Amplifier Problem - Need help! - 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 15th April 2011, 08:05 AM   #1
diyAudio Member
 
Join Date: Feb 2005
Location: India
Default Amplifier Problem - Need help!

Hi,

I use an 845 Tube based power amplifier and just got a brand new SS preamp to replace an ARC.
I am encountering a strange problem. When I take the volume control past 46 on the display I start getting a high pitched whine which becomes a loud hiss with intermittent cracking past 50. At 45 it's dead quiet. This happens regardless of there being an input connected to the preamp or not.

Could this be an impedance mismatch? Any way to diagnose/fix the problem? I've sent a message to both manufacturers and am yet to hear from them.

The specs are below:

PREAMP
Input resistance:
10Kohm RCA
600ohm XLR (This is the input I use)
Output resistance: 75 ohms
Maximum gain: High Gain 12dB
Max output voltage: +/- 20V

POWERAMP
Input sensitivity 1V for 200W output @ 6 Ohms
Input impedance 10Kohm
Output Impedance 4-8 Ohms

Thanks..
  Reply With Quote
Old 15th April 2011, 08:34 AM   #2
diyAudio Member
 
mastodon's Avatar
 
Join Date: Jan 2011
It's an oscillation. The first thing to do is to find out if it originates in the preamp or in the power amp. If you connect the new preamp to another power amp, does the problem still occur?
  Reply With Quote
Old 15th April 2011, 10:55 AM   #3
diyAudio Member
 
Join Date: Feb 2005
Location: India
Hi, I am going to try with another power amp later today to confirm if there's any problem with the preamp.
In case the preamp is OK, what can I do to remedy the problem?
  Reply With Quote
Old 15th April 2011, 03:53 PM   #4
diyAudio Member
 
Palustris's Avatar
 
Join Date: Aug 2006
Location: Cape Cod
Well, just from the specs you give above it is clear that you don't even need a preamp. Notice that the amp has an input sensitivity of 1 volt. That means that it will be driven to full power output (probably 20W not 200W) by 1 volt. Now look at the specs. for the preamp: maximum output is 20 volts!

It is no wonder that you can't turn up the preamp; you are driving the amp to full output with the preamp idling. You don't need a preamp with that amp. Does it have a level control on the input? If not get a passive preamp.
  Reply With Quote
Old 15th April 2011, 04:37 PM   #5
diyAudio Member
 
Palustris's Avatar
 
Join Date: Aug 2006
Location: Cape Cod
Just a reminder: standard specification for CD player output voltage is 2 volts. That's why you don't need a preamp with this amplifier.
  Reply With Quote
Old 15th April 2011, 05:15 PM   #6
diyAudio Member
 
Join Date: Apr 2011
Location: San Diego, California
Yes, it is an oscilllation, and yes, the output voltage of your preamp is way in excess of what you need. One solution here is to acquire a passive preamp, so that you'll have input-selection capability.
  Reply With Quote
Old 16th April 2011, 08:45 AM   #7
diyAudio Member
 
Join Date: Feb 2005
Location: India
Hi,
An electronics engineer friend suggested I put a 10K resistor in parallel at the Power amp input which has sorted the problem.

Palustris, I did try my DAC direct to the power amp (volume control from PC) to try and got the same (probably louder) noise as described above with no input on the DAC. I also do need a pre since I have 2 sources and my power amps have no controls.

Thanks
  Reply With Quote
Old 17th April 2011, 05:46 AM   #8
diyAudio Member
 
Join Date: Feb 2005
Location: India
I jumped the gun. The noise is not there if I select an input on the pre that's not connected. The moment I select an input connected to the source (DAC in this case), the noise starts although at a higher level.
On another note, I am getting a buzz/humm (like a ground loop). Could these be related?
  Reply With Quote
Old 19th April 2011, 06:14 AM   #9
diyAudio Member
 
Join Date: Feb 2005
Location: India
I have been recommended to add a Line input transformer at the power amp to sort the buzz and also increase the impedance.
Can someone suggest the right model from Jensen/Lundahl based on the specs above?
Thanks
  Reply With Quote
Old 19th April 2011, 07:04 AM   #10
Alkis is offline Alkis  Greece
diyAudio Member
 
Alkis's Avatar
 
Join Date: Oct 2004
With a passive preamp as suggested, you will overcome all of these problems. And you can add source selector too.In my opinion,this should be the first attempt.
Just remember, that volume control has to be far bellow 10k. If the source is 50R, then a 600R pot would do the job nicely.
  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
Problem with amplifier MisterTedster Car Audio 3 23rd October 2009 05:42 AM
problem with my amplifier marcojhp Solid State 3 24th January 2007 08:34 PM
sub amplifier problem testarossa2k Subwoofers 4 25th August 2006 06:36 PM
Amplifier problem empire Parts 0 4th April 2003 09:43 AM
Amplifier Problem !!! Dj BASS AMP Solid State 14 15th November 2002 01:12 PM


New To Site? Need Help?

All times are GMT. The time now is 11:43 AM.


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