not be possible to turn down the unused channels, and thereby improve the signal/noise ratio. Of course, in recording studios the mixer operator constantly juggles the signal levels and settings to obtain the best program quality.
In the amateur situation, the mixer is more likely to be initially set up and then left untouched for the entire program.
Our Playmaster mixer Incorporates automatic muting circuits which drastically cut the gain of unused input channels to improve the signal/noise ratio. As soon as the signal level to a given microphone rises above a given threshold, which is adjustable, the input channel is made fully operational.
When Initially setting up relative signal levels for each input channel, the automatic muting can be switched out of operation using the same control used to adjust the threshold level. More about this later. Now let us describe some of the physical features of the Playmaster mixer. The chassis is 400 x 150 x 180mra (W x H x D) and is' a simple U-shape with timber end-covers and a vinyl-covered top panel. Altogether it has fifteen Knobs, a toggle switch, headphone socket and dual meters on the front panel.
To avoid making the front panel appear too large, we have split it into two sections, one finished in black while the other has a natural aluminum finish.
The prototype front panel was made from photosensitive aluminum.
On the black section of the panel is the master level control and immediately above it, the dual level meters.
This seems to be a fairly logical arrangement.
On the other section of the front panel are the two tone control knobs, level knobs for the 8 input channel, and four knobs which control the muting threshold and also switch it out of operation if required. The muting facility is only provided on four inputs in the prototype but this can be extended to all inputs if need be.
The four input level and two mute threshold controls for each output channel are grouped together on the front panel. Thus, a group including knobs 1, 2, 5 and 6 on the LH side serves the left output channel.
Other panel layouts and chassis configurations could have been used. Sloping panels are common. However, we felt that for a universal mixer the configuration we chose was most appropriate. It can be placed on a shelf or on top of other equipment and all settings are visible from some distance away. This is not the : ( case with sloping panels. It wolild also appear to be more compatible with the vertical control panels of many tape and cassette decks now being retailed.
Slider controls have not been used, for several reasons: for a start, unless the better quality 60mm types are used, they give a poor control "feel": they are consider
ably more expensive than conventional rotary potentiometers; the need to provide brackets and slots in chassis and escutcheon panels makes the metalwork very expensive and dirt can quickly gain ingress to the track, to make it noisy.
To top it off, the author is not particularly keen on slider potentiometers.
On the back panel, we have standardized the input and output sockets. 6.5mm Jack sockets are used.
Inside the chassis are two relatively large PC boards, one we call the "mother" board and the other the main mixer board. Reference to the schematic diagram will show how it all comes together.
Twelve edge connectors are 'mounted on the motherboard.
The connecters mate up to eight small preamp boards and four mute boards. The preamp boards have the same universal pattern but can be wired up to suit quite a range of input sources. In this way, the mixer can cater for high level Inputs such as tape recorders, electronic keyboard instruments and synthesizers, and !- ' low level Inputs such as low or high Impedance microphones, magnetic cartridge, ceramic cartridge, electric guitar and so on.' The mother board not only provides a convenient method of plugging In preamp and mute boards but also eliminates the tedious job of wiring up and mounting all those eight-way edge connectors.
Dimensions of the mother board are 127 x 178mm while those for the mixer board are 100 x 20Onm., Each channel on the mixer board has four Inputs, each with a maximum sensitivity of 4CtaV at 10k input impedance for the nominal output level of 0.775V R11S. nominal output level of the mixer can be varied easily up or down by chaining a few resistors on this board. This enables the mixer to drive any amplifier to full power.
Refer now the circuit diagram of the main mixer board.
This contains the mixer stages, tone controls, metering circuits and the regulated power supply. The circuit shows one channel on the mixer board plus the common power supply.
Tr1 is the mixer stage which accepts the signals from the wipers of four level controls. Shunt negative feedback is applied from the collector of the transistor to its base - it could be referred to as a virtual earth mixer. The gain is unity, fixed by the ratio of the feedback resistor (10k) to the Input resistor (also 10k).
The I.
A 22k resistor provides the DC collector load for the mixer stage but because the considerable feedback applied around it the output Impedance is low, so that it can feed the 10k master level control without being unduly loaded.
A mixer stage such as this, with unity gain, can handle four inputs with negligible interaction, distortion and noise.
Following the master volume control is a direct-coupled transistor pair, Tr2 and Tr3, which provide a stage gain of 10. This circuit is interesting because it has two feedback loops, one predominantly AC and the other DC.
