Dynaco 120

[natesly]

My dad passed this on to me after cleaning out his basement where it sat for several decades. It had driven his AR 2ax and his klipsch quartets over the years. He built it in college with his buddies back when they were available as kits.


So this thing actually still turned on believe it or not but it looked horrible and after reading about its many shortcomings I reached out to Dan at update my dynaco dot com and got a full rebuild set for it.

I am halfway through and the power supply just passed voltage tests so I am soldering in the amplifier modules next. (I just finished the new grounding wires.) He still has his old Nikko beta preamp so after testing the output they will become the heart of a new stereo system. Anyone else do this project successfully. What are you driving with it? Finished pics to follow.

PS. that second pic is before I fixed my backwards polarity wiring from the pcb to the capacitor network. D’oh! Also I am not great with the soldering skills yet. By that is part of the journey.

 

[natesly]

Final voltage tests tonight but she powers on with no smoke or hysterics! Pretty excited to be at the end of this project and hoping it sounds sweet. All I have for test speakers right now is a cheap 6ohm set. Anyone have any advice on driving that load with this amp?

 

[ivandezande]

The rebuild kit just turns it into a LM3886 chip amp? That is anything but a Dynaco 120 now...sad.
Oh well guess it's too late.
Yeah that chip can go down to 4 ohms just fine.

 

[Lman]

I purchased a 120 and did the rebuild using Dan's kits and the result is a nice sounding amp. I've never heard a stock 120 and, based upon what I've read I would prefer not to, so I can't offer a comparison. A properly designed LM3886 board is capable of delivering very nice sound.

Please note that the amp is currently on the shelf as I prefer my WOPL 400 in my solid state rig.

 

[bbqjoe]

But, but...Where are the toobs?

 

[natesly]

Thanks for the info. Yes I know I am just using the transformer, heat sinks and the chassis but the goal was a restomod (to use car parlance). Honestly the burn marks around the resistors on the original PCB and the terrible state of the capacitors made the decision easy.

 

[natesly]

But, but...Where are the toobs?

Yeah if I could I would buy the new one instead

http://dynaco.com/

 

[s-petersen]

I once did a similar thing with a pair of HCT2500 RCA 7A op amps and it turned out great, My parents used it for many years wit a PAT 5 Bi-fet, and a SE10 equalizer

 

[homebrew]

As a former owner of 2 Stereo 120's in a bi-amp set up, I would venture to say there's not a lot you can do to one to make it sound worse than a stock unit. I couldn't get rid of mine fast enough so I could purchase a real amp like a Hafler DH-500.

 

[H/K crazy]

I worked for a Dynaco service center in the 70's and worked on many of the ST-120's. All I can say is that the stock 120's were designed bad, sounded bad and were a nightmare to repair.I'm glad to see that someone has designed a up grade with totally new circuits. Hopefully they sound good. No one should waste their time trying to restore one of these to the original factory concept. Hopefully the new design includes a bias adjustment pot.

 

[BinaryMike]

I developed a ST-120 upgrade circuit of my own way back in 1987. It deletes Dynaco's power supply regulator, which allows for increased output power, and I recall that it sounded much better than the original amplifier circuit. I'll attach the schematic below, but I see that it calls for a few transistor types that would have to be substituted if anyone decides to build a copy.

 

[fred soop]

When McIntosh was still doing amplifier clinics, we were able to access a pile of their distortion graphs from the previous days testing (They keep a copy). This was around 1968. The Dynaco 120 was one of the very few amplifiers that could maintain low measured distortion across the band until reaching about 30 Hz. Below that, it just shot straight up and off the graph. That's where the power supply simply ran out.

With todays higher voltage transistors being readily available and economical, there is no reason for the regulated power supply. They used selected 2N3055 transistors that exceeded spec and could withstand the 72 V but that 72 V had to be stiff. If it went higher, it could blow the transistors. If it was 72 V at no load and sagged, then the full 60 W per channel would not be available.

Also, being a capacitor coupled circuit, somewhat necessary at the time, there were additional problems.