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December 07, 2022, 07:53:03 AM

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Unicord Stage 65 schematic and layout

Started by galaxiex, March 25, 2018, 05:24:06 PM

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Here's a schematic and layout for the Unicord Stage 65 amp.  :)
Took me some time to draw this...

I found this on ebay as just the chassis, no cabinet.
Will post more later about this amp and mods I did to it.
If it ain't broke I'll fix it until it is.



I'll post soon some pics and a schematic with the parts value changes I made.
If it ain't broke I'll fix it until it is.


Most of the changes I made are to the tone stack.

With the stock values in the tone stack, there was too much bass at all times.
You just couldn't dial it down far enough. Even at "0" the bass was just over the edge of "enough".
This made the bass control useless. Leave it at 0 and still too much.

So I got out the Duncan Tone Stack Calculator and found that the "James" tone control closely matches the tone stack in this amp.

BTW, if you have never played around with the DTSC it's a fun tool.  :)


Here's some screen shots of the stock and modified tone stack values in the calculator,
showing the sweeps of the controls.

And a schematic with the changes made.

The bass is far more controllable now.
I've found putting the bass at about 2 or 3 and Treble at 6 or 7 works quite well.
Of course there is much more bass on tap if you need it.  ;)
If it ain't broke I'll fix it until it is.


Here's some pics of the chassis.

Added 3 wire power cord and a speaker jack too.

I think this amp may be biased a bit cold.
Even playing clean at low volume there is a decaying "fizz" to the notes.

Also the output transistors never get hot,
barely warm after playing at full volume and full clipping for 1/2 hour.

How would I change the bias on this circuit?
If it ain't broke I'll fix it until it is.




So is Q7 in the schematic the rubber diode?

Could bias be  adjusted by using a trim pot in place of  R38, R40?

I have re-drawn schematic output stage for clarity.
If it ain't broke I'll fix it until it is.


Yes, Q7 is "rubber diode", and like you guessed you set voltage drop across it with resitive divider R40/R38.

To increase voltage drop you either increase resistance of R40, or decrease resistance of R38. The circuit needs to generate a voltage drop of approximately 4 x diode forward voltage (ca. 2.4V) to compensate for the four base-emitter voltage drops introduced by the push-pull (darlington) emitter follower buffer.

Yes, a trimmer will work. Just make sure the circuit configuration is one that can't reach to a state where bias voltage would be too high. That would lead to catastrophic failure of the amp. Too low bias voltage won't hurt anything, but it just means more crossover distortion in the output signal.

AFAIK, the circuit - as is - should develop a voltage drop of approximately 2V so it's in the right range already.

I sense that your biggest problem is adequate thermal compensation because forward voltage of the transistor's base-emitter junction will vary in interaction with output device temperature, and insufficient compensation can lead to "thermal runaway", where bias current just keeps automatically increasing as the devices heat up. To the point of failure. You practically want an opposite effect: Increasing device temperature should decrease bias voltage.

I assume that such a simple design as this does not employ any "thermal tracking" schemes and is therefore biased fairly "cold" since the bias voltage will remain "fixed" regardless of device temperature. When the output devices heat up quiescent current will increase some but not in a magnitude to cause thermal runaway as the increasing starts from a state of moderately cold bias to begin with. Benefit is increased reliability with the expense of higher overall crossover distortion.

If you can make the bias circuit "thermally track" it allows biasing "warmer" since the bias circuit will self-compensate for effects of increasing temperature and lower bias accordingly. You often see bias diodes or the "rubber diode" transistor mounted to the heatsink (or close vicinity of the output devices) in order to make them track the operating temperature.

In this amp the bias circuit (and the transistor) are located in the circuit board. They will not track output device temperature and bias voltage remains in "fixed" value defined by ambient temperature around the bias circuit (which will be a lot less than the heatsink's temperature and subject to less thermal shifting during operation).

Much comes to application. Is the amplifier going to operate largely at high output volume levels producing ample output power? If yes the crossover distortion will be proportionally lower magnitude and less audible at those high output levels. If the amp is operated largely at low power output, at modest loudness levels, crossover distortion will be proportionally higher magnitude at such signal levels and therefore more audible and obtrusive.

Since audible distortion seems to bother you in normal use of the amp I would try increasing the bias voltage a notch while monitoring that the operation stays reliable and that the bias doesn't run off when the output devices start to heat up. It's probably going to needs just a tiny increase.

Output transistors should operate in a lukewarm temperature. If you burn your fingers to them they are running too hot. Due to practical inefficiencies of heat transfer the case or heatsink temperature will regardless be much, much less than the actual internal die temperature of the transistor, which is the thing one worries about concerning overall reliability.
So rather than worrying about whether the transistors seem to run too "cool" I would concentrate on checking out that the amp produces its rated output power at reasonably low level of distortion, as it should. If it does - and still runs cool - then you can just congratulate yourself for acquiring a very reliable amplifier. Output devices running "hot" is not a preferred state, it's just a nasty side effect one usually has to live with in practice. The cooler they operate the better.


teemuk, thanks so much for the detailed and thoughtful reply.  :)

As you noted there is no thermal compensation so I think I will leave well enough alone.
The slight crossover distortion that I hear at low volume is really very minor and not at all noticeable with a volume increase.
The amp runs "cool"  to "barely warm" and in all other respects, works quite well.

After you posted the pic of the rubber diode I did a search and found this video...


I found it very informative and well presented.
Even I could understand and follow alone!   :o

I found the video to nicely supplement your reply.

If I feel the need to tweak the bias, at least I know what to do now.
*If* that becomes the case, I may try moving Q7 to the chassis (heatsink) between the 2 output devices.

Thanks again!

PS, when I got this amp it was dead, no sound output at all.
I traced a signal thru the pre amp up to the output stage.

The 2SB507 was fine but the 2SD313 measured open B-E and B-C.
I replaced it with a TIP31C and the amp fired right up.

The original (as far as I can tell) 2SD313 showed no evidence of over heating.
No other components seem to have been damaged.

It's as if the transistor just "went open".
Maybe from a shorted output?

Whatever, I'm not worried about it, just curious about how it failed.
I suppose parts can just "go bad".

If it ain't broke I'll fix it until it is.