Electret Mic Amp in Frequency Analyzer Project

[nickelflippr]

I have modified an electret mic amp from the schematic of a frequency analyzer detailed in a **broken link removed**. This is my first look at this subject, and my guess is that the circuit includes a couple of buffers, with a gain stage, and a low pass filter at the end? So entertaining comments on the attached circuit. Everything seems to be working O.K. currently. Would it be better to start over with something else?

The electret mic (WM-54B) Op-amp (MCP609) circuit feeds a 7 band equalizer chip (MSGEQ7) which is controlled by a couple of PIC outputs, and read by a single analog input. The equalizer chip has the filters set for 63, 160, 400, 1k, 2.5k, 6.25k, 16kHz. So the circuit component changes came about, right or wrong, for various reasons stated below:

1) Needed to reduce gain because the MSGEQ7 adds 20 db to final output so reduced the pot to 50kΩ.

2) Low pass filter was set around 10 kHz so changed R7 to 180Ω for around 20kHz to nab the highest bandpass filter.

3) Substituted the MCP609 for the LM324. The MCP609 is rail to rail, but a relatively low bandwidth of 155kHz. Looking at improving that with a MCP6274 which is supposedly 2Mhz.

4) The current electret doesn't make the 63Hz band , so will upgrade to WM-61A which is supposed to be from 20-20kHz.

3)Didn't have the C6 150nf cap, so substituted a 220nf one without really knowing the consequences.

 

[ericgibbs]

hi,
The MCP6291 is a good R2R output.

 

[nickelflippr]

I will put one on next order, thanks. Lots more reading to be done. An automatic gain control circuit could prove interesting.

<Edit>Here is an interesting chip that covers the AGC aspect? That1280x. It looks like it flattens a log input to a linear output.

 

[atferrari]

You could also look at the SSM2164 from Analog Devices, a quad controller.

 

[nickelflippr]

Four channels of fun. Not sure if I need that now, but an interesting part, and opens up further possibilities. It may come down to the supplier, as Digikey and Mouser stocks one, but not the other.

 

[atferrari]

Limit of range.

I have no actual experience in all this but your electret WM-54b seems to be in the very limit with 16 KHz.

 

[carbonzit]

4) The current electret doesn't make the 63Hz band , so will upgrade to WM-61A which is supposed to be from 20-20kHz.

Heh; I would take that spec (20-20kHz) with a rather large grain of salt or three. Hardly believeable with a small, cheap unit like that. Besides, 20-20kHz +/- how many dB? they don't even say.

(By the way, your link didn't work; try this one instead.)

 

[nickelflippr]

atferrari:

Yes the WM-54B is borderline on the top end too. The MSGEQ7 does respond to tapping on a water glass. No top end response (or expected) from my laptop "SRS TruSurroundXT and Titanium" speakers...talk about crazy stupid marketing.

carbonzit:

Not looking to reproduce sounds faithfully here with the cheapo mic, but to output a rough sound level scale for the led display. Hopefully the new mic will supply the 63Hz and 16kHz bands. There is bound to be some sort of rolloff on the high and low ends I would think. Luckily the PIC can manufacture different conditional tests to account for the decreased sensitivity.

Huh, the link works for me in Firefox, I usually test before posting.

 

[carbonzit]

Huh, the link works for me in Firefox, I usually test before posting.

Probably works for you because your browser has cookies stored that can resolve it, from prior searches, while we don't. Mine should work unconditionally, since it's a direct link to the product page.

 

[nickelflippr]

O.K. I see what you mean, repaired the original link, thanks.

 

[Maikel]

Voice Frequncy Analyser Spectrum Lab 50hz-60hz

Hi guys this is very intesting topic for me, I use Spectrum lab software which anlayzes audio from a mic and plots it graphicaly in real time. To get it going I load the human voice configuration file which runs at 50 Hz I see you octect mic sensitvity starts at -35db up to 120db at 120 your eardrums are just about burst. I know that the softest whispers I have picked up with spectrum lab are at -75 db, check out the software DL4YHF's Audio Spectrum Analyser its supposed to be for amateur radio but has frequncy analyser componenets and human ranges fields you guys are talking about here. You could also a pre-amp circuit to the octet mic for ghost whispers, those changes would make the project suitable for my needs