Another Bias calibrating problem (D87 green pcb)

[camarada78]

Hello,

I'm about to calibrate my third U87 clone build, but this time its the older Dany v1.2 model (green PCB). The other two were D87M (blue pcb) and i had no trouble calibrating, both sounding great.

Without calibrating the third mic it works fine, but with less volume than the other two. 

So, i was ready to do the adjustments. But then I got very weird reading. I am probably doing something wrong or the circuits between models are a little different.

What am I doing:
-  no capsules connected.
- scope on drain of the FET (pin 1 of 2N3819)
- injecting 1khz  200mV sine on the C1 pin (same trail and easier than using reversed R6 as the usual instructions)
- putting variable resistor on max

When doing this i only get a flat DC voltage from scope.

- measuring input again with same scope calibration as the output = nice 200mV sine wave on input (with DC offset).
- measure pin 3 of FET on this calibration = DC voltage on output.

So i though, it is so hot that is clipping both ends of the sine. Than i

- change variable resistor to lower values = barely noticeble wave starts to appear. appears to be lower amplitude than input.

- i had to go up to 1khz 6v input on my sine generator to get a visible adjustable sine wave

- than i proceed to change resistor and going up the amplitude until no flat top/bottom appear

Am I doing something wrong or is my fet damaged somehow? I have the impression the output is lower than the input.

edit: my sine wave is amost a triangle/sawtooth in the output.

Thanks,

have a good day

 

[abbey road d enfer]

Without calibrating the third mic it works fine, but with less volume than the other two. 

Please post a schematic of the PCB; we can't know every type and every variant of the world's audio products.
Make measurements of DC voltages on the power rail and on the drain and source of the FET.

 

[camarada78]

Please post a schematic of the PCB; we can't know every type and every variant of the world's audio products.
Make measurements of DC voltages on the power rail and on the drain and source of the FET.

It's identical to the classic U87i circuit the difference is a pot in parallel with the bias resistor

 

[abbey road d enfer]

It's identical to the classic U87i circuit the difference is a pot in parallel with the bias resistor

You know the source resistor in the U87 is selected on test? What is the actual value? What is the pot's value? We can't guess it.
Is it so difficult to post an exact schematic of it? If you want help, you have to help us help you.
Can you please measure the DC voltages at both sides of R12 and at the source of the FET?

 

[camarada78]

You know the source resistor in the U87 is selected on test? What is the actual value? What is the pot's value? We can't guess it.
Is it so difficult to post an exact schematic of it? If you want help, you have to help us help you.
Can you please measure the DC voltages at both sides of R12 and at the source of the FET?

Thanks for the help.

I know the resistor is selected but in this circuit it is added a 25k pot in parallel with the source resistor (R11) so you can easily adjust your bias resistor without too much hassle. For now, my pot/bias resistor is 7.3k

The voltages are:

FET source = 1.9vdc
FET drain = 11.3vdc (with the r11/pot in 7.3k)
R12 side 1 = FET drain = 11.3vdc
R12 side 2 = 20.9vdc
zener = 23.1vdc

The phantom is only giving 45.2vdc, not 48vdc.

 

[abbey road d enfer]

Thanks for the help.

I know the resistor is selected but in this circuit it is added a 25k pot in parallel with the source resistor (R11) so you can easily adjust your bias resistor without too much hassle. For now, my pot/bias resistor is 7.3k

The voltages are:

FET source = 1.9vdc
FET drain = 11.3vdc (with the r11/pot in 7.3k)
R12 side 1 = FET drain = 11.3vdc
R12 side 2 = 20.9vdc
zener = 23.1vdc

The phantom is only giving 45.2vdc, not 48vdc.

So, in fact you're running the FET at about 0.22mA, which is nearly half of the operating point chosen by Neumann.
Gain of common-source stage is very dependant on Gm, which is dependant on Id.

 

[camarada78]

So, in fact you're running the FET at about 0.22mA, which is nearly half of the operating point chosen by Neumann.
Gain of common-source stage is very dependant on Gm, which is dependant on Id.

So is it normal? I just need to lower the resistor to this fet specification (its way different than the other two i tested).

 

[abbey road d enfer]

So is it normal?

There is no simple answer because you also run it at a lower voltage, so basically you have about 3-6dB less headroom. Why did you use a lower voltage zener?

I just need to lower the resistor to this fet specification (its way different than the other two i tested). 

FET's specs are all over the place, even when they are graded. Did you do some kind of selection before soldering it in?

 

[camarada78]

There is no simple answer because you also run it at a lower voltage, so basically you have about 3-6dB less headroom. Why did you use a lower voltage zener?
FET's specs are all over the place, even when they are graded. Did you do some kind of selection before soldering it in?

I only checked if it was working and basic testing using those "component testers":
i got:
1.4mA
Vgs = 1v

About the zener:
- I posted the wrong U87i schematic. Mine is the 1972 version (attached), the zener is 24 not 33v.  Sorry about that. they look the same on thumbnail.

 

[abbey road d enfer]

I only checked if it was working and basic testing using those "component testers":
i got:
1.4mA
Vgs = 1v

About the zener:
- I posted the wrong U87i schematic. Mine is the 1972 version (attached), the zener is 24 not 33v.  Sorry about that. they look the same on thumbnail.

I see on this "new" schematic that the op current is 0.23 mA. So it seems to be conform with your results.