Note: A tremendous amount of mail has been received with regard to this column. I wish to thank all for your confidence in what I am trying to do.
Because of this heavy volume, it will take some time to get to any given letter. I answer all letters in the order in which they are received.
Letters are answered regardless of whether or not we can use them in "Audioclinic." When material is to be so used, I always indicate this fact in my letter, giving you time to indicate that you do not wish your name used.
When I receive a request that a name not be used, I acknowledge this fact by mail so you can have the assurance that your request will be honored.
In closing, I wish to thank you for your patience while awaiting a reply to your questions.
-J. G.
Organ Tonal Balance
Q. My electric organ contains a stereo speaker system. I have installed jacks to which I can connect external speakers, which I also use for making stereo recordings. The sound produced by the left channel output is excellent. The right channel, however, carries the pedal tones and tends to over-power and blur the treble information, some of which is also carried on that channel. Can you suggest a circuit which would provide variable suppression of the bass frequencies without affecting the treble?
-Lee Ryer, Reseda, Calif.
A. You probably need very little in the way of special circuitry in order for you to attenuate the pedal tones. This can usually be accomplished via facilities contained in the instrument.
This might take the form of discrete pedal stops or by a potentiometer located right on the console. Some instruments employ pots located within the instrument which are intended for preset tonal balance.
Bass can be attenuated by placing the "hot" output terminal in series with a capacitor. The value of this capacitor will depend on whether the instrument is to feed directly into loudspeakers or whether it is to feed into a tape recorder.
In the event that you feed the recorder, perhaps you can find another point in the circuit from which to extract signal. This point may provide better tonal balance than is available from the speaker output you are now using. If you must use the speaker output, place a small capacitor in series with the "hot" lead. The value must be determined experimentally, but perhaps a good starting point is 0.005µF. The working voltage is no consequence because you won't have more than a few volts appearing across the capacitor. Adjust the value of this capacitor so that you obtain almost enough bass.
You can then shunt the capacitor by a potentiometer whose value will depend on the impedance of the circuit driven by the organ. A 0.5 meg pot is a good one to use for experimental purposes.
Fig. 1--Bass tone control.
The purpose of this potentiometer is to shunt low frequencies around the capacitor, providing you with a bass tone control. You should choose a value for this pot which will provide very little bass boost when open all the way. When you begin to close the pot, bass should gradually rise. If the value of this control is too high, the boost will take place only when it is nearly closed, making for difficulties in accurate balancing. If the value of the pot is too low, too much bass will be shunted around the capacitor, regardless of the setting of the pot. This bass boost circuit is shown in Fig. 1.
A similar arrangement can be constructed for driving external speakers.
In this instance the value of the capacitor is considerably greater, and the resistance of the potentiometer is considerably lower. A good starting capacitor value is 100 uF. This must be what is often referred to as a non-polarized electrolytic or motor-starting capacitor.
If such a capacitor is not available, a serviceable substitute can be fabricated as shown in Fig. 2. Note the polarities shown and be sure to observe them.
Fig. 2--Equivalent of non-polarized capacitor.
A reasonable starting point for the potentiometer would be 100 to 500 ohms. Here again, the purpose of this pot is to provide you with a means to adjust the tonal balance.
This potentiometer should be capable of handling at least 10 watts, though wattage for the potentiometer used to drive your tape recorder is not at all critical.
If the external speakers or a suitable dummy load is attached to the organ when you make your tapes, you can use the components needed for speaker tonal balance; you won't need the first network described here.
If your music system contains a separate preamplifier, you can use it as an alternative method by which to obtain proper tonal balance. Feed the organ directly to one of the high level inputs. The main outputs can be used to drive either a power amplifier or tape recorder. In either case, the bass and treble controls are available to enable you to adjust tonal balance. Do not use the "tape out" jacks because the tone controls will not be operative.
Phono Cartridge Aging
Q. Could you please tell me to what extent phono cartridges are subject to aging? Specifically, my present cartridge is three years old. The stylus was just replaced. When I received this stylus, I also received a brand new cartridge which I did not order.
Depending on what you have to say, I will either install the new cartridge or hang on to it for that rainy day when something will go wrong with the original cartridge
-Brian A. Bauer, Lake Forest, Ill.
A. You should not encounter aging problems with the cartridge itself.
Basically, the components inside the cartridge are magnetic material plus very fine wire. I have seen a couple of cases where the varnish covering this wire was of such a nature that it attacked the wire, causing it to open circuit. This, of course, resulted in a sudden, catastrophic failure of the cartridge. Until that time, the cartridge worked flawlessly.
I have seen stylus assemblies which employ damping material which can change in compliance with time. This will give rise to a gradual change in frequency response and overall smoothness. This is a rare condition.
Because you have installed a new stylus in your original cartridge, you should now obtain sound quality as good as was produced by the cartridge when it was first purchased. Chances are very good that you will never need to use that second cartridge.
Any change in the performance of your cartridge would be small. Probably the only way you could keep track of these changes is by checking periodically with a good test record. Check for both distortion and for frequency response. If distortion begins to rise, it is probably an indication of stylus wear rather than changes in the cartridge's performance.
Harmonic Distortion
Q. What is harmonic distortion?
-Ray Segura, New Orleans, La.
A. Suppose we feed a tone of 1 kHz into an amplifier. No amplifier is perfect. Therefore, in addition to amplifying the I kHz tone, it produces energy at frequencies which are multiples of 1 kHz, such as 2 kHz, 3 kHz, 4 kHz, etc. (These multiples are known as harmonics.) Each added harmonic, as a rule, is less in strength than the preceding one.
The fact that these harmonic frequencies were not present in the original signal indicates that the amplifier itself is generating them. These added signals are, or course, unwanted and are, therefore, considered to be distortion. Because the added frequencies are related harmonically to the signal fed into the amplifier, this phenomenon is called "harmonic distortion."
Multiple Use of Dolby Equipment
Q. The issue of noise reduction is confusing. This is especially true with the talk of Dolbyized FM broadcasting and the possible use of the Dolby system in home disc playback. If I purchase a Dolbyized tape deck, I assume the Dolby system will operate only for the deck in which it is incorporated.
Will a single, separate Dolby unit be able to serve as a noise reduction unit for all music sources: i.e. tape deck, FM broadcasting and eventual Dolby discs? Even with a separate Dolby unit, will I need an individual one for each music source?
-Samuel Neiditch, Highland, Calif.
A. If it appears likely that there will be extensive use of Dolby FM broadcasts and the like, tape machine manufacturers will probably arrange their circuitry so that the built-in Dolby circuitry will be available for use with external equipment such as FM tuners.
(It appears that phonograph reproduction would have to employ a variant of the Dolby B system; the spectrum of noise found in phonograph record reproduction is different from that of tape recording noise.) Let us assume that in your area there is to be experimental broadcasting using the Dolby system. Let us also assume that your tape recorder includes a Dolby system which is unavailable for use by external equipment. The only way to decode these broadcasts is to record them WITH NO DOLBY ENCODING and to play them back with Dolby decoding. Otherwise you will need an independent Dolby system.
If all broadcasters and record makers use the parameters of the Dolby B system, a single unit can be used for all applications. There would be no need to have separate units for each signal source, so long as you are willing to recalibrate it each time the source is changed.
The Dolby system does require a small setup procedure before it can decode the signals properly. At this time I wonder how the FM broadcaster or record manufacturer will provide the proper level tones for use in making these adjustments.
(Audio magazine, Dec. 1972)
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