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Preamp noisefloor

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magic:
With a scope you should be able to find out what's causing the pop...

Michaelaudio:
Well I can see it on the output of the amp. There r some strange things going on. I Biased Pin 3 (not C51 ofc thats wrong :palm:)from the Resistor devider i go with a 10k to Pin 3 but then the output signal is verry low.. when i replace the 10k with a diode the output is as high as it should be but the Sinus is a lil bit deformed and ofc i have a pop, but I can nothing see on the scope there is 12V every where as it should be, im confused.

Marco:

--- Quote from: Michaelaudio on July 27, 2020, 07:26:37 pm ---I made a really stupid mistake on the schematic and therefo re on the pcb, so I had to make some New traces (as u might see pn the pcb pic) which would short L and R chanel together, so I used the diodes that L and R havent an effect on each other.  Shorting the inputs? U mean to ground? I did that when I turned the pots down.

--- End quote ---

If you make the diodes 100K resistors even without a capacitor for the zener, crosstalk is limited. First the signal gets divided by the dynamic resistance of the zener over 100K and then that gets divided by the source impedance over 100K. Not a whole lot will remain.

The diodes make it an envelope detector ... it's silly.

graybeard:

--- Quote from: Michaelaudio on July 27, 2020, 07:26:37 pm ---I got it!! It were the zeners! Replaced every zener with a 10k and.. silence!  Never thought of that, although i read in an AD article that zener biasing is better than just with resistors..
Anyway, now I move on to another Problem (of course) when I turn the bass boost on (sw2) there is a huge visible pop and AWFULL crackling on the sub. I put some caps right before and after Pin 2 and 3 of the switch to keep dc after way but that change nothing. When the pop dissppears and i switch in right on and off the pop is almost gone, so has to be a loading issue or something like that, anyone ideas ?:-//

@Marco
I made a really stupid mistake on the schematic and therefo re on the pcb, so I had to make some New traces (as u might see pn the pcb pic) which would short L and R chanel together, so I used the diodes that L and R havent an effect on each other.  Shorting the inputs? U mean to ground? I did that when I turned the pots down.

--- End quote ---

Zener diodes are often used as noise sources.   Shunt voltage reference ICs are much quieter, but still noisy when compared with the input noise of many op-amps.

The resistors all contribute noise as well with their voltage noise be proportional to the square root of the resistance.  However how they contribute noise in a circuit is topology and sometimes bias dependent.

Michaelaudio:
@Marco  ok ill do that, but why would it be an envelope detector, there is no signal just DC?

@graybeard  can u explain why zeners are so noisy?

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