Still some problems with Crescendo. Oscillation or what?

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#41
Hi Joensd...

Sorry I don't have better picture than those which are above.


1. Which one is better option:
- I disconnect input cable shield from amplifier board and join that shield with short wire to starpoint or
- I disconnect input cable shield from input socket (RCA) and take wire from input socket ground directly to starpoint?

2. And should I add about 10ohm resistor to that shield?
 
#42
Also some problems with Crecendo ME

Hi all,

A few weeks ago I finished my two Crecendo ME amps (only the electronics, no enclosure). I bridged them to drive one speaker of 8ohm. I doubled the mosFET's. I was very happy with the sound coming out of it.

Here is the problem:

Three days ago I plugged the AC power cord in the net. First there came distorted music out of the speaker. And a few seconds later the AC power cord burnt throught.
So, I checked everything, but I didn't found anything suspicious.
I replaced the power cord and plugged the amp back in the net. At first the music sounded good and normal. But then I smelled something. One of the two amps got VERY hot :hot:. The 4 mosfet's of one amp were about 80°C within a few seconds. The other mosfet's of the other amp stayed cold.

Then I checked all the voltages over the resistors, but everything was fine.
So, I thougt that mine mosfet's were cooked. I replaced them with new ones. But these were alse heating up very fast. So, with the mosfets is everything OK.
Could it be the drivers (mje)? Between the gates off the N and P channel mosfet's I measure 4V. This should be OK, not??
I don't thing the amp was oscillating because the resistor in the zobel network was't hot at all. Also the source resistors were not hot.

Can anybody tell me wat the problem is? I have now idee??
I inserted a new shematic, that's easier to search the problem.

Greetz

Ben
 

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#43
Replace both compensation caps ( C4 , C5 ) and give capacitor ( M1 / 160 V ) between + / - rails - amp surely oscilated. If you have variac ( regulation transformer ), try start with him - oscilation will be probably depend on rails voltage, so start from zero.
 
#44
Thanks for the reply Upupa Epops,

You've said to replace C4 and C5. Probably you mean to replace them with other values, or not? Or do I just have to stick new ones in the amps? Caps like this can't be defect, can they? They are wima FKS caps.

And why is the behaviour off my amp appeared suddenly while he worked fine for about two weeks? The other amp just the same, and this one works perfect ont he same rails.

Greetz

Ben
 
#47
Bensen,

sometimes a fuse just after the ac power cord will help against smoke (or fire)😀 First I would put one in your amp!

Since your amp worked for a few weeks and the other channel is still working I would look for a bad component or a bad solder joint (wich gets worse with every heating/cooling cycle)

william
 
#48
Hi

I've put a fuse in the AC cord, So that is fixed. I replaced the caps and add a few more caps over the rectifier bridge and over the rails.
I've noticed that the output devices were heating up less fast than before, but still, much to fast when I look at the good working amp.

I think I found one reason that could be the cause of the oscillations.
I build two pcb's for the two amp's. One board with all the output devices and the bias network. The other board contains the diff. input...(voltage amplification)
NOW: those two board are connected with two bundels of three wires. These three wires are: the wire from the amp's output back two the diff input (feedback) and two wires from the drivers to the bias network.
BUT I twisted the three wires together:att'n:

Could this be the cause???

I recently bought a scope. Is it possible to see the oscillations on the scope? How to measure this, direct on the output.

Thanks in advance

Ben
 
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