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-   -   DC servo in Amp (http://www.diyaudio.com/forums/solid-state/46497-dc-servo-amp.html)

blu_line 25th November 2004 07:20 AM

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

Elso Kwak 25th November 2004 08:15 AM

Re: DC servo in Amp/Shooting in the Dark
 
Quote:

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,
Without schematic it is like shooting in the dark for me!
Did you incorporate an offset adjustment in your scheme?
:confused:

peranders 25th November 2004 08:30 AM

2 Attachment(s)
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.

blu_line 25th November 2004 11:09 AM

I will put it up asap !

grtz

Simon

blu_line 25th November 2004 11:38 AM

Powe amp schematic
 
2 Attachment(s)
Here is the poweramp !


grtz

Simon

blu_line 25th November 2004 11:39 AM

DC Servo
 
2 Attachment(s)
And the DC Servo

grtz

Simon

peranders 25th November 2004 11:42 AM

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

Elso Kwak 26th November 2004 12:25 PM

Re: DC servo in Amp
 
2 Attachment(s)
Quote:

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:

peranders 26th November 2004 01:03 PM

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.

blu_line 26th November 2004 04:36 PM

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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