DC servo in Amp

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Re: DC servo in Amp/Shooting in the Dark

blu_line said:
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
Hi Simon,
Without schematic it is like shooting in the dark for me!
Did you incorporate an offset adjustment in your scheme?
:confused:
 
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
 
Originally posted by blu_line 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
Hi Simon. Match Q9 and Q10 for Beta or HFE. Same for Q11 & 12. If that does not help enough install the attached circuit. Top and bottem are supply voltages. Connect the wiper of the pot to the input of your amp after the input cap.:cool:
 

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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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