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Re: Dynaco SS, the early days (long)

162.40.174.95

If I remember correctly, Dyna had the RCA folks design the Stereo 120. I can remember a lot of questions to Dyna as to when they would have a transistor amp and preamp. Their answer was they would have one when they could assure their customers of the same value and quality they were getting in the tube gear. When the ST-120 came out it was hailed by many as the greatest thing since sliced bread. 60 W/ch, regulated power supply, and zero dynamic headroom. I had a PAS-3x at the time driving a pair of MK II's. One of my frineds had a ST-120, so naturally we compared them on his A-25's and my KLH 17's using his PAT-4. Then we sat around conjecturing how two amps could sound so different. We did frequency response measurements on the two amps and both were flat 20-20,000, with low distortion.

Later the Stereo 80 came out as a lower cost alternative to the Stereo 120. Pretty much the same circuit, except no regulated supply, and no AC feedback around the output coupling cap. Dyna advertised a damping factor of 50 at 50 Hz, but with no AC feedback, there is no way to get that low a source impedance. So the low frequency DF of the ST-80 is pretty low. The ST-80 did have some dynamic headroom, so on music, it clipped at around 60 W/ch. About the same as the 120. If you over drove a 120, the thing cut off for a second or so until the peak passed and the circuit recovered. To compensate for the lack of AC feedback, the ST-80 used a larger coupling cap (5000 mF) than the ST-120 (3500 mF). Since the circuits are so similar, you can modify an ST-80 (or the SCA-80 or 80Q) to have AC feedback. This does tighten the bass somewhat and makes the ST-80 as good as the ST-120. Which is probably why Dyna didn't use AC feedback on the ST-80, it would undercut the 120.

Years ago I took two ST-80's that had one dead channel each, and rebuilt them as mono blocs. That doubled the heatsink capacity (one transistor per heat sink instead of two) and improved the power supply a lot. Since the output coupling caps are the same model used for the power supply filter cap, and there was now an extra cap per amplifier, I doubled the capitance in the power supply. I also modded the circuit for AC feedback. These worked very well, with much cleaner bass and a legitimate 50 W/ch rating into 8 ohms and 70 W/ch into 4 ohms both with good headroom. And being dual mono, I now had much better imaging. Bass was still a little "wooly", but not bad. I used these for the mids and highs in a bi-amp set up.

A friend had an ST-120 and sent it off to Jensen's Stereo Shop, where Frank Van Alstine put in his circuit boards and redid the power supply to be unregulated. This was driving a pair of Rogers LS3/5a's and sounded very good in a bi-amped set up with a pair of 12" subs. I kept the original ST-120 PC boards, and built up a stereo power amp in a larger chassis with a much larger power transformer and massive heat sinks from an Acoustech 1 power amp that had blown up. That worked really well, too.

While the power amps definitely had their problems, the basic circuits were not all that bad for the era. In fact, they were pretty typical of the capacitively coupled, single ended power supply, quasi-complementary symmetry designs that everyone was using, including the early Japanese SS amps.

The real problem with early Dyna SS stuff was the PAT-4. The PAS-3x was a decent preamp, but being a tube design, it didn't have a low enough output impedance to drive most SS amps. So a solid state preamp was needed, hence the PAT-4. This was a really primitive design and to me it looked like it was rushed to market. I don't know of anyone who liked the sound of these.

As I see this era of Dynaco, they waited too long to get into solid state, and by the time they did, there were low cost Japanese receivers starting to come into the country. Low enough that the cost of kits was only a little better (if any) than a wired receiver by the time you had a stereo amp, preamp, and tuner. So cost cutting was a major issue for Dyna and they had undersized power supplies and heat sinks. The later Dyna SS stuff (ST-150, 400, 416, PAT-5) was really pretty good, but by that time, the Fair Trade laws had been repealed, the Japanese stuff was now much lower in cost, had good quality, and it looked better. Besides, you could get a Phase Linear 700 for almost the same price. Eico and Heath got caught the same way.

If you think about it, the repeal of the Fair Trade laws really torpedoed the US industry, and opened the way for the mass market electronics stores. The independents really could not compete with the volume outfits, once discounting was allowed. So today, we're left with Circuit City and Best Buy, and a bunch of Audio Boutiques.

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