The interconnection of separate units to make up a complete audio system is not necessarily as simple as one might expect.
Apart from the need to provide the necessary cables and plugs, it is important to consider the level of output and input signals, and the possibility of hum and instability arising from inappropriate earth connections.
In a utopian situation, all audio home entertainment units would be completely compatible, irrespective of kind or brand. Any pickup or any radio tuner could be plugged into any amplifier and fed through any loudspeakers. The system could be cross-coupled to the input of any tape recorder or could pick up a signal front any tape player, and so on. All one would need would be the requisite number of standardized cables and plugs.
Unfortunately, this utopian situation is not immediately in view. To be sure, manufacturers have been making a conscious effort to reduce the variety of connectors in use and to provide amplifiers and tape recorders with multiple input and output facilities. But that is about as far as it goes.
The level of the output signal from some devices is just too great to feed directly to the input circuits of others; overload and distortion will occur unless steps arc taken to attenuate the signal within the interconnecting link.
The problem becomes even more difficult if, conversely, the available signal is not adequate to drive the second device.
Then there is the question of impedance levels in the respective units.
In very broad terms, this refers to the effective resistance across which an output signal is developed, or across which an input signal has to be impressed.
As a rule, there is no problem if the impedance of the signal source is lower than that of the circuit into which the signal has to be fed. But there is likely to be a problem if it is the other way round.
Again, complications can arise from the 'earthy' side of the interconnecting cables . In many amplifiers and tape players, the input and output connectors are kept clear of the metal chassis adjacent to where they are mounted. The shell of the connector is earthed instead to a point in the wiring appropriate to its function. Thus input connectors may be earthed close to the preamplifier circuitry and output connectors close to the output or power supply circuitry.
Wien the equipment is connected to ordinary peripheral components as, for example, a record player and loudspeakers, no common paths are established between the input and output wiring. However, if two or more units are interconnected with cables having a common earth-braid, this can set up an external link between tire earthy sides of the input and output connectors.
In certain cases the external link, which completes a virtual earth loop, can invoke problems such as hum injection, interference from the tape drive motor or instability. While by no means the rule, such problems arc always a possibility which has to be allowed for.
Unfortunately, a basic and thorough treatise on the compatibility of hi-fi units would almost inevitably end up as a long and tedious tome, which could test the patience of writer and reader alike.
In this article, we have adopted a completely different approach. We simply discuss the compatibility problems which were encountered with a particular group of units which we had occasion to interconnect in our laboratory: The problems may not be identical with those which might be encountered in another situation, but they are typical and the approach which we adopted to meet them may serve as a guide to other enthusiasts.
The three units involved comprised a record player, a completely self- contained stereo cassette player/recorder and a normal stereo amplifier.
The object was to interconnect the units so that the amplifier could reproduce stereo music from disc or cassette, or from the amplifier's own in-built radio tuner. Alternatively, it had to be capable of recording on to cassette program material being reproduced from disc or radio-a very common requirement.
The first problem encountered was an earth loop involving the power wiring and the signal wiring between the record player and amplifier.
An annoying hum was audible between tracks or behind soft music. The job of tracing and rectifying the trouble provided the inspiration for the article in an earlier issue. With the player wiring modified as described, the hum disappeared and the equipment performed to expectations when reproducing from disc.
At first glance, the job of interconnecting the amplifier and tape recorder looked to be a breeze. The amplifier had a pair of Tape In' and 'Tape Out' sockets; the tape unit had sockets for 'Aux In' and 'Ext Spkr'.
From a local supplier we were able to obtain a well finished equipment cable about 3 ft long containing four colour-coded and shielded leads inside an outer plastic sheath. The only modification was to equip two of the leads on one end with miniature phone plugs to fit the 'Ext Spkr' jacks in the tape unit. The other connectors were of the right kind to plug straight in.
But alas; it wasn't to be that easy! When an attempt was made to record from disc to cassette, the result was intolerable distortion. The signal level from the 'Tape Out' sockets on the amplifier was clearly far too high for the *Aux In' circuitry in the tape recorder.
This kind of problem arises because, in many tape recorders, the 'Aux In' socket connects to the ‘Mic' socket through a resistive pad, the signal then going to the microphone preamplifier stage. While the resistive pad permits a much larger signal to be accepted, it is still possible for the available input to be excessive, producing overload and distortion in the preamplifier stage.
The overall gain of the recording amplifier is usually controlled after the preamplifier stage, either by a gain control potentiometer, or by an automatic gain control circuit. While gain control can reduce the signal to an appropriate level for recording, it can do nothing to correct the distortion that has already been introduced by the overloaded preamplifier stage.
In the particular record player involved, the 'Tape Out' signal was derived from just ahead of the volume and tone controls, so that there was no way of reducing it, short of modifying the internal circuitry.
Equally, the tape recorder could not be setup to accept a greater signal input without internal modification.
The obvious course, in the face of such a problem, is to break the connecting signal lead and to insert a series resistor or resistive divider to introduce the required order of attenuation.
If the tape recorder has a manual recording level control and meter, the resistor(s) can be selected so that normal recording level is obtained with the control knob in approximately the same position as for recording via the unit's own microphone.
Where the tape recorder incorporates automatic level control circuitry, the resistor(s) can be selected so that the signal is recorded at normal level on tape without audible distortion.
