Reverse engineering Krell KMA 160

[Jozua]

Hi

Nice and very impressive but now the big question- how does it sound ?


Jozua

[jwb]

Pretty dang great. There is the slightest buzz if you stick your ear right in the speaker, but otherwise the performance is exemplary. I've been listening to Dire Straits, Stevie Wonder, Michael Jackson, Natalie Merchant, and Paul Simon. All have a very satisfying sound.

I ordered a pair of B&W 803 to go with this amp, so we shall soon see in earnest.

[cmjohnson]

Hi, newbie here.

This is a really great thread for me to start out here with, because of the unusual situation that surrounds how I got my amps.

I have a pair of Krell KMA-160 monoblocks that I got for 500 bucks plus shipping.

Why so cheap?

Because they'd been in a house fire, that's why!

But...they weren't burned, they had only been water damaged when the amps got hosed down and put out with the fire.

And then they ended up being left to rot in a basement for two years, without so much as a hosing down, brushing out, or any type of attempted cleaning whatsoever.

This is not particularly good for electronics.


"Fortunately, ah keep mah feathers numbered for just such an emergency.".... Foghorn Leghorn.


I have well developed electronic assembly and soldering skills, and I'm a decent technician.

Working on one amp and once circuit board at a time, I tore the amps down to a bare baseplate and aggressively cleaned and resoldered every last connection, using strong activated flux, and then cleaned the boards again to remove all residue. I cleaned, replaced, or refurbished ALL the hardware, to the last screw.
And I did out of circuit testing on every driver and every output transistor BEFORE applying power. Good news there, out of all the drivers and outputs in both amps, the total failures were ONE output transistor and its emitter resistor.

Both amps came up without a problem the first time I brought them up, using a variac so as to limit any potential carnage.

They've been working with absolute reliability ever since, for roughly the past seven years.

They drive a pair of Aerial Acoustics 10Ts. The combination is detailed, natural, punchy, relaxing to listen to, and very dynamic.



The new boards you've made for your amp are very intriguing.

I might as well ask straight out: Would you be interested in making a few more for me at a fair price?


BTW, I found one inaccuracy in your circuit description, at least, it's inaccurate compared to my amps.

The output devices in my amps consist of twelve pairs of complementary outputs that are much improved, high bandwidth versions of the Motorola MJ15024 and MJ15025 complementary output pairs.

15024s and 25s will work in these amps, but the devices that Krell used are a much improved custom device made for them by Motorola. Motorola required a minimum order of one million of these devices (for each type) and Krell went ahead and had them made.

The standard 15025/24 transistors have a 5 MHz bandwidth, while the custom devices have a 30 MHz bandwidth, and are superior in several other measurable parameters as well.

I think that if anyone attempted to load all 24 output sockets with 15025s, the amps would sound bad assuming they worked at all. Half the devices would be PNPs in an NPN circuit!


These amps are fully balanced differential amps from the input jack to the speaker terminals. Every transistor used (except maybe in the power supplies) will have a complementary partner.



CJ

[Elso Kwak]

Originally posted by cmjohnson

BTW, I found one inaccuracy in your circuit description, at least, it's inaccurate compared to my amps.

The output devices in my amps consist of twelve pairs of complementary outputs that are much improved, high bandwidth versions of the Motorola MJ15024 and MJ15025 complementary output pairs.

15024s and 25s will work in these amps, but the devices that Krell used are a much improved custom device made for them by Motorola. Motorola required a minimum order of one million of these devices (for each type) and Krell went ahead and had them made.

The standard 15025/24 transistors have a 5 MHz bandwidth, while the custom devices have a 30 MHz bandwidth, and are superior in several other measurable parameters as well.

I think that if anyone attempted to load all 24 output sockets with 15025s, the amps would sound bad assuming they worked at all. Half the devices would be PNPs in an NPN circuit!


These amps are fully balanced differential amps from the input jack to the speaker terminals. Every transistor used (except maybe in the power supplies) will have a complementary partner.



CJ

Hi CJ, I wrote about the transistors here:
KISS the Ultimate Amplifier

[jwb]

Hey Elso, I noticed the KISS amp includes THREE LEDs! As everyone knows, any amp containing fewer than TWO LEDs is ****. Originally, the KMA-160 contains NO LEDs! Well you can imagine my disappointment. Naturally I have added two (per channel) to the KSA-80 Mk. II.

I have heard that Krell's transistors were somehow special, but the ones in my unit look like plain jane MJ15024 in all respects. Perhaps they are special on the inside. The new ones (MJ21195/6) are indestructible perforated emitter units with huge safe operating area. The seem likely replacements, but I haven't blown any outputs yet.

