Audioclinic (Mar. 1984)

Reader Response:

Power Amps and Speaker Impedance

I feel that you did not do justice, and may have added confusion to, the question asked by Bob Robinson in the "Audioclinic" column in the March 1983 issue.

Although it is true that maximum power transfer occurs when impedances match (amplifiers and speakers rarely do), the answer would have been much easier to understand if it had been explained in terms of Ohm's Law, and its related power lines:

E=IR;P=I^2R=E^2R.

The amplifier's output impedance can be neglected because it is much lower than that of the speaker.

My explanation would have been along the following lines:

The amplifier will, at any given volume setting and program source, produce a voltage (E) across the speaker load (R), with resultant power being P = I x E, where I is the current determined by I = E/R.

Reducing the speaker impedance by half, and given the same program and volume setting, the current (I) will double, and, as a result, the power delivered to the load will double.

- Albert Reichel, Kent, Wash.

I was a bit surprised by an answer to a question about load impedance and amplifier power output which appeared in the March 1983 issue.

It is true that, when loads are matched to the inherent output impedance of a source, maximum power transfer occurs. But in order to achieve the high damping factors desirable for driving a loudspeaker, an amplifier's dynamic source impedance must be considerably lower than the loudspeaker's impedance, especially at bass frequencies. In this respect, a modern solid-state amplifier looks like a voltage source to the speaker. This explains why amplifier power ratings are higher for lower load impedances (which by the way, was not the case in the days of vacuum-tube amps and impedance-matching transformers). It is generally true that, for solid-state amplifiers, lowering the load impedance will allow the amplifier to deliver more power. If one could do this and maintain loudspeaker efficiency, the acoustic output would therefore rise, so that a listener would turn down the volume. This would lower the power output from the amplifier to an acceptable level, and, of course, prevent burnout. Even if one does not turn down the volume, the maximum power an amplifier will deliver will be limited to the maximum signal level that it can produce under steady-state conditions. As long as one can tolerate the acoustic intensity, any well-designed amplifier will be sufficiently ventilated to deliver its rated output under steady-state conditions without burning up. That statement must hold regardless of the load impedance, because the maximum power delivered will be subject to the limitations described above

-Edwin A. Karlow, Riverside, Cal.

Receiving Weak AM Signals

Q. My problem is that my receiver is weak in "pulling in" AM stations. Is there anything I can do to improve this condition so that I can hear distant stations?

-Name withheld

A. The AM sections of most receivers are little more than portable radios. If you live in a structure which has a lot of metal, this will attenuate the signals to a point where they cannot be received well, and the situation is not helped at all by the poor front-end performance of many receivers. Their performance is adequate for local signals.

Weak signals can often be received by using an external antenna, consisting of little more than a piece of wire perhaps 20 feet long, insulated from the structure in which you live. Some receivers have no provisions for connecting such an antenna. Even where such provisions exist, the addition of the antenna broadens the selectivity of the front-end, which introduces images and whistles which add further problems to weak-signal reception. As a first thought, give this a try and see what happens.

The alternative is to get a communications receiver designed for weak signal reception and which covers the broadcast band. It has good i.f. selectivity, a very important element in rejecting adjacent-channel signals.

Your only problem with receiving any given station is whether or not it is on a clear channel. You may want to experiment with some kind of directional antenna for this receiver. Try winding a loop of many turns and connecting this between the antenna terminals. Make the loop about 3 feet in diameter, and mount it well clear of surrounding objects. Provide some way of rotating the loop. This loop will be a bidirectional device and, hence, not completely satisfactory. For unidirectional applications, it will be necessary to erect a vertical antenna and phase it with the loop so the system can be calibrated in terms of compass points. This arrangement is the basis of the direction-finding apparatus used for certain types of marine navigation.

Firms which manufacture such equipment can perhaps supply the loop and vertical antenna assemblies suitable for your applications.

