Update for Elektor Crescendo Power Amplifier 1982 - diyAudio
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Old 3rd February 2004, 12:41 AM   #1
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Lightbulb 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 everyone who built this amplifier has or had some problem with high frequency oscillations and 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, and 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.

So, here is a listing of proven and unproven ideas:
This should really be a table!
|| No || What || Why || Reference ||
|| 1 || 47nF in parallel with the bias resistor P1 || HF osc || Proven effective by me ||
|| 2 || Non-inductive source resistors R27-R30 || HF osc || Recommended by Elektor and others ||
|| 3 || RC-chain R32 C16 moved from pcb to speaker posts || HF osc || Recommended by others, used in other constructions ||
|| 4 || 1 nF between n-channel source resistors R27 and R28 (alternative to no 2) || HF osc || Recommended by Elektor ||
|| 5 || Replace cascade transistors T8 and T10 BF469/470 with better MJE350/340 || Improved sound quality || Recommended and proven by others ||
|| 6 || Active DC regulation || Improved sound quality || Recommended and proven by others ||
|| 7 || Resistive collector load for the voltage amplifier (it sees a varying impedance in the gate of the mos fet's) || Improved stability || Unproven here, but used in other constructions ||
|| 8 || Different feedback paths for LF and HF || Improved stability || Unproven here, but 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.

Best regards,
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Bjorn
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Old 20th February 2006, 03:16 PM   #2
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I have problem whith oscillations in my Crecendo 250W mosfet, it oscillations is about 4 Mhz??
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Old 20th February 2006, 04:08 PM   #3
Bensen is offline Bensen  Belgium
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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.
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Old 20th February 2006, 04:16 PM   #4
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Is 4 Mhz no problem? When i take a my hand on t8 the oscillations is gone and the luodly noise?Why?
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Old 22nd May 2006, 02:50 PM   #5
Bgt is offline Bgt  Netherlands
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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.

Click the image to open in full size.
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Old 22nd May 2006, 09:05 PM   #6
maudio is offline maudio  Netherlands
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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

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.
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Old 23rd May 2006, 06:09 PM   #7
Bgt is offline Bgt  Netherlands
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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.

Click the image to open in full size.
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Old 24th May 2006, 12:25 PM   #8
joensd is offline joensd  Germany
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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
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Old 25th May 2006, 01:09 PM   #9
Bgt is offline Bgt  Netherlands
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Quote:
Originally posted by joensd
parallel 5*1Ohm 1W resistors)
thats what I did.
Oh, and gain is about 65x
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Old 25th May 2006, 01:36 PM   #10
Wynand is offline Wynand  South Africa
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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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