SIGNALS & NOISE (Letters to Editor) (Feb. 1989)

End of the Domain

Dear Editor:

I was sorry to see Ken Pohlmann's "Digital Domain" column come to an end (August 1988). I had read it faithfully from its very first appearance, when he boldly proclaimed that digital audio was the pinnacle achievement we'd been waiting for-or words to that effect. That statement did raise my hackles a bit, but I've kept reading and listening ever since! Of course, Pohlmann has other plans now, but I hope an emphasis on digital technology will be retained in Audio.

Pohlmann provided the foundations on which many of us came to understand what was happening in CD players, PCM video recorders, and other digital implementations. His "Digital Domain" column frequently found its way into my university lectures or into the outside readings I assigned students in my classes on analog and digital electronics.

I tend to agree that digital audio is, overall, the best thing that has happened in sound since the vinyl record. I am anxiously waiting for R-DAT recorders to come down in price. I'll be the first on my block to own one! And to Ken Pohlmann, sincere thanks for many stimulating and informative columns in Audio.

-Edwin A. Karlow, Chairman, Dept. of Physics. Loma Linda University Riverside, Cal.

Informal History, Formal Appreciation

Dear Editor:

I wish to thank you for your article, "An Informal History of Solid-State Amps," which appeared in the June 1988 issue. As a C.E.T. who repairs stereo equipment and sees the actual components daily, I enjoy articles which really explain circuit design. Perhaps in the future you could include an article explaining the circuit differences between the "generations" of CD players.

-Ross Rabuck Anaheim, Cal.

Amplifying History

Dear Editor:

I read with great interest author John Bishop's article, "An Informal History of Car Amps" (May 1988). However, I must disagree with his statement that no tuner/cassette car deck utilizing true preamp-level outputs existed in the late '70s.

In July of 1979, I became the proud owner of an Alpine 7307 tuner/cassette deck augmented by a Model 3002 power amp. The 7307 did indeed have true preamp-level outputs! Speakers couldn't be driven from this unit (no speaker output terminals were provided), as it was specifically designed to drive an external power amp, such as the 3002.

- Larry Hurst Salt Lake City, Utah

Author's Reply:

The Alpine 7307 was indeed an early indicator of the direction head units would take in the 1980s. It was, however, still a wonderful candidate for the signal-tapping procedure mentioned in my article. In fact, I produced a technical bulletin at Audio-Mobile describing a modification and preamplifier-bypass procedure for improving the performance of the 7307. The tape section's frequency response was limited at the low end to about 100 Hz by an undersized coupling capacitor at the Dolby IC's output buffer. Replacing the 1-uF cap with a 10-uF cap improved response down to the 30-Hz region. This output buffer stage could then directly drive an outboard preamp with a line-level signal having flatter response and lower noise and distortion than the inboard preamp section. A later-generation 7307 incorporated the capacitor of larger value after about 1980-81. Alpine was indeed an innovator in the world of car-stereo head units in the early 1980s.

There were, of course, other exceptions to my "late '70s" scenario. As early as 1976, the Nakamichi/ADS 2002 system used a preamp-output Nakamichi 250 tape player with Dolby B noise reduction and standard and chrome tape equalization. (ADS's contribution was a self-powered loudspeaker system.) Concord also had a number of in-dash cassette/tuners with preamp output before Alpine of America came on the scene. And Audio-Mobile marketed its short-lived Model ST770 cassette/tuner in 1978 and '79.

-John R. Bishop

Amplifier Classics and Classes

Dear Editor:

Just about every year, in recent years, I decide not to renew my subscription to Audio. Then along comes an article like "An Informal History of Solid-State Amps" (June 1988), and I'm good for another 12 months. The survey was excellent, if too abbreviated. Perhaps Sweeney and Mantz might have discussed the influence of IC-operational amplifier design on the input and gain stages of audio power amps.

As one who has built various kinds of audio amps and preamps over the years-starting in 1950 with vacuum tube types and continuing through what the authors term the classic era of transistor designs (Meyer, Bongiorno, et al.). I have drawn some conclusions about design versus audible qualities. These may be of special interest to that neglected, and possibly negligible, class of Audio readers who are inclined toward the "low end," out of necessity, opinion, or just old-fashioned thriftiness.

Second-generation amplifiers, as defined by Sweeney and Mantz, were a marked improvement over the first.

But for my ears, the "classic" full complementary design did not add anything in sound quality to what might be called a refinement of the second generation-complementary output, single-ended driver, and single-differential input. There was no cascoding, but the second stage had an active, constant-current load and the input had an active, constant-current source. (As far as I know, that was a Daniel Meyer design, possibly adapted from then current IC op-amp configurations.) Of course, amplifier configurations can be compared only if they use the same power supply and, consequently, have the same output rating. This is almost impossible to do with commercial examples, because, invariably, the manufacturer provides his more deluxe and complicated circuitry with more power output.

As for the qualities of later-generation amps, I have no direct knowledge, but I do have an opinion that applies to all amplifiers: More power means that greater flexibility in the selection of speakers is possible, and that is an advantage. Every other improvement is of doubtful advantage if one's present amp has little or no crossover distortion, low THD (perhaps less than 0.1%), and a full-power bandwidth that covers the range from 20 Hz to 20 kHz.

In addition, the amplifier should show no trace of instability with any value of capacitive loading up to 1 or 2µF. I never compared Class-A and Class-AB transistor amplifiers, but I have increased output bias in a Class AB amp to the point of thermal runaway. This provided Class-A bias up to a couple of watts of output, which is adequate for listening to string trios played on fairly efficient loudspeakers. No difference in sound quality was observed.

There are some amplifier changes that could make a difference detectable to us low enders (and have for me). The easy one first: Get an accurately compensated phono preamp if your present one deviates from RIAA specs by more than, say, 1 dB in the range from 30 Hz to 15 kHz. Current amps in all but the lowest price class probably meet this criterion, but some older models may not. The second change may not be as easy: Eliminate all speaker crossover networks and provide each woofer, midrange, and tweeter with its own amplifier (properly filtered at the input). This change is most practical, not to mention cost efficient, if you build your own speaker system and your own amplifiers. Some commercial speakers also allow the possibility of bi-, tri-, or multi-amping.

This is old advice to be sure, but it bears repeating: You will get the most improved sound for your money by buying better quality speakers and phono cartridges (or a CD player) rather than new and better amps or preamps. Until you win the grand prize in your state's lottery, keep that 50watt/channel Radio Shack receiver of any post-1970 generation and put your bucks where they will do the most good. However, because of your limited amplifier power, you should purchase quality speakers of moderate to high efficiency. They doubtless exist.

Then, when time and your talents (or the lottery) have made you the kind of upscale consumer advertisers love, consider that $1,000 amp. Chances are great that it will make your system sound better to you. Furthermore, you will now be able to participate confidently in learned discussions concerning the influence of oxygen in your wire and cable and Teflon in your capacitors. Enjoy!

- Lawrence Wallcave; Santa Rosa, Cal.

Editor's Notes:

Because Bert Whyte is recuperating from minor surgery, "Behind the Scenes" does not appear this month.

In the December 1988 "What's New," we stated that an Eclipse car stereo does not have auto-reverse; the Model ECE-101 does have it.

(Source: Audio magazine, Feb. 1989)

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