Reverse engineering Krell KMA 160

[jwb]

Originally posted by Zero Cool
Now if i could just get the schematics for a KSA-250, i would be in heaven!

My plan is to turn these KMA 160s into a pair of KSA 80. My friend is going to share the cost and he'll keep one. Full instructions will be included here, naturally.

[jwb]

I completed the reverse engineering and now I'm just making nicer drawings. Here's the power supply. No surprises, it's a basic zener-regulated supply. Outputs are ±85V and ±38V. This supply runs the frontend and the output drivers, but the output itself runs from a separate, unregulated, bulk supply.

[jwb]

Here's the output stage. Points A, B, C, and D are driven from the input, a pair of FET followers and dual differential voltage gain stages. There's also a DC servo. Both of those drawings to follow.

[jwb]

I decided to draw the input stage with the CAD program. Tired hands...

[jwb]

Here's the servo. That's all the info you need to build your own KMA-160. Just add a suitable power supply for the output stage.

[jwb]

Some notes:

C1 and C2 in the output appear to have been added as part of a rework. They are on the wrong side of the board and stuck in the holes for some other components.

R3 and R5 in the input were certainly added in the rework. Initially the bases of Q8 and Q18 were driven directly from the divider R1/R2. There is a trace under R3, which has been manually cut to accomodate the change.

R56 is used to adjust the output DC. The 5K pot in the output is used to trim the bias.

The value of the zener on the servo is unknown. It's less than 25V.

There are many places where a 4K7/4K7 divider is used to bias the base of a transistor. For example, R44/R45 in the servo. This divider is shown many times but in the real amp there's only one such pair of resistors for each rail.

The amp I am working with was destroyed when 87V zener in the power supply exploded. Apparently, the amp didn't shut down, and the DC feedback circuit quickly destroyed itself. It might be advisable to use two zeners in the high-voltage supply, for redundancy. Also there should be a shutdown circuit in case of power supply failure. The Krell has neither.

The input of this amp is DC-coupled without protection. You could easily destroy the input FETs with a hot-plugged cable. I would advise adding zener protection on the input, and possibly also coupling capacitors.

[X.G.]

The great works!

Thanks a lot,jwb

Could u share the PCB and internal wiring pictures of it?

regards,

X.G.

[jwb]

Originally posted by X.G.
Could u share the PCB and internal wiring pictures of it?

OK, here are some pictures. The first is the main circuit board, containing most of the circuits diagrammed here.



This is what a KMA-160 looks like when you take it to pieces.



The main power supply. Note the quad supply, for stereo operation.



Here's one of the four output boards.



And this is another output board mounted on a heatsink.



Enjoy.

[X.G.]

Thank you very much,jwb

Encouraging by you,I work hard at writing a post to share my experience/idea which repair/build a normal PP power amp.

But my English is poor.... wish to finish it in a week.

X.G.

[Sze]

nice job JWB

What will you use for the power Fets, Q1,2,3,4, in the output stage? Are the VN and VP0210N5 available anywhere?

YOu didn't specify the voltage of the Bulk supply for the output stage. Do you happen to know what it is.

I'd sure like to built this amp but I think I'd better try something a little simpler first. I'll be keep this info you posted for future reference.

Regards,