5-pattern mic circuit

[dhammer]

Figuring that the drawing is in good shape, I've spent time following up on Abbey ID'ing the input/amp section of the circuit as a discrete opamp.   

I think this means that:

• Q5 & Q4 are differential inputs (Q5 = + input & Q4 = - input) with a feedback network on the gate of Q4
• Q2/Q6 are controlling current on both FETs (identical current on both)
• Q3/Q11 & Q7/Q12 are amplifiers with followers

In which case:

• noise in the circuit is extremely low because Q4 is controlled by feedback from the output of the amp section
• which means component noise doesn't have a big impact on output like it would in other mic amp topologies
• the FETs must be very closely matched
• the BJTs must be closely matched

other questions:
• what does Q8 do?
• are the FETs biased by R23 & R28?
• why is D5 a 4V zener?  I'm used to seeing 6-8V zeners here.
• what does L4/C26 do?
• can the input scheme be changed as in Zapnspark's genchinamod to ground the backplate & connect the front diaphragm to Q5 or is this impossible because of multipattern?

Planning to tinker & appreciate ideas or stop-signs:
• capsule swapping – try a U89-style medium-diameter diaphragm and a Beesneez K7
• disconnect R43, L5 & C27 with these other capsules installed
• replace D5 with higher voltage zener or trimpot as in Schoeps CMC5

Thanks in advance!

 

[abbey road d enfer]

• noise in the circuit is extremely low because Q4 is controlled by feedback from the output of the amp section
• which means component noise doesn't have a big impact on output like it would in other mic amp topologies

NFB reduces noise due to the stages that follow the input stage; the input device is what governs the overall noise, so its performance is still critical.

• the FETs must be very closely matched
• the BJTs must be closely matched

You may find that it's not as important as it looks...

• what does Q8 do?

Q7 & Q8 do the same than Q2 & Q6, i.e. make a CCS (constant-current source).

• are the FETs biased by R23 & R28?

R23 is part of the CCS, as is R25. Not R28.

• why is D5 a 4V zener?  I'm used to seeing 6-8V zeners here.

Is it? HZ4B3 exists in 1.8 to 36V.

• what does L4/C26 do?

I already answered that; it creates a rather wide  but not deep cut centered at 5kHz

• can the input scheme be changed as in Zapnspark's genchinamod to ground the backplate & connect the front diaphragm to Q5 or is this impossible because of multipattern?

You would need to create a negative bias voltage in addition to the positive one.

 

[dhammer]

Thanks for this!

Q7 & Q8 do the same than Q2 & Q6, i.e. make a CCS (constant-current source).

How does Q12 fit in?

R23 is part of the CCS, as is R25. Not R28.

R28 is bias resistor?

Is it? HZ4B3 exists in 1.8 to 36V.

I have datasheet for HZ zeners that charts the 4B3 as 3.9-4.1V for zener voltage – maybe I am reading it wrong.  Attached.

I already answered that; it creates a rather wide  but not deep cut centered at 5kHz

I thought that you meant the RLC series R43/L5/C27 in the earlier post regarding the notch – L4 & C26 also there to EQ?

 

[Khron]

Q12 is the bottom transistor from the output stage.

R28 is the load for the class-A biased Q9.

L4 & C26, are part of the (selectable) low-pass filter.

Thanks for this!

How does Q12 fit in?


R28 is bias resistor?


I have datasheet for HZ zeners that charts the 4B3 as 3.9-4.1V for zener voltage – maybe I am reading it wrong.  Attached.

I thought that you meant the RLC series R43/L5/C27 in the earlier post regarding the notch – L4 & C26 also there to EQ?

 

[abbey road d enfer]

How does Q12 fit in?

The output of Q3/Q7 is high impedance; it is necessary to add a voltage follower to reduce the output Z. That's the role of Q11/Q12

R28 is bias resistor?

R28 is the collector load for Q9.

I have datasheet for HZ zeners that charts the 4B3 as 3.9-4.1V for zener voltage 

You're right. I think it was not necessary to have a higher voltage. Maybe it was over-engineered and they had to reduce the supply voltage...

I thought that you meant the RLC series R43/L5/C27 in the earlier post regarding the notch – L4 & C26 also there to EQ?

L4/C26 make a notch at about 23kHz. I don't have an explanation.