Threshold S300 offset issues

FootFungus · 3rd December 2009, 11:11 PM

I have a Threshold S300 (original model from '82) that have been refurbing trying to eliminate 70mv (left) and 130mv (right) of DC offset on the speaker outputs. I've matched and replaced the input diff pair. I did the cap replacements suggested in threads here (thud and offset threads). But at this point, the easy stuff has not done the trick.

Following general practice I put in new matched differential pairs. The 70mv channel showed no difference, as the originals were still dead on. The 130mv channel dropped from an original 150mv to about 80mv. However adjusting the bias up to 50ma brought that 80mv up to 130mv. And I was so excited.....

Going by tips in other threads here, I replaced the 470uf 6.3v electrolyte cap and the 0.15uf tantalum. That made little to no difference.

The DC offset increases almost linearly while increasing the bias.

This amp had 2N6428As as the differential input transistors. I replaced them with MPS6571s based on the schematic I have.

Anything else to check? Could the CCS for the diff inputs be out of spec enough to cause problems? Hfe on the outputs too uneven?

eyoung · 4th December 2009, 02:17 AM

Hola,

Did you read Mr.Pass' papers on matching transistors for current or voltage. Its on his(their) web site, build a simple jig and measure following his guidelines. Matching hfe is not a guarantee of a match. Its late and I do not have the link, but it is easy to find...

Regards, Elwood

FootFungus · 4th December 2009, 02:29 AM

I think I've read the pdf you are referring to. I'll go hunt that down again. Thanks for the tip.

Nelson Pass · 4th December 2009, 06:53 PM

Having done all the obvious things, the thing to do now is
to adjust either the resistor setting the bias current to the
input diff pair, or the resistor setting the load coming off the
drains of the diff pair - the resistor attached to the Base of
the VAS transistor and v+.

eyoung · 5th December 2009, 02:17 AM

Thanks Papa for Bailing me out...

Elwood

FootFungus · 15th December 2009, 12:58 AM

I ended up replacing the 82 ohm load resistor on the diff pair with a 200 ohm trim pot to find the right resistance. The DC offset seems to change about 40mv for every 10 ohms either direction. I used 93 ohm resistors on my S/150 as a test, and that dropped the DC offset down to less than 5mv. It was previously reading 40-45mv on both channels.

Thank you for the tip.

Out of ignorance induced curiosity, I was wondering why the emitter resistors on the input diff pair are different values? Every other schematic I recall has identical values.

Would making thermal contact between the diff pair transistors bring any tangible benefit? I'd use thermal grease and some heat shrink to bind them.

3rd December 2009, 11:11 PM	#1
FootFungus diyAudio Member Join Date: Oct 2008	Threshold S300 offset issues I have a Threshold S300 (original model from '82) that have been refurbing trying to eliminate 70mv (left) and 130mv (right) of DC offset on the speaker outputs. I've matched and replaced the input diff pair. I did the cap replacements suggested in threads here (thud and offset threads). But at this point, the easy stuff has not done the trick. Following general practice I put in new matched differential pairs. The 70mv channel showed no difference, as the originals were still dead on. The 130mv channel dropped from an original 150mv to about 80mv. However adjusting the bias up to 50ma brought that 80mv up to 130mv. And I was so excited..... Going by tips in other threads here, I replaced the 470uf 6.3v electrolyte cap and the 0.15uf tantalum. That made little to no difference. The DC offset increases almost linearly while increasing the bias. This amp had 2N6428As as the differential input transistors. I replaced them with MPS6571s based on the schematic I have. Anything else to check? Could the CCS for the diff inputs be out of spec enough to cause problems? Hfe on the outputs too uneven?

4th December 2009, 02:17 AM	#2
eyoung diyAudio Member Join Date: Aug 2007 Location: Sunny SC,USA 15 min south of Charlotte NC	Hola, Did you read Mr.Pass' papers on matching transistors for current or voltage. Its on his(their) web site, build a simple jig and measure following his guidelines. Matching hfe is not a guarantee of a match. Its late and I do not have the link, but it is easy to find... Regards, Elwood __________________ "Sometimes obstacles are put in our way to make us slow down so that we can enjoy the journey and appreciate the destination when we eventually get there"

5th December 2009, 02:17 AM	#5
eyoung diyAudio Member Join Date: Aug 2007 Location: Sunny SC,USA 15 min south of Charlotte NC	Thanks Papa for Bailing me out... Elwood __________________ "Sometimes obstacles are put in our way to make us slow down so that we can enjoy the journey and appreciate the destination when we eventually get there"

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
Threshold s300 Amp Power on Thud	nellson	Pass Labs	16	30th August 2010 05:57 PM
DC offset issues with GC	fishski13	Chip Amps	18	17th November 2009 12:33 AM
DC Offset on the input of a Threshold SL10	hifi-hospital	Solid State	10	25th February 2007 08:26 AM
DC Offset circuit for Threshold	johnsciolino	Solid State	15	27th September 2004 02:49 PM
Threshold S300 output transistors help	jewilson	Pass Labs	12	24th June 2002 07:44 AM


	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

4th December 2009, 02:29 AM	#3
FootFungus diyAudio Member Join Date: Oct 2008	I think I've read the pdf you are referring to. I'll go hunt that down again. Thanks for the tip.

4th December 2009, 06:53 PM	#4
Nelson Pass The one and only Join Date: Mar 2001	Having done all the obvious things, the thing to do now is to adjust either the resistor setting the bias current to the input diff pair, or the resistor setting the load coming off the drains of the diff pair - the resistor attached to the Base of the VAS transistor and v+.

15th December 2009, 12:58 AM	#6
FootFungus diyAudio Member Join Date: Oct 2008	I ended up replacing the 82 ohm load resistor on the diff pair with a 200 ohm trim pot to find the right resistance. The DC offset seems to change about 40mv for every 10 ohms either direction. I used 93 ohm resistors on my S/150 as a test, and that dropped the DC offset down to less than 5mv. It was previously reading 40-45mv on both channels. Thank you for the tip. Out of ignorance induced curiosity, I was wondering why the emitter resistors on the input diff pair are different values? Every other schematic I recall has identical values. Would making thermal contact between the diff pair transistors bring any tangible benefit? I'd use thermal grease and some heat shrink to bind them.

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