How much output offset voltage is too much?
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 12th October 2003, 01:19 AM #1 tubetvr diyAudio Member   Join Date: Jan 2003 Location: Sweden How much output offset voltage is too much? Hi I am experimenting with different amplifier circuits and have experienced that output offset voltage can be quite different in different circuits, also some circuits drift with temperature so even if offset is very low initially it can increase quite a lot when the amplifier gets warm Now my question, how much offset is too much? It is obvious that if offset is very high, i.e. several volts most electrodynamic speakers will react and the speaker membrane will be in a offset position either out or in from the resting position. In this case either the voice coil will go to the end position or the speaker will give excessive distorsion as the voice coil is not centered in the magnet gap. So how much offset is tolerable? 50mV, 100mV, 200mV, 500mV or 1V or even more? Regards Hans
 12th October 2003, 01:42 AM #2 Tube_Dude   diyAudio Member     Join Date: Mar 2002 Location: Aveiro-Portugal Hi Hans... For me , my target is less than 50 mV at the output...with the normal +- 5 Ohms speaker DC resistence ...10 mA is the of set current circulating in the voice coil... But some people can be less stringent... __________________ Jorge
 12th October 2003, 01:29 PM #3 Circlotron   diyAudio Member     Join Date: Jun 2002 Location: Melbourne, Australia My class A has a constant 325mV at the output. No problemo. Zero. __________________ Best-ever T/S parameter spreadsheet. http://www.diyaudio.com/forums/multi...tml#post353269
 12th October 2003, 04:45 PM #4 Richard C   diyAudio Member   Join Date: Jul 2003 Location: Nottingham, England I'd go for less than 50mV if possible. Moving coil loudspseakers are only reasonably linear over a few milimeters of cone travel so it pays to operate with as little offset as possible.
fmak
diyAudio Member

Join Date: May 2001
Location: London UK
Quote:
 Originally posted by Circlotron My class A has a constant 325mV at the output. No problemo. Zero.
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This is way too high and biases your speaker cone popsition.

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I'd go for less than 50mV if possible. Moving coil loudspseakers are only reasonably linear over a few milimeters of cone travel so it pays to operate with as little offset as possible. [/QUOTE]
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I agree; I'd go for <25mV and my systems have less than 10mV.

 12th October 2003, 05:57 PM #6 peranders   Electrons are yellow and more is better! diyAudio Member     Join Date: Apr 2002 Location: Göteborg, Sweden I think this question has a rather floating answer. Take a small 10 W speaker and a huge PA speaker. I'll guess that over 1 volt isn't good if we talk distortion but under 100 mV is good I think and normal also. __________________ /Per-Anders (my first name) or P-A as my friends call me Group buy: SSR03 Super Regulator Power Supply. Sign up HERE 53 pcb's in interest so far.
EchoWars
diyAudio Member

Join Date: Dec 2002
Location: Left of the Dial
Quote:
 Originally posted by peranders I think this question has a rather floating answer.
Probably sums it up. It's a personal thing. On amps I'm repairing, anything over 50mV warrrants investigation. On my personal amps, anything over 15mV needs attention. I like to have them at <2mV.

 12th October 2003, 11:31 PM #8 markp   diyAudio Member   Join Date: Oct 2003 Location: L.A., CA My amps are within 5mv without a servo. If you have lots of offset then try to match your transistors better or build a servo to control it. __________________ If it sounds good... it is good!
 13th October 2003, 12:09 AM #9 EchoWars   diyAudio Member     Join Date: Dec 2002 Location: Left of the Dial I'm with you Mark. But the problem with matching is that they aren't going to stay matched. Check your amp in a few years. I have only recently been introduced to the whole servo thing...I like. Long-term stability, no muss, no fuss. I do believe I am a convert.
fmak
diyAudio Member

Join Date: May 2001
Location: London UK
Quote:
 Originally posted by EchoWars Probably sums it up. It's a personal thing. On amps I'm repairing, anything over 50mV warrrants investigation. On my personal amps, anything over 15mV needs attention. I like to have them at <2mV.
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I agree with your numbers. However, this is more than a personal thing and doesn't dpend on the size of the speakers but on sensitivity and cone displacement.

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