Dynaco ST-410 Amp

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I am considering purchasing a Dynaco ST-410 200 wpc amplifier to run my Acoustat Spectra 1100s. I am not an expert on amps and I haven't found too many reviews of this amp. Has anyone ever used this amp? Is it a good amplifier? The price would be under $200, and I was wondering if this would be a good purchase?


The ST-400, ST-410 have only four outputs in series.The maximum rated Ic=16A and the Beta=15~60@8A .The RMS I^2=32 therefore the amp may put out 64W at 2 ohms, 128W at 4 ohms or 256W at 8 ohms before getting into trouble.The ST-416 has double the number of outputs and therefore can do four times the power into low impedance loads.A DH-500 can do RMS I^2=288, or 576W at 2 ohms, over twice what a ST-416 can do.Electrostats can and do drop down to 2 ohms at high frequency.One could install Semelab BUZ900D/905D in a ST-410 and get an RMS I^2=511 for about $120.This would involve re-wiring the output stage to parallel with the FETs.
Dynaco (Dynasaur) ST-410

I also have a Dynaco St410 (I actually have 3 and a ST416 that is a basketcase)and a pair of Acoustat 1100's. I have recently aquired a Dunlap-Clarke Dreadnaught 1000 but, it needs repair and I am trying to aquire information to repair it. So that leaves me with the Dyna's. My plans for the Dynaco is to double the power supply caps from 10,000mfd. to 20,000mfd. Change the driver transistors from TIP41C/42C to MJE15030/31, and change the outputs from 2N5630/6030 to MJ21193/94. My choices in transistors is based on posts that I have read in this forum and the SAE forum on Yahoo. I have read an article in Audio Amateur about adding servo control to the amplifier ,why? why not? I aslo plan to use the MJ21193/94 devices in the Dunlap-Clarke and an Adcom GFA1. Now back to the Dyna. Can I expect to see an improvement in low impeadence loads with the before mentioned changes to the Dyna? What kind of load can 2 pair of MJ21193/94 devices handle? Same question for 4 pair that would be used in the Dyna 416 and Dunlap-Clarke? Will the protection circuit in the Dyna 410 prevent the amplifier deliver the current required to drive a low impedience speaker? I have seen a post sugesting parallel outputs for the Dyna with the MJ21193/94, why? This is a learning experience for me. Please show me the math. I have built kits in the past but my electronics skill and knowledge doesn't go much farther than a basic electricity class in college, a life long curiosity about what is in the box, and how does it work.
I used to test the driver boards for the "Black Box 410". Probably tested over a 1000 of them. I hated the circuit, for no good reason. It was developed before there were good high voltage, high current BJT's. I don't see much advantage in changing outputs to the mj21xx.
Dynaco ST-410's

I am a life-long fan of Dynaco gear. However, Dyna's large solid-state amps were fires waiting to start. I blew up two 410's in a week while trying to drive a pair of Infinity RS IIIB's. The 416 was better, but still...

If you want a Hafler designed amp, ditch the Dynas and go with a Hafler DH 500. They are truly bullet-proof. The 500's benefit from larger caps in the P/S, but they are about as stable as big amps get. They sound pretty good too, especially if you upgrade some of the caps on the driver boards and do a few other basic updates. Do that and you will never hear the distinctive "WHAP!" a Dyna amp makes when it dies, or smell the unforgetably pungent odor of smoked electronics.
There is no SOA (safe operating area) on the central Semi datasheet for 2n5630. However using Imax in Class AB amp service is a recipe for disaster. Imax is for saturated switch service, 16 amps at about 1.5 Vce. Amp output transistors don't go that low in voltage.
Andrew has said using about a third or less of the rated wattage of a transistor is usually advisable. So 200 Pd, 1/3 is 66 w per transistor. I don't know the rail voltage of a ST410 but back up from there, split rail voltage then (V^2)/Z speaker tells you how much you can dissipate per transistor.
Then convert to amps.
5 amp transistors were a later generation, MJ15003 etc. 2n3055 was about 2.6 amps SOA according to Thompson datasheet. 2n5630 is probably somewhere in the middle.
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Per the manual, Dynaco specified the available power output for the ST-410 as 300 watts continuous average per channel at 4 ohms. Interestingly Hafler didn’t appear to offer a 4 ohm power specification for the DH-500, at least not that I could find.

Looking at the Motorola data sheet for the 2N5630 and 2N6030, the maximum collector current rating is 16A continuous and 20A peak. The required current for 300 watts at 4 ohms is 8.7A RMS. Appears to be plenty of margin.

Just to verify the numbers, I put my vintage ST-410 on the bench and fired it up with a 4 ohm load to see what it would do.

Result: over 300 watts per channel at the onset of clipping with less than 1% distortion, both channels driven (see attached pictures). Note that at these output levels the amp was pulling nearly 12A from the wall, and I was fighting to maintain 117 volts at the outlet. With one channel driven, I measured over 330 watts without clipping before I backed off for fear of blowing a fuse. The amp worked fine without any problems during these tests, with the only noticeable change being the internal fan occasionally switching to high speed mode every couple of minutes and promptly switching off again as the heat sinks cooled down.

Note that this is a bone stock ST-410, nearly 40 years old, with all original caps, transistors, etc. No doubt a new unit would have performed better back in the day.

Over the years I’ve used this amp to drive Magnepan 2167’s, Dahlquist DQ-10’s, and Martin Logan Sequel II’s without a hiccup. Occasionally the fan would switch to high-speed mode during heavy demand periods (as it did on the bench) but the amp has never let me down.

Bottom line: not sure how you justify the “128W at 4 ohms” statement but real-world data suggest that an ST-410 can reliably provide a bit more than that.


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"Appears to be plenty of margin."

The gain drops hard above 8A.

Crown uses the MJ15015/16 in many of their amps. The driver transistors fail at higher power as the Beta droops on the outputs.

I was a Dynaco dealer back in the day, I got tired of fixing them when installed in a club driving 4Ω.

"Hafler didn’t appear to offer a 4 ohm power specification for the DH-500"

Nominal, 400W/4Ω, the power peaked around 600W/3Ω, went down at 2Ω.
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