Volume drop when playing and hiss noise at turning off from one channel - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 2nd March 2015, 06:11 PM   #1
diyAudio Member
 
Join Date: Mar 2015
Default Volume drop when playing and hiss noise at turning off from one channel

Hi, my name is Giulio and I'm new to this forum, so hello everyone!

I'm starting this thread because I have one of those weird issues which don't happen all the times I use the amplifier, therefore it is difficult to make an accurate diagnosis.

My brother's Copland CTA405 (an integrated tube amp), which is about 7-8 years old but with very little listening hours on its back, has an intermittent problem: quite often, but not always, I hear a very noticeable and persistent volume drop from the left channel after a few minutes of operating (with no additional noises); if I turn the amp off (which is what I normally do when this happens), I hear a hiss with some little cracklings from that same channel, lasting about 3-4 seconds. If I turn the amp on again, the problem often disappears (with no noises when I turn the amp off, this time), or it appears again after some more minutes of listening.

I already excluded a speaker problem (I recently moved to another house with new cables and speakers, with no variations in the problem) or a tube problem (if I invert the left and right preamp or power tubes, the problem is always on the left channel, when present). Bias test point voltages are steady.
As the problem only appears after the amp has warmed up, and it seems to appear less often if I bias a little colder than suggested (41mV vs 45 mV), I suppose it is something related to temperature/overheating. In your opinion, what do I have to check?

Thank you in advance,
Giulio
  Reply With Quote
Old 2nd March 2015, 10:11 PM   #2
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 7
It's difficult to diagnose remotely, but it could be a couple of things. Cold solder joints could cause problems like this. It could also be a case of marginal stability. The left channel could be breaking out in oscillation at super sonic frequencies. That would account for the loss in volume. You'd need to o'scope to see the oscillation.

Marginally stable amps can frequently go unstable at power off. As the cathodes cool, and the voltage drops, so does the open loop gain. This could shift the operating point past the -1.0 + j0 Nyquist instability point. Yes, not having enough open loop gain can have detrimental consequences under gNFB. You might want to check out components in the feedback loop (phase compensating capacitor gone bad) or the Zobels across front end voltage amp plate loads, or compensating capacitors/Zobels paralleling the OPT primary. Duff parts there can compromise stability.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 2nd March 2015, 11:05 PM   #3
diyAudio Member
 
JonSnell Electronic's Avatar
 
Join Date: Dec 2012
Location: The Jurassic Coast, England. GB
Send a message via Skype™ to JonSnell Electronic
I would check for dry joints and poor connections.
__________________
Support for Flying Mole, Allen & Heath, Valve amps, older designs & 2CVs. www.jonsnell.co.uk
  Reply With Quote
Old 3rd March 2015, 04:46 PM   #4
diyAudio Member
 
Join Date: Mar 2015
Thank you very much for your help
  Reply With Quote
Old 3rd March 2015, 08:30 PM   #5
diyAudio Member
 
Join Date: Dec 2014
Location: London
Does the amp have manual bias trimmers?
If yes, try turning the same side trimmer a little back and forth a few times, putting it back where it was.
It could be that the wiper is corroded and this could clean it up.
It could be the bias drifting due to this.
  Reply With Quote
Old 11th March 2015, 10:07 PM   #6
diyAudio Member
 
Join Date: Mar 2015
Thank you for your smart suggestion: I tried it even though there is no measurable bias drifting, and in fact I had no results.

One thing I forgot to mention is that the noise coming from the left channel at switching off, when present, is independent from the volume level, and happens even if I switch on the amp with the volume closed, I wait, and I switch off without opening the volume at all.
  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
AKAI 4000DS MkII irriating hiss when not even playing PgrAm Analogue Source 10 22nd July 2014 04:09 PM
Philips CD-101 static noise in one channel while playing TBL Digital Source 1 12th January 2013 09:20 PM
Shanling PH100 - Non-volume/input dependant hiss through one channel danza Headphone Systems 2 21st October 2011 10:42 AM
Correct gain drop when playing one channel on two speakers halfgaar Everything Else 6 12th November 2006 06:53 AM
Noise thru speaker when turning off Dan2 Car Audio 4 4th November 2006 09:46 AM


New To Site? Need Help?

All times are GMT. The time now is 10:29 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright ©1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2