Top EQ for 'BM-700' type Chinese mics

[k brown]

Oh, those bright Chinese capsules - found an article online on how to add low pass to the circuit to tame the top end.

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

 

[RuudNL]

It works, but it will increase the self noise of the microphone.
(VERY old trick, by the way...)

 

[k brown]

According to the article, the increase in self noise is inaudible. The noise is from the 'added' series 10k resistors, but the BM-700/800-type mics already have these resistors. One of course needs to also do the added filtering after the voltage regs in these to make them usably quiet.

This seems the perfect fix for the brass-body capsules that KingKorg and I have been experimenting with, which sound quite nice, but are unusably bright for my purposes.

 

[gyraf]

..you may as well eq after the mic - there'll be no difference in the basic mic behavior regardless of internal eq'ing. What I mean is that you can't eq a bad-sounding capsule into sounding good anyway, so perhaps it's better to save the signal-alteration to some later stage in the processing where you vcan have more complex control.

/Jakob E.

 

[k brown]

In another thread you mentioned you and/or Tim was going to run some tests on these capsules - anything further in that?
https://groupdiy.com/threads/new-ish-electret-capsule.76460/

 

[rogs]

According to the article, the increase in self noise is inaudible.......

The increase in noise is acceptable, if you keep the the resistors below 2k2.
With resistors values higher than that, the noise definitely becomes audible.
At least that's what I found...

The problem I've found with trying to use a simple internal passive LPF to control excessive capsule HF is that it tends to be either too much or too little. The problem is often an unwanted 'lift' within a certain passband rather than just 'too much HF'.
I'm currently looking at an idea based on what Jakob describes in his post above. Looking at adding active EQ as an 'insert' in the preamp channel strip - at line level - to try and match the required correction more closely.
More control, less added noise.
Still in early the experimental stage at the moment....

 

[k brown]

True enough - most SDCs have a bump at 6-10k then back down to flat above that.

 

[RuudNL]

That is why a 'notch' filter would be better than a low pass filter.

 

[k brown]

A notch - natch'!

 

[Gus]

it also changes the emitter followers output resistance and makes the CMRR worse

 

[rogs]

I'm currently looking at an idea based on what Jakob describes in his post above. Looking at adding active EQ as an 'insert' in the preamp channel strip - at line level - to try and match the required correction more closely.
More control, less added noise.
Still in early the experimental stage at the moment....

I've taken this idea a little further, and built some cheap prototype filters to try it out. I think it does have has possibilities - although so far I've only built a few bandpass filters.
I like the idea of being able to create different band pass filter blocks - with different parameters regarding band pass, slope, Q, gain, etc - and being able to add them into the signal path in any order you like.
It's also easy to remove them individually by simply unplugging a single jack. Signal pass through is automatic from the vacated socket....

Some initial notes here: www.ceq.jp137.com

 

[Gus]

IIRC there is an AKG patent that shows inductor based EQ after a CF microphone.
https://patents.google.com/patent/US3393271
Should add it is nice write up I like the stripboard layout