Chinese mic with Schoeps circuit

[jp8]

D1 and D2 are usually zener diodes (mostly 6.2V)
C9 doesn't have a function, it would prevent any polarisation voltage.
I highly doubt if R13 is really 456 Ohm.
Zener diode D3 may introduce noise, even with an 22µF electrolytic capacitor in parallel with it.

Indeed. And D3 shouldn't have been there in the first place. That's why I proposed to connect it to the collectors of the output BJTs. OK, original Schoeps circuit has the zener also connected to the drain resistor directly, but that is a less noisy 6.2V zener, shunted by 220uF instead of just 22uF. In my opinion, this is a design flaw. Quickly simulated a slightly different Schoeps circuit that I had on my PC: moving the zener from the drain resistor node to the collectors node decreased output noise density at 1kHz from 30nV/sqrtHz to 20nV/sqrtHz...

Moving the zener to the collectors will inject some noise into the 150k resistors, but with sufficiently large coupling caps and low value drain and source resistors, will be strongly attenuated. And the noise will to a large extent appear as common mode noise on the outputs, so will largely be rejected by the mic preamp. Alternatively, take over the power supply scheme from the pimped Alice design, but forget about the useless 10nF and 100nF caps shunting the decoupling electrolytic capacitors. The multistage RC filtering will completely prevent zener noise from entering the audio path.

Jan

 

[Barry Farmer]

Stupid question but what is the function of R12 110 Ohms in series with R11, 1GOhm? Should there be a capacitor, etc hanging off the junction?

 

[RuudNL]

Yes, I saw that too! This resistor (if it is really there) has absolutely no function.
I didn't mention it because there is a possibility that it is used as a 'bridge' on the PCB.
But in that case they could also have used a piece of wire...

 

[RCAguy]

I recently picked up two of these (because i'm beginning to work on and modify microphones and figured since I didn't have any schoeps style circuits it would be a fun place to start) for next to nothing. As far as i can tell, they have the normal Schoeps Jfet feeding a pair of PNP transistors. I've traced the circuit and even drew up a schematic using LTspice to see what was going on circuit wise and to see the frequency response. I've only showed the plot from the positive side (XLR pin 2) as Pin 3 is nearly identical just at difference phase. Any tips or recommendations for mods on these? The capsule appears to be a K103 single diaphragm style copy, but without the correct backplate holes. They don't sound bad, but wondering on ways to improve?

Are you sure about C9 on the schematic? Seems to block 48vdc.

 

[Khron]

Are you sure about C9 on the schematic? Seems to block 48vdc.

You had the option of reading through the whole thread, y'know, it's not like it's 200 pages...

C9 doesn't have a function, it would prevent any polarisation voltage.

 

[Amled87]

You had the option of reading through the whole thread, y'know, it's not like it's 200 pages...

I was able to go back through and get a more accurate reading of everything I believe, and this is what I had come up with. I'm not sure how I got 456r the first time, i changed my batteries and etc in my meter to make sure I was getting accurate readings and it's actually a 12 Meg. It reads closer to 15 Meg, but it's bands identify it as a 12 Meg. I also had the location of the 10uF and 22uF wrong so i changed those. I do believe this is more accurate to what is actually in the microphone. I'll work on getting some photo's of the board (It's rather tiny actually) while I clean it with Isopropyl and swap out the source/drain resistors with 2k2 1%, swap the 0.1uF coupling caps with 470nF (Wima MKP) and change the 1nF capsule coupling cap with a 1nF polystyrene and I'll also be changing the electrolytics which aren't in the audio path with some higher quality Panasonic's (I'll be raising the 22uF's to 47-100uF and the 10uF to 22-47uF).

 

[jp8]

Can you add add an electrolytic cap on the R3, R4, R5, R12 node? Or jumper R5. This node should be on AC ground, i.e. should be decoupled to ground through a capacitor.

What kind of 1nF capacitor is currently in the circuit? If it is ceramic NP0/C0G, leave it in. You wont get any better, despite old wife tales about polystyrene Wundercaps (my opinion at least, there seem to be two camps in the audio society). Polystyrene could be microphonic, more than NP0/C0G, but I'd like to do some comparitive measurements once I get to it to prove it, or debunk this story. So this is more what others tell, not from own experience. Anyway, because there is close to zero AC voltage across this cap, it is physically impossible that this cap generates any significant amount of distortion.

Jan

 

[Khron]

Can you add add an electrolytic cap on the R3, R4, R5, R12 node?

Or move C3.

 

[Amled87]

Can you add add an electrolytic cap on the R3, R4, R5, R12 node? Or jumper R5. This node should be on AC ground, i.e. should be decoupled to ground through a capacitor.

Jan

If I'm not mistaken, c3 is attached to the r3, r4, r5 and r12 node. It's drew this way on LTspice because of a quick mock up (I could have made the simulation larger and moved it into the correct spot) but let's pretend it's attached to that r3, r4, r5 and r12 node and then to ground.

 

[Khron]

C13 (22u) is really between R13 and R11?

That would work out to a time constant of ~260 seconds to reach 63% of the max capsule bias voltage, and another 1000 seconds(!!!!!!) to reach max.

Oh, and that 48V supply won't be (or shouldn't be) 48V at the node you're feeding it into, in the schematic...

 

[jp8]

C13 (22u) is really between R13 and R11?

That would work out to a time constant of ~260 seconds to reach 63% of the max capsule bias voltage, and another 1000 seconds(!!!!!!) to reach max.

Oh, and that 48V supply won't be (or shouldn't be) 48V at the node you're feeding it into, in the schematic...

Good catch. Ech XLR line should go to 48V through a 6k8 resistor, indeed. These 6k8 resistors are in your mic preamp/mixer.

Off to work.

Jan

 

[Amled87]

I do believe i've got the schematic down this time or at least close enough. (I don't have a load connected for the outputs (but i have simmed with one).

What I'm planning on doing is changing out C4 & C5 with 0.47uF Wima MKP caps for better low end response, changing out R6 & R7 for 2k2 1% metal films (on my simulation, changing them brought the noise levels down, but also brought gain down). I'm also going to add some 560pF-1nF caps from R3/R4 like traditionally seen on Schoeps style schematics (LPF?) anything else I should be looking at? I'm also thinking about pulling the cheapo Electrolytics and replacing those with better Panasonics (I have tons of caps and etc on hand!)

 

[jp8]

For simulations V2 should be in series with Ccapsule.

Jan

 

[Amled87]

I'll change that, thank you Jan.

For simulations V2 should be in series with Ccapsule.

Jan