With the particular items being considered, we found that the signal level was suitable reduced by including a 120k resistor in series with each active lead from the record player 'Tape Out' to the tape recorder 'Aux In'. For the time being, the resistors were wired roughly between the inner conductors’ and the respective plugs at the tape recorder end. A more permanent arrangement would obviously be required later on but, for the moment, the objective was to determine what was necessary.
Sufficient to say that, with the 120k resistors in position, programs could be recorded on cassette free from any suspicion of overload distortion.
Next problem was to make the tape recorder play through the main amplifier and loudspeakers. To evaluate results we were careful to use a pre-recorded cassette of known good quality . In setting up equipment, it is very easy to be misled if reliance is placed on a recording which might itself contain imperfections.
Unfortunately, we ran into another pack of trouble. While the recorder would reproduce the cassette in normal style as a self-contained unit, any attempt to divert the signal through the main amplifier and loudspeakers resulted in a heavy pulsating hum being imposed on the music.
It sounded rather like a mixture of mains hum and ripple from the tape drive motor.
It transpired that the effect was due mainly to the fact that plugging into the 'Ext Spkr’ jacks left the recorder output stages without a proper load. The noise largely disappeared when low value resistors (actually 18 ohms) were soldered across the two plugs.
Thus two more resistors had to be strung temporarily in the once neat connecting cable.
At this juncture it was possible to record on to--and to replay from--cassette with passable results. Unfortunately, reproduction from cassette still contained a significant amount of hum, and ripple from the tape drive motor.
After a certain amount of fiddling, it became evident that the set-up was sensitive to configuration of the earthy connections. The hum and noise could be influenced by isolating certain of the plugs from the braiding or running separate earth wires. And here the problem of the single 4-in-1 cable became evident. All four braids made direct physical contact inside the outer sheath; thus an input/output earth loop was inescapable both at the amplifier and the tape recorder.
In some set-ups it might not matter. In this one it did! Facing the problem of earth loops and the need to include resistors in the interconnecting leads, we decided the time had arrived for more drastic measures. We would devise a little box-and-cable assembly which could well provide a pattern for solving all kinds of interconnection problems of this general kind.
The box we selected was a standard item measuring 4.25 x 2.25 x 1.5 inches and involving a folded aluminum base and cover secured by self-tapping screws . Inside the box we mounted two 8-tag strips, each providing 6 insulated lugs. The input and output cables were separate lengths of twin 'stereo' lead shielded with an outer PVC covering. These were clamped just inside the box and wired to the tagstrips so that, initially, all inner conductors and braids were insulated from each other and from the box. This provided complete freedom to select and mount series and shunt resistors and to choose which earth paths to establish or leave open.
The 120k series resistors mentioned earlier were wired so that they were in series with the signal lead from record player to tape recorder.
Had it been necessary to decrease the signal level still further, it would have been possible either to increase the value of these resistors or to form a voltage divider by connecting shunt resistors from the tape recorder end to braid.
The 18-ohm load resistors were connected across the tape recorder output and bridged across to player input. Had it been necessary to attenuate the level here, additional series resistors could have been added.
Finally the earth circuitry: This involved experimenting to establish which arrangement gave the best all-round results in terms of hum content and motor noise interference.
In this case, it transpired that the basic earth path was best established between the output of the tape recorder and the tape input of the main amplifier. These two braids were therefore linked to each other and to the box, as shown;
It was found that the braiding from the Tape Out socket of the amplifier could also be joined to the box but NOT tire braiding to the tape recorder's 'Aux In'. For reasons to do with the internal tape recorder circuitry and wiring, completion of this earth circuit invoked hum and motor noise. We did not try to pursue it further.
Our job was to do the best we could with the external connecting leads. It was not for us to get involved with the design in an effort to solve the problem at that level.
In fact, the end result, while acceptable, was still short of true high fidelity standards because of hum content in the tape recorder.
As a unit, with its small detachable loudspeakers, it was excellent in its intended role-providing pleasant stereo music in a small flat. But even then, with the unit operating and with no cassette in position, hum and motor noise could be discerned.
Through a large amplifier and fully baffled loudspeakers this barely noticeable background came up in direct proportion with the improved bass response. This was inevitable; our care with cabling could do no more than avoid aggravating the problem by undesirable earth loops.
One important point should be mentioned: As supplied, the tape recorder/player had only a 2-core flex to the power point. Hopefully the internal circuitry had been designed to ensure adequate safety without the third wire.
But had there been a third wire running back to the power point and joining the third wire from the amplifier, we would have been faced with yet another classic earth loop situation. It might have been necessary' to fit a mains outlet to the amplifier, so that the tape recorder could plug in direct into the amplifier rather than a separate power point. The lesson from all this would seem to be fairly obvious and not very palatable.
Portable tape recorder/players are very handy as self-contained entertainment units but not all of them are suitable for interconnection with basic high fidelity systems. One can achieve so much by close attention' to the interconnections but, beyond that, basic design limitations may
show up.
-------- When compatibility problems really stack up, the neatest
answer is a merging unit in which shunt and series resistors can be
mounted, and where earth connections can be arranged to minimize the effect
of earth loops.
It is reasonable to assume that the new generation of specialized cassette decks will be substantially free from the troubles described in this article. Being intended primarily for use with high fidelity installations, close attention should have been given to interconnection, signal level and low frequency noise content, as well as to other qualities expected of a high fidelity unit.
It is significant that no serious problems have been encountered with any of the cassette decks we have reviewed in recent times. After connection of the necessary cables, they have functioned as expected with whatever amplifier happened to be on hand.
Which is the way it should be!