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-   -   Update for Elektor Crescendo Power Amplifier 1982 (https://www.diyaudio.com/forums/elektor/27423-update-elektor-crescendo-power-amplifier-1982-a.html)

bjornagain 3rd February 2004 12:41 AM

Update for Elektor Crescendo Power Amplifier 1982
 
I have lived with this 180 W amplifier for almost two decades. It has been proven to be good sounding and reliable over the years, even though I must admit a little tricky sometimes. In the posts in the www.diyaudio.com I am glad too see that the Crescendo has good reputation, and many have built it and likes it a lot. The Crescendo is still very competitive I think, and should be a contender to the Hall of Fame of diy-amplifiers, if there is one!

My guess is that almost everyone who built this amplifier has or had some problem with high frequency oscillations and perhaps some hum induced from the power supply. High frequency oscillations can be hard to get rid of. There have been some tweaks published in the Elektor I have tried, beside some that I have collected from postings on the Internet.

My amplifier had a lot of high frequency oscillations, and the RC-chain on the output went up in smoke. But the solution was easy; a small capacitor across the bias resistor solved it! I think it should been there in the first place. It is common practice in many costructions.

So, here is a listing of proven solutions to kill or reduce HF oscillations:
  • 47nF in parallel with the bias resistor P1. This killed all mine HF osc.
  • Using non-inductive source resistors R27-R30 is recommended by Elektor and have worked for others.
  • Put 1 nF between n-channel source resistors R27 and R28 is also recommended by Elektor.
  • Move the RC-chain R32 C16 from PCB to the speaker chassi posts can kill some HF osc. This is a solution used in various builds for this reason.
Suggestions for improvements of stability and performance:
  • Replace cascade transistors T8 and T10 BF469/470 with MJE350/340. This may improve sound quality. Or not.
  • Active DC regulation for lower DC offset. Could also improve sound quality.
  • Resistive collector load for the voltage amplifier (it sees a varying impedance in the gate of the mos fet's) may improve over all stability. This is used in many constructions.
  • Different feedback paths for LF and HF signal could improve stability. This is used in other constructions.

I still have some problems with hum, and I have rewired a lot a tried many other things. Nothing helps. Stabilizing the input section didn't. To compact housing perhaps? Left and right channel does not hum equally either, and it is very irritating!

On the other hand, one of the channels have a defective mos fet, but still works...

Anyway, I think this amplifier deserves another decade using some of the tweaks. I was a bit disappointed with the new Crescendo ME, I can't use it to upgrade the original Crescendo, and there are problems with high frequency oscillations in that one too!

I will be happy to receive comments and help to update the reference columns with links to diyAudio forums.

danne_linkinpar 20th February 2006 03:16 PM

I have problem whith oscillations in my Crecendo 250W mosfet, it oscillations is about 4 Mhz??

Bensen 20th February 2006 04:08 PM

4Mhz is no problem. Just leave it like that if the amplitude is only a few µV.
But I think 4Mhz is coming from the cascade VAS. Take a decent 3.9V zener diode, make sure you measure 3.9V across it! And use a decent cap over this zener.

danne_linkinpar 20th February 2006 04:16 PM

Is 4 Mhz no problem? When i take a my hand on t8 the oscillations is gone and the luodly noise?Why?

Bgt 22nd May 2006 02:50 PM

used my Crescendo for many years and it still is a nice amp.
Did some mods on them many years ago. PCB's were hand made in the 80's so I am trying to redo them again. Never had oscillation problems BTW, only in the beginning.

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maudio 22nd May 2006 09:05 PM

I once built the mini-crescendo version for a friend of mine (the mini was a scaled-down 70-90W version, only 1 pair of output devices and lower rail voltages).

Strange enough it would oscillate when I increased the idle current to about twice the recommended setting. I tried to run it more into class A only to find the zobel network resistor going up in smoke. Restoring it to the recommended setting solved the problem and made it run stable even into highly capacitive loads :cannotbe:

I must admit I never investigated this phenomena any further but settled for this easy way out. Probably something to do with changing mosfet parameters.

I must agree that it's a very good sounding amp.

Bgt 23rd May 2006 06:09 PM

The original design by Elektor had some nasty switching glitches and started oscillating in the wild. After making a new PCB(by hand drawing) for it and modding it is a super stable amp. Not a powerhouse, 100W 8Ohms at +_60V), but enough. Still use them, but every 10 years change all the electrolyt. caps./wiring.
The amp.is build in complete symmetry. Crosstalk at 20Khz= -100DB. Best I ever achieved with an 1 transformer stereo amp.

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joensd 24th May 2006 12:25 PM

I haven´t built the original (but the newer ME)
but have read several times that the biggest oscillation problem are caused by the output resistors (0,22) if they´re of a "high" inductive type.
Many exchanged them with for example metal-oxyde resistors.
(if you can´t get them at that power rating, just parallel 5*1Ohm 1W resistors)

greets
Jens

Bgt 25th May 2006 01:09 PM

Quote:

Originally posted by joensd
parallel 5*1Ohm 1W resistors)
thats what I did.
Oh, and gain is about 65x

Wynand 25th May 2006 01:36 PM

would anyone have a copy of the pcb layout and schematic around? My boss built one and had the elector magazine, but lost it. I think he built the 198* version.


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