AC feedback is applied from the collector of Tr3 to the emitter of Tr2. The gain of 10 is set by the ratio of the 15k resistor to the 1.5k resistor. Note that the 15k resistor provides a DC path so that to some extent, the 15k resistor provides a second DC feedback path which tends to interact with the DC feedback path mentioned above. This means that an alteration to AC feedback requires an adjustment to the biasing conditions.
Output from the collector of Tr3 is fed via a 1-uF capacitor to the tone control section comprising Tr4 and Tr5. This circuit is quite different from those we have used in the past which have employed a single transistor - the common Baxandall negative feedback tout control.
Our tone control circuit is based on a design by P. U.' Quilcer. Basically, it consists of common-emitter amplifier stage with an emitter follower. The emitter follower provides an output buffer for the relatively high collector load of Tr4 and also supplies a bootstrap voltage to effectively raise the value of this collector load.
Bootstrap voltage, ie., positive feedback with almost unity gain from the collector of Tr3 (via the emitter- follower) is coupled from the emitter of Tr5 by a 10 uF capacitor to the Junction of two 10k resistors which form the collector load of Tr4. Since the AC voltage at the junction of the two 10k resistors is almost the same as at the collector of Tr4, a very little AC current flows in the "lower" 10k resistor and so Tr4 "sees" a very high value of collector load,' much higher than 20k.
This means that the open loop gain of the stage becomes very high and with the application of negative feedback, the distortion is very low. Thus the performance of this circuit is significantly improved over the common single transistor tone control stage. Distortion of this section is typically less than 0.01% over the whole audio range.
Another difference between our circuit and those published in previous years is that it has a "constant turnover, variable slope characteristic" whereas those published previously have a "variable turnover, constant slope". Slope refers to the rate of boost or cut in the circuit; this is a maximum of 6dB/octave for any typical tone control circuit.
Turnover refers to the frequency above which, in the case of treble control, boost or cut occurs, case of a variable turnover, constant slope tone control, the frequency above which treble boost or cut occurs varies with the setting of the tone control, while the slope above this frequency remains constant at 6d8/octave.
By contrast, in a variable slope, constant turnover tone featured here, the slope is altered by the In the By contrast control as tone control while the turnover frequency remains the same.
For the same time constants, both tone controls systems will give the same amount of maximum boost or cut but the variable slope control will seem to be more progressive, in its operation.
A look at the tone control performance curves shows why.
The solid lines show the amount of boost or cut available at maximum and half settings of the controls.
The dotted line shows, the amount of bass boost available at half boost setting for an equivalent "variable turn over" control.
------ Connects with drawing on Page 35.
------ Connects with drawing on Page 34.
As can be seen, what boost does occur Js below 100Hz and will not be apparent on much of music programs, contrast the variable slope control gives quite a reasonable amount of boost to frequencies above 100Hz at the half boost setting, and thus will sound quite effective.
In This means that while the variable slope tone control sounds quite progressive in its action, the effect of the variable turnover control "seems" to be compressed into the ends of the control rotation - nothing appears to happen over much of the control rotation, reason, "apparent effectiveness", we have used the variable slope control in the Playmaster mixer.
For this One small drawback with the circuit as we have used is a certain amount of interaction between the bass and treble controls; if the treble control is fully boosted and then the bass control is fully boost or cut, the amount of treble boost is reduced by about 2dB at the extreme highs. However, use of the treble control does not similarly reduce the amount of bass boost or cut available.
We feel that the drawback is a small disadvantage and it is seldom that full bass boost and full treble boost are applied simultaneously.
As the tone control curves show, generous bass and treble boost and cut is available at the extremes of control rotation, while there is a very little interation with the mid-frequencies. 10k limiting resistors are placed in series with both sides of the one control potentiometers. This prevents the boost and cut from being excessive at very low frequencies, and at very high frequencies. This improves stability and helps prevent acoustic feedback.
Since the tone control stage has a very low output Impedance due to the large amount of negative feedback (stage gain is two) it can drive a pair of stereo headphones via a resistor of only a few hundred ohms, 270 ohms in this case. This resistor is selected so that when the mixer is delivering slightly more than the maximum nominal output signal, ie., 0.775V RMS in the case of this circuit, the signal becomes distorted due to current limiting in the tone control stage.
This warns the person monitoring the mixer output signal with headphones that the power amplifier is being overdriven . If the nominal output of the mixer is changed to suit another amplifier, the 270 ohm resistor is changed accordingly to give an audible warning when the power amplifier is being overdriven.
With the headphones disconnected, the mixer output does not does not clip until the signal rises to about 5V RMS.
Output from the tone control stage is coupled to the output socket of the mixer via a 4.7k resistor so that it can drive amplifiers with a low input impedance'.
The 4.7 resistor can be reduced to lk, if necessary.
