JOSEPH GIOVANELLI
Driving Two speakers from Two Stereo Amps
Q. I have two amplifiers. 1 would like to feed both of these stereo amplifiers into two common speakers. How can I make this connection and, at the same time, eliminate the possibility of damage to the components if both of these amplifiers are turned on at the same time? I would like to make this connection without using switches.
-1st Lt. James M. Rose, Peekskill, New York.
A. I see no way by which you can have two amplifiers connected to a single set of speakers without switching unless you are willing to sacrifice some power which otherwise would be available from the amplifiers as a possible reserve margin. You could isolate the speakers from either amplifier by connecting them to the amplifiers through 8or even better, 16-ohm resistors. Power will be lost across these resistors and some additional power will be lost across the amplifier which is not in use.
I recommend switching, even if the switching is accomplished by a relay system. By using a relay connected to the convenience outlet on each power amplifier, you can arrange matters so that, when one amplifier is turned on, the other amplifier is disconnected from the circuit.
If wiring has been done correctly, simultaneously turning on the two amplifiers will result in neither amplifier being connected to the speakers. No damage can result. (I could not have said that when vacuum-tube amplifier circuits were king.) The relay for amplifier No. 1 will, when energized, connect the speakers.
However, it will interrupt a possible connection to amplifier two. If amplifier two is turned on and amplifier one is off, a relay associated with amplifier two will connect the speakers to it, but will disconnect the speakers from amplifier one.
If both amplifiers are turned on, each relay will attempt to connect the speakers to their respective amplifiers, but contacts will interrupt the opposite circuit.
Thus, no amplifier will be connected.
This would call for two double-throw, four-pole relays with 120-V a.c. fields.
Additional switch contacts might need to be provided in the event that one or both amplifiers do not use ground as one of their output terminals.
Dubbing Rented Tapes
Q. I am planning to purchase a stereo dubbing deck. But before investing my money I would like to know if it is legal to rent tapes and copy them for home use. What is your opinion concerning the renting of tapes and then copying them for one's own tape library? (Name withheld)
A. So long as one confines the use of a tape copy to oneself, the legality of copying a rented tape or rented phono disk tends to be academic. It is when you try to make commercial use of the copy or start passing the copy among friends that the trouble is apt to start. The above statements are based on some articles on the subject that I have read in the past, and in no way constitute a guide to your actions. For such a guide I urge that you consult an attorney. My opinion about renting tapes or disks and copying them is that this is unethical, even if not illegal.
Using VU meters with Disc-Cutting Systems
Q. I plan to install a VU meter across the 16-ohm output of my power amplifier, to be used in conjunction with my disc cutting system. I would like to know if there is any objection to using the circuit shown in Fig. 1, to drive the vu meter.
Fig. 1
Fig. 2, Fig. 3, Fig. 4, Fig. 5
Would installing a matching transformer between the output of the voltage amplifier and the meter help the performance of the meter? See Fig. 2.
-Walter Marion, Evergreen Park, Illinois
A. The circuit you have shown in Fig. 1 should work reasonably well. However, I do not like connecting a meter directly
in the plate circuit of an amplifier. The coupling capacitor will assume a sudden charge when the amplifier is first turned on, pinning the meter. Meters are, of course, capable of handling a certain amount of overload. However, there is no reason to tempt fate. Rather than using both halves of the 12AX7 to provide gain, why not use the second half of this tube as a cathode follower? The meter is coupled via a capacitor to the cathode of this circuit. The capacitor feeding signal to the meter would charge very slowly during the tube's warmup, as the electron emission from its cathode slowly builds up.
As an alternative you could use a matching transformer in the plate circuit as mentioned in your question. The meter should be connected in series with a 3.9-k ohm, 1-percent resistor, with this series combination being connected to the 600-ohm winding of this matching transformer. This transformer should be shunted by a 600-ohm resistor. See Fig. 3. The use of this resistor provides for proper damping characteristics of your VU meter. This is extremely important when a number of meters are used, and where they must all read alike when a given signal is impressed on them. This factor is not particularly important for your application.
As I have said, a voltage amplifier can be used to drive a VU meter. However, it is possible to drive the VU meter directly from the output of the power amplifier. Use a line-to-voice-coil transformer as shown in Fig. 4. Notice once again that we are using the 3.9 k-ohm resistor. This improves meter damping, but also isolates the meter from the power amplifier circuitry, thereby minimizing distortion. What we have here is a basic circuit, not calibrated to your needs. You will have to devise an attenuator network which can be used to calibrate the meter.
The amount of signal provided from the line-to-voice-coil transformer will be far too much when the cutting head is operated at normal recording level. When using the circuits described thus far, the VU meter "sees" the signal supplied to the disc recording head. This signal is pre-emphasized for proper RIAA treble compensation. Some people do not use a VU meter in this way. I, myself, have the VU meters in the output of a program amplifier, which, in turn, feeds the RIAA equalizer, whose output feeds the input of the disc recording power amplifiers. See block diagram, Fig. 5. Under these conditions the VU meter will "see" the same kind of signal as read on the VU meter in the tape recording chain. You can see that this makes comparisons between tape playback and disc recorder feed more meaningful.
In the arrangement where the VU meter is driven by the pre-emphasized signal, the meter will often read higher than it will with the arrangement shown in Fig. 5. With practice, either system can be used to advantage. The main thing to remember is that good disc recordings can be made best when you observe the VU meter in relation to the sound you hear from your monitor speaker. This implies that you must be totally familiar with the characteristics of your meter and those of your monitor speaker. Further, you must understand the relationship that these two factors have upon the cutting process.
Record Cleaning
Q. I am writing in the hope that you can solve a serious problem. Some time ago, I was given some electrostatic record detergent as a gift. I assumed from the blurb on the container that this fluid would reduce the surface noise, remove the problem of dust attracted by static electricity, lengthen record life, etc. As my investment in recordings is not inconsiderable (a library of about 1,000 albums), I thought that the use of the cleaner would materially prolong the value of my discs.
Unfortunately, I have discovered that the effects of the detergent have been quite the opposite of those claimed.
Many of my recordings now have an irritating level of surface noise. My problem, therefore, is what can I do about it? I'm hoping that you can advise me of some method by which I might remove the "glue" from my valuable recordings.
-Douglas Purkis, Toronto, Canada.
A. First, get rid of remaining "cleaner." If the material you used to "clean" your records has attacked their surfaces, changing them chemically, there is nothing which can be done to restore these discs to their original condition. However, if the detergent has only deposited a surface film, this should be removable.
I suggest that you rinse your discs in warm water, NOT HOT WATER. If you have a sink spray, this is ideal because it will provide sufficient water pressure to really wash out surface deposits. If you do not have a sink spray, use a garden hose.
You will next want to dry the records with a lint free cloth.
If you ever wish to try another type of cleaner advertising its ability to do all sorts of wondrous things, my general thought is, "don't do it." However, if you want to try it for "kicks," try the cleaner on just one or two discs, discs which you are not particularly interested in. If these discs are ruined, the loss will not be serious. Do not clean an entire collection until you know that the product you are going to use is safe and, of course, effective.
(Audio magazine, Dec. 1970)
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