DIY Amp Tweaking - 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 15th September 2004, 08:00 AM   #1
diyAudio Member
 
Join Date: Sep 2004
Location: Cambridge, MA
Default DIY Amp Tweaking

I have a pro class A/B amp that's outputting odd noise for small signals, like very quiet orchestral passagse. The noise is low in volume but noticeable. Increasing the input level causes the noise to also increase in volume, but eventually with a high enough input the noise go away. Decreasing the input level back keeps the noise away for a few seconds, but then it comes back. The noise isn't quite uniform; it sounds like it has more HF energy.

I suspect that the bias of the transistors might be wrong, causing them to slip out of the linear region, does this seem plausible?

Opening the amplifier, I see 2SA1302/2SC3281 transistors in the output stage; each channel has a pot which I assume is used to adjust the bias.

Does anyone have tips or advice as to how I can figure out what's wrong and fix it? I can get to an oscilloscope, but it's a lot easier for me to find a multimeter.
  Reply With Quote
Old 15th September 2004, 12:07 PM   #2
diyAudio Member
 
Join Date: May 2003
Location: quebec
The first thing I would check is dc level at the input of the power amp or dc level around volume control, is there is a dc signal at one of those spot it may unbalance or change the operation point of the amp an cause some noise.
  Reply With Quote
Old 15th September 2004, 02:05 PM   #3
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Hi choatie88

A noise that depends of the position of the volume pot....look like oscilation to me.(Oscilation dependent of the driving resistence).

Inspect with a scop...

Regards
__________________
Jorge
  Reply With Quote
Old 15th September 2004, 03:40 PM   #4
diyAudio Member
 
Join Date: Sep 2004
Location: Cambridge, MA
I'm not measuring any DC at the input or output.

Where should I look for oscillation?
  Reply With Quote
Old 15th September 2004, 10:16 PM   #5
diyAudio Member
 
Join Date: Jun 2004
Location: Knoxville
Quote:
Originally posted by Tube_Dude
Hi choatie88

A noise that depends of the position of the volume pot....look like oscilation to me.(Oscilation dependent of the driving resistence).

Inspect with a scop...

Regards
Yes sounds like RF osciliation, or just plain old "dirty potentiometer syndrome". If scope shows RF noise after cleaning, check the feedback loop, and miller feedback caps, or any caps in the signal path for that matter................bad solder joint maybe????
  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
Tweaking Mcintosh diyman Solid State 3 7th June 2006 05:48 PM
Tweaking B&W DM 602 s2 zverina Multi-Way 24 4th May 2006 12:12 PM
what CD buy for tweaking? LennyK Digital Source 0 29th July 2005 08:50 AM
Help with car amp tweaking sq_guy Car Audio 15 7th May 2005 04:14 AM


New To Site? Need Help?

All times are GMT. The time now is 05:48 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