KRELL KSA Amplifier series mods - 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 26th June 2009, 07:04 AM   #1
diyAudio Member
 
Join Date: Apr 2002
Default KRELL KSA Amplifier series mods

In the late 1980's KRELL brought out a series of amps, KSA-80, 80B and KSA-200. The mono-blocked versions of these were the KMA-160 and KMA-400 respectively.

The KSA series are regarded by many as the "sweetest" sounding amps Krell produced, even in comparison with their more recent stuff.

I've applied a few simple mods to these already very good sounding amps (at least they sound good on the Apogee Scintilla, a notoriously difficult speaker).

There are two power supplies, being supplied by a single 1.5kVA transformer. The one supply, for the output stage, uses around 90,000uF per rail per channel. Rectifiers are simple block based. These should be snubbered (100R in series with 330nF) which results in a lower noise floor.


For the input and voltage gain stage, Krell uses a generic rectifying diode (1n5408) and just 1880uF of capacitance per power rail!

I've simply raised this by 10,000uF, benefits include tighter and cleaner bass, especially the very low stuff.

Also replaced the rectifying diodes with MUR860, a fast recovery lower noise device. This results in enhanced overall detail across the spectrum.

Lastly, the gain stage uses just 470uF for additional power supply smoothing and this has been raised to 1500uf (space limitations and the stage benefits from upstream capacitance anyway). The benefits of the last mod are not immediately clear and I'll perhaps have to evaluate further, although a subtly lower noise floor seems to result.

So in summary mods that work are:

Replace all ERO caps (noisy red caps typically)

Main supply diode snubbers (100R in series with 330nF)

Raise the value of the input and gain stage power supply caps

Replace the generic rectifier diodes for the the input and gain stage power supply stage with MUR860 and snubber these.

I'd like to explore two more ideas:
1. Voltage regulation for the front end is provided by zeners. I've been told that zeners are noisy and may try using a constant current source on the zeners.
2. In the DC servo, there is a 10uF cap being used in a feedback configuration made by SAHA and apparently a high precision cap. Not sure if there will be sonic benefit in using a quality film cap here.


Any other mods out there?
  Reply With Quote
Old 26th June 2009, 10:22 AM   #2
Luke is offline Luke  New Zealand
diyAudio Member
 
Join Date: Sep 2001
Location: Wellington NZ
Send a message via AIM to Luke
Quote:
Main supply diode snubbers (100R in series with 330nF)
How did you get these values?
  Reply With Quote
Old 26th June 2009, 12:00 PM   #3
diyAudio Member
 
Join Date: Apr 2002
Previous experience with a different amp. Perhaps not optimised for this setup, but better than not using them at all.
  Reply With Quote
Old 17th December 2009, 12:41 PM   #4
diyAudio Member
 
Join Date: Apr 2002
Finally got around to implementing a constant current source on the zener regulating the the positive rail (70V) and this brought substantial benefit in terms of resolution across the frequency range.

Would it be worth doing the negative rail as well?
  Reply With Quote
Old 20th March 2010, 12:04 AM   #5
diyAudio Member
 
Join Date: May 2009
Default schematics on Krell 80B

Hello Dr. H,

I have Krell KSA-80B and KRC-2. If you have schematics for either one, I'd love to study them.

Would you also post some photos of your modded amp? I started modding about a year ago - I modded several dacs and a couple of CD players so far - I'm still a beginner. I have been replacing the capacitors and setting up the output stages using either capacitors or the transformers. Pictures would help me a lot as well as any tips.

You mentioned replacing the old ERO caps and the power rail. What did you replace them with? What were their values?