The output signal of the mixer is amplified by Tr6 and then fed to a bridge rectifier and meter to provide signal monitoring.
The power supply is derived from the mains via a small transformer, which has two 14V windings connected in series to give 28V. This is fed to a bridge rectifier to give a nominal 34V DC which is fed to the metering stages, i.e., Tr6.
All the other circuitry in the mixer is powered from a 19V supply. This is derived from the 34V rail via a zener diode network and emitter-follower regulator, Tr7. The 47 ohm resistor renders Tr7 short-circuit proof. This is a very worthwhile feature, as the author can testify from repeated experience.
As stated previously the main mixer board has four inputs in each channel, each with an input sensitivity of 40mV for the nominal output level of 0.775 volts RKS, and an input impedance of 10k. When combined with the -10k level control of each input, the input impedance is shunted down to a minimum of 5k.
Where a higher input impedance is required for a high level source such as a tuner, the input impedance may be simply increased by connecting a resistor in series with the input. This is done by making up a Jumper board which , is Inserted into the mother board in place of the preamplifier board. For example, the input impedance can be increased to 20k by using a Jumper board with a 15k resistor. This reduces the sensitivity to 16mV for rated output .
We will describe how to make a Jumper board later, in the section on construction.
Apart from high level sources such as broadcast tuners or tape recorders, greatly Increased sensitivity is required for all the low level sources the mixer is likely to be used with.
Accordingly, each input is provided with its own PC preamp board which is plugged into the mother board.
The preamplifier boards use a standard copper pattern and circuit configuration. Circuit constants are modified to .give the required gain, input impedance and frequency characteristic.
Refer now to the circuit of the preamplifier which we have designated as a "universal preamplifier". Two high-gain low-noise NPN transistors are used, in a direct-coupled feedback pair configuration.
Astute readers will notice that this circuit is very, similar to one following the mixer stage on the main mixer board. The voltage divider resistors in the anl tterclrcuit of the second transistor, Tr9, have been changed (from the circuit of the mixer board) to take into account different operating conditions.
The AC feedback components are R3, C3, R4, C2, R2, C1 and the 1.5k resistor in the emitter circuit of the first translator, changing bias feed resistor Rl.
The input Impedance Is varied by The values for the components nominated above are tabulated below the circuit. To make each type of preamplifier. Just refer to the appropriate values In the table.
For a guitar preamplifier, for example, R1 is 100k, R3 is 33k, R4 Is replaced by a wire link and the others (R2, C1, C2 and C3) are omitted. The resulting preamplifier has a gain of Just over 20 and an input impedance of 100k.
Note that the input impedance of all the low level preamplifiers, except that of the magnetic cartridge preamp, is 100 k. We have not attempted to "match" the input loads to the nominal impedance of the sources, we have the apparent anomaly of a 600-ohm microphone (say) feeding a 100k input load.
There is no point in providing a low impedance load for a low impedance source. If the load impedance matches the sources impedance half the signal is lost and residual noise produced by the amplifying stage reduced little, if at all. For optimum signal/noise ratio, an amplifier should be arranged to provide a nominal high Impedance load and be driven by a low impedance source.
At the same time, the operating conditions of the amplifier should be optimized to give the lowest possible residual noise considering the source it is to be used with. In general this means selecting the optimum quiescent current for the first transistor in the amplifier. This has been done in the universal pre-amplifier circuit, after considering the variety of sources it will be used with.
To make a corollary of the previous paragraphs: unless an audio source is required to operate into a stated load (such as 50k for a magnetic cartridge) in order to obtain its rated frequency response, the load impedance should be considerably higher than the source impedance, other-wise there will be a loss of signal and a reduction in the signal/noise ratio.
There are two preamplifier outputs, output 1 and output 2, the latter being fed via a 10k resistor. These two outputs are necessary to drive the mute board which is described later. Readers will also notice that there is no output coupling capacitor on the universal preamplifier board. This is located on the mute board or on the mother board.
Readers may wonder why the frequency response of the low impedance microphone preamplifier is better at the low end than for all the other preamplifiers. This is merely a function of the feedback network which causes a slight "hump" in the low frequency response.
Signal-to-noise ratios are quoted with respect to the rated output of 0.775V RMS and are unweighted (ie., measured with a millivoltmeter having a wideband response). Unless otherwise stated, the measurements were taken with a short-circuit input, ie.( with the appropriate input jack removed with automatically shorts the input.
In each case, connecting a typical input source causes a slight degradation in signal to noise ratio.
