2sk135 2sj50

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Hai Peranders,

I have a hybride amp wih the K135/J50 combo. It's running in Classe A with a bias voltage of round 1 Volt. I'm using them only for my woofers now (100Hz/24db crossover) and therefore I would like to change the operating point to Classe AB to run things less hot.

Have You any idea how low the bias voltage can go without sacrifising linearity. The amps are running without negative feedback.
 
The K135/J50 are rather special, they have constant gm over a certain current! This is good for linearity but bad for output impedance. Since it is about bass, just use your ears! Turn down the idle current until you actually can hear the crossover distorsion. Play at approx. 1 volt output signal. The sound will be like a burned voice coil, like it scratches towards the magnet.
 
It always worked pretty fine without feedback. I used two pairs of per channel. I don't worry to much about output impedance, up to 1 Ohm is okay, my loudspeakers don't mind. The Fets sound very powerfull in combination with the mu-stage I used as a driver, but it simply just does not have the "magic" of a good tube amp. I've tried them with overall feedback but I found that the gain in control and speed didn't weight against the loss in air and detail. It sounded somewhat nervous with the feedback and began to sound more like an all transistor based amp.

Personally I think there are better sounding Mosfets nowadays.
 
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K135/J50 work fine without feedback. Never had any problems and I didn't notice coloration.

I don't use any resistors, the MOSFETS are coupled directly ( I do use "gridstoppers", resistors to the gate ). Made quite a few amps with them that still run after 10 years of uninterrupted use ( always on ).

In my amps I ran them with 400 mA- 500 mA idling current. When not used curent was switched to a low value for prolonging the life of the devices. They do suffer from aging when used in class A.

Still like them very much despite TO 3 case and the trend of thinking that newer devices are better. Technically maybe right but soundwise K135/J50 really are OK. Never saw one that failed BTW ! Even saw an amp that had the devices "swapped". It didn't work but the FET's survived :bigeyes:
 
I have also a special feeling for these transistors. My first MOSFET amp was built in 1983 and is still working. The original transistors delivered 2 x 285 W at 4 ohm but later I changed them and then got 2 x 500 W!

I you are pleased with the sound, no reason for changing but I have a friend which had difficult speakers and together with a high output impedance amp they sounded peculiar. They sounded allright with a "normal" amp.
 
Hi,

I also build 4 or 5 amps with 2sj50/2sk135. The oldest must be almost 20 years old by now and it´s still working.

I also experimented with bias. Big improvement up to 200mA, after that still getting better but more subtle.

I also used them without source resistors but found it a bit difficult to check the bias per device so now I´m using 5x1watt resistors per fet.

Here´s a picture of my current amp

william
 

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Hello.

I need help with an amp that i have received, it was a kitset DT MOS 250.

An externally hosted image should be here but it was not working when we last tested it.


this included a working PCB, filter caps and center tapp transformer (50V 0V 50V).

the main problem if the wire up of the power transformer.
the design is this:
An externally hosted image should be here but it was not working when we last tested it.


but i dont know exactly where the output goes.

could someone please help me.
also what does the +ve and -ve on the PCB repersent, is this the speaker output? And where does the power +ve GND and -ve go?

thanks.
 
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Joined 2002
Patrick said:

also what does the +ve and -ve on the PCB repersent, is this the speaker output? And where does the power +ve GND and -ve go?


Hi Patrick

It looks like your website won't allow external linking to images:(

A long as they are under 100k you can an image to each post by using the button at the bottom of the reply page. If this fails, email them to me and I will post them for you.

I will reserve further comments until the pics are up, cos its easy to make mistakes without all the evidence;)
 
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Joined 2002
Ok

Input looks ok, +&- ve are the power inputs, and the connections at the top are probably earth and speaker output. To pin down the earth a little better could you post a pic of the track side of the PCB?

The return from the speaker will go to a star earth point,( as will the other earth connections), and that is something that will need to be made when the enclosure is built.
 
I've got more images on the same web site www25.brinkster.com/pbridgman/main.htm

I also have another one of these amps which is half wired up (these were Salvaged and some wires were cut but they do work)
and if you look carefuly at the power wires they do go into the two close unmarked wires (ignore the colour coding i think it is incorrect) and the wire that i have marked as disconnected had come from a power on delay unit. And the two -ve and +ve wires are not connected to any thing.
but i am still confused about which is +ve power, -ve power and GND.
 
It looks like:

The one marked +ve is positive power, the one marked -ve is negative power.

The wire to the right, above that resistor with a coil on the outside is speaker out, The left one is speaker and power common. (this seems to be right looking at unit number 2)

Input signal is the right one and common the left one. Is the screen on the input cable connected to the left one? (if it isn't that is weird)

What is the resistance between input common and output common? You might need to connect them externally.

Ugh... the wire colors on the second unit are totally messed up :) Red for ground??? Never seen that before... Black wire for speaker out? Both positive and negative power red :)

But are you sure what you are doing? Be careful at least...
 
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