Hi

I need a some help to interface an Accuphase MOFI3850 pre-amp, to an old Sony pwr-amp.
The situation is this STK3102 is obsolete, and dozens of attempts to provide another stable solution has failed.
So - I found this MOFI3850Se pre-amp, which I imagine could do the job instead.
BUT - the output of the MOFI is just this one DC neutral straight ahead, whereas the old STK3102 had two lines with an offset (pins 5/6 + 10/11)

Could that be fixed by rebuilding the interface?

 

That does not look like a change that I would be at all comfortable with. What I think that I am seeing is part of a direct coupled amplifier. BUT the output section is not shown so it might not be. The big feature of direct coupled amplifiers is that it is very simple to destroy a lot of expensive output stage devices with a failure of a single earlier stage device. I have seen that happen on a few power amplifier sections.

 

That's exactly the problem. The old STK3102, and a few other attempts of "similar STK3102s" just destroyed the output stages, as was the initial fault. Can see that many others have had the same battle.
Obviosly I/we must fight some DC offsets, and trimming Idle currents, if this MOFI amp could work.
The big question is; Could it be done?

 

The situation is this STK3102 is obsolete,

I have you tried Ebay
IC

 

Interfaces between audio components are generally ground and an AC signal going + an - around it. Are you saying you have a power amp that requires a signal with a DC offset from ground?

 

Interfaces between audio components are generally ground and an AC signal going + an - around it. Are you saying you have a power amp that requires a signal with a DC offset from ground?

Short answer "YES."
The amplifier shown looks to me like one of the "Direct Coupled" amplifiers that were promoted a while back, in which they were indeed DC coupled, and adjusted for zero DC offset. The problem was, and still is, that this mode is difficult to keep balanced, even more-so when it has to be optimized for minimum cost as a consumer product.
Industrial servo amplifiers usually achieve this goal, but it is never simple nor easy.
Look at the circuits in post #1 and see that there are no capacitors in the forward signal path, and only one capacitor in the feedback path. (it might just be for stability).

 

Given the apparent direct coupled design of that Sony amp, and the specific design of the preamp board it would seem the best chance of replacing it with something that would work, would be to build a discrete transistor design identical to the original.
Do you have an old board to determine all the component values?

 

It may be worthwhile to experiment with the "Accuphase" circuit driver portion instead. That is, if the intention is to still have a DC coupled amplifier.
My choice would be to save the power supply portion and the input sections that we do not see, and replace the DC amplifier with a more common type, which should be available.

 

Interfaces between audio components are generally ground and an AC signal going + an - around it. Are you saying you have a power amp that requires a signal with a DC offset from ground?

See Sony Schematics "Main Board" (right section)

 

It may be worthwhile to experiment with the "Accuphase" circuit driver portion instead. That is, if the intention is to still have a DC coupled amplifier.
My choice would be to save the power supply portion and the input sections that we do not see, and replace the DC amplifier with a more common type, which should be available.

DC coupled or not. Just looking for a few components, which should make the proper "handshake".
This "Accuphase" MOFI C3850 board I have, also has it's own PS. No problem to supply a seperate 2 x 15 VAC transformer.

 

Donby,
Did you see my link in post #4?

 

Donby,
Did you see my link in post #4?

Yes I did. I've just lost confidence in STK3102's.
Have tried a few solutions, and read about other peoples poor experiences.

 

What the actual benefit of a totally DC coupled "HIFI" amplifier is, is subject to argument. The claim that it eliminates "Transient Inter-modulation Distortion" is vague at best. The fact that they are more subject to expensive failures is proven. That they are more difficult to service is obvious.

The advice to the TS has already been offered.
I do recall reading about another DC coupled driver IC back around 1976 or so. So there is a slight chance that a different IC might work. But possibly that IC may also be unavailable.