I have build Elektor mini crescendo bach in the 90's. last year i re-caped it and re-wired it.I also use a separate and stabilized power suplly for the first stages. The amp is very qiuiet and sounds great. Two months ago left channel started to sound very low. I had to turn on and off a few times in order to recover. Some days ago this trick did'nt work. Left Channel continues to sound low. Any ideas?
Hi,
i own an Mini Crescendo too, nice amp with failures. But i got it stable. Nostalgia 😉
Have you measured the voltages in the amps to get a comparison ? First measuring i would make at the input stage. The original zeners deliver about 3,9 volt. The second measuring at the bias pot. Perhaps there is a different. The rail voltages are equal ?
My supposition is a problem with a capacitor. If you could recover for some times and now the fault remains, may could be a faulty capacitore ore something else. May a bad connection let the voltage drop...
Peter
i own an Mini Crescendo too, nice amp with failures. But i got it stable. Nostalgia 😉
Have you measured the voltages in the amps to get a comparison ? First measuring i would make at the input stage. The original zeners deliver about 3,9 volt. The second measuring at the bias pot. Perhaps there is a different. The rail voltages are equal ?
My supposition is a problem with a capacitor. If you could recover for some times and now the fault remains, may could be a faulty capacitore ore something else. May a bad connection let the voltage drop...
Peter
Attachments
Wathever the problem you should add a proper compensation with two 47pF/150-200V capacitors, one from T8 collector to T7 base and the other from T10 collector to T9 base, Elektor has been quite uncautious in this respect, they explained in the 140W
crescendo article that they relied on the FETs input capacitances as shunt compensation, but that doesnt work for many reasons,
the amp will oscillate more often than not.
crescendo article that they relied on the FETs input capacitances as shunt compensation, but that doesnt work for many reasons,
the amp will oscillate more often than not.
The osciltation is gone if the zobel get a 100 nf capacitor instead of 22 nF. 22 nF is to small. Next trick is to solder two resistors 22k from driver collector
T8/T10 to ground and between the drain and the gate an extra 33pF , and between the source and the gate a 330pF capacitor at 2SK 135 or similar. No oscillation anymore. I changed the transistors BC546/BC556 to 2SA970/2SC2240, MJE 340/350 for drivers. Instead of the 3,9 volt zeners you can choose 2 matched red led. 1µf capacitor parallel to the pot. The pot may a 200 ohm trimm pot, perhaps 47 ohm in row. With matched SJ 49 and 2 SK135 i have 0 mV offset.
More Tricks...😉
The mini runs tuned...😊
T8/T10 to ground and between the drain and the gate an extra 33pF , and between the source and the gate a 330pF capacitor at 2SK 135 or similar. No oscillation anymore. I changed the transistors BC546/BC556 to 2SA970/2SC2240, MJE 340/350 for drivers. Instead of the 3,9 volt zeners you can choose 2 matched red led. 1µf capacitor parallel to the pot. The pot may a 200 ohm trimm pot, perhaps 47 ohm in row. With matched SJ 49 and 2 SK135 i have 0 mV offset.
More Tricks...😉
The mini runs tuned...😊
I have added two 47pF capacitors as walab explains, i also changed the zobel capacitor from 22nF to 100nF and putted an 100nF across the pot The oscillations got better but not dιsapeared. They dissapeared when i changed the pcb decoupling capacitors with 1000 μF// 1μF//100nF according to guidelines from Andrew C. Russell https://hifisonix.com/wp-content/up...B-Design-Guidlines-for-Minimizing-Hum-1-1.pdf. BUT the reason for my post is that left channel sounds very low. I measured voltages all over the pcb. The are OK and at least equal to right channel. DC-offcet 11mv, the power mosfets are warming up equal each other, the rail voltages equal. (45 volt for the output stages and 50volt stabilized for input and driver stages). But the problem remains. Left channel sounds very low.
Sure, that the problem occurs in the amp, not in the preamp ? A defect at the input ? RCA measured ?
If you have tested, forget about my questions...
The pot resistor seems to be ok, drivers too, because of the 11 mV offset... Voltage is similar in the boards...
I am not an expert in electronics....the recover problem is strange to me. As i wrote, my supposition is a problem with a capacitor.
Honestly no other ideas in the moment...🙁
Peter
If you have tested, forget about my questions...
The pot resistor seems to be ok, drivers too, because of the 11 mV offset... Voltage is similar in the boards...
I am not an expert in electronics....the recover problem is strange to me. As i wrote, my supposition is a problem with a capacitor.
Honestly no other ideas in the moment...🙁
Peter
Please connect the input terminals of both channels temporarily. If this »cures« the issue, it's cause is somewhere in the preamplifier. If not, check capacitors C4, C5 for shorts.
Best regards!
Best regards!
Problem solved!! And the winner is.... Kay Pirinha. There was no connection between C4 and C5 because of bad soldering. Thanks to all for your recommendations.