Hi everyone,
I have a dead Zoom H4 and I want (for the exercice) to transform its two electret microphones into two distinct standard phantom powered mics.
What I've done so far:
I tested the circuit and "yes" it kind of works, but I have to push my preamp to its limits and thus have a lot of noise. I'm using a breadboard so part of the noise eventually comes from that.
How can I improve the mic gain ? (I have no specs on this particular capsule)
Maybe adjusting values for the two 2k2 (or 5k6) resistors ?
Thanks a lot for any advice !
I have a dead Zoom H4 and I want (for the exercice) to transform its two electret microphones into two distinct standard phantom powered mics.
What I've done so far:
- extracted the two electret microphones. According to the Zoom PCB, I think the FET is directly inside the capsule (that seems quite common), since what I can observe on the PCB looks like the "Alice" circuit (see below).
- build a circuit based on Scott Helmke’s “Alice” (as shown here: https://audioimprov.com/AudioImprov/Mics/Entries/2015/4/23_Basic_FET_Microphone_Circuits.html )
- used two 2N4126 transistors instead of 2N5087, since I have them in stock
I tested the circuit and "yes" it kind of works, but I have to push my preamp to its limits and thus have a lot of noise. I'm using a breadboard so part of the noise eventually comes from that.
How can I improve the mic gain ? (I have no specs on this particular capsule)
Maybe adjusting values for the two 2k2 (or 5k6) resistors ?
Thanks a lot for any advice !
Last edited:
Day 2:
I fixed the microphone gain issue, there was something wrong in my circuit.
Now I still have a very strong white noise, basically at the same level of what I'm recording.
What I've tested so far:
- Switch 5k6 resitors with 2k7 => Just a small effect on gain (and on the white noise level too)
- Connect the capsule metal body to ground => Reduces the gain a lot -6dB (and the noise level too)
At this point I'm wondering if the capsule really has an integrated FET (but if not, I guess the signal would be extremely low, right ?)
Again, any help would be appreciated 🙂
I fixed the microphone gain issue, there was something wrong in my circuit.
Now I still have a very strong white noise, basically at the same level of what I'm recording.
What I've tested so far:
- Switch 5k6 resitors with 2k7 => Just a small effect on gain (and on the white noise level too)
- Connect the capsule metal body to ground => Reduces the gain a lot -6dB (and the noise level too)
At this point I'm wondering if the capsule really has an integrated FET (but if not, I guess the signal would be extremely low, right ?)
Again, any help would be appreciated 🙂
Electret capsules with the FET inside are generally easy to use, I have many that I use with a Zoom H4 and Tascam recorders. Of course those recorders have 2.5V power to supply the mic and pretty good mic preamps, which makes it simple. Would you not be better off using a good IC mic preamp, like something from AD or THAT? They work surprisingly well.
@wintermute - who is one of our mods - has built a lovely discreet mic preamp for just this usage, it's phantom powered. I've heard his test recordins and they are super clean and low noise. Maybe he can help you.
@wintermute - who is one of our mods - has built a lovely discreet mic preamp for just this usage, it's phantom powered. I've heard his test recordins and they are super clean and low noise. Maybe he can help you.
Hi Pano, Thanks a lot for your help!
Actually, it's my "christmas vacations project", so I'm trying to achieve this with what's available at home 🙂
THAT mic preamps looks nice that's true, but I'd like to embed on the microphone only what's needed (microphone electronics, before the actual preamp). I have some nice preamps on my studio that I'd like to use!
Actually, it's my "christmas vacations project", so I'm trying to achieve this with what's available at home 🙂
THAT mic preamps looks nice that's true, but I'd like to embed on the microphone only what's needed (microphone electronics, before the actual preamp). I have some nice preamps on my studio that I'd like to use!
From my quick web research the H4 microphones have a lot of self-noise - better to get some decent large electrets if that's an
issue, 25mm style are available with FETs. Smaller electrets will usually be noisier because they are much less sensitive, and
old electrets may lose output due to the electret fading/discharging too, although I believe this is less of an issue than it used
to be.
