Psu will only go down to 13.5 - it seems to have got quieter. Tried 3 lithium lifepo4 cells - 9.3v at the amp terminals - noise was still there, fades in/out for 2-3 min then faded away, completely silent. Power light still on, amp not in protect - still produced low audio.
Looking at the duty cycle of the PS, does it change noisy to quiet with the batteries?
Is the duty cycle steady (not constantly, slightly pulsing)?
Is the duty cycle steady (not constantly, slightly pulsing)?
With the 9.x v input the pwn duty cycle is very steady noise or not. If it’s changing I can’t tell
With 12.x v input low pass for the respective front or rear channels will significantly lower the noise to those channels - to the point where you can barely tell it’s there
Well the bias has to be “normal” (below 5-7mv) for the noise to be present, and low pass eliminates the noise
No signal source - noise is present with no rcas connected or with rcas connected and source off or turned down.
No signal source - noise is present with no rcas connected or with rcas connected and source off or turned down.
Home audio denon receiver with rca outs at the moment. Can switch back to the car deck again. Sorry didn’t think it mattered if it happens with no inputs at all.
With noise, it's difficult to understand how bad it actually is unless you're there and know the speakers being used (video or audio recordings of it are useless).
How bad is it? If you had it installed in a car, would it be audible from the normal listening position at the lowest volume or much louder? There is no amp that's absolutely quiet.
How bad is it? If you had it installed in a car, would it be audible from the normal listening position at the lowest volume or much louder? There is no amp that's absolutely quiet.
This is beyond normal noise floor. Yes it would be heard from normal listening position at casual or moderate levels. If you had to crank the gain to the top 1/3rd to match a low voltage source it would be down right annoying.
So this amp had all the to220’s except the diodes swapped. 3205 in place of the 1010e, 540/9540 in place of the buz’s. I wonder if some other passives need optimizing? Something around the 494?
My hunch is it’s psu related - the more load the cleaner it is. I wonder if I can put a load across the rails directly to see if it makes a difference.
So this amp had all the to220’s except the diodes swapped. 3205 in place of the 1010e, 540/9540 in place of the buz’s. I wonder if some other passives need optimizing? Something around the 494?
My hunch is it’s psu related - the more load the cleaner it is. I wonder if I can put a load across the rails directly to see if it makes a difference.
The increased current making it quieter seems to be (partly) a PS issue. That's why I asked about the pulse width stability when it was noisiest. The regulation (which varies the pulse width) can sometimes be under-damped and cause noise. The preamp section may be a secondary problem and cheap op-amps could be slightly noisier than expensive replacements.
Are all of the rail voltages and the output of the voltage regulators straight DC (no ripple)?
What's the resistance between the RCA shields and the primary ground?
What signal level does it take to drive the amp to clipping (driving a load) at the minimum and maximum gain?
Are all of the rail voltages and the output of the voltage regulators straight DC (no ripple)?
What's the resistance between the RCA shields and the primary ground?
What signal level does it take to drive the amp to clipping (driving a load) at the minimum and maximum gain?
RCA shields to battery ground were 950ohm and 980ohm. This amp doesn’t have resistance between front/rear shields like some other Fosgates.
I’ll verify the drive levels and regulators tomorrow perhaps.
I know you said I can keep the 10ohm gate resistors but I wonder if that’s leading to unexpected behavior with the 3205’s?
I’ll verify the drive levels and regulators tomorrow perhaps.
I know you said I can keep the 10ohm gate resistors but I wonder if that’s leading to unexpected behavior with the 3205’s?
I've never had a problem with the 10 ohm resistors but you can go to 47 ohms if you'd like. There's a very slight chance that a softer switching could make a difference.
So I had another round at it probing the rails and regulators when I noticed that if I bump the power terminals the noise changes. I would lift or press on them and at a point the noise went away. I thought metal or debris got under the board so I took it out of the heatsink again. I’m going to run out of compound soon…..
At this point in the round it makes the noise for 30sec or so after powering it on and slowly fades away then stays off until cycled. My suspicion is it’s related to heat as the longer it stays on, the 30 seconds becomes shorter. If I quickly remove power and reapply it’s only a few seconds. No amount of pressing on various parts of the board or components brings the sound back.
Also noticed - it only makes the sound after the turn on delay. I’m going to let it cool completely overnight again and reprobe tomorrow. Nothing is really heating up other than the to92’s….(and the regulators, but those always burn hot as I’ve come to learn with Fosgate amps….) all the mps06/56 become too hot to touch. Also noticed when it is making the sound shortly after turn on touching the 6521 leads by a finger changes the noise. If they are that sensitive I wonder if that tiny amount of rosin flux residue left behind from the swap is affecting things… I’ll have to clean it throughly.
At this point in the round it makes the noise for 30sec or so after powering it on and slowly fades away then stays off until cycled. My suspicion is it’s related to heat as the longer it stays on, the 30 seconds becomes shorter. If I quickly remove power and reapply it’s only a few seconds. No amount of pressing on various parts of the board or components brings the sound back.
Also noticed - it only makes the sound after the turn on delay. I’m going to let it cool completely overnight again and reprobe tomorrow. Nothing is really heating up other than the to92’s….(and the regulators, but those always burn hot as I’ve come to learn with Fosgate amps….) all the mps06/56 become too hot to touch. Also noticed when it is making the sound shortly after turn on touching the 6521 leads by a finger changes the noise. If they are that sensitive I wonder if that tiny amount of rosin flux residue left behind from the swap is affecting things… I’ll have to clean it throughly.
The amp can't make any noise until after the delay because there is no bias to the differential amplifier until the end of the delay.
What if you apply compound to the bias transistors and mate them to the MEHSA strip?
Do you have any canned air or component cooler?
I'm sure you have a heat gun or blow dryer to heat various areas to see exactly where the heat makes the biggest difference.
Notice how much of a difference there is in idle current when you touch the bias transistors to reduce the noise.
What if you apply compound to the bias transistors and mate them to the MEHSA strip?
Do you have any canned air or component cooler?
I'm sure you have a heat gun or blow dryer to heat various areas to see exactly where the heat makes the biggest difference.
Notice how much of a difference there is in idle current when you touch the bias transistors to reduce the noise.
No bias = no audio? Ok. I thought bias was only to help with crossover distortion
By bias transistors I assume you mean 6521’s? Sorry if I mislead. Those weren’t getting hot to touch. The 06/56 were and those are too far away from mesha strip.
Yes to heat gun and canned air, I’ll use those to see if I can accelerate or make the noise go away. I had replaced all the 6521 and 06/56 though when changing the fets and source resistors.
By bias transistors I assume you mean 6521’s? Sorry if I mislead. Those weren’t getting hot to touch. The 06/56 were and those are too far away from mesha strip.
Yes to heat gun and canned air, I’ll use those to see if I can accelerate or make the noise go away. I had replaced all the 6521 and 06/56 though when changing the fets and source resistors.
There are multiple biases in the amp. The bias that the pot controls is for the idle current.
There is another bias that controls the current to the differential amplifier at the front-end of the power amplifier itself through Qx00 which is controlled by the LM339.
If you (or others) don't know, you invert the canned air to dispense liquid refrigerant.
There is another bias that controls the current to the differential amplifier at the front-end of the power amplifier itself through Qx00 which is controlled by the LM339.
If you (or others) don't know, you invert the canned air to dispense liquid refrigerant.