Bias adjusting on Threshold SA4/e

Mywing · 28th March 2010, 02:44 PM

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

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.

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.

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.

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.

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

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

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

28th March 2010, 02:44 PM	#1
Mywing diyAudio Member Join Date: Mar 2007	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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Adjusting the BIAS for Threshold SA3	sonic22	Pass Labs	9	16th May 2011 12:38 PM
Threshold SA4/e bias	nchelazzi	Pass Labs	2	16th June 2008 08:10 PM
400A bias adjusting	Mywing	Pass Labs	7	25th May 2008 07:53 PM
Adjusting BIAS	barmesan	Tubes / Valves	2	10th January 2007 12:18 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

28th March 2010, 06:34 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	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.

28th March 2010, 07:17 PM	#3
Mywing diyAudio Member Join Date: Mar 2007	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?

28th March 2010, 07:27 PM	#4
Nelson Pass The one and only Join Date: Mar 2001	I would not adjust the channel which is at 49 deg. You would probably want to reduce the bias of the other channel slightly.

29th March 2010, 03:21 PM	#5
Mywing diyAudio Member Join Date: Mar 2007	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

29th March 2010, 05:15 PM	#6
Nelson Pass The one and only Join Date: Mar 2001	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.

30th March 2010, 01:18 PM	#7
Mywing diyAudio Member Join Date: Mar 2007	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

30th March 2010, 07:08 PM	#8
Nelson Pass The one and only Join Date: Mar 2001	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.

30th March 2010, 07:39 PM	#9
Mywing diyAudio Member Join Date: Mar 2007	Ok, which voltage value I would see on the voltmeter?