AUDIOCLINIC (Jan. 1986)

New-Year Greetings

It's come again-another year!! As I have said before (and am glad to be able to say again), thank you for your encouragement. It really does help.

Thank you for your criticism, too, both positive and negative (and this includes corrections to some of my math-a field in which I am not expert). Thank you to those who have taken time to share ideas which I, in turn, can pass along to the rest of you.

With the November 1985 "Audio clinic," I began my 31st year of service to you. With your help, this can be the best one ever. Keep the questions coming, and I'll keep the answers coming. I'm not on an ego trip; there are times when I don't have the foggiest notion of how to answer a question.

When this is the case, I will just tell you that, or, when possible, refer you to someone who can help.

What I offer is a love of music, long experience as a recording engineer, a gang of time spent on the service bench, and an interest in helping others. I look forward to hearing from you during 1986.

Turntable Isolation

I have a suggestion for readers with turntable isolation problems. Take a piece of foam rubber approximately 1 1/2 inches thick. On top of the foam, place a 1-inch-thick piece of flagstone or any other stone. Then place the turntable on top of the stone. I have found that this added mass significantly improves the isolation, particularly from footsteps and vibration.

-Todd M. James; Reading, Pa.

Speaker Frame Ground

Q. My portable stereo system contains two loudspeakers connected in parallel. From one of the terminal lugs, there is not only a connection to the other speaker in the pair, but also a wire which is directly attached to a rivet that holds a terminal strip to the speaker frame. Why is this connection made?

-Richard Roy, Downsview, Ont., Canada

A. If you check the wiring of these speakers, you'll find that the terminal which is wired to a rivet on the speaker basket is also connected to the ground side of the system's amplifier. The manufacturer must have found a need to ground the speaker assembly. In a portable system, the speakers would be close to other components; if the assembly's frame were not grounded, capacitive coupling could result in oscillation.

It is also possible that the manufacturer purchased a quantity of loudspeakers prewired in the way you have described, and simply used them without unwiring the frame ground.

Output Tube Bias Problem

Q. I have a Dynakit Mark Ill power amplifier which keeps blowing fuses.

After the amp has been on for about 15 minutes, the bias voltage for the KT88 tubes will gradually rise from its normal 1.56 V to about 2.6 V -- at which time the fuse blows. I'd appreciate a clue to the source of this trouble.

-Harry Anderson, Chicago, Ill.

A. As I remember it, the bias voltage readout is really an indication of the output stage's cathode current. This stage's gradual drawing of higher and higher current during operation can result from a number of factors.

First, your output tubes may be gassy. The ionization within the tubes will result in excessive current. Second, a defective component in the bias supply, such as the bias rectifier, may cause the bias supply voltage to drop.

If so, the decreasing bias will cause an increase in plate current and cathode current. Since you are measuring voltage drop across a cathode resistor, rather than measuring bias voltage directly, this causes an apparent voltage rise. Third, a leaky coupling capacitor feeding one output tube may cause that tube to draw excessive current, with the same apparent voltage rise.

Low Volume from a Mixer

Q. When I play my components through my mixer, I must set the mixer's volume very high--to approximately 8 on a scale of 10. The amplifier volume must also be set to near maximum (though this is not necessary when the amp does not go through the mixer). The level meters on the mixer indicate a low output, 15 dB. Other than this need for high volume-control settings, the mixer works well.

-Elmer F. Maye, Jr.; Coram, N.Y.

A. You mentioned the "output" of the mixer. If, by this, you mean what we often refer to as the "master pot," it may simply be that you have set your individual "mix pots" too low. The lower the individual input controls are set, the higher the master gain will have to be in order to make up for this. If, however, you do have the mix pots set near their maximum positions and still obtain the results described, something may be wrong with the power supply (the only component common to both channels). Another thought is that perhaps there is a sensitivity switch or pot which affects the overall gain of the mixer. If the mixer was designed to handle "professional" signal levels as well as those usually encountered in home audio equipment, there is certainly a likelihood of such a sensitivity adjustment, or perhaps one such adjustment for each input channel. If so, you may have set these too low. If the mixer was intended solely for professional service, it may be that no sensitivity adjustments are possible.

Levels used in professional recording systems are on the order of 1.2 V, whereas voltages produced by home entertainment equipment are in the range of 0.1 to 0.3 V, a minimum of 12 dB below professional standard levels.

Subwoofer/Satellite Balance

Q. For the past year I have been considering the purchase of a sub woofer to complement my present system-thus making the present loudspeakers act as satellites. I have noticed, however, that the subwoofer in which I am interested has a higher SPL than my present loudspeaker system. Could the integration of this subwoofer (or similarly rated ones) with my original loudspeakers result in audible unbalance as a result of this?

-Name withheld

A. The answer to your question depends on how you plan to wire the subwoofer and on your taste in terms of low-frequency response.

Your best bet would probably be to bi-amplify the system, using a separate power amplifier to drive the subwoofer.

If there are volume controls on either power amplifier, or the electronic crossover which apportions bass to one amp and higher frequencies to the other, you can use them to lower bass gain till you obtain flat frequency response. If your two amplifiers are of different power, use the lower-powered amp for the satellites.

If cost or other problems force you to use a simple, passive crossover network to separate the highs and lows, you have a problem. Chances are, if there is a balance control associated with this crossover network, that it would adjust the midrange/treble, leaving the SPL produced by the woofer unaffected. If the subwoofer has so much output (compared to that produced by the rest of your loudspeakers) that it is intolerable, you have a few decisions to make. If you have an equalizer, you can lower all frequencies below the crossover point to balance the system.

Notice that I did not offer you the choice of adding an L or T pad to attenuate the signal feeding the subwoofer. Because of the power required to produce bass, heat buildup in the pad would be excessive, thereby requiring an expensive and hard-to-find unit. Further, the use of the pad would lower the damping to the woofer cone.

This, in turn, might produce poor sonic results with some woofer designs.

Amplifier Power Output at 16 Ohms

Q. My loudspeaker systems have impedances of 16 ohms. I note that power-amplifier output is never quoted at this impedance. If a typical amplifier produces 150 watts at 8 ohms, will the output be 75 watts at 16 ohms? And are there any variables which would change this picture?

-Eugene L. Bershad; Freehold, N.J.

A. The power output from an amplifier will certainly be lower when feeding a 16-ohm load than it will when it feeds an 8-ohm load. The amount of falloff will depend on feedback and upon the internal impedance of the output stage. The reduction at 16 ohms may well be less than half (3 dB), but it is determined by the particular amplifier being used.

Notes:

In the July 1985 "Audioclinic," I made a statement to the effect that all turntables produce audio output when their surfaces are tapped. One reader wrote to tell me that his B&O 1800 is practically free from such unwanted output, and he believes some other units are also relatively insensitive to mechanical shock. I can't imagine how this is possible, but I surely want to investigate it.

In the October 1985 issue, in the item "Cable Length and Signal Degradation," I said that a 6-dB loss occurs at the frequency where capacitive reactance equals the output impedance of the diver. Another reader reminds me that this loss is really 3 dB. My thanks to both readers.

(Source: Audio magazine, Jan. 1986, JOSEPH GIOVANELLI)

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