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Sennheiser KAT 15 modification


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Hello all.


I was wondering if the vintage Sennheiser Kat 15 preamp designed for NagraIII, IV and E, , is good enough to make a project, to work this days for any recorder.

Dismantling one i've seen a big cilinder component inside, is a jensen transformer?

How can be the sound?, i've never listen throug one, any comments, experienced sound mixers?!


Thanks a lot!



kat 15.tiff

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They were kind of noisy.   It won't power a 48v phantom powered mic--only T-power condensor mics that are kind of hard to find these days.  It will work with a dynamic mic ok, but it was designed to be powered from a Nagra's accessory jack, I think with 10 volts.  If you have a Nagra reel machine it might be worth it to fix it, as a preamp to use with other gear probably not.  The cylinder inside is probably a capacitor, not a transformer.

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Hi Philip, thanks for your answer.

Today I dismantle the unit, and it's a transformer, as marked o the pcb. I guess it should be a good one.......?

I have a pair of T powered Schoeps ( and another CMC6 pair), so I will be able to test it, if I arrive to make it work.

I don't have a Nagra to test it.....

Here are the specs.

I'm afraid, to feed it with 12V ( from a T powered mixer, after building a correct cable) these old caps.....I need to look at caps voltage values, may be the unit can handle it ( as I see now, some tantalums are 35V)

Someone can have a circuit diagram? I cannot find it in the web.

This coronavirus enable us to make things, we never do, ja!


kat 15 data.jpeg78af4fb4-53f5-413d-a7f0-cc572131dc69.jpgd6868f92-e012-4b9e-a0e4-cace9d3a17a6.jpg


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Yes, you are right that is a transformer, although not one from Jensen.  I never took my Kat 15s apart in the old days, they just always seemed to work ok.  By now they are very old--I bought mine used in 1979 and they were old then!   If you are handy you might consider recapping it, but I wouldn't do any serious work to it until I could test it and see what you really have.  It's good you have T powered mics to try it with, maybe you could borrow or rent a Nagra to hook it up to for a few minutes to asses its condition.  That said, I'm, not sure the effort is really worth it, if what you are after is a good sounding and reliable portable mic pre for recording.

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Hi Philip,

thanks for your opinion, you are right, I don't know if it worth the pain, but, is just because I've time now, it's like a toy, a fun project.

I've just finished to recap a Uher 4000IC, imagine....

I've repaired a Dual 1019, and a 1229, and past friday, I suspected that one of my CMC4's ( dead in a film 2 years ago), maybe had a simple cap fix, and bingo!, a 1 $ cap solved the schoeps problem (220uf/10V) , amazing good new to me.


I'll try to power the preamp from a T power supply, and see what happens.

I keep in touch with results.


Have a good Sunday Philip, here, it's sunny.





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Hi David, thanks for your reply, yes I'm taking care of.

The problem I faced today, is that the connector (Nagra side, 6 pins), has 3 of the 4 cables unsoldered...so I don't know exactly ..

Dismantling the xlr3 mic side, I see ground+ one cable in pin 1 ( standard) blue pin 2 yellow pin 3.


So, my problem now is to wire the recorder side, so, if the signal is balances ( as I suppose), I wire pin 2 with blue and pin 3 with yellow, as mic conector ( decision I take, good or bad?). But I need to apply voltage in the recorder side, so i tested with a 9V batterie: + in the ground cable, and - in the other cable ( the one weld with ground in pin 1 of the mic connector).

I run a jumper cable from pin1 recorder side to ground in the pcb.

I sent the signal to a line in in a Mackie mixer. No sound, solo hum. Sensibility, hum level changes as I move With from dynamic to condenser, and bass changes, when I switch low freq cut.


I know this is confused to read, but any ideas are welcome!


Thanks for your patience.








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Hi Fabian,

The NAGRA 4.2  instruction manual says pin 2 on the Tuchel 6 pin plug is common to the chassis, pin 3 is the 3.73 uA input (audio input), pin 6 is  -10v stabilized and filtered for powering an accessory unit and 100mA may be drawn from it.

