Compensate Bias for DC offset? - diyAudio
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Old 24th April 2015, 08:08 PM   #1
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Default Compensate Bias for DC offset?

I know that Bias and Offset play a part on each other. So my question is,

I would guess that Bias specifications for an amp are derived from a manufacture that is basing it on a perfect 0 offset.

In an imperfect world, have an offset +/- 20-30. Would the Bias need to be adjusted 20-30 also?

My Hafler amp has a negative offset, which I found common, and a pain to correct (without changing components). So would the Bias need to be increased to compensate?
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Old 24th April 2015, 08:20 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

In typical usage of the terms bias and offset are not related.

Optimum bias is an output loop voltage unaffected by DC offset.

rgds, sreten.
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Old 24th April 2015, 08:35 PM   #3
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I always thought of bias as a volume knob,

Too high and it distorts.. too low and nothing. But right in the middle theres a sweet spot.

Dc offset, I thought was, considered the voltage the Bias was riding (piggy backing) off of, thus for affecting its outcome. Having a 0 offset ment a smooth ride for the Bias.
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Old 24th April 2015, 09:25 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Bias works in an inner loop at all output voltages.
Consequently any DC offset has no effect on it.

rgds, sreten.
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Old 24th April 2015, 09:27 PM   #5
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Quote:
Originally Posted by sreten View Post
Hi,

In typical usage of the terms bias and offset are not related.

Optimum bias is an output loop voltage unaffected by DC offset.

rgds, sreten.
But since bias affects offset, can optimal bias then be a different setting for each channel? I mean in order to maintain 0 offset.

Last edited by Discopete; 24th April 2015 at 09:33 PM.
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Old 24th April 2015, 09:33 PM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Discopete View Post
But since bias affects offset, can optimal bias then be a different setting for each channel?
Hi,

No. Bias levels do not affect output DC offset.

Offset is determined by the balance of the input.

rgds, sreten.
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Old 24th April 2015, 09:48 PM   #7
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Quote:
Originally Posted by sreten View Post
Hi,

Bias works in an inner loop at all output voltages.
Consequently any DC offset has no effect on it.

rgds, sreten.
Then why make sure DC offset is at or close to zero before adjusting bias?

"To bias it, you insert a milliampmeter in series with the positive rail to the channel. A convenient place is at the rail fuse (it has separate pos. and neg. rail fuses for each channel). Connect the meter across the fuse holder, than remove the fuse.

You set to 250-275 mA idle current, no load/no signal. Make sure the DC offset is adjusted to 0V out first."

So its alittle confusing.
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Old 24th April 2015, 09:49 PM   #8
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Originally Posted by sreten View Post
Hi,

No. Bias levels do not affect output DC offset.

Offset is determined by the balance of the input.

rgds, sreten.
Hi. I have read that if bias is unequal in both channels an offset can result as HFE in the outputs rises unequally among them. Should I assume this condition will disappear when the amp reaches full operating temp? And if so, can temp be used to set quiescent current? Thank you in advance for your response.
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Old 24th April 2015, 09:56 PM   #9
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Originally Posted by jakelm1975 View Post
Then why make sure DC offset is at or close to zero before adjusting bias?

"To bias it, you insert a milliampmeter in series with the positive rail to the channel. A convenient place is at the rail fuse (it has separate pos. and neg. rail fuses for each channel). Connect the meter across the fuse holder, than remove the fuse.

You set to 250-275 mA idle current, no load/no signal. Make sure the DC offset is adjusted to 0V out first."

So its alittle confusing.
Hi. I by no means mean to hijack your thread. I just think my questions are pertinent. FWIW, my Acoustat TNT200 manual directs bias to be set first. Directions for my other amps is similar.
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Old 24th April 2015, 10:05 PM   #10
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Originally Posted by Discopete View Post
Hi. I by no means mean to hijack your thread. I just think my questions are pertinent. FWIW, my Acoustat TNT200 manual directs bias to be set first. Directions for my other amps is similar.
Its quite alright.

Thats why I asked.

Some say DC offset has no effect on Bias, and vice versa... but it clearly states to make sure offset is adjusted first. So my question was, if offset cannot be set (or more.. cannot be set to zero...more like +/- 20 or so), do we still go by the manufactures specs for Bias?

Last edited by jakelm1975; 24th April 2015 at 10:08 PM.
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