How do we remember 1970s amplifiers?


I would be curious to hear some of the memories and impressions associated with the following short list of 1970s amplifiers:

- McIntosh "first generation" SS amps, MC2105, MC2505, MC2300, MC250, MC2100
- Dynaco Stereo 400 and Stereo 120
- Phase Linear 400 and 700
- Bang & Olufsen "slide rule" receivers (i.e. especially blackface Beomaster 4000)
- Original Ampzilla (not Son of Ampzilla)

I've chosen this list mainly because they cover a wide range of approaches to solving the issues of early semiconductor technology, and they were all pretty mainstream products in the U.S. I'm excluding the Japanese receivers/amps not out of predjudice; it's simply that the circuit designs varied quite a bit with each model, and thus harder to broadly classify their characteristics.

I'm interested in impressions of both sonic and non-sonic attributes, and a preferred ranking of the above, if you like.
kirkus

Showing 6 responses by kirkus

Wow, I'm really enjoying everybody's comments, thank you. I particularly like the venomous reactions to the Phase Linear amps . . . as I think it's in so many ways the poster child of 1970s solid-state amplification.
Yet, it's earlier repuattion does not appear to have survived time. Just a question: why is that? I admit that I have not critically listened to Crown gear recently nor compared it to my current gear. Perhaps some of the oldheads out there can tell me why the old vintage Crown gear has fallen out of favor.
Bifwynne, these sorts of questions are what I had in mind to discuss when I started the thread . . . and I'd like to try as much as possible to avoid the subject of NFB, as it has a way of hijacking threads. Specifically, I think the "1970s transistor sound" is related much more to poor linearity in several key circuit areas, principally the "quasi-complementary" (all-NPN) output stage and its inescapable notch distortion.

But the Crown gear is an excellent subject, especially as it compares to Phase Linear . . . I have my suspicions that Bob Carver actually derived his designs from the Crown DC-300 schematic. They both use quasi-complementary output and driver stages, although there's a subtlety in the Crown design that's not immediately obvious: the drivers operate in Class A and in parallel with the output stage through the crossover region, smoothing out the notch distortion to a significant degree. It also makes the whole thing far less temperature sensitive for its bias conditions.

Although I don't know the exact chronology, I find it interesting that when Crown added an opamp front-end to the DC-300A (and D150) that Phase Linear followed suit by doing the same thing with the 700 Series II and 400 Series II. But Carver's amps still show the earmarks of a cheap copy, especially how he cut costs significantly by simply using an electrolytic to bootstrap the voltage-amp load, rather than the true current-source transistor powered off a separate supply rail in the Crown design. Carver was also obviously having stability issues with all these amps, as shown by the ferrite beads and picofarad-value capacitors "liberally sprinkled throughout the schematic" (to borrow an expression from Douglas Self).

All in all, from today's perspective both are dated designs and have a couple of the same flaws, but the Crown is a thoroughly well-optimized and beautifully engineered amplifier, and I think this shows in its sonics and its remarkable reliability. I'd say that with easier loudspeaker impedances, one could still do much worse than a DC-300 today.
Lacee, thanks for bringing up the little 20W NAD . . . this is the perfect 1980s equivalent of the Phase Linear amps. I serviced these back in the day, and even had the "Power Envelope" decendent (3225PE?) as a test-bench amplifier for several years. In contrast to the big, "all technical" 1970s aesthetic, it was small, lightweight, low-powered, and simple-looking. But like the Phase Linear, it was cheaply made and unreliable . . . and the sonic signiture was filled with exactly the opposite types of problems.

I will say that the preamp sections were pretty decent, maybe even good considering the low price . . . but the power-amp section was an absolutely atrocious piece of engineering. The key "feature" was that it didn't have emitter resistors on the output transistors, and I assume that this was an attempt to reduce notch distortion. But even if this had been a good idea (which it wasn't) . . . they then chose the absolutely cheapeast big power transistors available (2N3055/MJ2955) that have poor linearity even by 1970s standards. The result is that even with enough idle current to make it run pretty toasty, it was still really operating in underbiased Class B, and put out a whole slew of nasty harmonic and intermodulation distortion products. This may have been "redeemed" somewhat by the single-transistor input stage, which guaranteed a big helping of even-order distortion as a big sugary coating for that rancid-piece-of-meat of an output stage.
Not as good or as pleasing a sound as the tube stuff, but still better than the solid state amps I had been using in the 1970s.

