Solid State Power Amp Reviews

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.

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.

...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.

... the dawn of the transistor era might have been the Sony TA-3120:
http://www.thevintageknob.org/sony-TA-3120.html