KRELL KSA-50S - Noise distorsion HF - 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 30th September 2014, 03:56 AM   #1
diyAudio Member
 
Join Date: Feb 2014
Location: Riviere-Rouge, QC
Default KRELL KSA-50S - Noise distorsion HF

Hi, thanks for reading,

This is my first post, even if I'm a regular read-only guest since many years!

I read previous post concerning the KSA-50S and found no answers, tried google too, etc.

My 50S play well the first few minutes then noise is heard from both channel specialy with high frequencies like voices, piano, etc.
I looked inside and visually, everything seems perfect, no burst caps... only 1,5'' long brownish circuit tracks at one legs of the 1k2/20W WW tube-shape resistor on the faceplate board.

I already planned to recaps (alum. elec.) all boards, changed relays... regular maintenance after 20 years I thought.

I'm asking you members, do you have any more suggestions or advices (more than what I planned to do) that can be related to high sound noise ?


Thank you!
  Reply With Quote
Old 2nd October 2014, 09:05 PM   #2
diyAudio Member
 
LineSource's Avatar
 
Join Date: Sep 2003
Location: SiliconValley
"My 50S plays well the first few minutes then noise is heard from both channel specially with high frequencies like voices, piano, etc"

I would ground one amp input at a time and put a 10Khz sine wave on the other channel input and measure both amp outputs with an O'scope. If you do not have an O'scope, test one channel at a time with a speaker while the other input is gnd.

Guesses:
1) One channel oscillates and transmits high frequency noise to the second channel input. This could be a faulty feedback or Miller capacitor, maybe a transistor. ALSO... the Vbe thermal bias circuit in one channel can drift and create ClassB crossover distortion that is broadcast on the shared gnd. Easy to see with an O'scope. Testing one channel at a time might uncover these fault.
2) A faulty ground or soft start circuit.
The KSA50S uses dual mono amp packaging which just shares the AC plug ckt, power switch with soft start circuits(relay in latest, thermistor in original), and three grounds... AC plug gnd, chassis gnd, signal gnd which connects through a resistor to the transformer shield. Check lose mechanical connections, and diodes+resistors+caps associated with power.
  Reply With Quote
Old 2nd October 2014, 11:01 PM   #3
diyAudio Member
 
Join Date: Feb 2014
Location: Riviere-Rouge, QC
Thank you LineSource,

I don't have a scope... so I'll skip and go to guess #2 for the moment!
All new caps, relays, resistors are there or on the way.

The soft start circuit is on the pcb call Faceplate board and there is 2 relays, one small 12V dpdt (nte R74-11D1-12SM) and a bigger one that we also found on the driver boards (omron 4pdt 24v)... and a thermistor!
Both relays will be changed and the thermistor on R23, but there is nothing write on it.
Do you have suggestions?
I have schematics for almost all pcb's, but I only have the ''component side'' drawing for the Faceplate board, with no indication on values! grrr

I saw elsewhere on forum guys building a KSA-50 clone and talking about a 10-20ohms, 5W thermistor, is that makes sense for the 50S too?
  Reply With Quote
Old 3rd October 2014, 05:46 AM   #4
diyAudio Member
 
LineSource's Avatar
 
Join Date: Sep 2003
Location: SiliconValley
Quote:
Originally Posted by 819jeans View Post
Both relays will be changed and the thermistor on R23, but there is nothing write on it.
Do you have suggestions?
Your safest path for the thermistor is to measure the resistance COLD and HOT. It could be a resistor. Krell used CL60 thermistors in early KSAxxx amps. GE SENSING / THERMOMETRICS|CL-60|Thermistor | Newark.com

Without a O'scope, you could ground the input of each channel and listen for distortion on the other speaker.

Without a O'scope you could ground both inputs and use a DVM to measure the voltage drop accross the output emitter resistors.... Vbe bias test.... should measure 15-23mv accross a PNP and NPN emitter resistor for optimal ClassAB bias which gives the lowest crossover distortion.
  Reply With Quote
Old 4th October 2014, 01:41 AM   #5
diyAudio Member
 
Join Date: Feb 2014
Location: Riviere-Rouge, QC
Good for the thermistor but when he failed, I should read open or he can give me good readings but still generate noises when on duty?
I wrote to RCD Components inc., they made and maybe still make custom parts for Krell, the emitter resistors (RCD160XD) on the 50S for sure.
I'll try to get more info concerning other parts like thermistor, resistor, and so on...
For the moment my machine is on the bench and partialy dismantle. You tell me the voltage drop accros the emitter should measure 15-23mv for AB bias but the KSA-50S is class A (or kind of -sustained plateau...-) but still class A?, the values still apply?
  Reply With Quote
Old 4th October 2014, 03:16 PM   #6
diyAudio Member
 
LineSource's Avatar
 
Join Date: Sep 2003
Location: SiliconValley
Thermistor thinking: If one soft start relay is defective, then all the transformer current in that channel goes through the soft start resistor/thermistor.

You should measure about 400mv across the 0.68 ohm emitter resistors to get 50 watts class A output.

My comment on 15-23mv Re drop was about the lowest Vbe bias level before very nasty ClassB crossover noise would get generated, and might affect the other channel through the common ground.

After your re-Cap, it would be useful to gound one input and listen to the other channel alone for distortion. Your ears are the O'scope.

Last edited by LineSource; 4th October 2014 at 03:22 PM.
  Reply With Quote
Old 7th October 2014, 01:43 AM   #7
diyAudio Member
 
Join Date: Feb 2014
Location: Riviere-Rouge, QC
Ok, thanks for details, I'm on the job for the next few days!

To be continued...
  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
Help repair Krell KSA-50S Wizard Solid State 11 28th May 2014 04:14 AM
KSA-50S Krell fragmental Solid State 3 22nd October 2011 06:00 AM
KSA-50S ruined? Patrik Floding Solid State 3 19th August 2008 05:48 PM
Noise in one channel of krell KSA-80 Deoreo Solid State 2 30th March 2004 07:22 PM


New To Site? Need Help?

All times are GMT. The time now is 03:36 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