fed up with bosoz

[serengetiplains]

Originally posted by cowanrg


what the heck is that?

Hi Cowan, the bosoz circuit operates differentially. One beneficial element of differential operation is that noise appearing on the power supply rail will be fed to both + and - legs of the circuit, and to the extent everything is equal on those legs (equal amplification per frequency, equal resistances, equal capacitances) will that noise be cancelled, including as amplified by the fets, on the output (think of combining two sine waves of same magnitude with 180 degrees phase difference between them). Any imbalance (inequality, if you will) in the operation of + and - legs will allow noise, to the extent of that imbalance, to pass into the output. With both hiss and hum, I would suspect a faulty fet, but you say you've replaced the fets. I wonder if you have a faulty resistor on either the + or - leg? With the amp idling, check voltage drop across all resistors. A faulty resistor could be a source of hiss and could allow one channel to amplify psu hum more than the other.

[poobah]

Check again,

I just sent it...

[cowanrg]

ok, more testing.

i found this out, which could be the issue. my coupling caps, the 10uf guys are only rated to 50v! i dont know why i have those in there, but i do. and they are non polar electrolytic black gates. i think they were a suggestion of peter daniel back when i listened to him

anyways, could they be causing some issues? i measured all the resistors after them in the circuit, and none are getting any voltage. they all just measure 0v at both ends.

i have some 3.3uf wima's laying around. can i pop those in to test them in the circuit? is the value ultra-critical?

[poobah]

Hey Cowanrq

When the circuit is idle (no input) all resistors except for R16, R17 & the 750 Ohm's should have zero volts across them.

[poobah]

Does your meter measure inductance (L)?

[cowanrg]

well, i replaced them with the WIMA's, and you were right, they werent the problem. i still got the nasty nasty noise on startup, and the hum when it was running.

[cowanrg]

Originally posted by poobah
Does your meter measure inductance (L)?

nope, just the usuals, capacitance, and diode tester.

[BDP]

If you have +/- 60 volt rails each drain should see about 30 volts.

The source resistors should see the Vgs drop minus the neg. rail voltage. Example: -60 volts - 4 Vgs = -56 volts.

-56 volts / 750+750 source resistors = ~37mA.

37mA flowing through the mosfet and through the 750 ohm drain resistor = 37mA x 750 = ~28 volts. 28 volts minus the pos. 60 volt rail voltage = 32 volts. This would be the voltage measured between the drain and ground. The gates of the mosfets need to be referenced to ground. This is accomplished through R13,14 or an input pot. if used.

If you continue to have problems and the pain gets to great, you could ship it to me for repair. You pay only shipping.

It is a very simple circuit and you must be overlooking the problem.

BDP

[cowanrg]

believe it or not, im getting almost EXACTLY those values. ive measured them all.

im getting 29.9v at the drain, 4.006 at the source. and the 750ohm resistors are 57 and 26 or so respectively. everything adds up, but im getting nasty noise on turn on, and noise when its on and inputs shorted...

ive tried different power cords, interconnects, wiring (multiple times), amp, source, etc. every single thing has been swapped. all fets have been replaced twice. 3 different power supplies, with 3 different transformers at different voltages.

poobah seems to think it might be the inductance of the 750ohm resistors. i hope he's right. its not like i have a bad fet, everything measures as it should. so, one of the parts is acting up. and BOTH channels are doing the same thing. so, its not like i have two bad parts in the exact same place doing the same thing. thats too unlikely.

edit:

thanks for the offer. i MIGHT take you up on it. however, this is a phantom mystery board. look how many threads there are. and i have hundreds of personal emails from members of this forum helping me via email. after about a year, its still doing the same thing. i dont know if i would want to subject you to that kinda frustration.

[poobah]

OK,

Now put your meter on AC RMS, on the lowest setting (mV), and see if you can measure some AC across ANY of the 750 Ohms...

With the input shorted (+ to -) you should see nothing...