meanman1964 said:
Below follows the mods as Elektor - April/2001-pg.58 issue presents regarding the article Hexfet Power Amp from September/1995 issue:
- at the solder side of the board, fit a 27nF capacitor in parallel with R31;
- connect a 20k resisitor between the collector of T8 and ground;
- connect a 20k resisitor between the collector of T9 and ground;
- replace R20 by a 1.8k. ( here, the value may be other. It depends on the output power transistor that you use - IGBTs are not available and power mosfet, some has low VGSth and others has higher VGSth);
- replace R17 and R18 by 390 ohms;
- replace R3 and R4 by 33 ohms.
Re: I Fixed it, FINALLY.
Doesn't R3-4 need to be 33 Ohm and not 330 Ohm?
raise0 said:Hi! My arm is free again, after 6 months of immobilization. So last night I tried to continue my work on this hexfet amp. I started by replacing resistors and caps as suggested by elektor mag. and other members from this forum. Placed some decoupling caps, changed fuses to 5Amps, replaced trimmer P2 and T7 and R3, R4 to 33ohms. But while I was soldering the 20K resistors to the collector of T8 and T9, I FOUND IT.
My PCB after a lot of part replacements lost a lot of traces. So my circuit slowly transformed, into a point to point connection. Something went wrong during that and I couldn’t find. It took several months but I saw the base of T7 connected to feedback loop (where R25,R28,R30,R31 meets)
I stopped everything, fixed that trace and I powered up the amp…
220mA and 4mV to output! FINALLY …waited 20~30 minutes and connected speakers… IT WORKS!
Cannot believe I was so stupid and BLIND before.
I am not sure what this connection did to the circuit and which part was “cancelled”. I mostly get it, but not quite sure.Any comment on that is welcomed. Anyway… Although it works now I will complete all the following mods to both circuits.
F1, F2 = 5A Fuse
100nF decoupling caps
27N over R31
20kohms resistors from collector T8 & T9 to ground
R3, R4 to 33Oohm
R20 to 1k8
R17, R18 to 390ohm
R25, R28 to 22ohm
R26, R29 to 120ohm
Furthermore I will place heatsinks on T8 and T9… They get extremely hot even with no signal applied.Probably they are oscillating… Also BOTH transformers make a slightly audible sound. That has no effect to output, but it wasn’t there some time ago.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Doesn't R3-4 need to be 33 Ohm and not 330 Ohm?
Re: Re: I Fixed it, FINALLY.
R3 and R4 are parts of compensation network of complementary differential input stage and originally they were 100 ohms ( 100 ohms with 2.7nF in series - Elektor September/1995 ) but now the mods change both, R3 and R4, to 33 ohms each and not change the capacitors. As I said, it is compensation network and with the others mods it helps to avoid oscillations.
meanman1964 said:
R3 and R4 are parts of compensation network of complementary differential input stage and originally they were 100 ohms ( 100 ohms with 2.7nF in series - Elektor September/1995 ) but now the mods change both, R3 and R4, to 33 ohms each and not change the capacitors. As I said, it is compensation network and with the others mods it helps to avoid oscillations.
meanman1964 said:
In this amplifier, the output stage has voltage gain ( this is not source follower - is a commom source stage ). So, it may oscillates and the 27nF capacitor will be there to avoid this. This capacitor is not critical since you keep it in safety range. There is a value below 27nF where that stage may to oscillate ( I don´t know wich ) and above that value it will narrow the bandwidth of amplifier. I believe that 27nF is a optimal value arrived in measurements made by author of the article.