Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Pass Labs This forum is dedicated to Pass Labs discussion.

BA-3 As Preamp
BA-3 As Preamp
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
Reply
 
Thread Tools Search this Thread
Old 19th March 2017, 12:09 AM   #1221
6L6 is offline 6L6  United States
diyAudio Moderator
 
6L6's Avatar
 
Join Date: Oct 2010
Location: Denver, Colorado
BA-3 As Preamp
Place a couple of Kohm across the output, it makes setting bias a lot easier.
  Reply With Quote
Old 19th March 2017, 02:08 AM   #1222
dimkasta is offline dimkasta  Greece
diyAudio Member
 
dimkasta's Avatar
 
Join Date: May 2010
Location: Athens
Thanks mate

How come? Does it make the change faster?
__________________
Too many projects... so little time...
  Reply With Quote
Old 19th March 2017, 02:38 AM   #1223
6L6 is offline 6L6  United States
diyAudio Moderator
 
6L6's Avatar
 
Join Date: Oct 2010
Location: Denver, Colorado
BA-3 As Preamp
I just remember that it was easier with a load, it's been a while since I've messed with one on the bench...
  Reply With Quote
Old 19th March 2017, 03:28 PM   #1224
nashbap is offline nashbap  United States
diyAudio Member
 
Join Date: Jul 2012
Location: New Jersey USA
Quote:
Originally Posted by dimkasta View Post
Thanks mate

How come? Does it make the change faster?
Dont forget to short the inputs. Big difference in DC offset.

nash
__________________
F5TV3, AVC, BA3b pre, Salas DCG3b
  Reply With Quote
Old 19th March 2017, 05:01 PM   #1225
nashbap is offline nashbap  United States
diyAudio Member
 
Join Date: Jul 2012
Location: New Jersey USA
[QUOTE=dimkasta;5023624]Here we go. Some progress.

The pre seems to bias nicely even with random unmatched 610/9610. I am using them for tests so that I do not blow my matched Toshibas.

BTW I really hate how twitchy the pots are (Currently using 1k).


I finally go tired of this and came up with the following:

Having experimented with different bias levels and the resulting sound I found that a bias level of around 50ma sounded best to me. Besides 60ma was producing too much heat. I measured the pots P1 and P2(+ to end of R6 not joined to R8 for P1, and - to R7 for P2). I re-measured the pots after removing P1 and P2 from the boards. In my case with R6 being 56R I measured around 348R for P2 and around 6R higher for P1, this with very well matched Toshiba's.

I then installed a CMF 55 348R resistor in place of P2 on the board. For P1 I replaced it with a 20R pot in series with a 348R resistor since I wanted a measure of adjustability. Since this has to be in parallel with C1 (otherwise the AC gain of the amp is affected) I bent the pin of the pot closest to R6 so that it wrapped upwards along the pot, soldered the 348R resistor to it, wrapped it around the pot and now made that end of the resistor effectively the third pin of the pot. I also put a bit of insulating material at the bottom of the pot to prevent any contact between the resistor and the pot pin that was bent.

DC offset setting is so easy now. And you get similar results the next day after you have switched it off which never happened before. And there is plenty of adjustability. One turn of a 20R multiturn pot is around 0.65R and I find that you get around a 60mv offset change from just 1 turn. A 500R pot is around 16.5R per turn. No wonder it is so twitchy. I just did this mod a week ago. Maybe it even sounds slightly more detailed. The only change in DC offset I am now expecting is with a change in room ambient temperature.

nash
__________________
F5TV3, AVC, BA3b pre, Salas DCG3b
  Reply With Quote
Old 25th September 2017, 08:44 AM   #1226
kpsthakur is offline kpsthakur  India
diyAudio Member
 
Join Date: Sep 2016
Location: Delhi
[QUOTE=nashbap;5024290]
Quote:
Originally Posted by dimkasta View Post
Here we go. Some progress.



The pre seems to bias nicely even with random unmatched 610/9610. I am using them for tests so that I do not blow my matched Toshibas.



BTW I really hate how twitchy the pots are (Currently using 1k).





