KRELL KSA Amplifier series mods

[Dr.H]

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?

[Luke]

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

How did you get these values?

[Dr.H]

Previous experience with a different amp. Perhaps not optimised for this setup, but better than not using them at all.

[Dr.H]

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?

[kgodfather]

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.

[h_a]

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 -

[Dr.H]

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

[Dr.H]

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