Cheers, Lloyd

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Hi Lloyd thanks for the information.

As the wires were unsoldered in the connector, I need to figure out what cable goes to each pin.

For that, the shortest way is a diagram, but I cannot find it on the web!


If anyone has a KAT 15 on the shelf, can take a look on what color cable goes to what pin in the Nagra side connector?


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Looking at the open Tuchel plug: pin 2 is yellow, pin 3 is blue, pin 6 is clear, braid is soldered to the metal outer shell which is permanently connected to the threaded back part. On the circuit diagram, pin 3 is connected to the unbalanced banana plug input on the left side of the Nagra 4.2.


If you can find someone with a 4.2 or IV Instruction Manual its a great read generally as well as a beautifully laid out Synoptic Diagram which details what is connected to what.




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Lloyd, thanks for your invaluable info

ok, now mic side :

XLR connector was soldered in a way it doesn’t seems to be good.

reading your info XLR can be:

pin 2 :  blue

pin 3 clear

pin 1 yellow ( or yellow+ braid) ?


I’m good with that?


I’ve the 4.2 manual at home,  i”ll put more time in the reading.

As I will “build” this preamp to use with another recorders ( I need to build a battery pack) my signal unbalanced out will be: 

imagine a banana out

Red banana conector : + blue cable  

Black banana conector ground yellow cable 


you agree with that as a way to start?


thanks again for your time and patience!





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Hi Fabian,


I have two KAT 15-2 units and they each have different brands of cable on the Tuchel side. The XLR side on both units is: pin 1=braid or braid and white,

pin 2=red, pin 3=blue. This cable has a black outer sheath. In T-power mode I read +9.89v on pin 1 and - on pin 3. 


The Tuchel connector I described to you has a cable with a grey outer sheath like your photos.


I also looked for KAT 15-2 circuit diagrams with no luck!


Not sure what else I can say except page 19 of the manual discusses the NAGRA line input and why they use current feed for accessories instead of voltage feed.

The banana plug input still wants a normal voltage feed.



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hello Lloyd, how are you.

Thanks for your answer, and info.

Two days ago, i bought from germany, a Diagram of one KAT-15 ( i thing the unit in question has been modified for 48V), and i've recieved from spain, some pictures of another unit, with black cable as the one you described.

Ok, now i'm good on wiring, codes on my unit doesn't works because it was repaired, and someone changed the mic side cables.....

But now i'm good, but anyway no signal.

I follow your comment, and make a read on page 19 about current feed and voltage feed.

I think here is the problem now.

My asymetrical out ( 6,3 mono plug, wired tip with equivalent of pin 3 in the tuchel, and ground with equivalent with pin 2, or pin 2+ braid in the tuchel) is current feed , and i'm connecting it to a Mackie mixer, that is waiting for a voltage feed.


I'll try to find how to stay in voltage feed out of the preamp ( as i said, my idea is to use it as a stand alone 12V preamp).


Good read, and important one, this page 19. i upload it here,as the diagram.

May be i can connect my outputs at a stage before in the preamp, cause i assume the preamp, converts the voltage feed to current feed.

Any ideas where i can try to do that in the diagram?


Thanks Lloyd!!!!, thanks all.







Schematic KAT15 - 3.pdf




diagrama tiff.jpg

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Hi Fabian,


While going thru my NAGRA accessories stash I found a KAT 15 P48. 

My no.1 KAT 15 works with a dynamic mic it does not work in T- Power mode, even though it puts out 9.89v,whereas the other two units work pefectly.


I emailed Sennheiser asking for a KAT 15-2 circuit diagram. They emailed a job no. so I wait to see if one turns up. Where did your circuit diagram come from?


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Hi Lloyd, thanks for your answer. You have a stock there!

Is from one seller that has this diagram along with his unit is a KAT 15-3 diagram ( is written in the lower part of the diagram, not present in my picture)

I'm reading now about a way to convert current to voltage.

If you have any news , I'm here!



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