While the universal preamplifier will cater for low level sources which are likely to be required, it will not provide for a ceramic cartridge. Here, we have used a quite different circuit. Two high gain NPH transistors are employed. Tr10 is connected as a common-emitter stage with a high-value collector load of 100k and a bootstrapped bias network to give a high input impedance.
A .047uF capacitor connected from the emitter of Tr10 to the junction of the three resistors in the biasing network provides the bootstrapping. Bootstrapping increases the input impedance in the following way: Without the above- mentioned -047uF capacitor, the input impedance is determined by the product of the 22k resistor multiplied by the AC current gain of Tr10, shunted by the sum of the 330k resistor and the parallel combination of the 1M and 150k resistors. This long-winded expression adds up to about 450k, which is not adequate for good bass response from a ceramic cartridge. ' Bootstrapping here makes use of the fact that the voltage gain from the base of a high-gain transistor to its emitter is very close to unity. In fact, It is typically 0.98 or more in the stage under discussion. So with the aid of the .047uF capacitor, 98% of the input signal is coupled to the Junction of the three bias resistors.
This drastically reduces the signal current flow in the 330k resistor. By way of illustration, if IV RMS was fed to the input of the preamplifier, the signal current flowing in the 330k resistor would just 2.2 microamps, without bootstrapping. With the bootstrapping capacitor in circuit, the signal current flow in the 330k resistor is typically reduced by more than 30 times. In other words, the input signal "see" the 330k resistor multiplied by more than 30 times, or about 10 megohms. When combined with the product of the transistor's AC gain multiplied by the 22k emitter resistor, we find that the bootstrapping has raised the input impedance to an adequate value of about 3 megohms or more.
An emitter-follower stage, Tr11, acts as a buffer for the high collector load (100k) of Tr10 and thus provides a low output impedance for the preamplifier. Voltage gain of the preamp is about 4 times.
As mentioned in the first article, automatic muting is provided to improve the apparent signal-to-noise ratio of low-level sources such as high or low-impedance microphones. This is particularly desirable with low impedance microphones because they have such a low output signal, but it can Improve the signal-noise ratio with any microphone used in a noise environment. So besides accommodating the eight preamplifier PC boards,
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the mother board has provision for up to eight mute boards. In the prototype mixer, only four mute boards are provided: we imagine that this will satisfy the needs of most constructors.
If all eight mute boards are required, the extra controls will mean a larger control panel. Either this or they will have to be installed on the rear panel.
Refer now to the circuit of the mute board. It uses three transistors and two diodes. As can be seen, the circuit requires two inputs, from 1 and output 2 of the associated preamplifier board.
Basically, the circuit works as follows: Signal is fed from the preamplifier output 2 via a 10k resistor and 4.7uF capacitor to the collector of Tr14. The 68k resistor connected to the 19 volt supply line forward- biases the base-emitter Junction of this transistor and causes its collector emitter path to become a low resistance to AC signals.
In this way, the transistor can be used similarly to a FET, as an audio switch. Note that the transistor does not 'give as progressive a control characteristic as can be obtained with a FET, but this is of little importance in this application, since the muting action need not be progressive.
To recap then, when the base-emitter Junction of Tr14 is forward-biased, -the collector-emitter path becomes a low resistance, typically 100 ohms or so, depending on beta. This low resistance forms a voltage divider with the series 10k resistor (which for calculation purposes is effectively in parallel with the 10k mixing level control) to reduce the signal level by about 30dB, dependent upon the setting of the level control. In this condition, the preamplifier is muted. In order to get the mute circuit un-muted, Tr14 must be changed from the low resistance state to a high resistance state, i.e., it must be turned off.
by overcoming the forward bias provided by the 680k resistor with a negative voltage, developed by the mute amplifier comprising Tr12, Tr13, D1 and D2.
Tr12 and Tr13 form a complementary voltage amplifier stage with negative AC and DC feedback from the collector of Tr13 to the emitter of Tr12 via a 10k resistor. Signal from output 1 of the preamplifier is coupled to the base of Tr12 via a' 100k resistor.
Besides signal coupling, the 100k resistor also provides the required bias voltage for the base of Tr12 (from the collector of Tr9 on the preamplifier board). This is one of the reasons for not having an output coupling capacitor on the preamplifier boards. The 100k resistor also prevents Tr12 and 13 from being driven hard into clipping-when presented with a very loud signal.
Thus it prevents a "click" being heard when Tr11 is un-muted.
This is achieved Tr12 and Tr13 amplify the signal and feed it to a halfwave "voltage doubler" rectifier formed by D1 and D2 which develop a negative voltage across the 470uf capacitor. If the input signal to the mute amplifier is sufficiently high, the resulting negative voltage is sufficient to turn off Tr14, which stops it from shunting AC signal to the negative supply line.
