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Update for Elektor Crescendo Power Amplifier 1982

Ok ..i had already planned to follow the advice bjornagain has posted and go a little futher :smash:
Among others re-locate the gate stoppers so they are soldered directly to the gates of the MOSFET's, the distance from there original placement and for the gates are in my pov./experience to far away to be any good, the optimal RG value has to be determinated by messure and audition tests
compensation of the Cg by add of a 300pF cap. betwin GD of the 2SK135 will also add some stability,
there is difference betwin SJ50/2SK135 (Ciss = 900/600pF)
R1/C2 will be changed for 1K/1nF (160KHz -3dB)
R32/C15 altered for: 5x47R-2W MP/100nF
Removel of the C1,C2 :dodgy:
Change of the R1 value to perfect fit of my passive pre-amp :angel:
Parr. of all the el-caps will make wonders for the stability and the sound of the Crescendo :D
 
Long time, no see

The nice thing with the Crescendo is the pure simplicity, I think, and any modifications must keep it that way. I would like to share one idea from a fellow member.

First a more proper way to bias the cascode T8/T10, which will enhance almost everything.

Second R3 is moved and both differential inputs references meets the signal ground at the board entry.

Then I can not see the purpose of C12/C13 more. Suggestions?

The mods is quite simple. Unfortunatly I have not tried them yet, but I will.

Schematics attached!
 

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And again

Very fast I entered a simple schema of recent proposed mods, using ECX-devices for which I had SPICE-data for. With shown values calculated THD is 0.005% at rms 40 W 1 KHz, no overshots using square wawe, and nice falling spectrum (2nd larger then 3rd and so on). This is theory.

In practice supply and zener must be bypassed, proper HF-protection at input applied, output filter and HF-impedance loading applied, output device doubled with proper source-resisitors, current limiting applied, and an active dc-servo inserted controlling output offset.



It is still close to the original, with some sonic improvements in theory, and hopefully more stable function.

Still, it all remains to be proven in practice!
 

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I have an Elektor amp I built a long time ago, maybe 1983? I have since misplaced the original Elektor magazine (German version) that contained the schematic. Attached is a picture of the board. At the time I made a single board containing 2 channels for a stereo amplifier. IIRC the original had an individual board per channel.
 

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Just a closer view of mostly one channel.

And my question, would anyone still have the schematic for this amplifier? I would like to replace the 220V transformer with a 110V version. Would be nice to have documentation on the amp in case of troubles...
 

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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,

Thank your for your hints. I build the Elektor Crescendo with good success, but most of all your hints are not working as expected. I changed the design in five points:

1. Non inductive metal film resistors for R27/28/29/30.
2. Lowpass filter with 6k8 and 330pF attached to the input.
3. Better driver transistors MJE350/340 instead of BF469/470.
4. A seperate stabilized power supply with +/-80V for the pre-stages.
5. The 10 Ohm resistor R3 needs to be removed to prevent ground loops.

No hum and no noise is audible. The amplifier works stable. Since the power transistors 2SJ50 and 2SK135 are available again, the Crescendo can still be build.

Holger
 
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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