Updating the Dynaco Stereo 120

[djoffe]

Bridged Mode Experiments

With a little tweaking, the updated version of the Stereo 120 will put out clean as a whistle 160 Watts into 8 Ohms running in bridged mode. It almost makes it to 200 Watts, but the power supply gets a bit unhappy, and the distortion rises a bit.

Here are some more details...

Bridge Mode Beast

 

[djoffe]

Dynaco Stereo 120 Power Supply Replacement Kit

There's now a power supply replacement/upgrade kit for the Stereo 120:

Power Supply Replacement

 

[djoffe]

Here's a link to some nice work updating a Stereo 120:

Refurbishing a Stereo 120


Update My Dynaco

 

[Bob Richards]

As a kid about 10 years old (1965?) I remember thinking the Dyna ST120 was a very cool looking amp. I all but drooled over it. Now as a seasoned audio circuit engineer, I have to laugh. My brother got one and it blew itself up by the end of the first song, playing a little above "room volume", but not at all pushed to it's limit. He got it repaired, and it lasted about a day before blowing again. The heat sinks are not good enough, and the phase margin may well have been an issue as well.

It appears that phase margin wasn't well understood by many engineers in the audio world in those days. Bob Carver admitted in an Audio Magazine interview, that he didn't learn about that until many of his Phase Linear power amps blew up for no apparent reason.

I think it's fantastic that someone turned landfill into a great amp of today! I built a 4 channel 3886 poweramp and am very happy with it.

You might want to put some 1uF polypropylene caps across the giant electrolytic output caps (use appropriate voltage rating). Probably not necessary, but large electrolytics can have a little bit of inductance and/or funky electrolyte that could cause a rolloff at high frequencies and/or distortion. If the electrolytics are of good quality I doubt if you'd hear the difference.

You may want to include a turn-on time speaker decoupler circuit and relays. I'm guessing that the turn-on transient is significant (the output caps have to charge to roughly half of B+ in a few seconds).

 

[indianajo]

You might want to put some 1uF polypropylene caps across the giant electrolytic output caps (use appropriate voltage rating). Probably not necessary, but large electrolytics can have a little bit of inductance and/or funky electrolyte that could cause a rolloff at high frequencies and/or distortion. If the electrolytics are of good quality I doubt if you'd hear the difference.
You may want to include a turn-on time speaker decoupler circuit and relays. I'm guessing that the turn-on transient is significant (the output caps have to charge to roughly half of B+ in a few seconds).

My 1970 built Dynakit ST120 was bought with burned output transistors, drivers on one PC-15, and many burned parts on the regulator board (PC14) . I repaired it in about 1987 with all new electrolytic capacitors and output transistors, three new transistors on the one PC15, etc. With no schematic available online, my first approximation of the PC-14 current regulator circuit was limiting at 2.5 amp, I discovered when I discovered the schematic and 6.5 A specification on diyaudio.com about 2010. The amp never had a turn on thump, pop, or noise, I see no need of a turn on delay circuit. I put it away due to the heat problem in 1990, a four hour PA session filling a 120 seat church with electric keyboard had melted the solder on the output capacitor and the wire sprung up and shorted against the cover.
The installation of the Greg Dunn documented "TIP mods" in 2010 didn't improve sound, but the NTE60 output transistors I had used in 1987 probably did due to the increased Ft over the RCA 5 digit original transistors. The TIP mods I installed were a RC network across the output terminals, and anti-oscillation 50 picofarad capacitors across the b-e of the driver transistors. That arrangement was further updated in 2011 with the djoffe designed servo bias current control ciruit, which did something to improve cold low volume crossover distortion. http://www.diyaudio.com/forums/solid-state/156627-dynaco-stereo-120-can-beautiful.html
I also installed 2 PCAT fans on the heat sinks, which eliminate the chance of overheating in a typical 18 hour day I ran it.
The original transformer is quite robust for a 120 w unit, and in fact has withstood without damage a lightning surge that blew up the anti-pop capacitor over the power switch.
Mr. Joffe's LM3886 conversion is quite conventional, but as my unit sounds great (about the same as a Peavey CS800s at the 1 W level) and is not failing, I won't be converting. With the low levels of high freq intermodulation distortion I'm getting (test track top octave solo Steinway piano, and cymbals and bells) I don't need the poly capacitor parallel either. The unit sounds much better, especially at high frequencies, than the Dynaco ST70 tube amp with new electrolytic capacitors, rectifier, and 6CA7 output tubes.

 

[tauro0221]

Hi,
I agreed with indianajo about the Dynaco 120. Attached it is a picture of my Dynaco rebuilt. Once you replaced those cheap components with today quality the sound it is super excellent. My Dynaco 120 sound as good as the best tube amplifier. I called the "Beast" and it is completed new with the exception of the switch and the chassis. Everything it is new.

 

[Bare]

Yes and No Update sound is reasonably good, not frontline use suitable but wayyy better than stock .
BUT the Transformer sound is Loud and overpowering.
Rendering MY 'updated' st120 to Garage uses.
Unless I can remove and distance the crap transformer, via a Loong cable to live behind the garage.