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-   -   Bias adjusting on Threshold SA4/e (http://www.diyaudio.com/forums/pass-labs/164006-bias-adjusting-threshold-sa4-e.html)

Mywing 28th March 2010 02:44 PM

Bias adjusting on Threshold SA4/e
 
I finally managed to buy a SA4/e used and in excellent condition. I only have a small problem: the temperature of the two heatsinks differ by about 4 degrees: 49 to 54 left and right sides. I should probably adjust the bias but the instruction manual in my possession does not say how ...
Among other things I do not know at what point precisely measure the temperature ... I did that in the screw holes of the top cover; would be right?

All the advices about the exact procedure of bias adjusting will be welcome.
Thank you :cool:

Nelson Pass 28th March 2010 06:34 PM

Screw holes in the top, 50 deg.

There are a couple of potentiometers on the front end board,
one controls the bias, the other controls the offset. It won't
take long for you to figure out which.

:cool:

Mywing 28th March 2010 07:17 PM

Thanks,
...but if the potentiometer to bias adjusting is unique for both channels, how can I remedy the temperature difference between left and right channel?

Nelson Pass 28th March 2010 07:27 PM

I would not adjust the channel which is at 49 deg. You would
probably want to reduce the bias of the other channel slightly.

:cool:

Mywing 29th March 2010 03:21 PM

I think I have identified them.
Should be two small blue trimmers one for each channel.
I would not be wrong, but to decrease the temperature I should UNSCREW, right?

Thanks again

Nelson Pass 29th March 2010 05:15 PM

You should not touch either pot unless you are monitoring
the current draw. Usually counter-clockwise will reduce the
bias, but don't assume anything.

:cool:

Mywing 30th March 2010 01:18 PM

I thought it was enough to decrease the bias of the channel with the warmer heatsink...
Btw, what is the correct procedure of bias adjusting? My owner manual does not say that.
Also does anyone know where I can find a more comprehensive manual?

Thanks again

Nelson Pass 30th March 2010 07:08 PM

There is a lengthy procedure, but it is simple enough to put
a voltmeter on the power Emitter resistors (one for each output)
on the channel and look at the DC voltage across it. You want to
turn it down slightly and then wait a couple hours with the lid on to
see what temperature you get. When you get 50 deg for a
room temp of 25, you are done.

:cool:

Mywing 30th March 2010 07:39 PM

Ok,
which voltage value I would see on the voltmeter?

MashBill 31st March 2010 02:05 AM

Quote:

Originally Posted by Mywing (Post 2137100)
Ok,
which voltage value I would see on the voltmeter?

That which gives you 50 degrees.......


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