Leach amp powering up trouble

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congo

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Joined 2010
#1
Hello everybody,
first to introduce myself, i`m Nikolay from Bulgaria ( excuse me for my not good English).
I have a small problem with one leach amp and my try to get it running.
The problem is that everything seems to be ok when i powered it up for the first time, but if i shut off the power and then turn it back on there is no sound and the two 100 ohm resistors in series of the supply starts to smoke. I believe that it is oscillating but i cant figure it out why. the amp is build following this scheme http://zosilnovace.eu/200W/Leachaktivsch.png , i builded and the original amp from Dr. Leach site but the result is the same. I use bc546 and bc556 in the input stage, MJE340/350 for VAS, MJE15032/33 as drivers and MJ21193/94 in output stage. The power supply is +-52V coupled with two 10000uf capacitors. From the test i made i think that the problem is in the input stage but i cant narrow it down. I don't have access to scope

I will be very happy if someone can help me to get it right.

Thank you all in advance
 
#2
Do you mean the two 100 ohm resistors that drive the protection circuitry for the first driver stage? I don't see 100 ohm resistors in series with teh power supply.

One thing with the schematic - for the 10 ohm series output resistors, there should be a coil of wire in parallel. Did you remember to wrap the wire around that resistor?

But, if those two 100 Ohms resistors are smoking, it might be due to some blown output or driver transistors.
 
#3
Jeff R said:
Do you mean the two 100 ohm resistors that drive the protection circuitry for the first driver stage? I don't see 100 ohm resistors in series with teh power supply.
Click to expand...

The two 100 ohm resistors is acting like a fuse now (ie instead of fuses on supplay rails i`m useing 100 ohm resistors for test)

Jeff R said:
One thing with the schematic - for the 10 ohm series output resistors, there should be a coil of wire in parallel. Did you remember to wrap the wire around that resistor?
Click to expand...

The 10 ohm resistor has wire in parallel.

Jeff R said:
But, if those two 100 Ohms resistors are smoking, it might be due to some blown output or driver transistors.
Click to expand...

As i told, the amp is playing ... if and only if all the capacitors are discharged before plugging it in. There is not blown transistor ... i checked twice.

🙂
 
#5
AndrewT said:
Have you set the output bias to the value stated by Leach?
Are you passing that output bias plus the amp quiescent current through those 100r resistors?
Click to expand...

It is alive. Thank you Andrew. It was all due to 100 ohm resistors in place of the fuses. Is is a little bit strange because i never hit that kind of a problem till now. The amp performs great. It is not that good as this one i build with LME49810 but it still kick some butt 🙂

Thank you all
 
#8
10p Miller compensation capacitors are far too small. 47pF would be better. Really, you'd need an oscilloscope to look at the output to see what it's doing. Does the Zobel resistor (the 10R resistor in series with 100nF capacitor on the output) get hot with no signal?
 
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