Update for Elektor Crescendo Power Amplifier 1982

[HumNSmoke]

Thanks man. I'll try this first. 0.22ohm is critical? I thought I could also use Caddock's MP9100 (3.5W each air free) 2*0.1Ω=0.2 instead ... but very expensive

[Bensen]

No, the 0.22 is not critical in this design.

I'm having 0.25Ohm there. I've chosen to go for 0.25 instead of 0.2 (4resistors in parallel instead of 5) because I haven't matched those otput devices.

[HumNSmoke]

No luck with PR02 resistors. I don't know where to dig, I will try to recap the amp now...

[HumNSmoke]

Before I recap (I miss the 100µF/10V ones), I tested the 47nF (just some ceramic from the drawer) tip over the bias resistor: it did half the job : no more oscillations neither when I plug the output power cables alone or with the fully loaded passive filter.
Although the oscillations remain when I load the amp only with the bass speaker (Z=8Ω, R=5.7Ω L=2.2mH), or with the passive filter when I remove the tweeter.

[Esperado]

Originally Posted by HumNSmoke

or with the passive filter when I remove the tweeter.

Not enough load at hf ? Increase the zobel cap value ?

[HumNSmoke]

~ 68µF is ok? I found in wikipedia that zobel C is L/R², for me 0.0022/5.7² ? Thank you for been there for me man.
BTW, I found from here your fine protection circuit.Did you do one for mono block?

[Esperado]

Originally Posted by HumNSmoke

~ 68µF is ok? I found in wikipedia that zobel C is L/R², for me 0.0022/5.7² ? Thank you for been there for me man.
BTW, I found from here your fine protection circuit.Did you do one for mono block?

The Zobel: 0.022µ. You can try 0.033 or more.
Did-you have 0.22Ohms non inductive resistances at sources of your power FETS ?
About my protection, just remove the non used parts form the schematic.

[HumNSmoke]

Hi 'Toph
Yes I had. But now they are 4*5 Vishay-Dralotic non inductive 1Ω metal film PR02.
I will try your tip to increase the zobel cap, con!
The strange is that the amp ran in oscillation just when I plugged a simple-XLR-cable-without-no-load-at-all on the outputs (USE<HAR>H07RN-F 3*2.5mm², 5 meters long, 1 conductor between 2-2 and the two others between 1-1 to get some 2*3.75mm² equivalent)... maybe a strange load for the amp from a HF point of view?

[HumNSmoke]

Cool Esperado, I added // a 15nF over the 22nF, no more 2MHz oscillation, even when I remove the 47nF from the bias P1. Although, not enough with bottom door shield mounted just close to the input. 47nF removed oscillation with the shield mounted, and 56nF removed all issues with shield and the passive filter and no tweeter or with just with the boomer or not load at all
Would you advise to increase the rated power for the 10Ω Zobel now the capa is more than twice the initial value? e.g. 3W PR03 (I don't know how to calculate this).
I shall now tweak the 2nd channel and check if hum is still here. Do you know a small audio frequencies generator for PC please?
Thanks again for your help man!

[Esperado]

Originally Posted by HumNSmoke

Would you advise to increase the rated power for the 10Ω Zobe

No need. if your amp is stable, there will be very little power in the resistance.
Condenser in // with the zeners and bias potentiometer are just here to reduce zener and pot noise and impedance. You can // any value you want, but remember, as the 100µf is an electrolytic, it has an inductive impedance in it. Paralleling with film caps can high the result impedance at hf, this self // with your film cap is a resonating circuit. If your value makes your amp stable, everything OK.
Test your amp with 10Khz square waves to be sure no ringing on the front edges.