A communications receiver includes items which can be useful, such as a notch filter. (This filter rejects some of the odd whistles which often ruin distant-signal reception.) Further, such receivers are equipped for short-wave listening. If you have not experienced this, you may well find it interesting.

Playing Vertically Cut Discs

I noticed your recommendation to readers in the March 1983 issue about playing laterally and vertically cut records. I'd like to tell you about something that I stumbled into some time ago about "vertical" grooves.

The idea of reverse-phasing stereo cartridges presumes that the grooves are truly cut on the perpendicular, because the two outputs are converged at unity. It seems, however, that many, if not most cylinders and discs were cut on some diagonal plane. Therefore, optimal signal-to-noise ratio is not achieved on many such recordings by wiring the cartridge as you have described.

What I have been doing is wiring the cartridge reversed, but mixing in a different manner. I pre-amplify the two outputs independently but identically. I then mix them, via a two-in/one-out mixer. Either input is set to nominally correct line level. The other is then adjusted until there is a maximum noise null. The settings are rarely at unity gain and vary from record to record.

This scheme also works for the short-lived "compatible" discs, such as Emersons, which ostensibly could be played on lateral or vertical reproducers. The process resulted in better S/N ratios with much less filtering required. This method was also applied to the Edison "Kinetophone" synchronous soundtrack cylinders which I have been transferring, along with their matching films, to videotape.

-Art Shifrin, Douglaston, N.Y.

FM Pre-Emphasis

Q. Can you tell me why FM transmission requires pre-emphasis and why reception calls for de-emphasis (75 µS in both instances)?

-Michael D. McCormick, Tampa, Fla.

A. FM could have been left strictly alone, with neither pre-emphasis during transmission nor de-emphasis during reception. The received frequency response would be flat.

The problem is that the noise present is audible mainly at high frequencies. If these frequencies are boosted during transmission and correspondingly lowered when the signal is received, the noise is reduced, at high frequencies, by the amount of de-emphasis.

The pre-emphasis used in the U.S. for non-Dolby broadcasts is 75 µS, which corresponds to a boost of about 13 dB at 10 kHz. For broadcasts using Dolby NR, a milder pre-emphasis of 25 µS is used. When played back with normal de-emphasis and without NR, this compensates for the high-frequency boost otherwise heard from undecoded Dolby signals.

The reason pre-emphasis was considered possible by those who set standards for FM broadcasting was that highs found in typical program material are low in spectral distribution.

Boosting them, therefore, will not create over-deviation at high frequencies or excessively wide sidebands which could be a source of interference to adjacent-channel services. As time has passed, more and more high-frequency content has been introduced into program material. To compensate for this, it is often necessary for the station to use a high-frequency limiter to reduce the level of such frequencies, at least on peaks.

FM Signal Dropout

Q. About a year ago, a friend and I each purchased stereo receivers of the same make and model. Since then, we have both noticed, with the receiver set to the FM mode, that the volume of sound will suddenly decrease dramatically. This has happened on all the stations I listen to. We both live in a rural area. Would that make a difference?

- Josh Jaeger, Iowa City. Iowa

A. I wonder if the mysterious drop in volume has to do with your being located so far from the stations to which you listen that their strength is influenced by atmospheric conditions. If I am correct about this, you will experience most of these difficulties during the spring and fall, while winter is the least affected time of the year.

You did state that you experience drops in volume, but I hope this more correctly translates into loss of signal strength. Normally, of course, changes in signal strength, although creating added noise, do really cause a drop in volume level. I have no other ideas at this time, except for possible defective components in the receivers. It just seems too much of a coincidence that you both should be plagued by the same problems.

Improved Record Playback

Q. I read that sonic quality in creases during the first few "plays" of a disc and then declines. Is this true?

-Rudi Schmid, Kensington, Cal.

A. I do not believe sonic quality improves during the first few plays of a disc except in one respect: The noise background will sometimes be reduced as a result of the polishing of the groove walls by the playback stylus.

(adapted from Audio magazine, Mar. 1984; JOSEPH GIOVANELLI)

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