Dynaco Stereo 120...can be beautiful

[djoffe]

I have always had a soft spot for a Dynaco Stereo 120 solid state power amp. It was the first solid state amp with real power in wide production. It did an awful lot with just 6 transistors per channel. Still, it probably did a lot of damage, too.

The output stage was notoriously under-biased, creating lots of crossover distortion at low levels. If you measured the distortion and didn't look at or think about the harmonics, the numbers seemed pretty good...in reality, it was pretty miserable classic notch (crossover) distortion...

Of course, if you let it warm up long enough, the bias current would creep up, and some of the worst distortion wouldn't be quite so bad...Perhaps this one amp is responsible for a world of audiophiles warming up equipment for hours before listening!

Then, there were a few out there, that probably sounded great, but the output stage bias was on the hairy edge...play them loud enough, long enough, and the magic musical smoke would escape from the amp as the quiescent current ran away...

Anyway, with this major shortcoming in mind (purists might point to a number of other shortcomings...), I designed an analog bias controller for the output stage quiescent current. It nails the output stage quiescent current wherever you'd like (I've settled on around 50 mA), and keeps it from varying over temperature, no matter how hard you beat on the amp.

You can kluge it onto existing working amps with very little fuss. The result is distortion behavior much like a tube amp...almost no distortion at low levels (e.g. unmeasurable harmonics with 10 mV rms input), and then as output amplitude picks up, you get more 2nd harmonic than 3rd. The distortion increases gradually until you approach clipping.

I figured I would just post this much to see if there's interest...if so, I can write up the circuit, and post some information...

All for now...comments invited...

Dan

[djoffe]

To establish a baseline, here is a simulation model for the Dynaco Stereo 120 amplifier channel. Play with it some...put in some small signals, 10 mV or so...look at the output spectrum...lots of harmonics, and not too far down...classic symptoms of not enough current in the output stage...look at the output stage currents...output transistors really aren't even conducting.

If you change one of the diodes (say D1) to a 1N5817, (a quick way to model a slight change to the output stage bias) see how much worse the distortion gets.

In an upcoming post, I'll add the bias controller, and show its effect.

The included files are:
1. stereo 120 standard schematic.jpg
2. dynaco stereo 120 stock with TIP Mod.txt - change the .txt to .asc and you can run it as a simulation file.
3. standard.bjt.txt - rename it to standard.bjt. After you have backed up your existing LTSPice standard.bjt file to another name, you can copy this new one in its place. It has all the models you'll need.

[Mr. dB]

Would your bias scheme work on, say, a Naim NAP140-type circuit?

Like the Dyna, the Naim is quasi-comp with NPN outputs, but it uses a dual-rail supply and requires no output cap.

[Bamalama]

I am not nearly smart enough to mess with simulation files but I do have a thought in the back of my mind to fool with early SS amps and see what the experience is like. I got a garage sale Mitsubishi discrete transistor preamp a few weeks ago and I am having fun with it.

B

[djoffe]

Something like it would probably work...It would require some adaptation, because the Dynaco original circuit was kind of "baroque"...If you can post a schematic, I can think about it a bit...

[wahab]

it would be relatively easy to upgrade the amp to a current feedback
amplifier with vasly improved performance..
here a link..

Simple 60 Watt Power Amplifier

use the base version, the only thing is to adapt a better bootstrap
than the one in this schematic, you just have to add a resistor
and a capacitor...you can also go complementary for the final stage
rather than quasi complementary...
i saw this schematic first in the 70s, i did built one at the time,
and it really sounded very good...
distorsion is about 0.03 at most in my current simulations...

regards,

wahab

[djoffe]

Wahab,

Thanks...that's an interesting link...I enjoyed reading it through...
It's interesting to note that the old Dynaco design is also current mode feedback.

Dan

[wahab]

Originally Posted by djoffe

Wahab,


It's interesting to note that the old Dynaco design is also current mode feedback.

Dan

yes, this the reason i proposed this schematic which has
the advantage of DC coupled interstages...
you even wont have to drill the pcb, the current
layout being enough, just be careful in components connections...

regards,

wahab

[djoffe]

Here are two simulations of the amp: both are at 10 mV rms input at 2 kHz.
The first is stock, the second is bias control...harmonics drop from 46 dB below the fundamental to nearly 80 dB below the fundamental...actual measured performance is even better than the simulation!

[Krisfr]

Question, Is this the amplifier that had the output inductor wrapped around the electrolytic capacitor? I think I built about 3 of these...

Thanks for the memories....