Cure for HF oscillation on positive cycles only - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Pass Labs This forum is dedicated to Pass Labs discussion.

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
Thread Tools Search this Thread
Old 26th June 2002, 02:30 PM   #1
diyAudio Member
Join Date: Jan 2002
Default Cure for HF oscillation on positive cycles only

Last night I hooked my oscilliscope to the outputs of my power follower. There looked like there was an oscillation (>100Khz) on the positive increasing part of the sine-waves.

I decided to increase the cap I have across the base and collector of the NPN transistor used to sense voltage on my CCS. Essentially the same as the Pass CCS using the MPSA18. My CCS is connected to -12v using a IRFP140N.

This cleared the oscillation.

After listening to this though... it seemed that the highs were either A) more detailed or B) more "etched".

Is this the correct way to cure this oscillation? Or would it be better to use a 5uh inductor and a 1 ohm resistor in parallel on the outputs?


p.s. my circuit is a IRFP140N connected to +12 then another IRFP140N used as a CCS to -12. Gain is ~1. DC coupled output.
  Reply With Quote
Old 26th June 2002, 02:54 PM   #2
diyAudio Member
Join Date: Feb 2001
Location: Columbia, SC
Before you start playing with caps, try dressing the power leads. For instance, when I was building the prototype Mini-A, the placement of the negative power lead was critical for getting rid of a little glitch very much like what I think you're describing. The positive power lead had no effect whatsoever.
Also try moving around the input leads, etc. as your circuit may have other sensitivities.
I had something else in mind, but it fluttered away before I could pin it to the Riker mount. I'll get out the butterfly net and see if I can recapture it.

  Reply With Quote
Old 26th June 2002, 02:58 PM   #3
diyAudio Member
Join Date: Jan 2002
Default power leads

dressing power leads???

I haven't heard that one.

I did try physically moving them around... also I have a 100uf cap across each +12/ground and -12/ground at the amp (not at batteries) that should take care of that issue.

What is your take on paralleling batteries with caps? Is this a non-issue since the amp IS pure class A?

  Reply With Quote
Old 26th June 2002, 07:54 PM   #4
The one and only
Nelson Pass's Avatar
Join Date: Mar 2001
I think increasing the caps on the NPN reference for the
current source is probably the ticket. Also, you always
need 100 to 300 ohms in series with the Mosfet Gates.

It is common for parasitic oscillation to muddy the sound,
and removing it will sharpen up the top end.
  Reply With Quote
Old 27th June 2002, 12:20 AM   #5
diyAudio Member
Join Date: Feb 2001
Location: Columbia, SC
Dressing in this case means moving them around, in the sense of where they fit within whatever you choose to use for a chassis.

  Reply With Quote


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
RF is the cure for cancer?!? gain The Lounge 11 4th September 2016 02:47 PM
Sinusoidal, Pink Noise & Duty Cycles. OMNIFEX Solid State 14 13th March 2009 12:59 PM
Hertz or Cycles? Bob2 Everything Else 10 9th June 2003 11:50 PM

New To Site? Need Help?

All times are GMT. The time now is 02:16 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2