I finally go tired of this and came up with the following:



Having experimented with different bias levels and the resulting sound I found that a bias level of around 50ma sounded best to me. Besides 60ma was producing too much heat. I measured the pots P1 and P2(+ to end of R6 not joined to R8 for P1, and - to R7 for P2). I re-measured the pots after removing P1 and P2 from the boards. In my case with R6 being 56R I measured around 348R for P2 and around 6R higher for P1, this with very well matched Toshiba's.



I then installed a CMF 55 348R resistor in place of P2 on the board. For P1 I replaced it with a 20R pot in series with a 348R resistor since I wanted a measure of adjustability. Since this has to be in parallel with C1 (otherwise the AC gain of the amp is affected) I bent the pin of the pot closest to R6 so that it wrapped upwards along the pot, soldered the 348R resistor to it, wrapped it around the pot and now made that end of the resistor effectively the third pin of the pot. I also put a bit of insulating material at the bottom of the pot to prevent any contact between the resistor and the pot pin that was bent.



DC offset setting is so easy now. And you get similar results the next day after you have switched it off which never happened before. And there is plenty of adjustability. One turn of a 20R multiturn pot is around 0.65R and I find that you get around a 60mv offset change from just 1 turn. A 500R pot is around 16.5R per turn. No wonder it is so twitchy. I just did this mod a week ago. Maybe it even sounds slightly more detailed. The only change in DC offset I am now expecting is with a change in room ambient temperature.



nash


Which points are you using to read DC offset..a pic would be helpful.

Cheers
  Reply With Quote
Old 4th October 2017, 10:11 AM   #1227
dimkasta is offline dimkasta  Greece
diyAudio Member
 
dimkasta's Avatar
 
Join Date: May 2010
Location: Athens
I don't think a photo would help since I am using my own pcbs, but it should be quite easy to do
DC is measured between the output before the Capacitor (inside lead of the cap) and ground
__________________
Too many projects... so little time...
  Reply With Quote
Old 4th October 2017, 11:48 AM   #1228
dimkasta is offline dimkasta  Greece
diyAudio Member
 
dimkasta's Avatar
 
Join Date: May 2010
Location: Athens
Here is a photo if it helps. You should be able to extrapolate where to attach the probes on your pcb.
The probes closer to the cap measure offset (back side of cap and output return)
Attached Images
File Type: jpg cvb.jpg (950.4 KB, 226 views)
__________________
Too many projects... so little time...
  Reply With Quote
Old 4th October 2017, 12:07 PM   #1229
kpsthakur is offline kpsthakur  India
diyAudio Member
 
Join Date: Sep 2016
Location: Delhi
Quote:
Originally Posted by dimkasta View Post
Here is a photo if it helps. You should be able to extrapolate where to attach the probes on your pcb.

The probes closer to the cap measure offset (back side of cap and output return)


Thank you so much for reverting, meanwhile I used the 330R resistor (R13 as per schematic), put probes around it an set DC offset...Have biased the amp to about 80%, will use it for some time then attempt a full bias...
  Reply With Quote
Old 4th October 2017, 12:38 PM   #1230
dimkasta is offline dimkasta  Greece
diyAudio Member
 
dimkasta's Avatar
 
Join Date: May 2010
Location: Athens
Quote:
Originally Posted by kpsthakur View Post
Thank you so much for reverting, meanwhile I used the 330R resistor (R13 as per schematic), put probes around it an set DC offset...Have biased the amp to about 80%, will use it for some time then attempt a full bias...
Yes sounds correct. R13 is connected to the back of cap and output return
__________________
Too many projects... so little time...
  Reply With Quote

Reply


BA-3 As PreampHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
ADA Audio Design Associates Millennium - preamp - MILLEN-2000+BA - control center Jimmy154 Swap Meet 1 16th November 2012 05:19 PM
BA-C buzzforb Pass Labs 60 23rd October 2011 08:58 PM
WTB: Burning Amp BA-1 or BA-2 jdubs Swap Meet 0 23rd March 2011 03:16 AM
FS: two burning amps (one each BA-1 and BA-2) without drivers Eric Weitzman Swap Meet 15 17th February 2010 02:35 AM
Ba-1 Rembulan Pass Labs 0 15th May 2009 01:09 AM


New To Site? Need Help?

All times are GMT. The time now is 06:03 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki