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Old 22nd April 2003, 02:54 PM   #1
jjj84 is offline jjj84  Finland
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Default DC-current

I checked my amplifier speaker outputs for DC and i found +100mV DC.. is this bad? Can it do any damage to my speakers? Amplifier is made of stk4231II.
Is there any way of removing this DC?

ps. sorry bout my bad english, i hope you get the point..
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Old 22nd April 2003, 10:09 PM   #2
moamps is offline moamps  Croatia
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Hi,

this voltage on output is on the borderline value, IMHO. If amp is old, check input caps and caps in NFB line; C5, C6,C9 and C10 according original manual.

My english is also poor, but ...

Regards
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Old 22nd April 2003, 10:54 PM   #3
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I've read archived data from this forum that indicated
that some individuals tolerated as much as 1000mv of
DC offset.
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Old 23rd April 2003, 05:27 AM   #4
moamps is offline moamps  Croatia
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Hi,

I use batery with 1,5V to detect what is + on some bass speaker. With 1V too, you see clearly membrane moving. With this DC bias speaker work for my point of wiev in inproper condition (also is 125mW on 8E).

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Old 23rd April 2003, 05:39 AM   #5
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Default Re: DC-current

Quote:
Originally posted by jjj84
I checked my amplifier speaker outputs for DC and i found +100mV DC.. is this bad? Can it do any damage to my speakers? Amplifier is made of stk4231II.
Is there any way of removing this DC?

ps. sorry bout my bad english, i hope you get the point..
Your english is fine...

Caps are a possibility, but more than likely you have a differential amp in the input stage that is unbalanced. Solution: Replace input pair transistors. You may match them for optimum results.

My own experiance is that offset of as little as 25mV degrades sound quality noticeably. Others may argue, but I hear what I hear. 100mV is worth tracking down and fixing.
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Old 23rd April 2003, 09:02 AM   #6
jjj84 is offline jjj84  Finland
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Default Re: Re: DC-current

Quote:
Originally posted by EchoWars
Caps are a possibility, but more than likely you have a differential amp in the input stage that is unbalanced. Solution: Replace input pair transistors. You may match them for optimum results.

In the picture u can see that i cant change those transistors..
In my amplifier input caps are changed from 2.2uF to 10uF, can this cause it? I think not. The amplifier is only 2 months old.

datasheet stk4231II

Click the image to open in full size.
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Old 23rd April 2003, 10:06 AM   #7
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Hello to you all.
The DC on the output should be below +/-10mV (my amps runs with app. +/-0,1 mV after 15 min. warm up
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Old 23rd April 2003, 11:55 AM   #8
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I think we all can conclude this to:

1 How much DC voltage before we get distortion due to displacement of the woofer (not tweeter)?

2 How much DC voltage before a smell can be detected?

1) < 0.5 V (guess), <0.1 V feels good (or safe), <20 mV very OK

2) Depending on how big the woofer is, < 5-10 W I'll guess

100 mV creates a click when the speaker is switched on and off and this can be irritating but hardly harmful for the speaker.

Together with a DC-servo you can reduce the offset to almost nothing, around 1 µV (mean value).

(My new SMD headphone amp with AD8610/BUF634 has only 0.04-0.08 mV at the output! Amazing!)
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Old 24th April 2003, 07:06 AM   #9
jjj84 is offline jjj84  Finland
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Now i think that i know where the problem is..

Input coupling capacitors are in the original schematics 2,2uF and the input bias resistors are 56k ohms. This way dc current should be zero.

I my schematic these capacitors are changed to 10uF and the bias resistor is 10k ohms. I think that the reason for this 100mV of dc is caused by wrong value of the input bias resistor.

Does anyone now how to calculate right value for this resistor?

Thanks a lot if someone could help me.
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Old 24th April 2003, 08:56 AM   #10
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jjj84,

Despite all the expert advice, you're doing fine! I would like to help also, but can't find the schematic. Is it somewhere on the net?

Jan Didden
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