That said using the indicated 2N5087's will probably drop the noise several dB compared to those unmarkable general purpose PNPs.
In low noise circuits its a really good idea to fit signal diodes backwards across the base-emitter junctions, otherwise power-up/down transients can reverse bias/breakdown the Vbe junctions, (ruining the noise performance of any BJT). This is more important when high voltages are around like phantom power 48V. Reverse breakdown of EB junction can increase noise generation orders of magnitude and must be prevented in low noise circuitry. This is why audio opamps have back-to-back diodes across their inputs for instance. All Alice-style mic circuits need this as a matter of course.
issue, 25mm style are available with FETs. Smaller electrets will usually be noisier because they are much less sensitive, and
old electrets may lose output due to the electret fading/discharging too, although I believe this is less of an issue than it used
to be.
That said using the indicated 2N5087's will probably drop the noise several dB compared to those unmarkable general purpose PNPs.
In low noise circuits its a really good idea to fit signal diodes backwards across the base-emitter junctions, otherwise power-up/down transients can reverse bias/breakdown the Vbe junctions, (ruining the noise performance of any BJT). This is more important when high voltages are around like phantom power 48V. Reverse breakdown of EB junction can increase noise generation orders of magnitude and must be prevented in low noise circuitry. This is why audio opamps have back-to-back diodes across their inputs for instance. All Alice-style mic circuits need this as a matter of course.
Hi Mark,
Thanks for your help!
I'll order some 2N5087 and see if this fixes the noise issue.
I'd also like to try the diodes trick across the Base/Emitter junctions, I've updated the schematic accordingly (see below), can you confirm I understand you well?
Thanks again !
Thanks for your help!
I'll order some 2N5087 and see if this fixes the noise issue.
I'd also like to try the diodes trick across the Base/Emitter junctions, I've updated the schematic accordingly (see below), can you confirm I understand you well?
Thanks again !
Thanks Pano 🙂 here is the schematic for the mic line driver I made. It is pretty simple and performs well. Thanks to Pano and Kevinkr for the input during the design of this!
couple of mp3's attached, and also a link to a helicopter recording. It may sound a bit boring but stick with it 🙂 Helicopter and more 🙂
Mics used are Primo EM258 matched pair.
I really need to do a new version of the schematic. I went down to 100nF caps. I should check if I had to make any other component value changes for that. 1uF is much more prone to popping and very low frequency overloading. I've built a new pc and not transferred everything over, need to do some digging.
couple of mp3's attached, and also a link to a helicopter recording. It may sound a bit boring but stick with it 🙂 Helicopter and more 🙂
Mics used are Primo EM258 matched pair.
I really need to do a new version of the schematic. I went down to 100nF caps. I should check if I had to make any other component value changes for that. 1uF is much more prone to popping and very low frequency overloading. I've built a new pc and not transferred everything over, need to do some digging.
I've been looking at MAX9812H Microphone Amplifier Modules on ebay, but havent pulled the trigger. Any good? I mean, as long as you're ordering parts anyway...
Thanks for the schematic Tony, and that's a good helicopter recording. Those Primo capsules are nice.
Hi everyone,
So.. I finally fixed my noise issue!
For the 0,47uF capacitors I used polarized ones, making sure they were properly oriented, and I doubled checked just in case.
Next time I'll triple check: one of them was inverted 🙂
Now the board works just fine! I'll try with 2N5087 once I've received them and I'll post pictures of the final microphones!
Thanks again!
So.. I finally fixed my noise issue!
For the 0,47uF capacitors I used polarized ones, making sure they were properly oriented, and I doubled checked just in case.
Next time I'll triple check: one of them was inverted 🙂
Now the board works just fine! I'll try with 2N5087 once I've received them and I'll post pictures of the final microphones!
Thanks again!