Preamp mics - pre sound card

Thread Starter

Olddumandnew

Joined Jun 6, 2019
23
I'm trying to give a little kick to some electrets before going to a USB soundcard (see attached circuit).micsamp.jpg I think I'm missing someting in my circuit (although I haven't built it yet).
I feel there should be something more before the input to the LM386. Any views?
(sorry for the dodgy circuit diagram)
 

Thread Starter

Olddumandnew

Joined Jun 6, 2019
23
Just to rule out any misunderstanding - the sound card should be drawn jumping the ground rail (it's not meant to look like it all goes to ground at that point. Likewise the +5v going from the USB to the on/off switch circuit).
 

Thread Starter

Olddumandnew

Joined Jun 6, 2019
23
A power supply for the electrets would help.
BTW, a 386 is a power amp. What you need is a low-noise pre-amp instead.
The mics & LM386 have a power supply from the transistor Emitter and the 555 direct from the USB doesn't it? (from the 2nd USB coming out of the splitter). What's wrong with using the LM386 in this way (using it as a preamp)? It should boost the mic gain 20x (which is what I want to get to the appropriate levels)
 

MrChips

Joined Oct 2, 2009
19,768
Your circuit shows three electret mics connected to each other.
This is not a good idea. Each mic is a FET opamp providing an active output. You need to sum the outputs, perhaps using a resistor network or a summing amplifier (AC coupled at the inputs).
 

Thread Starter

Olddumandnew

Joined Jun 6, 2019
23
Your circuit shows three electret mics connected to each other.
This is not a good idea. Each mic is a FET opamp providing an active output. You need to sum the outputs, perhaps using a resistor network or a summing amplifier (AC coupled at the inputs).
.
I've tested the mics (there's actually 4 of them) and they work fine. Just a little quiet (I could have added a resistor in series with each to average the inputs but there was no need on testing)
 

Alec_t

Joined Sep 17, 2013
10,609
The 386 pinout shown at the bottom of the pic doesn't agree with the one above it, which seems to have been inverted.
If you're not interested in audio quality then yes, a 386 will do for providing gain.
 

Thread Starter

Olddumandnew

Joined Jun 6, 2019
23
This setup has been tested and works (each is very clear) BUT I have to take the levels on the sound card up to 87% (hence I want to amp them)
micstested.jpg
 

Thread Starter

Olddumandnew

Joined Jun 6, 2019
23
The 386 pinout shown at the bottom of the pic doesn't agree with the one above it, which seems to have been inverted.
I inverted it to make the circuit easier to draw. (mostly because of pins 3 & 4 being upside down to convention of +ve at the top).

The audio quality won't be that bad surely? I know the 386 is prone to hiss but I can get rid of that at software side of things. I'm using the 386 as I have a couple but happy for suggestions.
 

Thread Starter

Olddumandnew

Joined Jun 6, 2019
23
I think there's only one thing for it - solder it - test it. At the end of the day, 20x gain will take my levels up to top end of line in (of the sound card) levels rather than bottom end - that's kind of the definition of a preamp so what can be wrong there? ;-)
 

MrChips

Joined Oct 2, 2009
19,768
As already pointed out, LM386 is not the right amp for this application.
LM386 is meant to drive a loudspeaker.
What you need is a regular run-of-the-mill opamp.
 

Thread Starter

Olddumandnew

Joined Jun 6, 2019
23
As already pointed out, LM386 is not the right amp for this application.
LM386 is meant to drive a loudspeaker.
What you need is a regular run-of-the-mill opamp.
I'll order a suitable OA (SO many to choose from - any recomendations?) but I might build two circuits (one with an OA and one with the LM386) and compare the two.
One thing I think I've got wrong is the electrolytic cap powering the mics. I have a suspicion this should be just a ceramic cap (my tired head is thinking now it should allow discharge in both directions). I'm not fussed too much about the capacitance or resistance at that point ; can't see it making too much difference tweaking those. Luckily I have both a spare USB sound card and a spare set of mics so things can't go so badly wrong. :rolleyes::eek:. (if it does go wrong I'll stick to the simple version and just up the levels on the sound card to 100%).
 

MrChips

Joined Oct 2, 2009
19,768
That 1KΩ resistor and 450μF capacitor powering the mics are not connected correctly.

Remember, DC current will not flow through a capacitor.

Here is an example of a proper circuit. The values of the resistor and capacitors are not super critical.
The resistor controls the FET current. The resistor and capacitor will affect the low frequency response.

 

Thread Starter

Olddumandnew

Joined Jun 6, 2019
23
That 1KΩ resistor and 450μF capacitor powering the mics are not connected correctly.

Remember, DC current will not flow through a capacitor.

Here is an example of a proper circuit. The values of the resistor and capacitors are not super critical.
The resistor controls the FET current. The resistor and capacitor will affect the low frequency response.

Now you see why my user name is what it is :oops:
Cheers for your help. I knew there was some relationship between the Cap : Res ratio & frequency response but wasn't sure what. I'll look into it more. I'm currently trying to get my head around op amps (to choose which one to try out). Most seem to operate at quite a high Vs so my 5V limits choice a bit. Beginning to think I should stick to things I understand better (microbiology) but it's quite fun trying new things :).
 
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