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Old 12th October 2003, 12:19 AM   #1
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Default 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
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Old 12th October 2003, 12:42 AM   #2
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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...
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Old 12th October 2003, 12:29 PM   #3
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My class A has a constant 325mV at the output. No problemo. Zero.
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Old 12th October 2003, 03:45 PM   #4
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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.
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Old 12th October 2003, 04:56 PM   #5
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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.
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Old 12th October 2003, 04:57 PM   #6
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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.
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Old 12th October 2003, 06:45 PM   #7
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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.
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Old 12th October 2003, 10:31 PM   #8
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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.
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Old 12th October 2003, 11:09 PM   #9
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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.
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Old 13th October 2003, 07:11 AM   #10
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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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