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25th November 2004, 06:20 AM
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#1 |
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diyAudio Member
Join Date: Jan 2002
Location: The Netherlands
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DC servo in Amp
I finally soldered my amp this week.
I did not include a DC-servo since i was not sure i needed one or not ! Well, it turned out i do it. So i prototyped one. The problem is now that it does not completely adjusts the output DC to 0. it is around 150mV. Any one suggestions how come ? grtz Simon |
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25th November 2004, 07:15 AM
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#2 | |
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Banned
Join Date: Dec 2001
Location: Zamboanga, City of Flowers, Mindanao
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Re: DC servo in Amp/Shooting in the Dark
Quote:
Without schematic it is like shooting in the dark for me! Did you incorporate an offset adjustment in your scheme? 
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25th November 2004, 07:30 AM
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#3 |
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Electrons are yellow and more is better!
diyAudio Member
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If you had done it right you ought to have less than 5 mV or the value the servo opamp has.
Schematic please. Meanwhile you can check my simulation files. I have used LTSpice from www.linear.com freeware.
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/Per-Anders (my first name) or P-A as my friends call me |
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25th November 2004, 10:42 AM
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#7 |
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Electrons are yellow and more is better!
diyAudio Member
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In your case the input bias current may cause extra DC shift.
SonnyA has made a similar amp, check his A1 http://mirand.dk Your servo must have an additional cap at the + input otherwise the gain won't be less than 1. Notice also that you must tune the servo so it won't clip at 20 Hz and full output power of the main amp. The is even more important since the opamp has lower supply voltage than the main amp. Your emitter resistos and the inverting input are also pretty large compared to feedback. The emitter resistors should be a small as possible. Not also that you can have a current gain in your main current mirro
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/Per-Anders (my first name) or P-A as my friends call me |
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26th November 2004, 11:25 AM
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#8 | |
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Banned
Join Date: Dec 2001
Location: Zamboanga, City of Flowers, Mindanao
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Re: DC servo in Amp
Quote:
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26th November 2004, 12:03 PM
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#9 |
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Electrons are yellow and more is better!
diyAudio Member
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This is a good advice but practially rather hopeless task, the matching thing. It's better to inject the missing base current. You could have a servo for it! I have this in my RIIA discrete amp. I have the same problems there with unequal base currents.
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/Per-Anders (my first name) or P-A as my friends call me |
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26th November 2004, 03:36 PM
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#10 |
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diyAudio Member
Join Date: Jan 2002
Location: The Netherlands
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Thanks for all the advice !
The problem is solved. The whole problem was related to measurement issues. I measured 150 mv over a supply wire to ground and not from output on the pcb to ground on the PCB ! It actually is just 2 mv. All components are matched within 2 % except for the output power devices. I could not match NPN's and PNP's in the same range. grtz Simon |
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