AUDIOCLINIC (Dec. 1987)

Bigger Bass from Smaller Woofers

Q. Is it true that some 8-inch woofers can produce more bass than 12-inch woofers can?

-Wonseok Lee, Tarrytown, N.Y.

A. Yes, some can. Bass output depends not only on the size of the woofers but upon their compliances, their free-air resonances, and the characteristics of the enclosures in which they are housed.

More on Sibilance

Q. My problem is as follows: High frequency sounds--especially "s" sounds--are harsh, more so than when I began using my system, about 15 months ago. If l use the "high" filter or switch to mono, the problem is less noticeable. What can I do to cure it altogether?

-Name withheld

A. Sometimes, the sound quality of an audio system may appear to change over a period of time when, in fact, it is the listener's ears that are becoming more critical. Assuming this is not the case, there are several things you can do to determine the source of the problem.

If the sibilance is apparent only when playing phonograph records, it is possible that the stylus is worn and should be replaced, This is especially true if you have inadvertently dropped the cartridge onto a record or perhaps onto the turntable at some point.

Is the harshness you describe heard from both loudspeakers? If it is heard from only one, then there could be a defective tweeter in the "bad" channel. It is also possible that the amplifier/ receiver has a problem in one channel.

By interchanging loudspeakers, you can determine what is taking place. If, after switching speakers, the harshness is still found on the same channel, you will know that the problem is not in the loudspeakers. If, on the other hand, the harshness moves to the other channel when you switch speakers, you will know that the problem lies in the loudspeaker system.

If this kind of distortion is heard only when listening to FM broadcasts, I would suspect that the problem is multipath distortion. The distortion would be heard on both channels but could be eliminated by switching to "mono" as you have described. The problem might not have been present when you purchased your equipment. If there has been building construction in your area, the new buildings might cause reflections of the desired signals, leading to multipath distortion. Along these same lines, perhaps you have moved your antenna to a less desirable location. This is especially likely if you use the folded dipole supplied with most equipment.

Problems of this latter kind are probably more likely than loudspeaker problems, considering your sound improves when you use a filter or switch to mono. Still, it's perhaps worth exploring the loudspeaker situation too.

Balancing Channels

Perhaps your readers would be interested in the simple way I obtain perfect channel balance:

If the preamp, amp, or receiver in question has a mono switch which controls all input sources (not just the tuner), switch it in. Then connect a loudspeaker between the two "+" terminals of your amplifier output (but don't do this if it has "floating grounds"). This speaker connection effectively cancels out all but the differences between the two channels' signals. Adjust the balance control for minimum sound, and channel balance should be perfect.

If there is no mono switch on your equipment, don't despair. You can use the same procedure with the help of two Y connectors, one to merge the left and right outputs of a program source into one output, another to feed this output into both the left and right inputs of the amplifier or receiver.

-Mike Prachar, El Sobrante, Cal.

Amplifier Classes

Q. Could you tell me what the difference is among Class-A, Class-AB, and Class-B amplifiers?

-Kiumars Boubour, Carlsbad, Cal.

A. The differences among these amplifier classes relate to the ways in which bias is applied to their output stages. The current flow through a tube or transistor depends on the voltage or current applied to it. This current or voltage can come from the input signal, a fixed bias from the amplifier's own power supply, or, more usually, a combination of the two.

It is possible to bias the circuit to a point where, in the absence of input signal, little if any current flows in the output circuit; this is Class-B operation.

It is also possible to set the bias volt age so that output current is always flowing, with or without signal being applied to the input; this is Class-A operation. In Class AB, bias voltage may be set so that output current al ways flows when a signal is applied and sometimes flows when there is no signal.

In a Class-A amplifier, the operation of the tube or transistor is always linear. In other words, a change in the input signal's amplitude will produce a corresponding proportional change in the amplitude of the output signal. Current flows in the output circuit of such an amplifier even when there is no signal present. When an a.c. signal is applied to the input, the output current rises and falls above and below this idling current.