The signal threshold above which the mute circuit amplifier and its associated diodes develop enough negative voltage to Un-mute Tr14 is dependent on the gain of Tr12 and Tr13.
This is adjusted by the 1k switch/pot in series with the 400 ohm resistor and 10uF capacitor.
When the pot, which is wired as a variable resistance, is set to its minimum resistance condition (fully anticlockwise) the gain of the mute amplifier is at maximum and therefore provides the lowest threshold for un-muting. In this condition small signals will un-mute Tr14. When' the pot is fully clockwise, gain of the mute amplifier is at a minimum and so relatively large signals are required for un-muting to occur.
Since very low frequencies are of little Importance to the mute amplifier, a relatively small capacitor, 10uF, can be used to roll off the low frequency response.
Similarly, high frequency response. Similarly, high frequency response is rolled off by the 0.001uF capacitor in parallel with the 10k feedback resistor.
When the mixer is used in a typical set-up it is necessary to be able to disable the muting circuits while gain levels Otherwise it would be possible to step up to a microphone with the appropriate gain control fully advanced but the system sounding relatively quiet. As soon as you spoke into the mic, the system would un-mute and then break into the ear-shattering how of maximum acoustic feedback are set.
Consequently, the switch section of the 1-k pot is connected in series with the collector of Tr14. Rotating the switch-pot fully anticlockwise to operate the switch disables the muting function and enables gain levels for all channels to be set in the normal way.
Having described the major circuitry features, we can now talk about construction of the mixer, can begin with assembly of the PC boards, with the mixer board.
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Construction
Start
Assembly of the mixer board is
straightforward. }W or
If resistors of low noise cracked carbon or metal film construction may be used throughout. The board has been designed to suit PC electrolytic capacitors so these should be used if at all possible. Three 1uF capacitors in each channel should be tantalum types as marked on the circuit diagram, In several places on the board where a 4.7uF capacitor is specified we have also used tantalum types but this is not mandatory.
Quite a diverse range of transistors may be used on all the boards in the mixer, encapsulated types are to be preferred.
From a cost point of view plastic
A plastic encapsulated BC137 (or TIP31A) is specified for the supply regulator transistor. This is operated without any heat sink. Note that the center lead of the transistor must be bent to insert it.
PC stakes are used for all connections to the board.
Any type may be used, provided they are a tight fit in the PC board holes before soldering.
There are several wire links on the board. Note the two associated with the output to the dual meter. These are required with the meter we used in the prototype but must be replaced with suitable resistors if a more sensitive meter movement is used and/or if. the rated output of the mixer is increased.
Assembly of the mother board, is equally straightforward.
Twelve eight-way edge connectors with mounting brackets are required. These have one closed and one open mounting bracket. The closed mounting bracket (i.e., not slotted like the Individual connectors) functions as a polarizing key for the P.C. board inserted into it.
Screws, nuts and washers or pop-rivets and washers may be used to secure the edge connector brackets to the mother board before soldering. If pop-rivets are used, take care not to crack the board when they are applied, especially when close to the edge.
Assuming that only four muting PC boards are required, as in the prototype, the space for the additional four edge connectors may be left vacant. In their place, 4.7uF tantalum or aluminum PC electrolytic capacitors are installed as shown on the mother board diagram.
As with the mixer board, all connections to the mother board are made via suitable PC stakes.
After assembly of the mixer and mother boards is-complete, attention can be turned to the preamplifier and mute boards.
A keyway must be cut in each of the plug-in PC boards to polarize them and so prevent them from being plugged in to the edge connector incorrectly. This is arranged by cutting along the length of the connector contact nearest the input capacitor using a hacksaw.
Ensure that all boards plug in easily without fouling of the keyway, before mounting any components or soldering.
No problems should be encountered with assembly of the plug-in boards. If constructors are not able to obtain PC mounting 220uF capacitors, C1 can most easily be mounted by interchanging its position with R2 (where C1 is required). On the mute board, the 4.7uF capacitor in series with Tr14 should be a tantalum type. The other 4.7uF capacitor associated with the output from Tr13 may be either a tantalum or PC mounting aluminum electrolytic.
Since the circuit of the ceramic cartridge is quite different from the universal preamplifier circuit, the standard board pattern has to be modified to accommodate The wiring diagram of the ceramic preamplifier board shows where the copper pattern is cut and the position of the wire links-three on the topside and two on the copper side of the board.
Having assembled and carefully soldered all the PC boards, attention may be turned to the chassis. Mount the four rubber feet first. Then carefully follow the details given in the following paragraphs for mounting the transformer and associated hardware.
