120w elektor amp sees the daylight again

[Tekko]

Hello!

Anyone remember the 120w elektor amp published in 1981 ?

I have got mine running again, i got it about 2-3 years ago in very crappy condition. It was someones diy project and it was full with cold solders so i had it apart to redo it but it never happened so it have been in a box since then.

Until the day before yesterday when i tried to make it running again but blew the output transistors after 2 secs, it happened because the amp would start motorboating heavily as soon as a load were connected.

As i had broken a leg on one of the original transistors used in it i had to make my own darlingtons, i happened to find a BD911 and a BD912 as well as a 2SC2238 and a 2SA968 that were driver transistors, i thought *perfect* and it sure was. Hooked up power and the amp started playing for the first time in years.

In the original schemo the ps voltage should be 44.4v, i give it 25v but it doesent seem to be a problem yet.

Heres the pics of my amp, i wanna redo the board as its crappyly made by someone:

[peranders]

It's a virtue to have short wires! 1 nH per mm plus all other possiblity to pick up noise.

[Tekko]

You mean the wires between board and output transistors ?

This is just the test setup to see if it would work, and it did.

[peranders]

This is a general rule and it concerns all wideband circuits. The faster, the more important.

Strive to have max 1-5 cm between the driver and the output stage. If it's possible it good to have everything on a single pcb.

[Tekko]

If i have had the possibulity to make own boards that would have been done already. I was thinking og redoing the thing on a piece of veroboard.

The bias pot is not a good choice either, if the wiper looses contact it sends the amp into self destruction.

And that i run it at just half the voltage, can that cause problems ?

[ChocoHolic]

You should connect the pot as a variable resistor by adding a connection from the wiper to R7 in the elector schematic.

If the wiper loses contact, then bias will be small.
But be careful, after adding the connection you should
move the wiper to R7, before turning on.
... and then readjust the bias, because the added connection will
shift the bias set up.

Have fun
Markus

[Tekko]

Ok i'll try it.

[jacco vermeulen]

I remember making one on a piece of perf board a century ago.

A lot of people will be envious seeing original Toshiba 2SA968/2SC2238s, and B-versions to make it worse.
And Roederstein EYS capacitors, and Fukujima Futaba 0.22 Ohm meanderband resistors.
These parts are meant for an amplifier which is far superior than ancient Elektor circuits that should be forgotten.

(Oops, i am beginning to sound like UpuPavel)

[ChocoHolic]

As you are using a very low supply voltage also the
stabilisation effect of the 9V1 zener diode will be lost.
Bias of the input stage will be little low, but should still be
acceptable.
(Or could this be the reason for motorboating? Not very probable,
but possible.)

In order to get back zener stabilisation, you could reduce R9.
For 25V supply any value between 3k3 and 1k8 should be fine.
Keep R5 as it is.

[Tekko]

Hum that mod dident work very well, i had to turn the pot all way to the 680 ohm resistor to get any descent sound at all bur even then it doesent take mutch bass to make the treble click and crackle, otherwise it just sound ****

Also it makes a whisteling sound at startup that it doesent do without the connection between R7 and the pot wiper.

Could be because i just run it at half the voltage tho.

Chocoholic: Actually i have a nice 9.37v at the zener.