I will consider building up boards for you, but I gotta be honest, the parts cost is pretty high. Are you planning to build two more channels from scratch, or replacing your old ones?

[jwb]

Originally posted by cmjohnson

BTW, I found one inaccuracy in your circuit description, at least, it's inaccurate compared to my amps.

Are we talking about this drawing?

I think I drew it with 12x15024 and 12x15025, did I not? Or perhaps I don't understand your point. Obviously I wouldn't load all NPNs into a complementary amplifier

In any case I didn't touch the outputs on mine. They are the original Krell factory assemblies. I also used the four original driver boards.

[Elso Kwak]

Originally posted by jwb
Hey Elso, I noticed the KISS amp includes THREE LEDs! As everyone knows, any amp containing fewer than TWO LEDs is ****. Originally, the KMA-160 contains NO LEDs! Well you can imagine my disappointment. Naturally I have added two (per channel) to the KSA-80 Mk. II.

I have heard that Krell's transistors were somehow special, but the ones in my unit look like plain jane MJ15024 in all respects. Perhaps they are special on the inside. The new ones (MJ21195/6) are indestructible perforated emitter units with huge safe operating area. The seem likely replacements, but I haven't blown any outputs yet.

I will consider building up boards for you, but I gotta be honest, the parts cost is pretty high. Are you planning to build two more channels from scratch, or replacing your old ones?

Here are the right typo numbers.
http://www.diyaudio.com/forums/showt...381#post493381
I have this information from pictures in German audio magazines. The latest Krell amps are using these.

[cmjohnson]

You wrote this in your second post on this topic:

PS, the output stage consists of 24 MJ 15024 transistors...

That was what I was referring to.


I am interested in your boards as retrofits for my existing amps. Roughly how much do they cost to make?

At the very least, I'd be interested in the boards themselves (unpopulated) and a parts list of what you put in them. I can source
them myself, and assemble the boards myself.


Krell did use the MJ15024/MJ15025 devices for a long time. It wasn't until a bit later than the KMA-160/KSA-80 that they bought the newer, updated custom versions.


A related project that I want to get into would be rebuilding PS Audio 200C power amps, which are notoriously delicate, depend on obsolete, almost impossible to source Darlington pair transistors (MPSA95, if I remember right), and have an annoying habit of blowing everything from the input all the way through every output device. They've got a great power supply, a robust current delivery system, great heat sinking, and lousy reliability. I'd like to come up with a RELIABLE and RUGGED output board for retrofitting to these amps.

Their first problem is that they're DC coupled, hence their sometimes spectacular explosions. The second problem is that they run on 75 volt rails but only use three pairs of TO-3 cased output transistors per channel, meaning that their current delivery capacity falls far short of their theoretical output capacity (by voltage) at lower mpedances. At four ohm loads, the 75 volt rails (53 VRMS) suggests a 700 watt max output, but the power handling of just three pairs of devices falls far short of that, so they blow easily. As
soon as any device hits about four amps, they die horribly.

Those newer devices with the perforated emitters sound like a good step in the right direction. But really, the amps just need some real DC protection between stanges and they need more pairs of outputs....and a circuit board that wasn't drawn out with a Sharpie!

CJ

[Zero Cool]

Now see, this is a thread i love. that is some SERIOUS craftsmen ship right there! a man after my own heart! So many people want to just destroy these classics and put whatever is popular at the time into them. It is just a shame to walk up and see a 69 Hemi Cuda that someone hacked apart and put whatever mods they thought was cool on a car that was cool to begin with. or seeing a 69 les paul goldtop guitar that someone decided to sand down and repaint pink metallic sparkle. what a huge shame.

Now these Krells, may not be the worlds best amp. but these are history man! this is the beginning of the High end revolution. a generation or two after the plastic age 80's split of audio into high end and crap Best Buy sells.

Here is a true craftsmen that has restored a piece of audio history to like new condition. This i admire and respect. Mega kudo's to you JWB!


Zc

[cmjohnson]

Hi. It's been a long while since I posted here.

Those boards look great!


My KMA-160s are still running strong, with no faults of any kind since I rebuilt them, right when I got them, coming up on...nine years now? Has it been that long?


I'm seriously considering building up a set of your boards to upgrade my amps. And the output board has some limited life parts that are probably due for replacement anyway.


Incidentally, my amps are a bit different than yours started out as. Mine have PC boards in the brute force rectification system, mounted
over the reservoir caps, which are screwed to the board via standoffs
and machine screws. Possibly mine are later production models.


I'd sure like for the 160s to have sustained plateau biasing, or at least dynamic bias, because they really can heat up a room! But
the sound is simply fantastic, to this day. I have never heard
anything objectionable about their sound unless someone was
playing a rap CD on my system....and that's justification for a severe beating!





CJ