Audioclinic (May 1982)

Continental Divide

Q. While stationed overseas, I purchased a European model of an AM/FM stereo receiver. After setting up my equipment back in the United States, I noticed that, although the receiver has a voltage selector switch for use at 117 volts, there is no provision to switch from 50 to 60 Hz. Will the use of this receiver with 117 V, 60 Hz current damage it in any way?

-Geddes MacLaren, Cascade, Md.

A. You do not need a means of switching between 50 and 60 Hz powerline frequencies. It is safe to say that a device which has been designed to operate at 50 Hz will also operate properly at 60 Hz. The power transformer will run slightly cooler when operated at 60 Hz than at 50 Hz.

However, it may not be possible to operate a device designed for 60 Hz at 50 Hz. Where a power transformer runs very hot at 60 Hz, chances are that the transformer will burn out if run at 50 Hz. Sound reproduction will not be affected, regardless of powerline frequency.

While what I have said holds true for tuners, amplifiers, preamplifiers and other signal-processing equipment, it may not be true of tape recorders, turntables, or other devices containing motors.

Tracking Down a Step-Up

Q. I am making my own electrostatic speakers and have questions about step-up audio transformers. How is it possible to have a transformer step up an audio signal to a sufficient voltage to drive an electrostatic speaker and not have the transformer cause distortion and loss of frequency response? I would like my electrostatic speakers to cover the entire audio range and therefore need a transformer which will permit this. Where can I get such a transformer or how can l make one?

-William Hird, Jr., Pawtucket, R.I.

A. I suggest using a good output transformer, the kind used with tube amplifiers, between the plates of the output stage and the loudspeaker system. This transformer will have the center-tapped winding needed for a balanced speaker system.

Good output transformers have always been capable of producing a very wide frequency response, even up to frequencies as high as 100 kHz.

I have measured the output voltage of such transformers for possible use in projects just like yours and found that many of them could produce in excess of 1,000 volts of audio frequency signal with a relatively modest 15 watts of drive. These transformers were typically 8 to 10 kilohms plate-to-plate.

It is possible that manufacturers of transformers may produce units ,specifically designed for your application, so checking their catalogs should be most helpful. Newark Electronics, in particular, offers a wide variety of transformers and they have offices in major cities across the country.

If at First You Don't Succeed ...

Q. I have a problem with my solid-state amplifier. This unit replaced a dual 35-watt tube unit, and about three months after purchase, it failed.

The service center that repaired the amplifier said the power transformer burned out. The transformer was replaced, but about three months later the amplifier failed again, showing the same symptoms as before.

I have a multi-speaker setup controlled by a six-speaker selector switch. During the first three months of operation I used a combination of speakers just as I had done with my tube amplifier. The lowest impedance used was 3.2 ohms, and most of the time I was operating with about a 5 ohm load. When the unit failed, I was running a pair of 8-ohm speakers and a 16-ohm large speaker connected to one of the channels.

The large speaker is home built, consisting of a horn midrange, tweeter and a 15-inch woofer, impedance unknown, with two crossover networks.

When the amp came back from being serviced the first time, I never operated it with less than 5-ohm loads, just in case that was the problem. I also changed the connections of my large speaker system. By using the six-speaker selector switch, I paralleled the two channels. When the amplifier failed the second time, I was running only the large speaker at low volume.

I have used this same setup for almost 20 years with my old equipment and never had a problem.

-Richard W. Curdo, Ayer, Mass.

A. It may be that the original problem stemmed from running your amplifier at nearly full output; the 3.2-ohm impedance is less than the 4-ohm minimum recommended load for many amplifiers. This added load will result in increased heat, leading to premature failures. I suspect that the output stage blew, taking the power transformer, but we will never know for sure.

I also get the impression that, at times, you have two channels connected in parallel. This arrangement might well damage your amplifier. With some program material, one channel would try to drive the other channel in addition to driving the speakers. The impedance presented to the output stage could be very low at such times.

Unless there are specific provisions for doing so, never parallel the two outputs. I suspect that this is what led to the trouble the second time.

The large speaker may also be suspect. Although you state that its impedance is 16 ohms, you also say that the woofer has an unknown impedance. Should it happen that the woofer's impedance is really 8 ohms or 4 ohms, the impedance of your total system at low frequencies will not be 16 ohms, but 8 or 4 ohms or even less.

Thus, the amplifier may have been loaded more heavily than you suspect, both in your original installation and in the installation which ultimately produced the second amplifier failure.

Tube equipment is much more forgiving of overloads. Solid-state equipment is often more forgiving of being operated without a load.

Ratio Rationale

Q. I recently read an old review of the Bose 301 speakers. However, the efficiency was stated as 3.9 watts input for 94 dB at 1 meter. All current speaker reviews state acoustical output referenced to 1-watt input. Is there a simple conversion formula to use?

-Denis A. Bryan, New Orleans, La.

A. At 1-watt input, the Bose 301 would produce about 6 dB less than it would when driven by the 3.9 watts you mentioned. Therefore, if the speaker, as reviewed, would produce 94 dB SPL, with 1 watt feeding into it, it would produce something on the order of 88 to 89 dB SPL at 1 meter. Please note that the 1-meter distance must be held constant for all measurements. All this assumes linearity of output with input.

To make conversions of the kinds you need, set up the 1 watt in a ratio to the power used in the old measurement, 3.9 watts. The ratio is, of course, 3.9 to 1. If you look this up in a decibel table, such as is given in many reference books, you will find that the dB difference is 6 dB. This 6 dB is then simply subtracted from the 94 dB SPL measurement obtained with 3.9 watts input, and the result is 88 dB.

May the Force Stay with You

Q. Is it necessary to adjust the anti skating and/or tracking forces when playing records which have been treated with a record preservative?

-Richard Torres, APO N.Y.

A. There is no reason to adjust either the anti-skating or tracking force because of the use of a record preservative. Even if such adjustments were necessary, they would be small and would require the use of very special test records and test equipment to make accurately.

Right and Wrong Angles

Q. I just mounted a cartridge in a turntable recently purchased and I noticed that the headshell, cartridge, and the stylus don't appear to be set up properly relative to the record. When viewed from the front, the stylus isn't at 90° to the disc, and the top of the headshell isn't parallel to the record's surface.

Is this normal? I thought that the stylus should be at a 90° angle when viewed from this perspective and that the headshell should be parallel to the record plane. Is this a defect?

-James Eggerman, Seattle, Wash.

A. It is true that the cartridge should be parallel to the surface of the disc or, to put it another way, the stylus should be 90° with respect to the surface of the disc when viewed from the angle you mention.

Apparently there is something wrong with the way the shell is mount ed inside the arm, or perhaps the shell itself was not made correctly. Try to gently turn the shell; if this doesn't work, then try a new one. If the new shell does not solve the problem, the arm will probably have to be replaced.

Failure to remedy this will result in added record wear and degraded channel separation.

(Audio magazine, May. 1982; Joseph Giovanelli )

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