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Old 25th November 2004, 06:20 AM   #1
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Default 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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Old 25th November 2004, 07:15 AM   #2
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Question 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?
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Old 25th November 2004, 07:30 AM   #3
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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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File Type: zip qrp01 dc-servo.zip (5.2 KB, 119 views)
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Old 25th November 2004, 10:09 AM   #4
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I will put it up asap !

grtz

Simon
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Old 25th November 2004, 10:38 AM   #5
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Default Powe amp schematic

Here is the poweramp !


grtz

Simon
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File Type: pdf power_amp_build.pdf (20.2 KB, 183 views)
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Old 25th November 2004, 10:39 AM   #6
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Default DC Servo

And the DC Servo

grtz

Simon
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File Type: pdf dc_servo_build.pdf (9.6 KB, 183 views)
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Old 25th November 2004, 10:42 AM   #7
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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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Old 26th November 2004, 11:25 AM   #8
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Lightbulb Re: DC servo in Amp

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.
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Old 26th November 2004, 12:03 PM   #9
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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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Old 26th November 2004, 03:36 PM   #10
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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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