What sets the sensitivity of a condenser mic?

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camshash

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Apologies in advance if I'm posting this in the wrong discussion.

What sets the SPL limit of a condenser mic? Is it the capsule or the circuit, or both?

I have an Apex 460b modded with RK47 capsule and micparts circuit, and the thing does not have a high SPL rating. For vocals it works great, but for drums I get nasty overshoot distortion if I bring it too close to the kit. Mic'ing it as a kick out mic results in extreme overshoot distortion.

On the other hand, mics like the U47FET are more than capable at handling high SPL like a kick drum.

Any insight is appreciated, thanks
 
I suspect it's related to the a) tension of the diaphragm, and b) the width of the gap between the diaphragm and the backplate, and c) the acoustic 'resistivity' of the air chamber in the backplate. A higher tension, wider gap will deflect less under higher SPL, however will react less to higher frequencies, and require more electronic gain.
 
What sets the SPL limit of a condenser mic? Is it the capsule or the circuit, or both?
Both. The capsule will tend to crap out progressively, until you reach a limit where the diaphragm hits the backplate, which produces a very nasty sound. Electronics will tend to clip in a more abrupt manner.
I have an Apex 460b modded with RK47 capsule and micparts circuit, and the thing does not have a high SPL rating. For vocals it works great, but for drums I get nasty overshoot distortion if I bring it too close to the kit. Mic'ing it as a kick out mic results in extreme overshoot distortion.
I don't know what you mean by "overshoot distortion" but are you sure it's not your mic preamp that clips?
 
Both. The capsule will tend to crap out progressively, until you reach a limit where the diaphragm hits the backplate, which produces a very nasty sound. Electronics will tend to clip in a more abrupt manner.

I don't know what you mean by "overshoot distortion" but are you sure it's not your mic preamp that clips?
Pre's are not clipping, nor is the signal in DAW. Its a very aggressive sound and is audible at low gain settings aswell but only in high SPL settings, so It seems to be localized in the mic


Ya know, Ive heard the term "overshoot distortion" a few times when reading microphone reviews and such, so I just assumed it was the first situation you described where the diaphram makes contact with the backplate, but I cannot for the life of me find an audio example of what it actually is, so just assumed it was that.
 
The capsule is a micparts RK47, I asked Matt at micparts and while he didnt quote the SPL rating for the capsule, I showed him an audio example of the distortion and he mentioned that you can pad it down to be able to withstand higher SPL at the cost of a raised noise floor. Not sure what he means by pad down though, I assume its the polarization voltage that Tim Campbell mentioned. I will reach out the other RK47 users and see their experiences.

With that in mind, would a mic like a U47FET, which can handle a kick drum, be a good choice for delicate vocals?
 
The capsule is a micparts RK47, I asked Matt at micparts and while he didnt quote the SPL rating for the capsule, I showed him an audio example of the distortion and he mentioned that you can pad it down to be able to withstand higher SPL at the cost of a raised noise floor. Not sure what he means by pad down though, I assume its the polarization voltage that Tim Campbell mentioned.
Decreasing bias is one way of decreasing sensitivity, but generally, padding down implies the use of a capacitor in parallels with the capsule; for a LDC, 150-200pF for 10dB attn, 500-700pF for 20dB.
With that in mind, would a mic like a U47FET, which can handle a kick drum, be a good choice for delicate vocals?
U47FET is highly recommended as a vocal mic. However, it depends on expectations and what voice.
 
If your new build is a tube mic, can you measure Pin 1 and 2 on the PSU's 7-pin XLR connector? Mic unplugged, just PSU on its own.

Also which tube are you using? Tube gain factor is important.
 
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If your new build is a tube mic, can you measure Pin 1 and 2 on the PSU's 7-pin XLR connector? Mic unplugged, just PSU on its own.

Also which tube are you using? Tube gain factor is important.
Tube is an EH 6071a type. IIRC xlr pin 1 is B+ , correct? If so it reads ~125VDC. Could running B+ at a lower voltage reduce bias voltage, thus lowing sensitivity.
 
Tube is an EH 6071a type. IIRC xlr pin 1 is B+ , correct? If so it reads ~125VDC. Could running B+ at a lower voltage reduce bias voltage, thus lowing sensitivity.
Reducing B+ does not reduce significantly the gain, however results in limited headroom. Whatever would be gained by reducing sensitivity would be lost in headroom.
Reducing bias voltage is not hard to do, generally changing one or two resistors.
 
Reducing B+ does not reduce significantly the gain, however results in limited headroom. Whatever would be gained by reducing sensitivity would be lost in headroom.
Reducing bias voltage is not hard to do, generally changing one or two resistors.
I see, Which method would you recommend I try first, adding cap in parallel with capsule or reducing B+ in the PSU?
 
Tube is an EH 6071a type. IIRC xlr pin 1 is B+ , correct? If so it reads ~125VDC. Could running B+ at a lower voltage reduce bias voltage, thus lowing sensitivity.
Ok, did you mod the PSU?

B+ in Apex 460 PSU's is usually very high (160-200V). 80-90V polarization voltage would have explained your problem. But if you're already at 125V and using a 6072, dropping B+ further to 110V will only make a small difference.

Do you know where to measure your polarization voltage? Resistors Abbey is referring to are there (voltage divider).
 
Ok, did you mod the PSU?

B+ in Apex 460 PSU's is usually very high (160-200V). 80-90V polarization voltage would have explained your problem. But if you're already at 125V and using a 6072, dropping B+ further to 110V will only make a small difference.

Do you know where to measure your polarization voltage? Resistors Abbey is referring to are there (voltage divider).
The circuit Im using is the Fox Audio 460 mod, which includes swapping out 2 zener diodes in the stock PSU.

Checking B+ means testing points D5, D6, and R6 reference to ground in PSU, IIRC, which all came back with expected numbers in the build guide.

I'm not sure what voltage was running before the mod. I assume it was much higher like you mentioned. However, during my initial swapping of the diodes, I had a very hard time getting the stock diodes out and the new ones in, which resulted in me overheating and destroying one of the new diodes, which left B+ to be 71VDC, I confirmed that the capsule was indeed getting 71VDC.

I do have samples of this. The samples are not a 1:1 comparison but notice how on the loud snare hits with 71VDC there is no distortion like there is on the 125VDC one.

Again thanks everyone for the help, I am learning a lot from these, this was my first build I did earlier this year and have since done many other builds but all this info is very helpful.
 

Attachments

  • Drum sample 125VDC.wav
    2.6 MB
  • Drum Sample 71VDC B+.wav
    6.9 MB
The circuit Im using is the Fox Audio 460 mod, which includes swapping out 2 zener diodes in the stock PSU.

Checking B+ means testing points D5, D6, and R6 reference to ground in PSU, IIRC, which all came back with expected numbers in the build guide.

I'm not sure what voltage was running before the mod. I assume it was much higher like you mentioned. However, during my initial swapping of the diodes, I had a very hard time getting the stock diodes out and the new ones in, which resulted in me overheating and destroying one of the new diodes, which left B+ to be 71VDC, I confirmed that the capsule was indeed getting 71VDC.

I do have samples of this. The samples are not a 1:1 comparison but notice how on the loud snare hits with 71VDC there is no distortion like there is on the 125VDC one.

Again thanks everyone for the help, I am learning a lot from these, this was my first build I did earlier this year and have since done many other builds but all this info is very helpful.
Also worth pointing out in the samples, mic is in the same placement in both, raw audio, no eq compression.
 
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