poobah said:
well, the input cabling hasnt been on there in about a year. back when i actually had it all together and "working", i was using synergistic research alpha sterling. sure, its an audiophile cable, but its just a basic 22ga (or thereabouts) silver-plated copper stranded wire. its non-shielded. it terminates nicely, thats all.
right now, there is nothing where the input connectors are. i just have phoenix connectors and thats it. no rca jack, nothing.
it WAS being driven by multiple things. i used musical fidelity cd players, denon cd/dvd players, etc. i tested it with numerous players all with the same results.
the buzz/hum is created without an input connected.
i just tried shorting the input. i just took a wire and put it from + in to GND IN. it was a bit quieter, but still noisy. the scope didnt read a bump like it did before, but the whole line was really jumpy and noisy. it just looked like static.
for what its worth, when i had JUST the + IN connected, and left the other end hanging, noise was about 10-20 times louder, and noticeable from 10+ ft away. without anything at all in the input, it was louder than with the input shorted...
edit:
i just saw what you wrote about not connecting input to ground, but connecting + IN to - IN. i will try that now and see what happens. i had - IN and GND IN shorted because im using single-ended.
i replaced the jumper at the - and GND on the input and replaced it so ground was left untouched and - and + IN were connected. it sounds the same as when - and GND are shorted, and + and GND are shorted. there isnt a difference in noise level.
for grins, i shorted - and GND, and pluged an RCA into + and GND. i hooked up my test cd player i have in my shop (it has a volume control on it) and played a cd. wow, noisy noisy noisy. its even louder with the cd player plugged into it.
for grins, i shorted - and GND, and pluged an RCA into + and GND. i hooked up my test cd player i have in my shop (it has a volume control on it) and played a cd. wow, noisy noisy noisy. its even louder with the cd player plugged into it.
pinkmouse said:
not an obvious question. i was wondering that myself. im very new to scopes, this is the first ive EVER used, and ive only had it for a month or so. and its OLD.
but, the fact remains, the preamp is doing something very funny. my scope appears to have a signal generator of some kind. is that useful, or do i need an outside source? i also have test cds (not MP3's) with signals on them, could i use that? or would i just be testing my cd player...
cowanrg said:
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.
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?
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?
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
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
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.
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.
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