One of the problems we had to contend with early in the development of the Playmaster mixer was the possibility of earth loops, when the mixer was connected to a tape recorder or power amplifier that was itself earthed back via the mains wiring. The problem is compounded because even the very low level input sockets are earthed directly to the chassis.
(1) Use a two-section bobbin-wound transformer conforming to the insulation and winding construction requirements of BS1.
it.
(2)Do not earth the chassis of the mixer back via the mains cord. Instead, mount the transformer on two insulating pillars. In the unlikely event that the transformer insulation does break down to the cote, no hazard would result.
(3) Sheath the active leads from the on/off switch with suitable plastic sleeving.
(4) Use an on/off switch of all plastic construction, and sleeve the spade lugs of the switch to prevent accidental contact of the user with the mains.
(5) We found it necessary to earth the core of the transformer back to the mains cord earth, in order to reduce the induced hum in the low level pre-amplifiers, particularly those for the magnetic cartridge.
Twist the two wires from the terminal block to the on/off switch and sleeve them - the outer sleeve of the mains cord will be quite suitable. Before soldering the wires to the switch, push a length of suitable sleeving over the wires and after soldering, push the sleeving over the spade terminals of the switch. All three terminals of the switch should be sleeved.
Ideally, the transformer should have a wraparound cover for the mains primary connections. The cover should be made of Presspahn or Elephantide (trade names for a vulcanized insulation material used in transformer and electrical motor construction). Unfortunately, we were unable to obtain a supply of this material during construction so we had to be content with wrapping two layers of Insulation tape around the transformer to cover the terminations, after soldering short leads to them.
Note that ordinary cardboard is not suitable for the job, as it is hygroscopic.
Before actually mounting the transformer on the insulating pillars, mount the six-way terminal block and terminate the transformer primary and secondary leads to it.
the red and blue secondary leads together to form the' center-tap of the series connected windings.
The mains cord should be passed through a grommeted hole in the rear of the chassis and anchored with a cord clamp.
Terminate the mains active and neutral leads to the terminal block and solder the earth wire to a solder lug to be secured under one of the transformer mounting feet screws.
Connect
Make sure that two screws in each insulating spacer do not touch in the center and thus negate the intended isolation of the transformer core, a suitable length and check with a multimeter to ensure that the transformer core is actually isolated from the chassis yet is in good electrical contact with the earth wire of the mains cord.
In other words, use screws of The headphone socket must also be isolated from the chassis and be provided with an earth return lead back to the mixer board. Otherwise, the headphone earth return current flows in the chassis and hack via the earth paths for the input signals. This causes instability when the low impedance microphone preamplifiers are used.
Wrap the threaded bush of the headphone socket with insulating tape or a suitable piece of plastic sleeve, a fiber washer on both sides of the front panel and check with a. multimeter to see that tile socket is correctly isolated from chassis.
Use The dual meter is a rather attractive unit. It has blue-tinted scales (but not calibrations) with red pointers.
It is intended to be illuminated by a light shining through the translucent rear section. At the same time, mounting of the meter presents problems as there are no screws or lugs of any sort provided.
Our approach was to make up a bracket which holds the meter in place and also positions a small lamp in the correct position for lighting. The bracket itself Is held in position by the master level control dual potentiometer.
Illumination is provided by a Sato miniature bez61 with the red lens removed. A 12V lamp is fitted, which draws about 100mA from one of the 14V windings of the transformer, and the regulation is such that only 12V is applied to the lamp. When combined with the 60mA DC drawn by the mixer circuitry, the total load is excess of the nominal transformer rating, which is 2 x 14V at 60mA.
In short, while the transformer is running in excess of lt3 ratings, it does not represent a situation of undue stress.
After an extended period of running, the transformer was found to be warm to the touch, but not unduly so.
--- These curves show ,he performance of the tone controls at half
and maximum settings
Four plug-in mute boards ere used in the prototype Playmaster 145 miter.
MAY BE CONSTRUCTED ON VERO-BOARD
Before the master level control potentiometer is mounted to secure the meter bracket in place, cut the shaft to 13 mm length and solder the figure-8 shielded cable to it.
Two lengths of 20Cmm of figure-8 shielded cable will be adequate.
All the potentiometers should have their shafts cut to 13mm length before being installed.
All the input sockets and the output sockets may now be installed. The input sockets have a shorting contact to earth the Input when not in use. All the chassis connections together with a length of tinned copper wire.
All interconnections to the mixer and mother boards should be made using pieces of flat multiconductor cable (otherwise known as "rainbow" cable) apart from the shielded input cables. Using the rainbow cable obviates the need for cable lacing and makes circuit tracing easy. We found two meters of 10-conductor cable to be more than adequate.