The Class-B amplifier is biased just to the cutoff point, where current stops flowing in the output circuit. If an a.c. signal is applied to the input of a tube or transistor with Class-B bias, current will flow in the device for one half cycle but not during the opposite half cycle.

In this latter half cycle, the signal being applied to the input adds to the bias already present, keeping the device's output current cut off. You can imagine that the loss of every second half cycle of an audio signal would result in very severe distortion. There is no way that a Class-B output stage consisting of a single device could be usable for any audio application. To overcome this, a second device is added, also biased to the point just sufficient to cut off output current. The polarity of the signal feeding the first device in the pair is opposite that of the signal feeding the second device in the pair. Therefore, when the first device is being driven beyond cutoff, the second device is drawing current, and vice versa. A circuit of this type is known as a "push-pull" circuit.

If a very small signal is applied to this configuration, current will flow as described, but it will not be linear: The output waveform will not be a faithful reproduction of the waveform of the original input signal. To overcome this, massive amounts of negative feed back are applied, reducing the nonlinearity of the Class-B output stage to such a degree that distortion becomes very low; such a circuit can be used for high-fidelity applications.

The Class-A amplifier can also be (and usually is) made as a push-pull stage. This results in more power out put than is available from a single de vice, as well as a decrease in the circuit's already very low distortion. The addition of negative feedback lowers distortion still further.

The Class-A amplifier can be made to produce somewhat lower distortion than a Class-B stage. However, the Class-B amplifier can produce more power than a Class-A circuit when both circuits employ the same devices.

Also, because the Class-B amplifier does not draw output current in the absence of input signal, it operates much more efficiently than a Class-A stage (which draws output current at all times).

The Class-AB amplifier stage is made by biasing the input of the de vice halfway between the linear portion of the input/output curve and current cutoff. It will, therefore, draw current at all times but will draw much less cur rent than the Class-A circuit. This makes it more efficient than a Class-A stage but not quite as efficient as a Class-B circuit. As you might guess, distortion in the Class-AB stage is higher than for a Class-A circuit but less than for a Class-B circuit. In all these biasing arrangements, when the rest of the circuit is designed with high fidelity applications in mind, distortion can be very low.

Multiplying Mixer inputs

Q. I do a lot of mixing and recording. My difficulty is that my mixer does not have enough inputs to handle all of my equipment. What I need is a mixer having perhaps a dozen inputs (rather than the six I now have). I'd like to keep the cost under $1,000.

-Al Brown, Melrose, Mass.

A. You did not say whether you need more microphone or more line inputs. Also, you did not mention whether you want to be able to mix a dozen inputs simultaneously or whether you mix only a few signals at a time but need to be able to choose from a large number of sources.

If you want to mix 12 simultaneous signals or require more than six micro phone inputs at a time, I don't know of any mixers in your price range which have enough inputs to satisfy you. If your needs are not that demanding, you will find a number of nice mixers for less than $1,000. My suggestion is that you stay with your present mixer but add to it.

You might consider adding a second six-input mixer to your present unit. If both your old and new mixers have bus extension jacks, you'll be able to link them so that one mixer's master controls will affect all signals coming through the mixing system.

Even without such bus connections, you can gang both mixers' controls by routing one mixer's output through a line input on the other mixer. If you can't spare that input, you can still use the two mixers simultaneously, but without having the convenience of ganged master controls.

If you primarily use line-level sources and don't need to mix more than six at a time, then you only need to add some sort of switching system so you can conveniently select the sources you want to mix. You might try using some of the sound-routing boxes that are made to permit a single set of in puts/outputs to serve many components. Two or three of these boxes, each designed to work with a particular pair of mixer inputs, could solve your problem. To make the best use of all this equipment, make a list of all of your gear and the various mixing arrangements you use. You will likely discover that some jobs require a particular arrangement of components and that other projects require different arrangements. Assign the components to inputs on the routing boxes which can permit the mixing you need. Merely by pressing a few switches, you will be set up for the next project.

