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Amplifier - Bias Adjustment.

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Amplifier - Bias Adjustment.

Post by STC on Mon May 12, 2014 12:46 pm

Hello DIYers,

Can someone help me to get the correct reading of the bias voltage? According to my manual the measured value should be 22mV across the emitter. The trouble is the reading changes when I leave the probe connected for about half an hour. Generally, it rises to 30mV. (Note: Amplifier was left on for more than 24 hours before adjustments made).

I am told that the value fluctuates due to EMI interference and temperature but at what point should I even consider that the reading is correct?

Thanks in advance.
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STC
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Character sheet
Source(s): Ambiophonics setup.
Amplification:
Speakers: Electrostatics

http://goo.gl/CF1C6B

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Re: Amplifier - Bias Adjustment.

Post by mthoi on Wed May 28, 2014 4:13 pm

Hi STC,  you would need to provide more information or circuit diagram/project you are benchmarking.

For example, in my earlier DIY PASS Aleph and F5 projects, the bias voltage measurement for the transistors at different  time has different values - probably due to item heating up after extended use, or mains voltage fluctuations (if the PSU is not regulated, transformer heats up, etc), EMI (by fixing a probe to the emitter, it would have effected the reading due to leads inductance, etc.).  I also have several voltmeters that measures to different level of accuracy and produces different readings.  

Bias voltage on my tube amp projects has shown greater variance. 

Earlier I was perhaps more particular to "perfection" on specs.  Now, I accept that electronic parts are within " a tolerence range". That includes voltage readings.  

Personally, I would say a 10mV variance range after more than say 30 min warm up, as "OK"; and over 24 hrs as "very good".  

I have a few cold brews and everything is more acceptable.

mthoi
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Source(s): Marantz SAKI Pearl, Michell Gyro, Garrard 401, Lenco L75, Technics SL1200II
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Re: Amplifier - Bias Adjustment.

Post by STC on Wed May 28, 2014 8:49 pm

MTHOI, 

Thank you very much for your post. That is the most comprehensive answer so far that I can find. I remeasured the bias of the Classe Amp connected to my voltage stabiliser and they are more stable.

The one point I am still uncertain is whether to take reading immediately or wait for a few seconds? When I turn on my Sanwa DMM it would show 22mV but after one minute it would gradually rise around 26mV ; leaving them(DMM) for 24 hours it would still show 26mV and occasionally up to 30mV.

Should the correct reading taken immediately or after about 1 minute? If I were to bring down the bias to 22mV after 1 minute I find the sound'a bit dull and Amp is bit cold.

I think at 26mV the sound is to my liking but the amp is warmer than it should be. With load it may shoot up way above 30.
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STC
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Number of posts : 391
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Character sheet
Source(s): Ambiophonics setup.
Amplification:
Speakers: Electrostatics

http://goo.gl/CF1C6B

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Re: Amplifier - Bias Adjustment.

Post by cslim on Tue Nov 22, 2016 10:39 pm

Hi STC,
Normally the correct reading should taken after the temperature of the amp stabilize, about 5 minutes or above.
During measure the bias reading, you should not connect any load and no signal input too.

If 26mV is to your liking, just keep to this value. I think it is still a safe value (guess so since I do not know your amp's design).
What happened when you adjust to higher bias is that your amp will be bias to more class A behavior (assume that ur amp is class AB design). That explain why ur amp is warmer than it should be after you increase the bias. Care should be taken not to adjust too high the bias as you amp might not design to operate in such operating condition.

cslim
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Amplification: Creek 4040S3
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Re: Amplifier - Bias Adjustment.

Post by STC on Tue Nov 22, 2016 10:50 pm

Thank you, CS Lim. 

The bias reaches 26mV at equilibrium no matter what value I put. Classe emailed me to set it at 22mV after 20 minutes of warm up. 

Thanks again for your input.
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STC
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Number of posts : 391
Age : 52
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Character sheet
Source(s): Ambiophonics setup.
Amplification:
Speakers: Electrostatics

http://goo.gl/CF1C6B

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Re: Amplifier - Bias Adjustment.

Post by cslim on Wed Nov 23, 2016 12:14 am

No problem, you are welcome.

cslim
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Amplification: Creek 4040S3
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