As a bonus it gives a wide range of wire colors and is cheaper than an equivalent quantity of hookup wire.
Take care to wire the boards, pots and sockets exactly as indicated in the chassis wiring diagram. Otherwise, hum and instability will almost certainly result.
Having checked all connections, fit a three-pin plug ;to the mains cord and apply power to the mixer. Check all the voltages shown on the mixer circuit diagram All voltages should be within IV or 10% of those on the circuit. If not, check the components.
If all the voltages check out, now is an appropriate time to make a "Jumper" board which enables the mixer to be operated without a preamplifier, i.e., direct. You will need a piece of Veroboard with 0.15in conductor spacing, at least eight conductors wide and at least 30mm long.
Now refer to one of the assembled preamplifier boards and identify which of the contacts is the input connection (this is actually the contact next to the keyway) and which is the output 2 connection. The idea is to make a dummy board which bridges the above two contacts together, with a wire link.
With the jumper (or dummy) board in place, the input sensitivity is 40mV with an input impedance of 5k.
Alternatively, if you wish to provide higher input impedance and/or reduced sensitivity than the mixer board provides, the input and output 2 connections should be connected by a suitable resistor. For example, by using a 22k resistor, the input impedance is increased to 27k and the sensitivity is reduced to 216mV which would be adequate for most high level sources.
---- Above is the circuit and component table for the universal
preamplifier boards.
MAY BE CONSTRUCTED ON VERO-BOARD
By making a slight alteration to the PC board for the universal preamplifier, the circuit at right can be accommodated.
CERAMIC CARTRIDGE PREAMPLIFIER MAY BE CONSTRUCTED ON VERO-BOARD
Now, with the Jumper board in one of the edge connectors for inputs 5,6, 7 or 8 (refer to the photograph of the front panel to identify these), check out the mixer operation with a set of headphones and a suitable signal source connected. With no signal applied, the mixer should be almost completely silent, even with all level and tone controls at maximum settings.
Note that the jumper board will not allow the mixer to function correctly when used in place of preamplifiers for inputs 1 to 4, unless the associated mute board is in position and the mute control is switched to off.
Now plug each of the preamplifier boards, one at a time, into an appropriate edge connector on the mother board and check the voltages on the circuit. Now plug preamplifier boards into edge connectors for inputs 1 to 4 and plug in the associated mute boards and check voltages on these (i.e., at the emitter of Tr11 on each mute board).
Note that the mute board will not work unless an associated preamplifier board is plugged in. This is because the mute board obtains its bias from the output of the preceding preamplifiers, as explained previously.
With a microphone connected to an appropriate input, you can now check that the mute function is operational.
Check this with all the mute boards.
When all checking and de-bugging has been completed, you can install the front panel, this step until now, otherwise there is a strong chance of scratching the panel while fiddling about.
It is better to leave Scotchal was used to make the front panel of the prototype mixer. The panel was made in two parts, with the join being concealed by a piece of pressure-sensitive tape.
We assume that metalwork suppliers will have front panels available shortly after this article is published.
Fifteen knobs are used in all. As can be seen from the photographs, we have used several different types of knob, each selected to suit the control function. It is desirable to have the individual mixer level controls small while the main level control knob can be fairly large.
On the prototype, the timber end panels were attached by screws from inside the chassis, but this is a very awkward method. A better procedure would be to have recessed holes in each panel and attach them by self-tapping screws to the chassis, from the outside. Before mounting the panels, staple a sheet of aluminum foil (as used for cooking) and position it oh the panel so that it will make contact with the chassis and so provide a degree of shielding.
The top panel can be given a practical finish by covering it with vinyl, using contact adhesive.
For best results, do not stack the mixer on top of power amplifiers with large transformers, otherwise hum will be a problem. Power amplifiers need plenty of ventilation anyway. At the same time, keep low level input leads away from power transformer fields.
List of Component Parts
Chassis and hardware
1 chassis with cover
2 timber end panels, 205 x 165mm
1 front panel 1 power transformer with two 14V secondaries.
1 dual level meter, 500uA sensitivity.
1 meter bracket (see text) 1 miniature bezel with 12V lamp
15 knobs; select types to suit.
1 10k (log) dual ganged potentiometer
2 100k (lin) dual ganged potentiometers
8 10k (log) potentiometers
4 1k (lin or log) potentiometers with switch
2 6.5 mm stereo Jack sockets 8 6.5mm jack sockets with shorting contacts 1 all-insulated toggle switch 2 12 mm insulating spacers, tapped 1/8in Whitworth 1 solder lug <
1 mains cord clamp 1 six-way insulated terminal block 1 three-pin mains plug and three-core mains cord.
