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-   -   resistors and Bias (https://www.diyaudio.com/forums/solid-state/340085-resistors-bias.html)

dj_holmes 11th July 2019 12:14 AM

resistors and Bias
 
Hi guys,

This may sound like a stupid question but to set the bias of an amplifier to 0 for adjusting would one increase the resistance or lower it?

I have mosfet amplifiers and transistor amplifiers and they all use 3 pin trimmers but on different sides of the board. Is it the same for both? The ones I've seen one pin in connected to another so only 2 of the pins actually give reading.

Most of my online searches are either for tubes or too difficult to understand. I assume its all the same for all technologies and it has to be set to 0 ohm.

Can someone with experience please shed some light on this? Its been on my mind for months :scratch2:

Thank you

Ylli 11th July 2019 12:18 AM

A well designed amplifier will 'fail safe' if the adjustment pot opens up. So to reduce the idle current one would normally increase the resistance of the adjustment.

dj_holmes 11th July 2019 01:07 AM

If the pot is 100r I should increase it to 100r for 0 bias? Is that right for both channel? Thanks

ubergeeknz 11th July 2019 01:11 AM

Quote:

Originally Posted by dj_holmes (https://www.diyaudio.com/forums/solid-state/340085-resistors-bias-post5848319.html#post5848319)
If the pot is 100r I should increase it to 100r for 0 bias? Is that right for both channel? Thanks

Depends on the design and most allow adjustment within a range only, which might not include zero.

dj_holmes 11th July 2019 01:13 AM

I mean if the pot is X value should I increase the X value to maximum resistance for the value of the bias to be as low as possible?

ubergeeknz 11th July 2019 01:23 AM

Quote:

Originally Posted by dj_holmes (https://www.diyaudio.com/forums/solid-state/340085-resistors-bias-post5848322.html#post5848322)
I mean if the pot is X value should I increase the X value to maximum resistance for the value of the bias to be as low as possible?

It depends. Do you have a circuit diagram?

dj_holmes 11th July 2019 01:27 AM

I dont. I thought it was a general rule for all bias adjustments. I have been wondering about his for a long time.

I have had many different technology amplifiers and just wondered how could you without a circuit diagram safely reduce the bias to adjust it. Thats all. Its even keeping me up this late trying to figure it out :(

rayma 11th July 2019 01:31 AM

Quote:

Originally Posted by dj_holmes (https://www.diyaudio.com/forums/solid-state/340085-resistors-bias-post5848329.html#post5848329)
I thought it was a general rule for all bias adjustments.

Depends on the particular circuit, it could be done either way.
It's more intuitive for clockwise rotation to increase the current.

dj_holmes 11th July 2019 01:50 AM

I can't measure current but can resistance.

You say it depends that some you have to increase resistance to set bias low and some you have to decrease resistance to lower bias?

3am here will reply tomorrow. Thanks

Ylli 11th July 2019 02:29 AM

1 Attachment(s)
Attached is a typical Vbe multiplier. The pot (rheostat) is normally installed at location R2. As you can see, as the value of R2 increases, the voltage at v (the amount of bias) decreases.

If R2 would totally fail and become an open circuit, v becomes 0.93 volts.
But the pot *could* be installed at R1, or it could replace both R1 and R2 with the wiper on the transistor base. But what I posted seems to be the most common, and is fail-safe.


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