This is the reason I find the new fascination for 70's solid state stuff so perplexing.
From someone who was there and listened to that stuff when it was new and all the rage, and then moving on to better things,I just can't see the interest in any of it.
Well, selective memory seems to work both ways . . . we overwhelmingly shun what was in vogue in some eras, and romanticize others. One of the reasons why I mentioned the 1970s B&O receivers was because (like the NAD amps of the 1980s) they offered a modestly-powered, domestically sensible alternative to the "high knob-per-dollar" mainstream aesthetic. They were vastly better engineered than either the Phase Linear or the NAD, but had a few quirks of their own (esp. cultural?) . . . I'm wondering whether they're remembered similarly to some of the more high-profile [sic] pieces of the decade.
SAE was also founded by James Bongiorno who founded G.A.S. and also co-designed the Dynaco 400.
I thought Bongiorno was just one of several people who designed different SAE products (not one of the founders), but I may be wrong. I had forgotten his connection to the Stereo 400; thank you.

One thing about the Dyna 400 and the GAS products was that they didn't skimp on the quality of the silicon . . . these were all fully-complementary output stages that used the transistors' characteristics very effectively. The GAS amps extended the complementary-pair thinking through to the input and voltage-amp stages, which IMO deserves respect for its conceptual elegance, even though there are some fundamental problems with its implementation. This fully-differential approach is also a hallmark of many of John Curl's designs, though I would hesitate to imply that the Levinson designs bore influence from the GAS.

But the Ampzilla was hugely influential . . . it did start out as a project amplifier with the schematic published in a magazine, and like the Williamson amp published 25 years or so before, bits of it seem to turn up everywhere. The current crop of McIntosh SS amps (from approx. the mid-1990s) use a front-end that strongly resembles Ampzilla's, with a few refinements.
Currently my #1 power amp is just past the '70s--the 1981 Heathkit AA-1600. I got it used for $239 and it makes my jaw drop. Just two years into the '80s, it is so far beyond the amps of the '70s. It reminds me a lot of the 1990-ish Jeff Rowland Design Group amps.
I remember these from the Heathkit catalogs, and dug up a schematic . . . and from the way it looks on paper, I'm not surprised that you like the way it sounds. It's kinda like a simplified Dynaco 400, but with the biggest flaws fixed (esp. the intermediate stage with its current-mirror).
Lacee, you're probably right that I was a bit too harsh on the 3020 . . . NAD's manufacturing quality steadily declined through the 1980s, then plummeted under the ownership of KH America in the early-1990s . . . and this is when I serviced them. It seemed that the late-1980s stuff failed at least twice as often as that from the early-1980s, even though they were newer. The current product of the time (I'm thinking of the 505 and 705) had out-of-box failure rates of at least 25%.

The 3020 was indeed cheap and it did have a nice phono preamp, this was probably the best part. And after two failures in two years with my 3225PE bench amp, I added a pair of 0.33-ohm emitter resistors and switched to 2SC3281/2SA1302 output transistors, then re-biased. It completely transformed the performance . . . the distortion was reduced by something like 75% leaving overwhelmingly even-order products. The little NAD then sounded quite nice with my KEF C25 bench speakers . . . which I guess was exactly the kind of setup for which it was intended.

Still, I don't know whether to praise the design because it didn't take many changes to make it a nice little amp, or to be really annoyed because these changes would have cost less than $2.00 per unit in production . . . probably cheaper if you factor in volume pricing of the day and the better reliability during the warranty period. With the Phase Linear 700 it was far more clear-cut -- the circuit was wholly unsalvageable. The best thing for one of these is to use the case and heatsinks (but NOT the transformer) for a project amp.
Pubul57, aside from tons of repair and modification, my audio design experience revolves mostly around small-volume or one-off custom stuff for pro audio applications . . . various iterations of analog and mixed-signal "preamp"-type stuff. This has included mic preamps, electronic crossovers, long-line drivers and receivers, active bandwidth-limiting filters, small low-noise mixers, etc. etc., and various combinations of these building blocks combined into a single chassis. I used to get a lot of requests like "We need these PA systems to be at least 8dB quieter at the inputs to the mid- and tweet-amps, and get rid of that 'clippy sound' in the bass when it's really loud. Can you build something to do that?" And I would . . . and all kinds of similar stuff in FM broadcast and recording.

But I'm not an EE - my education is in classical music, and the way we learned about harmony, part-writing, counterpoint, form, etc. was to study the works of the masters (and the also-rans, too). I tend to take this approach to the study of audio design . . . since it's much more informative to analyze the harmony in Schumann's "Ich grolle nicht" than a silly textbook example, I'd rather listen to i.e. a DC-300 or whatever, then measure it and study its circuit design, than simply read about hypothetical circuits (and equations simply for their own sake) in a typical undergraduate EE text. The repair process is a natural method for analysis and measurement, and since I like to listen to music while I work . . . I've always kept a well-set-up amp/speaker system on my test bench, so I'm always trying to correlate what I hear to what I see and measure.

And I'm also generally a sucker for history, and love hearing/reading people's stories and narratives . . . hence this thread. Again, thanks to all for the contributions!
Eldartford brings up an interesting subject in that many of the amplifiers with more humble aspirations in the 1970s were capacitor-coupled . . . Johnnyb53 mentioned that his Altec integrated stereo had issues with reduced power output at 20Hz, which is a strong indicaion that it had an output capacitor. Most self-respecting "high-end" amplifiers, on the other hand, were DC coupled, possibly taking this to an extreme in the Kenwood L-07 that actually passes DC, and has DC gain.

Crown was proud of this in the late-1960s; I think that the DC-300 got its moniker because it was "DC Coupled". And nowadays, this power-supply configuration is standard, and an output capacitor is an anachronism . . . ah, progress. But wait a minute . . . a quick re-examination of the current path in a typical "DC coupled" power amplifier (including even the L-07M), and the speaker is still coupled via capacitors - it's just that now they've moved from the positive side of the speaker terminal to the negative, and we call them "power supply filter capacitors". If their only purpose was to smooth ripple, there could be just one tied between the + and - rails . . . but then there'd be no way to return the loudspeaker current to the supply. The power-transformer's center tap does this in the case of faults and on startup (and for extreme subsonic noise/content on the L-07M), but most power amps can in fact purr along nicely with this disconnected, and no DC ground for the power supply at all.

It's true that the "DC coupled" topology doesn't (or at least shouldn't) suffer from nonlinearities associated with the coupling/supply capacitors, the reason is that they are effectively inside the feedback loop, whereas in the "Capacitor coupled" topology they typically aren't. In the "DC coupled" scenario, there's still usually an electrolytic grounding the feedback loop (keeping DC gain to unity), and this can be a measureable source of distortion.

So that leaves me wondering a bit why it was cheaper to build a capacitor-coupled amp in the 1970s? The extra cost of front-end transistors is tiny. You still need two big electrolytics, or three for a stereo amp, so it's actually more expensive in that regard. There is a bit of savings in the lack of an output relay and protection circuit, but that doesn't seem like enough to offset the extra big capacitor.

This leads me to the conclusion that the real big chunk of cost was the talent to design an input stage with good offset characteristics, and perhaps the production technician to adjust the offset on every single amplifier, making up for the inconsistency in the semiconductors of the era. With an output capacitor, the whole thing is much more tolerant of both substandard parts and design mistakes. Now that transistor quality has improved dramatically, and there are plenty of existing designs to plagarize . . . those costs are much less significant for inexpensive systems, and capacitor-coupling has fallen by the wayside.
Some of those old designs were pretty good and while the art has moved forward on a lot of amp features (protection, thermal and short monitoring, metering) sound was quite good in many of them.
Indeed it was . . . and the idea of swapping in fancier parts to "improve" them to today's standards has some appeal. There's of course usually plenty of room for sonic improvement by simply correcting the ravages of 30-40 years of age, and technology and manufacturing of all electronic parts has moved forward innumerably in the past few decades. But the parts themselves are certainly NOT the real story, and the real improvement to be have comes from applying a modern design perspective to the older circuits, and making major or minor tweaks as needed. Some of them end up being pretty amazing just as they are, and others . . . well there simply aren't enough Black Gates and teflon caps in the world to help.