Dynaco Stereo 120 transistor power amplifier

One by one, the major amplifier manufacturers have acceded to the pressures of the marketplace and introduced "solid-state" models, whether or not these happened to sound as good as their previous tube-type units. Dynaco was one of the last of the hold outs, preferring, according to their advertisements, to wait until they could produce a solid-state unit that was at least as good as their best tube types. Now, they've taken the plunge at last, with their Stereo 120.

Price-wise and power-wise, the Stereo 120 is the equivalent of a pair of Dyna Mark IIIs, but there the comparison ends. The 120 occupies no more shelf space than a Dyna Stereo 70, it is only about ¾ the height, and it runs as cool as one of Dyna's preamps.

Its 100k ohm input impedance is compatible with the matching requirements of any standard American preamp with the exception of Dyna's own earlier PAM-1, PAS-2, and PAS-3 units, which required matching into 470k or, with a minor modification, into 250k. Conversion kits are available to adapt them to the new arrangement used in the current-model PAS-3X, which is uncritical of output impedance loading.

The Stereo 120 is designed to deliver its maximum power of about 65Wpc channel into a load of 6 ohms, which allows it to feed its rated 60Wpc into either 8 or 4 ohms. Practically all low-efficiency speaker systems are 8- or 4-ohm units, whereas 16-ohm systems (with at least one notable exception, to be mentioned later) vary from moderately to very highly efficient and, hence, will not require the amplifier's full power potential. With a 16-ohm load, the Stereo 120's power capability drops to around 35Wpc, but its low-level distortion characteristic—one of its most unusual aspects—remains essentially unchanged.

Every other transistorized amplifier we have encountered (and read test reports about) hits a minimum of distortion at between 1/3 and 1/2 rated power and then shows a distortion rise below that figure. The Stereo 120's low-level distortion curve is almost exactly like that of a top-grade tube amplifier. Our measurements showed IM distortion down to 0.1% at 1/3 power, and below the 0.06% residual of our IM meter at all levels below about 10 watts.

The above distortion measurements were made after we had listened to the amplifier at some length, in order not to prejudice our judgment. They confirmed what we heard. This is the only solid-state amplifier we've encountered that does not add a trace of roughness to the sound at low listening levels.

We compared the Stereo 120 with the best of the tube types we could round up: a pair of Dyna Mark IIIs (also 60Wpc), a Dyna Stereo 70, and a Marantz 8B, both of the latter rated at 35Wpc. Speakers used were a KLH Model Nine full-range electrostatic, Janszen Z-600 dynamic-woofer/electrostatic-tweeter system, Acoustic Research AR-3s, Bozak B-310s, and KLH Model Seventeens.

The KLH Nine is rated at 16 ohms impedance, the AR-3 at 4 ohms, and the others at 8 ohms. Hence, the Model Nine system, one of the lowest-efficiency systems available, was operated at a marked disadvantage to the others, in that it was getting only about half of the Stereo 120's rated power capability.

Here's what we found. On the KLH Nine, the Dyna Stereo 70, the Marantz 8B and the Dyna Stereo 120 sounded virtually identical except at levels where some peak clipping started to occur—ie, at levels approximating that of an orchestra crescendo, heard from a row-M seat. At marginal-overload levels, an interesting thing occurred. On program material that was free from high-frequency impulse distortion (as occurs when a phono pickup starts rattling in the groove), the Stereo 120 seemed able to produce slightly higher subjective levels than the other 35W amps without sounding offensive. Thus, in reproducing tapes, for instance, it seemed to have a shade more reserve power than either of the other amps, both of which are actually capable of delivering as much power through most of the audio band as the mismatched Stereo 120. When reproducing discs, though, slight amounts of groove breakup, representing relatively weak but very-high-frequency energy, caused the Stereo 120 to produce noticeably more annoying shatter from the KLH Nines at marginal overload levels than did either of the other 35W amplifiers.

Below the overload point, groove breakup sounded identical—that is, barely noticeable—from all three amps. In this respect, the Mark IIIs did better than any of the other amps, yielding a small but perceptible margin of reserve clean power that allowed us to move from row M to, say, row H without incurring significant peak overload. Bass, too, appeared to be a shade deeper and tighter when the Mark IIIs were used, but at output levels well within the capabilities of the 35-watters, the Marantz and the Dyna Stereo units had a somewhat more liquidly transparent sound.

We are uncertain as to why the Stereo 120 should behave as it did with the KLH Nine, but we're open to suggestions. Meanwhile, suffice it to say that there are other amps better suited for the Model Nine.

On the other speakers tested, it was quite a different story. Here the Stereo 120 was competing with the Mark IIIs on an equal-power basis, and at almost twice the power of the other two amps. Yet the differences noted could not possibly be explained purely in terms of power capability.

For instance, the Stereo 120 seemed able to elicit noticeably deeper, and tighter, bass from all cone-type woofers than did any of the tube amplifiers, although there was less difference between the 120 and the Mark III than between the 120 and the other amplifiers.

Outside of this, the differences noted between the Stereo 120 and the other amps seemed to depend on the complexity of the crossover networks used in the speakers. Except for the low-end improvement, the 120 through the Janszen Z-600 sounded exactly like a higher-powered Marantz 8B or Stereo 70, and was a shade more transparent than the Mark IIIs. The exaggeration of groove breakup at marginal overload was observed from the Janszen Z-600s, but was less pronounced than from the KLH Nines and, of course, occurred at a slightly higher listening level, allowing fairly relaxed listening at row-H levels.

On AR-3s and the big Bozaks, it was hardly any competition. The Stereo 120 seemed to smooth out the response of both of these, making them sound more natural than we've ever heard them. The fact that both are three-way systems, with crossovers, may have something to do with this, but Lord knows why. The KLH 17s seemed a little less forward and somewhat more detailed than before, but except for the low-end improvement, the difference here was slight. In the case of all the dynamic-type speakers, though, the net result was an improvement over the sound from the tube amplifiers.

We have observed much the same sort of improvement when dynamic-type speakers were used with other high-quality transistor amplifiers, but the tendency for those amps to add roughness to the sound at low levels, and to deteriorate over a period of time in normal use, made us hesitant about recommending them. We have no idea how the Stereo 120 will stand the test of time, but it is clear that Dynaco has gone to great pains to ensure that not even the most clod-headed user can damage the Stereo 120. We did our best to pop something, by running the unit continuously at full power and at overload levels, with shorted outputs, partially shorted outputs, open outputs, capacitive loads, and outputs in parallel. The amp made unhappy noises, and it warmed up quite a bit, but nothing got really hot, and nothing let go. If the components will stand the test of time, this should be an extremely dependable unit.

Construction appears to be quite simple and, though we didn't build ours from a kit, we got the impression it would be as easy to put together as any other Dyna power amp.

Summing up, then, we are finally forced to do an about-face on our long-held conviction that transistor amps are not for the perfectionist. Not only does this one seem to have no sound of its own, it also makes most loudspeakers sound better than do tube amplifiers. This kind of performance, finally, justifies switching from tubes to transistors. It's a sad commentary on the industry that the justification had to come three years after the switch.

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.

es347's picture

..and also paired with the PAT-4 preamp both in kit form. Speakers were AR-2ax’s...source was a Dual 1019 with Shure M91E cart. This was in 1969 so perhaps Dynaco had replaced the early transistors with better ones...never had a minutes trouble with either amp! Those were the days my friend, those were the days!

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: