Modify a Schoeps luckalike

I have a microphone with a board called, mp v57. The circuit looks like a Schoeps derivative. It's fairly good for some applications, but a bit harsh in the high frequency range when it comes to song recordings. Has anyone here tried to attenuate the amplitude at higher frequencies in this circuit, around 7-12 kHz or so? I'm not confident enough to just get in there and get my hands dirty... Would like some pointers before I go in.

Pros and cons with:
Adding variable capacitance on the base to collector.
Adding a capacitance on the drain resistor.
Adding a capacitor between drain and gate.

liquidfuzz said:

Pros and cons with:
Adding variable capacitance on the base to collector.

Click to expand...

Will yield inconsistent results when the load changes

Adding a capacitance on the drain resistor.

Click to expand...

Probably the best since it creates a shelf instead of a LPF

Adding a capacitor between drain and gate.

Click to expand...

Second best IMO

There's another way:

http://audioimprov.com/AudioImprov/Mics/Entries/2014/2/8_EQ_Pt.3_-_Transformerless_Mics.html

ChinaMod+U87hybrid.doc in my Yahoo MicBuiders Files discusses this.  You have to join.

Henry (audioimprov)'s method works but probably inappropriate if you are striving for really low noise.

ricardo said:

Henry (audioimprov)'s method works but probably inappropriate if you are striving for really low noise.

Click to expand...

Prove it He's got measurements - capsule / air noise swamps the circuit noise by a large margin.

http://audioimprov.com/AudioImprov/Mics/Entries/2014/2/10_EQ_Pt.4_-_Noise.html

liquidfuzz said:

I have a microphone with a board called, mp v57. The circuit looks like a Schoeps derivative. It fairly good for some applications, but a bit harsh in the high frequency range when it comes to song recordings. Has anyone here tried to attenuate the amplitude at higher frequencies in this circuit, around 7-12 kHz or so? I'm not confident enough to just get in there and get my hand dirty... Would like some pointers before I go in.

Pros and cons with:
Adding variable capacitance on the base to collector.
Adding a capacitance on the drain resistor.
Adding a capacitor between drain and gate.

Click to expand...

First thing!
What's the body of microphone and what capsule you are using?
You can work a  little with headbasket if its design isn't great.
What's the polarisation voltage?
You can also change polaristion voltage, to get a little different response, but this depends on capsule.
I assume that  the bias of FET is correct!?!

You can try this:

- remove both caps from base to ground (lpf caps)
- C1, C2 use 10nF-15nF
- both input resistor change for inductors 47uH
- instal capacitor between drain and ground  10-22nF

Eh, did you just step into something here..?

The drain by-pass capacitor seems to be to most light weight when it comes to physical modification. Specially compared to the audioimprove mod. Ricardo, what's up with all the hush-hush? Do I have to join the secret order..? 

Edit, In76d, the mod you're suggesting seems pretty extensive... And there's no A/B for this mod, when it's there the original response is gone. Like I said in the first post, to some applications the mic is really good and if I can mod it such that I can A/B the mod, well I think it would be more in line with what I want.

Thanks all for the input so far!

Khron said:

ricardo said:

Henry (audioimprov)'s method works but probably inappropriate if you are striving for really low noise.

Click to expand...

Prove it He's got measurements - capsule / air noise swamps the circuit noise by a large margin.

http://audioimprov.com/AudioImprov/Mics/Entries/2014/2/10_EQ_Pt.4_-_Noise.html

Click to expand...

I said "if you are striving for really low noise."

Low Noise design is all about detail design.  In a good condensor mike design,  the 'capsule noise' should dominate.  I think this is what Henry has in his example so the 'electronic' noise is less important.  Or there may be other 'electronic' noise sources there.

The approach I show in ChinaMod+U87hybrid.doc is quieter .. which will show up in some applications.  Also simpler with less parts which may be more important.

If you want a tutorial on Low Noise design, I have LNprimer.doc on the same website with loadsa references if you want to pursue the subject.

liquidfuzz said:

The drain by-pass capacitor seems to be to most light weight when it comes to physical modification. Specially compared to the audioimprove mod. Ricardo, what's up with all the hush-hush? Do I have to join the secret order..? 

Click to expand...

No secret.

If you look at all my documents, I treat them like Engineering Documents and record their history.

I sometimes clarify & change things .. especially when it becomes obvious that things could be better explained or expanded.  (check at the end for HISTORY)

It's fundamental for this type of controlled data that there is only ONE master document.  Mine  just happen to be on MicBuilders 

How do I calculate the cut frequency on the filter rendered by the bypassed drain resistor?

ricardo said:

It's fundamental for this type of controlled data that there is only ONE master document.  Mine  just happen to be on MicBuilders 

Click to expand...

Okay, semi hush-hush.  ;-)

liquidfuzz said:

How do I calculate the cut frequency on the filter rendered by the bypassed drain resistor?

Click to expand...

This arrangement produces only a maximum of 6dB attenuation of high frequencies. The -3dB point occurs at approximately 1.5 times the cut-off (-3dB) frequency of the LPF constituted by the drain resistor and the cap that's put in parallels.
I recommend you use a circuit simulator such as LTspice to visualize all the possibilities. Although the circuits is simple, the calculations are quite tedious.

abbey road - Doesn't the bypass capacitor induce more noise than the modifications further down the signal path?

liquidfuzz said:

abbey road - Doesn't the bypass capacitor induce more noise than the modifications further down the signal path?

Click to expand...

Why would it? A capacitor in itself does not generate noise; the only thing there is that, because the response drops in the HF, the noise of the following stages (the common-collector outputs) would be slightly more significant, but in a well-designed circuit, they are already non-significant compared to the first stage, and principally the acoustic impedance noise, so that wouldn't change much.

abbey road d enfer said:

liquidfuzz said:

abbey road - Doesn't the bypass capacitor induce more noise than the modifications further down the signal path?

Click to expand...

Why would it?

Click to expand...

It was a question, not a statement. ;-)

abbey road d enfer said:

A capacitor in itself does not generate noise; the only thing there is that, because the response drops in the HF, the noise of the following stages (the common-collector outputs) would be slightly more significant, but in a well-designed circuit, they are already non-significant compared to the first stage, and principally the acoustic impedance noise, so that wouldn't change much.

Click to expand...

This is how I reasoned, but like second opinion. Thanks! (Normally I tinker with guitar amps, mods like these are sort of done without anyone ever considering the level of noise. ) I'll try  the bypass capacitor on the drain resistor in series with a spst switch so I can toggle between mod and off the shelf setting. I think 6 dB is spot on regarding the maximum attenuation.  Not sure where to place the cut frequencies. The problematic interval seems to be 5 to 10 kHz, for vocals.

I'm trying to figure out where the drain resistor sits on the pcb.  The general sag in the circuit is down to 30V from 48Vdc, if my shabby meter is okay. What range of voltages should I see at the drain resistor? I only found like 1 to 2 V on the resistors in proximity to the jfet in the circuit... Advice please! 

Is this the drain resistor..?

I tried to find the schematics for this circuit on the e-net. No luck! Is it that uncommon or what? ??? Anyone seen it?