HELP with an amp please!

darkfenriz · 27th April 2005, 12:56 PM

well
it is not working

as you may have supposed
simple, maybe a bit experimental mosfet one burns one resistor and I don't know why
can this be caused by output mosfets failure??
output voltage is ~3V
please help me wiser ones

best regards

richie00boy · 27th April 2005, 01:03 PM

Put a very high value resistor across C5 to reset the integrator when power is off.

If the 470R resistor is burning then both upper and lower driver transistors must be conducting, try and trace what could be causing this.

Nelson Pass · 27th April 2005, 08:39 PM

try shorting across Q7 and see if the 90 volts is still there.

darkfenriz · 27th April 2005, 09:37 PM

thank you
is the whole project 'acceptable' ?

Nelson Pass · 27th April 2005, 10:40 PM

Quote:

Originally posted by darkfenriz
is the whole project 'acceptable' ?

That is a metaphysical question.

fab · 28th April 2005, 12:06 AM

if you want to avoid wasting money in destructed mosfets, use Zener protection diodes. One between emitter of Q10 and output. Another onebetwwen the output and the emitter of Q? (PNP complementary to Q10). You can use Zener voltage about 2 volts lower than the max rated VGS of the IRF mosfets.
If you destroy zener diodes it costs less than the IRF...

anatech · 28th April 2005, 01:42 AM

Hi darkfenriz,
Check R24, R25 for open as well as the 0.22R resistors. If you have that much across the 470R resistor, Q10 and it's compliment are either on or could be in reverse E-B breakdown. I'm fairly sure you will find a few transistors shorted. An open PCB connection could change things though.
-Chris

darkfenriz · 2nd May 2005, 03:33 PM

thanks all
one more question: what will be minimal zener current thru 0.5W 47V zener diode? I have less than 1mA I'm affraid.

BTW: all I wanted from this project was to keep output at low feedback with minimal number of stages. I used inverting mode making summing junction the slowest part of everything by means of pole-zero compensation.
This should work nice in terms of low TIM and little interference with speaker back EMF, especially HF component of this signal. What do you think?

regards

Ultima Thule · 2nd May 2005, 05:01 PM

Mr darkfenriz,

you have shown such a "guru" approach on this board when answering other posts, and now you need help?

Anyhow, your design is going to have an awfull PSRR

Redesign it and then we can think about the "problems"!

Regards Michael

PS. the "other issue" is somewhere between U4 and Q1//Q3 stages, and is also part of the "problem" you mentioned!

darkfenriz · 2nd May 2005, 05:12 PM

thank you Ultima Thule

Quote:

Originally posted by Ultima Thule
Anyhow, your design is going to have an awfull PSRR

that's why I use for VAS separate transformer, 2x1000u caps filtering +-74V rail, regulated down to +-53 by lm317/337 and filterd by 2x10u caps.
Making it is so inexpensive and simple, and makes amplifier "don't worry" about PSRR aside from efficiency issue.
Thank you for suggestion, time will tell if it is going to have low noise or not.

best regards

27th April 2005, 01:03 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Put a very high value resistor across C5 to reset the integrator when power is off. If the 470R resistor is burning then both upper and lower driver transistors must be conducting, try and trace what could be causing this. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

28th April 2005, 12:06 AM	#6
fab diyAudio Member Join Date: Mar 2004 Location: Canada	over voltage at mosfets gates if you want to avoid wasting money in destructed mosfets, use Zener protection diodes. One between emitter of Q10 and output. Another onebetwwen the output and the emitter of Q? (PNP complementary to Q10). You can use Zener voltage about 2 volts lower than the max rated VGS of the IRF mosfets. If you destroy zener diodes it costs less than the IRF...

2nd May 2005, 05:01 PM	#9
Ultima Thule diyAudio Member Join Date: Jan 2004 Location: Koskenkorva Land	Mr darkfenriz, you have shown such a "guru" approach on this board when answering other posts, and now you need help? Anyhow, your design is going to have an awfull PSRR Redesign it and then we can think about the "problems"! Regards Michael PS. the "other issue" is somewhere between U4 and Q1//Q3 stages, and is also part of the "problem" you mentioned! __________________ "If transistors are blueberries and FETs are strawberries, then tubes must be.. pears" Michael 29th January 2010

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27th April 2005, 08:39 PM	#3
Nelson Pass The one and only Join Date: Mar 2001	try shorting across Q7 and see if the 90 volts is still there.

27th April 2005, 09:37 PM	#4
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	thank you is the whole project 'acceptable' ?

28th April 2005, 01:42 AM	#7
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi darkfenriz, Check R24, R25 for open as well as the 0.22R resistors. If you have that much across the 470R resistor, Q10 and it's compliment are either on or could be in reverse E-B breakdown. I'm fairly sure you will find a few transistors shorted. An open PCB connection could change things though. -Chris

2nd May 2005, 03:33 PM	#8
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	thanks all one more question: what will be minimal zener current thru 0.5W 47V zener diode? I have less than 1mA I'm affraid. BTW: all I wanted from this project was to keep output at low feedback with minimal number of stages. I used inverting mode making summing junction the slowest part of everything by means of pole-zero compensation. This should work nice in terms of low TIM and little interference with speaker back EMF, especially HF component of this signal. What do you think? regards

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