8 PC board supports 2 meters of 10-conductor rainbow cable 2 meters of figure-8 shield cable 2 fiber washers for headphone socket i Mixer Board 1 Vero board 37 PC stakes Semiconductors 4 EM401, BY126/100, IN40O2, RS 276-1136 silicon rectifier diodes.
8 1N4148, RS276-1136, 1N914A, BA 127-209, BAY 39-61 1 x B2X79/C20 zener diode 4 BC109, BC549, BC184L, RS 276-2013/2031, 2N930 low noise NPN silicon transistors 6 BC108, BC548, BC183L silicon NPN transistors, RS276-2009, 2N929-930 2 BC107, P.C547, BC182L silicon NPN transistors, RS276-2009/ 2031. 5K3020. 2H929-930 1 BD137, TIP31A RS 276-2018, 2N2102-4922. SK3054.
Resistors (JW or i'll, 5% tolerance) 4 x 1M, 2 X 220k, 2 x 150k, 4 X 100k 2 x 82k, 4 x 22k, 4 x 15k, 26 x 10k, 2 x 4.7k, 5 x 2.2k, 2 x 1.8k, 2 X 1.5k, 8 x 1k, 2 x 820 ohms, 2 x 390 ohms, 2 x 270 ohms, 1 x 47ohms.
Capacitors 1 470uF 63VW PC electrolytic 2 220uF 10VW PC electrolytic 2 100uF 25VW PC electrolytic 2 47uF 25VW PC electrolytic 2 22uF 25VW PC electrolytic 2 10uF 25VW PC electrolytic 4 4.7uF 25VW PC or tantalum electrolytic
2 1uF 25VW PC or tantalum electrolytic 6 1uF 25VW tantalum electrolytic 2 0.22uF 100VW metallized polyester or polycarbonate 2 0.1uF 100 VW metallized polyester or polycarbonate 2 ,047uF 100VW metallized polyester
. 4 .0015 100VW metallized polyester 2 47pF 125VW ceramic or-polystyrene Mother Board 1 Vero board 26 PC stakes 12 edge connectors, eight-way, with one open, and one closed bracket.
4 4.7uF 25VW tantalum electrolytic capacitors.
4 .047uF 125 VW ceramic capacitors 24 poprlvets and washers or screws, nuts and washers to mount edge connectors.
Preamp Board (8) with gold flashed or tib plated edge 1 Vero board contacts.
2 BC109, BC549, BC184L, RS 276-2013/2031. 2N930 low noise silicon NPN transistors.
Resistors <JW <Jr iW, 5% tolerance) 1 x 100k, 1 x 22k, 1 x 10k, 1 x 2.2k, 1 x 1.5k, 1 x lk, 1 x 390 ohms, 1 x 220 ohms.
Capacitors 1 220uF 10VW PC electrolytic 1 22uF VW PC electrolytic 1 0.39uF 100VW metallised polyester or polycarbonate 1 47pF 125VW ceramic or polystyrene.
Plus R1, R2, R3, R4, C1, C2 and C3 as required.
Mute Boards (4)
'1 Vero board 1 BC 148.BC548, BC183L, RS 276-2009/2031 silicon NPN transistor 1 BC158, BC558, BC2131, RS 276-2021 silicon PNP transistor 1 BC149, BC549, BC184L, RS276-2031 silicon high gain NPN transistor 2 1N4148, 1N914A, RS276-1136. BA127-209. BAY 39-61 Silicon signal diodes 1 680k, 2 x 100k, 1 x 22k 2 10k, 1 x 100 ohms (1W or 1W, 5% tolerance resistors) 1 47uF 10VW PC electrolytic 1 10uF 6VW PC electrolytic 1 4.7uF 25VW PC or tantalum electrolytic
•1 4.7uF 25VW tantalum electrolytic 1 .001uF 100VW metallized polyester or polystyrene capacitor Ceramic Preamplifier (Optional 2 required) 1 Vero board 2 BC109, BC5'19, BC184L, RS 276-2013/2031. 2N930 low noise silicon NPN transistors 2.047uF 100VW ceramic or metallized polyester capacitors 1 x Ira, 1 x 150k. 1 x 22k, 1 x 10k. 1 x 4.7k, 1 x 100k, 1W or 1W, 5% tolerance resistors).
Miscellaneous spaghetti Tinned copper wire, electrical insulation tape, sleeving, vinyl covering material, contact adhesive, self-tapping screws, solder.