1st Completed Podcast-style mic + Questions on Passive Filter circuits in Dynamic Mics

[KZhack14]

Hey there, as I have been pondering about passive filter designs in passive dynamic mics (was intrigued by the new Audio Technica ATS-99, and upon googling, found schematics on the MD441 & E945) to improve the frequency response of the capsule.
Just some questions to clarify my understanding (image taken from Sub-continental.com):



So in essence, a dynamic mic by itself is already a resonant LR circuit by design (L from the capsule, R from the input impedance). Am I correct in saying that any components added in between the circuit will change its resonance to a certain degree? I understand from MD441 that the filters are there to do low cuts and "presence" boost, but I'm not sure about the implementation in the E945 (heard from somewhere its doing a 10kHz boost, and if yes by how many dB?). Is this circuit a series LCR in parallel with the capsule?

Anyway, as a practice, I assembled a podcast style dynamic for myself (inspired by DSB-1: A DIY dynamic microphone for $30 -, to do something like a SM7B on a budget lool) I tried ordering the same capsule as the blogs, but it came looking different. Implemented a bit of low-pass filter ard 35Hz and RF filter with 1.5MOhm resistor & 3.3nF caps (on hindsight i think the 1.5MOhm resistor may have been unnecessary), added some foam to dampen the internal chamber : )

Pics in attachment and lmk what you think of the mic! (I actually have more of the mic body shell kept aside for future builds, I'm planning to implement the passive resonant filters in the next dynamic mic)

 

[kingkorg]

These filters are usually designed to adress certain bumps in the response of the capsule, or shape the response for certain source like Shure Beta52. You need to know where the frequency of interest is and adress it with the filter.

Passive circuit can't have boost, the capsule itself has the boost, the filter removes it. It's just that often companies make it look the other way around, probably to make the mic look flatter in its "natural" state which is often desirable.

 

[KZhack14]

These filters are usually designed to adress certain bumps in the response of the capsule, or shape the response for certain source like Shure Beta52. You need to know where the frequency of interest is and adress it with the filter.

Passive circuit can't have boost, the capsule itself has the boost, the filter removes it. It's just that often companies make it look the other way around, probably to make the mic look flatter in its "natural" state which is often desirable.

Im looking to target the highs, mainly betwee 12k-18kHz, as i noticed most dynamic capsules in general tend to have roll off in these ranges

 

[kingkorg]

Im looking to target the highs, mainly betwee 12k-18kHz, as i noticed most dynamic capsules in general tend to have roll off in these ranges

Not sure i understand. To boost 12-18k?

 

[abbey road d enfer]

So in essence, a dynamic mic by itself is already a resonant LR circuit by design (L from the capsule, R from the input impedance)

An LR circuit hardly qualifies as "resonant". It's either a low-pass or a high-pass filter. Typically, the mic preamp input impedance is high enough to not create a significant low-pass.
As someone said, resonances are mechano-acoustical in a dynamic mic.

. Am I correct in saying that any components added in between the circuit will change its resonance to a certain degree? I understand from MD441 that the filters are there to do low cuts and "presence" boost, but I'm not sure about the implementation in the E945 (heard from somewhere its doing a 10kHz boost, and if yes by how many dB?). Is this circuit a series LCR in parallel with the capsule?

In such circuits, "boost" can only be achieved by cutting other frequencies. In oder to boost the highs, you need to attenuate the lows and mids.

Implemented a bit of low-pass filter ard 35Hz and RF filter with 1.5MOhm resistor & 3.3nF caps (on hindsight i think the 1.5MOhm resistor may have been unnecessary),

Doesn't make much sense to me. 1.5 Meg is doing nothing with a nominal 200 ohms mic.
Can you post a schematic?

Nice build, though.

 

[KZhack14]

Not sure i understand. To boost 12-18k?

Yea sort of, to make up for the steep roll offs, also thinking of doing a low pass at 18kHz onwards

Doesn't make much sense to me. 1.5 Meg is doing nothing with a nominal 200 ohms mic.
Can you post a schematic?

Something like this (the capsule is 600ohm imp btw):
+1uf----------------------> XLR 2
|----1.5M Ohm---|
|---- 3.3nF--------|'''''''''GRD
-1uf-----------------------> XLR 3
|----1.5M Ohm---|
|---- 3.3nF--------|'''''''''GRD

 

[mhelin]

I do think passive circuits can be kind of "resonant" like with magnetic guitar pickups where you can tune the Q of the LP in tone control by selecting the value of capacitor. Those are high impedance circuits though...

 

[kingkorg]

Yea sort of, to make up for the steep roll offs, also thinking of doing a low pass at 18kHz onwards

You can't boost with a passive circuit. All you can do is attenuate everything below roll off frequency, but this will increase the noise of already weak output of a dynamic. You can go with active circuit, or just use an eq after the mic. You also need a capsule with reliable published response. Those cheapie china dynamics, although often decent, don't come with reliable spec sheet.

 

[metalockpick]

Low frequency passes more easily through L, high frequency more easily through C. The values of L and C , and the load impedance/resistance after the L/C set the frequency rolloff.

If you use L or C (and R) in a load circuit (across the diaphragm), you will absorb some of the audio, giving a reduction in output in the frequency range set by the combination of components.

The easiest way to roll off bass in a dynamic mic would be an in line capacitor, though it would only work consistently if you plugged it into the same preamp every time, or, you had a transformer in the microphone circuit and put the capacitor(s) in before the transformer.

 

[KZhack14]

Ah great tip. Thanks everyone! Will try out different combos then

 

[KZhack14]

So, I've been playing around with passive filtering a little bit, and I found that if I place a transformer after the filter circuit, I am making some kind of "resonant tank" ??

I've got my hands on some small 1:1 (600ohm) transformer apparently spec-ed @ 1.6H. Putting it in between my measurement it looked like this:


A bit of phase-shift going on at the low and very top end, but what's interesting is there's a 1db lift at 20 & 20kHz respectively (not sure if this is caused by harmonic distortion/noise, or resonance from the tranny)

I knew putting a series cap at the +ve of the signal output will give me a HPF, but I'm surprised by the resonance "Boost" its giving me at corner frequencies. Next pic is the magnitude of various capacitance from 22uf-1uF. (The green one has a series LC filter between + & - pretransformer, hence the slight dip between 1k-20k). I also found that adding a parallel resistor can help fine tune the magnitude to a certain degree.


Granted this is tested with running pink noise through my interface into the circuit. I've yet to test this with actual mic capsules to hear the difference. Usable or no? Here's the drawing of the filter btw (I might implement the LC HP filter as an on/off "presence boost" switch in a future project):