So I drew out this Chinese mic schematic - need a little help making sense of it

[living sounds]

Here's the schematic of a cheap transformerless mic:

I changed some caps two years or so ago (and didn't always know what I was doing), but this is what the components are:

C1 100n
C2 100n
C3 100n
C4 100n
C5 100u
C6 220u
C7 220u
C8 33p
C9 8.2p
C10 1n
C11 22n
C12 22n
C13 1n
C14 1n
C15 1n



R1, R2 22R
R3, R4 150k
R8, R9 1G
If more reistor values are needed I'll unsolder them, currently I cannot determine the values because the resistors look symmetrical, so there are always two possible values.

L1 150uH
L2 62uH


T1, T2 2N5401
T3        2SK170
T4        2N5551


All diodes are zeners.

I also did all the obvious mods (and may have also substituted bigger caps where it's not advisable). With all the mods (including grill and capsule) this mic now sounds very good, albeit pretty bright (but the C37 is a bright capsule, too).


Now I compared the schematics to the Schoeps one (in an excellent modding article by board member Rossi) and realized that C1, C2 form a low pass filter. Getting rid of those caps predicatably improved the high end response.

But I've still got some questions:


1. What is the purpose of C14, C15 here? From my very limited understanding these would result in some phase shift, but for what purpose? I took them out of the mic and couldn't find any difference. They're not in the Schoeps circuits, I think.

2. How high would be the polarisation voltage at the capsule? Is this a common circuit with predictable outcome (from the schematics)?

3. Would upgrading any caps besides C13, C3, C4 make an audible difference? What about C6?

4. Is there an uncomplicated way to balance out the high end vs. the low end a little? The low pass at the output takes away too much 16k and too little 6k, so to speak.

5. Would substituting T1, T2 with matched BC556 make a difference?

6. Any other suggested changes? Any mic previously discussed that matches the schematics

Thanks to anyone taking the time!

 

[abbey road d enfer]

1. What is the purpose of C14, C15 here? From my very limited understanding these would result in some phase shift, but for what purpose? I took them out of the mic and couldn't find any difference. They're not in the Schoeps circuits, I think.

They're here for some more low-pass filtering at the base of the transistors. Helps RFI rejection and stability.

2. How high would be the polarisation voltage at the capsule? Is this a common circuit with predictable outcome (from the schematics)?

Looks like a standard Hartley oscillator. According to the values of L1 & L2, the peak voltage at the top of L1 is very roughly equal to 3x the supply voltage, which is the rated voltage of Z3. Then the voltage doubler D4,D5, C10, C11.
While we're on this, I think there may be something missing in your schematic, since the DC path to power the oscillator seems extremely convoluted. (from the transistors to D3 via R7, the FET, R13, R14, R10, is that right?)

3. Would upgrading any caps besides C13, C3, C4 make an audible difference?

They are all in the signal path. If you have evidence that they impair the quality of the signal, then replacing them with a type that doesn't should change the sound. Electrolytics and ultra-cheap ceramic caps are supposed to be in the category of those which impair the sound, so you may try replacing them. For me it is more a religious subject than anything.

  What about C6?

C6 is a decoupling cap. Some say they don't have an impact on sound because the signal doesn't go through them. This is a false assumption. Decoupling caps ARE in the signal path; the signal is referenced to them. So they may affect the sound quality as much as coupling caps.

4. Is there an uncomplicated way to balance out the high end vs. the low end a little? The low pass at the output takes away too much 16k and too little 6k, so to speak.

Unfortunately, with the simplicity of this circuit goes the lack of control of its performance. The only simple way of taming the offending HMF would be to use an LC circuit or an active LC filter. I don't see how this circuit could be adapted to provide such a functionality. An LC circuit may not be very difficult to implement (series RLC between bases of T1 & T2), but you would need to experiment a lot.
What's wrong with some EQ?

5. Would substituting T1, T2 with matched BC556 make a difference?

Not a significant one; being emitter-followers, they are under tight control of the external elements.

 

[Gus]

Why did you waste your time? The schematic is on the web if you search for it.  Cap changes as well there is even some good changes to the circuit if you search.

 

[ubxf]

Thanks for the drawing and the analysis
francois

 

[living sounds]

First of all, abbey road d enfer - thank you very much! I'll look into it all tomorrow and will report back.


Gus, I asked for the schematics here in the Lab two days ago but didn't get an answer. After a thorough search here and on 'the internets', of course. The only MXL schematics I could find was for the MXL603. If you could point me to where to find the schematics and upgrade information I'd be very gratefull!

 

[living sounds]

Experimented some more. I put in the matched BC556Bs which, interestingly enough, cleared up the frequency imbalance. The upper-mid/lower high end boost dissapeared, as did the low end attenuation. Maybe there was an oscillation problem after all? I put in 330p mica caps for C14, C15 to be on the safe side. No sonic difference as far as I can hear it.

Anyway, the mic sounds great now and I'll keep it this way. Very detailed, very clear, but smooth as well, not much sibilance as long as you don't sing right into it. Uncoloured. Even though I removed a lot of mesh the capsule is pretty insensitive to pop noises.

Dale's capsules are great, and I like this one in particular.

I can post some clips later if anyone is interested.

 

[dmp]

great thread - I don't think you've wasted your time at all!
Thanks for the effort - I've learned something

 

[living sounds]

great thread - I don't think you've wasted your time at all!
Thanks for the effort - I've learned something

I've learned something, too, just by drawing the schematic. So it's all good.

I'd still like to know about the circuit changes Gus was talking about though.

 

[tommypiper]

I'd still like to know about the circuit changes Gus was talking about though.

It's unlikely he'll tell you, or even point to a link... though there's always hope...   ;D

I 2nd DMP and the thanks for your thread.  We all learn.  Definitely NOT a waste of time!

And thanks to abbey road for the generous reply!  People like him make this place open and a forum for sharing and learning.  

 

[zapnspark]

Hint.
Consider joining the Yahoo Micbuilders group 

Enjoy.

ZAP

 

[living sounds]

There's an error in the schematics, C6 doesn't connect to R6, R10, R14, but to ground. I'll upload the corrected drawing soon. I also found some information about mods in the Yahoo group, thanks alot! It seems one of the 1G resistors as well as the capsule-to-fet cap can be ommited with the J305.

 

[xaidi]

Hi there i need the schematic about what you guys are talking. Please can anyone help me with this. I don't see any link to schematic here.