Turntable Intermittents

Q. I have an automatic, direct-drive, quartz speed-controlled turntable which will stop playing in the middle of a record for no apparent reason. If I hit the start button, it will go to the beginning and play just fine. This failure occurs about once a month. What can I do about it?

-Robert Mayberger, Rosedale, N.Y.

A. I do not think the problem is serious. In all likelihood, it is only a matter of dirty contacts on either the stop or the start switch. Other contacts which can cause problems are those be tween the sliders and elements of the speed-setting potentiometers. My turn table has speed adjustment controls on its operating panel; your turntable might also have these, as well as internal speed-adjust trimpots. Go over all of these contacts with suitable contact cleaner.

If cleaning the contacts does not solve the problem, perhaps you should change the transistor or IC which controls the clock frequency; low feed back may be causing oscillator drop out. There is also the possibility of a hairline crack in a circuit foil. Examine all circuit boards with a strong light and solder any suspicious-looking foils. Don't overlook poorly soldered connections between the foil and the components.

Odd Volume Control Action

Q. When turning the volume/gain control on my receiver from "0" to "6," there is a gradual and minimal change in volume level. From "6" up to "9," however, there is a great change.

Please explain why this occurs.

-June E. Thomas, Wantagh, N.Y.

A. This behavior is probably a function of your volume control's "taper," the rate at which its resistance changes as it's turned, and the way this control interacts with your amplifier's other circuitry.

Volume control action is dependent on the particular potentiometer used by the manufacturer. Perhaps this odd action is an intended characteristic of the make and model of equipment you are using. Check a dealer's "demo" unit to see if its volume control acts the same way yours does. If it does not, try cleaning the control on your unit with a suitable contact cleaner. It is possible that the action of the control is being influenced by dirty contacts. If cleaning doesn't help and you find the condition hard to live with, replace the volume control on your unit with the correct factory replacement.

I have seen instances in which the action of a volume control changed when loudness compensation was switched on. Does the volume control's action you describe take place only when this circuit is being used? If so, there's not much you can do about it.

High- and Low-Level Phono Cartridges

Q. What is the difference between a high-level phono cartridge and a low level one?

-Name withheld

A. The difference between high-and low-level cartridges is in the output voltage they produce. I am not at all sure that there is a definite line drawn between the two types, though these days I tend to think of it as being around 2 or 3 mV. Incidentally, most of today's moving-coil cartridges fall into the low-level classification.

Remote Speakers For a Biamped System

Q. My audio system is biamped, using the following amplifier configuration: The left and right bass drivers are powered by separate stereo amplifiers, each in its bridged mono mode.

The mid/high drivers are powered by a single stereo amplifier in its two-channel mode.

I would like to use extension speakers at a different location and to power these extension speakers by my existing amplifiers. I need the ability to select either the main or remote speakers and would also like the remote speakers to have separate level controls. Is this setup possible?

-R. Johnson, Kendall Park, N.J.

A. If the remote speakers are biamped, your system can work fine.

The only problem will be that of independent level control for those speakers. Switching between main and re mote will involve switching two sets of speakers rather than one, so a four-gang switch will be needed.

If the remote speakers are not biamped, you will need a third power amplifier to feed them. You'll also need a second preamp output or Y connectors in each output channel of your preamp to feed signals both to the remote amplifiers and to the electronic crossover in your main system. The third amplifier must be a power amp with input level controls or an integrated amp; the latter would also give you control over tonal and left-right balance. This amplifier should be located in the same room as the remote loud speakers. It is unlikely that the extra cable run would adversely affect the system's sound quality.

If the remote-speaker amplifier has no level controls, passive controls can be connected between its output and the speakers. However, these waste a lot of amplifier power. Additionally, they can change the remote system's sound by reducing the amplifier's damping effect.

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(Adapted from Audio magazine, Dec. 1987, JOSEPH GIOVANELLI)

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