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Old 6th March 2017, 03:27 PM   #1
xXBrunoXx is offline xXBrunoXx
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Talking Need some help :D

I have some questions about the schematic bellow.
Since the "output stage " uses darlingtons and not drivers and power output transistors ( for thermall stability they must have been put on the same heatsink ). Just the TIP142-147 on the heatsink would be ok ? no thermall runaway ? Do i need to put the bias diodes on the Heatsink as well ? or replace them with a Transistor which i put on the heatsink ?.
Do i need to add a zobel for stability ?
50-100p caps between the base and collector of the TIP's for oscillations ?.
Thank you.
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Last edited by xXBrunoXx; 6th March 2017 at 03:30 PM.
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Old 6th March 2017, 03:41 PM   #2
multisync is offline multisync  Canada
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where is schematic?
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Old 6th March 2017, 04:01 PM   #3
xXBrunoXx is offline xXBrunoXx
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here
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Old 7th March 2017, 05:43 AM   #4
PRR is offline PRR  United States
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> 50-100p caps between the base and collector of the TIP's for oscillations ?

No. These are emitter followers. A cap there would just annoy the opamp.

> no thermall runaway ?

If the un-specified diodes are single diodes, it will never run-away.

Unless the un-specified bias trimmer value is set too high!

With some assumption about that un-specified bias resistor, the output stage will run stone-cold, with measurable crossover, greatly masked by opamp gain. I have seen/heard stages like this work "well". But it is not a "fine" design.

Build it. Simple, and doesn't cost much.
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Old 7th March 2017, 09:20 AM   #5
jaycee is offline jaycee  United Kingdom
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This circuit is simply a bad modification of ESP P113: Headphone Amplifier
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Old 7th March 2017, 10:49 AM   #6
sgrossklass is offline sgrossklass  Germany
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Add another regular diode in series with D1L, maybe a Schottky in series with D2L, mount all the diodes on the heatsink with the Darlingtons, and see what kind of idle current adjustment range you get. I don't think the pot needs to be much over 100 ohms.

BTW, in sim I have never seen much difference between the two electrolytics as shown and just one electrolytic between the two bases. Might save some space.

Since nominal supply is reasonably high as-is, I would also consider moving the opamp connection up to Q1L base while omitting R5L, slightly reduces maximum output but gives you some Class A bias for the opamp output stage along the way.

(The above two items are pretty much unavoidable when going for a Vbe multiplier bias instead.)

A 47-100 ohm series resistor at the opamp output may be advisable. Can't hurt to have a Zobel either.
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Old 7th March 2017, 03:08 PM   #7
xXBrunoXx is offline xXBrunoXx
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I modified it... Why you mean " bad " ?
Wanted something simple , 15-20 W
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Old 7th March 2017, 03:09 PM   #8
xXBrunoXx is offline xXBrunoXx
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Diodes are 2x 1n4148 and a trim pot
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Old 7th March 2017, 04:34 PM   #9
vzaichenko is offline vzaichenko  Russian Federation
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Need some help :D
The main reason for calling it "bad" - ESP 113 is a working circuit, your modification - is not really a well-working one

As sgrossklass already mentioned - biasing circuit must be more "sophisticated" and thermally coupled with the heatsink. What is enough for a headphone amp is not enough for a power amp. As soon as you reach the right bias one way or the other, you will need a thermal feedback to keep it stable and avoid the thermal runaway.

Compensation is the other point of concern.
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Old 7th March 2017, 04:49 PM   #10
xXBrunoXx is offline xXBrunoXx
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if i glue the diodes on the heatsink or use a transitor on the bias attached on the heatsink , would I still have problems with thermal runaway >?
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