I would like to summon the diyaudio forum`s circuit experts.
I need some advice, if it can be done of course. I have a few dead amps with STK00xx series darlington powerpacks as the power stage. In my country these chips are pure unobtainium. If there is any chance of buying them, then prices for these hybrid chips are more than the amps are worth.
I was thinking of remaking the STK00x0 on a perfboard with standard transistor as it`s one of the simplest, and since they are compatible between them it would be a substitute for any other STK00xx chip.
I need some advice, if it can be done of course. I have a few dead amps with STK00xx series darlington powerpacks as the power stage. In my country these chips are pure unobtainium. If there is any chance of buying them, then prices for these hybrid chips are more than the amps are worth.
I was thinking of remaking the STK00x0 on a perfboard with standard transistor as it`s one of the simplest, and since they are compatible between them it would be a substitute for any other STK00xx chip.
STK0030 trough 0080. The circuits are all the same. Probably the transistor are of different power.
Edit: here is the datasheet http://pdf1.alldatasheet.com/datasheet-pdf/view/89334/ETC/STK0040.html
I disassembled one STK0040. I was able to measure all but one resistor (the one between TR1 and TR2 emmiters) here is what i`ve got :
R1 2 kohm
R2 400 Ohm
R3 7.5 kohm
R4 320 ohm
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The shipping would cost more than the chip. Still, I think it would be a fun project to see if it can be done.
Unfortunately those are guaranteed to be fake. The appearance is wrong (as with all fakes), they are unused and they are too cheap. I suggest avoiding fakes completely, as their functionality can be unpredictable. Most likely though, they will blow up instantly, or very easily, or simply not work right whatsoever. If you bother with them, you'll lose a lot of time and still end up empty handed in the end.
I've compared fake STKs to the real deal and the fakes are always poorly made, obviously designed by someone without the necessary expertise.
The only way to get a real STK these days is to take one out of functional equipment. I usually harvest and discard amplifiers with one bad channel, and keep the working STK so that it can save a future lost cause.
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Yes, that is another problem. That is why I think substituting them with a discreet transistor circuit would be an ideal option.
Possibly, but only if the original circuit was merely a power amplifier for the last 26-30 dB. If it's a ~46 dB pre+power construction as they are commonly found, you're screwed. (The '3886 would also struggle to replace the highest-powered parts.)
@OP: Are you familiar with how discrete (EF2) output stages in audio power amps are constructed, complete with bias, thermal and insulation aspects? If so, coming up with a replacement should be quite doable (though thermal stability may prove a bit tricky). Otherwise, there's a fair bit of reading ahead of you.
I am pretty sure that the STK took advantage of its hybrid construction in order to ensure thermal stability. At the very least, I would expect thermal coupling between D1, D2, TR4 and TR5 to be necessary (possibly TR1 as well). You will typically also find "naked die" power transistors in these things for better thermal conductivity - heat dissipation when using already packaged parts may suffer unless you find a way of more or less directly mounting them to the original heatsink. Obviously driver transistors TR2, TR3 will usually need some cooling as well.
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The power supply is ±45V which is the first problem. The second problem is that the tone control is built in the negative feedback of the main amp.
I have very limited electronics knowledge. I can build a circuit after an already existing schematic, but designing my own is way above my skills.
Here is what I would do:
TR1,2=BD139, TR3=BD140, D1,D2= Diode-connected BD139
TR4=MJL21194, TR5=MJL21193
Base stoppers = 100 ohm, R5=220 ohm, R2= pot
Mount TR1,2,3 right up against one another
Epoxy D1 directly to TR4 case, D2 to TR5.
Adjust bias for 10mV across external emitter resistor, like any other amp
Done this many times. Usually on lower power units where I can get away with lower power outputs, but it's the same idea.
TR1,2=BD139, TR3=BD140, D1,D2= Diode-connected BD139
TR4=MJL21194, TR5=MJL21193
Base stoppers = 100 ohm, R5=220 ohm, R2= pot
Mount TR1,2,3 right up against one another
Epoxy D1 directly to TR4 case, D2 to TR5.
Adjust bias for 10mV across external emitter resistor, like any other amp
Done this many times. Usually on lower power units where I can get away with lower power outputs, but it's the same idea.
Wow! Thanks! The datasheet says that MJL21194/TR5=MJL21193 can push a lot of power though.
A pair of MJL21194, MJL21193 costs as much as an STK0040. Can I substitute them with lower power ones?
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There aren't many alternatives for lower power transistors. You can however use cheaper (but still as good) parts, for example 2STC5242 and 2STA1962. These should only cost you about €1 each.
I have a bunch of 2sc5196/2sa1939, from a project i never got to finish, I believe they are 6A. What can you tell me about these.
I see... Thanks for your help. Another question, and this will probably make me sound like a complete noob but the 2 diodes, why do they need to be epoxied to the transistors? And another question... Can I get away by making this in free air with pin to pin soldering?
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