Thank you very much.
  Reply With Quote
Old 20th March 2010, 07:48 AM   #6
h_a is offline h_a  Europe
diyAudio Member
 
h_a's Avatar
 
Join Date: Sep 2006
Location: Graz, Austria
Quote:
I've been told that zeners are noisy
No issue for power amps - by the way, current sources are noisy as well

Have fun, Hannes

PS: for sake of beauty, always do both sides + and -
  Reply With Quote
Old 5th July 2010, 03:14 PM   #7
diyAudio Member
 
Join Date: Apr 2002
OK, finally did the Constant current source mod on the 75V zener. Used a simple setup (3 diodes, 1 transistor, 2 resistors, about $0.50 worth of parts).

100V
l
l
1N914
l
l
1n914
l
l
MPSA92 (base to 1n914, emitter to 100V line via 150R resistor)
l
----1n914----75V zener
l
47k resistor
l
l
ground


Another good mod! This seems to remove the hardness I noted in my previous post and lowers the noise floor further.

In comparison, the unmodded channel is now sounding quite "muffled", with lower levels of detail.

Will listen more tonight.

Lastly, if this mod is good for the 75V zener, it will be good for the 39V zener as well. I'll be trying that soon.




I added a CCS (set for 8mA) to the 39V Zener. This zener reference feeds the input stage of the amp.

There were small gains in detail, not as much as with the 75V zener mod.

I think that's about it from my side, but would be curious to see/hear of any other mods that may bring benefits.

FWIW, I should add that I have been pleasantly surprised by this amp. Despite its 140W rating, it has been more than capable of driving my Apogee Scintilla's to comfortably loud listening levels, with bass remaining deep and detailed and no signs of distress in treble or mids being evident.

Now to mod the other stereo KSA80B and then run the system in vertically biamped configuration....
  Reply With Quote
Old 26th November 2010, 07:25 AM   #8
diyAudio Member
 
Join Date: Apr 2002
I've replaced the polyester 10uF cap being used in the DC servo with a polyprop. Once again, this brings more detail to the fore. No brightness or anything, just more "ambience".

I've also added a separate 300VA transformer for the input/gain stage (100vDC)-appears to lower noise floor even further.

If I were to rank the mods, with best mods first:

1. Add constant current source on 75V DC supply to input stage
2. Replace 10uF cap in DC servo and additional 300VA transformer on input stage
3. Add 10,000uF caps on input stage power supply

Last edited by Dr.H; 26th November 2010 at 07:29 AM.
  Reply With Quote
Old 11th July 2012, 11:30 AM   #9
diyAudio Member
 
Join Date: Apr 2002
Of course that CCS on the zener could be done by simply tying gate and source together on a JFET and tying this to a resistor, say 100R. The drain of the JFET would go to the input Voltage rail. Two components solution...
  Reply With Quote
Old 24th January 2013, 02:12 PM   #10
yairf is offline yairf  Israel
diyAudio Member
 
Join Date: Nov 2003
Location: Israel
Hi
I'm now contemplating a modding project for a KSA-200.
There are 6 main caps, and the cost of replacing them is outrageous
As for the secondary PSU - I usually give it more attention than main caps as it is critical for the micro-dynamics of the amplifier.
The original arrangement of Axial caps and Zener diodes, is rather crude. Yes - increasing the caps value could be useful, but that means you need to switch to Radial-shaped caps rather than Axial ones, which means mechanical changes as well.
I usually prefer to use shunt voltage regulators, replacing the zeners, which will also require mechanical changes.
More updates coming up soon
Cheers, Yair
  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
Krell KSA 100 type amplifier leffe Solid State 135 26th March 2010 07:44 PM
SME Series III (3) Mods & Suggestions Thomo Analogue Source 47 1st February 2010 09:57 PM
Audio Innovations series 800 reactivation + some mods Basill Tubes / Valves 3 11th December 2008 02:03 PM
Krell KSA S series corbelli Solid State 9 10th November 2007 04:00 PM
KRELL KSA300S Amplifier wilfredo Solid State 8 18th September 2007 05:05 AM


New To Site? Need Help?

All times are GMT. The time now is 02:45 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