Bypassing bias stage with caps - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 20th November 2003, 11:01 PM   #1
diyAudio Member
 
EchoWars's Avatar
 
Join Date: Dec 2002
Location: Left of the Dial
Default Bypassing bias stage with caps

I've seen large values such as 100f used to bypass the bias trimpot and diodes (basically joining the bases of the next stage, usually the drivers or pre-drivers), but I've also seen very small values used, such a .01f.

Certainly it depends on the input impedence of the next stage as to what ought to be used, but the low value above seems real undersized, and 100f seems to be real overkill. Voltage seems not to be a real factor, as there is unlikely to be must more than 3-4 volts across the cap.

What do you all do?
  Reply With Quote
Old 20th November 2003, 11:04 PM   #2
markp is offline markp  United States
diyAudio Member
 
Join Date: Oct 2003
Location: L.A., CA
I use 10uf accross the base-emitter and .1uf between the C and E.
__________________
If it sounds good... it is good!
  Reply With Quote
Old 20th November 2003, 11:20 PM   #3
diyAudio Member
 
EchoWars's Avatar
 
Join Date: Dec 2002
Location: Left of the Dial
OK...how about a visual?

This is about the most common bias arrangement I run into. Bypassing the DC bias arrangement would mean a cap from base of Q9 to base of Q10. In a scenerio like this, what would you use?
Attached Images
File Type: jpg image1.jpg (54.2 KB, 206 views)
  Reply With Quote
Old 20th November 2003, 11:28 PM   #4
markp is offline markp  United States
diyAudio Member
 
Join Date: Oct 2003
Location: L.A., CA
Sorry, I use a bias transistor not just varistors or simple diodes.
__________________
If it sounds good... it is good!
  Reply With Quote
Old 21st November 2003, 05:18 AM   #5
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Gteborg, Sweden
Blog Entries: 4
Default Re: Bypassing bias stage with caps

Quote:
Originally posted by EchoWars
...., but the low value above seems real undersized, and 100f seems to be real overkill. Voltage seems not to be a real factor, as there is unlikely to be must more than 3-4 volts across the cap.
100 nF is an average (=normal) value but the value isn't critical and if you really want to determine the value I think you must have a distorsion analyzer. I would have settled for 100 nF to start with and 100 uF seems overkill.

To be true I have never investigated how much I need in this position so I gather everything from 1-100 nF will do probably.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Tube Buffered Gainclone in work |Thread || Diamond buffer |Thread for the group buy | Wiki
  Reply With Quote
Old 21st November 2003, 08:16 AM   #6
diyAudio Member
 
EchoWars's Avatar
 
Join Date: Dec 2002
Location: Left of the Dial
Well, I cracked open a few service manuals to see what they did. Sansui uses a lot of 1f electrolytics for this. The one Kenwood that bothered to bypass had a 0.47f film cap. Several Marantz amps I have manuals for use 0.1f caps, and a largish Sansui BA-F1 has a 3.3f electrolytic in parallel with a 0.047f film cap. Seems to run the gamut.

On the circuit above, I stuck in a 0.56f 50V stacked film cap (since that's what I got laying around). Seems to have made little to no difference in distortion measurements, but also hasn't seemed to hurt anything. I'm inclined just to leave it there.

Maybe at higher frequencies it might actually do some good?

(BTW peranders, I always appreciate your replies...seems when no one else is interested, you always have something to say. Thanks.)
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
bypass caps for bypassing mylar caps? crippledchicken Multi-Way 4 27th February 2008 04:24 AM
Tweaking: Bypassing spindle motor with caps: Yay or Nay? Scorpio(DDMF)! Digital Source 5 25th March 2007 12:58 PM
Are Polystyrene caps suitable for digital circuit decoupling/bypassing? homemade Parts 8 29th September 2006 03:37 PM
BIAS and B+ caps WKZ or NH TBM Tubes / Valves 0 26th January 2006 12:00 PM


New To Site? Need Help?

All times are GMT. The time now is 02:47 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2