Dynaco Stereo 120 transistor power amplifier Manufacturer's Comment

Manufacturer's Comment
The Stereophile's subjective observations point up the importance of listening critically to any proposed amplifier-speaker combination because there may possibly be subtle differences which are significant to the perfectionist. However, subjective impressions are influenced by listening environment and other variables, and rarely reflect reproducible concurrence by different listeners. We have expended considerable effort in trying to formulate test procedures which equate general listening impressions with measurements, with some success.

Conventional specifications are important (there has never been a good amplifier which did not measure well by these standards), but there are other criteria whose results are quite difficult to measure. They help to separate the superior from the "almosts," and these criteria may affect subjective impressions. We do not have the reservations about using the Stereo 120 with the KLH Model Nine that The Stereophile has (and we note the observation that this was at least partly dependent on source limitations), and can only suggest that anyone contemplating this expenditure make such comparisons for himself, but we have always contended that the Stereo 70 is an ideal amplifier for that speaker.

Any good transistor amplifier should include protective circuitry to guard against the likelihood of trouble with unconventional loads. While tube amplifiers are much more tolerant of abuse than are transistor amplifiers, even they are not immune to damage. The Stereo 120 could have been released much earlier if we had not considered full protection to be important. Its protective circuitry will reduce the power output or shut down the amplifier if it encounters a severe overload, with instant restoration when the overload is removed. Although The Stereophile has found this protection fully adequate, the surest safeguard is avoidance of abuse. Undue activation of a safety device is like testing a parachute; it works fine almost 100% of the time, but most of us prefer not to put it to the test.—David Hafler

Reviewer's Addendum
For those who can stretch their finances that far, a speaker setup comprised of two complete KLH Nines (four panels, with each pair coupled together) will provide the most nearly-perfect reproduction of sound that it is possible to get from commercially available components, regardless of price. With each pair connected in parallel, 8-ohm impedance would provide a nominally correct match to the Stereo 120, and would allow full utilization of its power output capabilities.—J. Gordon Holt

Dynaco, a division of Radial Engineering Ltd.
1588 Kebet Way
Port Coquitlam, BC
Canada V3C 5M5
(604) 942-1001

partain's picture

I paired it with the Dynaco PAT-4 preamp , which , as I recall , was not as good as the previous tube models. Large Advent speakers , AR turntable , Shure cartridge.
I could have done worse.

dc_bruce's picture

It's funny to consider that, back in it's day, this was a high powered amp. I heard this demoed with a pair of AR 3as in 1968. I recall the combination as being "forceful"-sounding, not necessarily pleasant or natural. The wisdom at the time was that the better sounding amplifier was the Stereo 80, which differed from the 120 only in having a less stiff power supply. Dyna put the Stereo 80 in the same box with the PAT-4 preamplifier, which i did own and sounded very good with my AR 2ax's -- an older but better sounding speaker than the AR3a, albeit with a little less bass extension. The "acoustic suspension" woofers of AR and similar speakers were a bit under damped, so, as a general matter, the transistor amps gave a better bass quality.
Having owned a Stereo 70, before I got my SCA-80 integrated, I can't agree with the sainted JGH that there was little sonic difference between the Stereo 70's tubes and the Stereo 120's transistors. In my system at the time, replacing the Stereo 70 with the nominally 5 wpch more powerful transistor amp gave better bass and a more extended top end . . . pretty much what you would expect when moving from a classic tube amp to solid state.

Hafler's accomplishment with the Stereo 120 was not only building a transistor amp that sounded better than its contemporaries, as JGH notes, but also one that would not self-destruct as so many did at the time.

jmsent's picture

...I can tell you from personal experience that the Stereo 120 was anything but a reliable amplifier. I serviced many of them. The earliest ones used the ubiquitous 2N3055 output transistor and some wimpy TO-5 driver transistors. Not only were there failures, but also serious design problems causing high frequency oscillation under certain load conditions. If the series pass regulator failed (common), the B+ voltage shot up close to 100 volts, and then both channels became highly prone to failure. Dynaco made many production changes, finally changing out all of the transistors for more robust types. This was the infamous "TIP mod" from the early1970's. The ST 120 was Dave O'Brian's favorite amp to test at the McIntosh clinics, because it truly demonstrated the huge difference between an amp with conservative specs and one with "optimistic specs" It measured horribly, and as often as not, didn't come close to meeting its published specs. We always got a few ST120 trade-ins off one of those clinics. To be fair, a 1966 solid state amplifier is a pretty early design, and Ed Laurent, the designer of this amp, pretty much pulled it right out of the RCA transistor application manual . At least it used silicon transistors and not germanium. My biggest issue with it is that the basic design had become obsolete within a couple of years of its introduction, yet Dyna continued to sell it well into the 1970's. By that time, most run of the mill Japanese receivers were thoroughly outperforming it.

Ortofan's picture

... the dawn of the transistor era might have been the Sony TA-3120: