Hi,
R12, R22 .. one of these should be variable to adjust bias current of the power stage.
This current should be somewhere between 10mA and 100mA per transistor.
This means a voltage drop of 1mV ... 10mV across the source_resistors.
Be careful. Too high current will cause a lot of heat. Don´t overheat the transistors.
Klaus
You need to make them higher ohmic, not lower.Yes, are the value of R12 and R22 that I've changed, lowing seriously down, but nothing changed
Down from 180 to 47 ohm R12 and R22 to 220
Hi,
You need to make them higher ohmic, not lower.
Klaus
Measure or simulate the DC bias points! Don't just change values without understanding what is going on.
You could go all the way to full class A mode. Which is not a bad thing from the audio perspective, but could cause excessive output transistor heating.
According to R12,R13,R22,R23 and theoretical bias voltage for M1,M2,M3,M4 you should read approx. 8Vdc between opposed MOSFET gates. That is theoretical. In practice, although I have not reversed engineered that schematic completely, it appear at first glance that this circuit would be quite unstable when overall temperature is raised. Because Q10 and Q44 DC bias is controlled by temperature dependent 5 transistors KSA992 (assumed to be 2SA992 and complementary 2SC1845).
A minute change in Base bias voltage for BD140(and BD139) will dramatically change the bias voltage for M1-M2-M3-M4. Hence, dramatically changing bias current on R16-R18-R39-R42. So, my first observation is: BD139 and BD140 Emitter must be in series with an appropriate value resistor to lower its base bias voltage variation effect.
According to manufacturer specs, a change of 0.1V on BD140 VBE will change its collector current by 100 folds. This could be catastrophic to both the output MOSFET and BD140 itself, (Caused by excessive heat). Notice that those BD140 and BD139 MUST be heat sink.
Approximation attempt of calculating BD140 functional collector current leads to a value in the vicinity of 50mA, which is bare minimum to be able to decently drive those 4 MOSFET gates. So, loose a little voltage from an Emitter resistor, something like 0.1V/.05A= 20ohms, and it should substantially help bias current regulation. R1 and R2 should then be raised to compensate this 0.1V change. Values that I cannot calculate because the original values are not marked on your schematic.
Over and above those modifications, R12 - 180ohms resistor could be changed for a 800 or 1k ohms precision 10 turns TrimPot and a 250 ohms parallel resistor to prevent catastrophic current if the pot ever become intermittent. This way you will be able to accurately set the bias current.
Further improvement could be implemented but I would need all parts values.
Where did you get this schematic? I'd love to reverse engineer this thing and add it to my collection of power amp designs. It's a decent setup.
Hope this help
Cheers
With 100mA you surely are in class A mode for quiet listening.This current should be somewhere between 10mA and 100mA per transistor.
In the simulation I read 4Vdc for the mosfet gate, should I increase this?
About the heat developed in BD139-140 I don't care, because I already have an amplifier like this (a prototype) and those do not exceed normal room temperature without the emitter series resistor. In addition, at least according to simulation, put a series resistor the distortion would increase considerably, and that's exactly what I want to avoid.
About instead the voltage bias of the transistor, you said 8Vdc? R13 and R23 where changed with 220ohm resistor, and R12-22 with an unique 10k res. In the simulation I read 4Vdc for the mosfet gate, should I increase this?
Your modification with 220ohms and 10k dont seem to make sense to me. It would raise the R13 current to 15mA. Considering the 10K would only see 1.3V between the two bases (Q13,Q20) a 130microAmp. that means your two Q13 Q20 are taking some 14.9mA into base. Enough current to saturate the two Qs and bring bias to a minimum, like 0.1V , which will shot your output mosfet bias to almost zero. Cant work. Unless you mistakenly said 220ohms and instead you plugged in 22k resistors for R13 and R23. In that case it may work fine. But since your Mosfet Gate bias current is in the range of mAmps, I see no problem with the original values. My suggestion with the 1k trimpot and 250 ohms resistor was to give you an opportunity to perform a very precise bias current adjustment. But your original resistor choice were correct to start with.Over and above those modifications, R12 - 180ohms resistor could be changed for a 800 or 1k ohms precision 10 turns TrimPot and a 250 ohms parallel resistor to prevent catastrophic current if the pot ever become intermittent. This way you will be able to accurately set the bias current.
Further improvement could be implemented but I would need all parts values.
Absolutely, the LT1166 is an excellent choice for this job.Can I in this case use a LT1166 to replace completely the bias? For exampe
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