# [Q] How to modify mic pre-amp to higher gain?

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by zrbarnes, May 1, 2011.

1. ### zrbarnesNew Member

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I'm currently trying to modify the input of a Velleman MK140 audio kit to accept an electret condenser style microphone (generic computer mic). The kit comes ready to accept a dynamic mic. I could build a simple condenser preamp and feed it into the input, but I thought it might be better if I just modded the current input circuit.

The audio quality does not have to be great, so the simplest solution is probably the best.

Does anyone have any pointers?

I've attached the schematic that I need to modify below.

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• ###### 45081011_MK140schem.png
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2. ### audioguruWell-Known MemberMost Helpful Member

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Don't you know that the voltage gain of an opamp is the ratio of two resistors?
In the first opamp the max gain is 22k/560= 39.3 if the mic has zero impedance. If the 22k pot value is increased to 220k then the max gain will be 393 but then the high frequencies will be cutoff by the opamp.

The voltage gain of the second opamp is 4.7k/1k= only 4.7 and can be increased by increasing the value of the 4.7k resistor.

3. ### zrbarnesNew Member

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Of course I know that!

Haha, thanks for the explanation. It's been too long since I actually did anything practical that I'm usually at a loss when it comes to the basics.

Remind me again... What are the advantages of changing the gain on the second opamp vs. the first opamp (or vice versa, if that's more appropriate of a question), since they both can be manipulated to control the total gain?

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5. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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You probably don't need to increase the gain, simply add the power supply components for the electret mike, there's an example in the 'sticky' at the top of the General forum.

6. ### zrbarnesNew Member

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Alright, did some tests today and it seems like I'm close to the end.

A couple more questions though:

I have switched to a stereo jack, as shown in the schematic... good or bad idea? Should I short pin 2 and 3 (right now, pin 2 is not connected)?

The internally mounted pot I changed because I didn't like the 22k one I had.

If anyone could take a look at the schematic and tell me how bad I messed it up , that would be great!

Also, this kit was designed for a 9v battery. I have 12v, 5v, and 3.3v buses available. What would be the recommended way to get the appropriate voltage? (Or can it run off of 12v w/o significant risk?)

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7. ### audioguruWell-Known MemberMost Helpful Member

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Your mic preamp has a very low input impedance of only 560 ohms (for a very low impedance dynamic mic?). But most dynamic mics (coil and magnet) spec a minumum load of 1.2k ohms. Most electret mics spec a minimum load of 10k ohms.

Maybe you are shorting the input to +8.3V with the red line as a strange way to mute the circuit or to burn out a dynamic mic?

The datasheet for the NE5532 shows its minimum and maximum supply voltages.

8. ### zrbarnesNew Member

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Yes, the original kit was built to accept dynamic mic's only, and I am trying to use a cheap generic computer condenser mic. I have no idea why the original schematic was designed with a 560 ohm resistor, though

I actually had forgotten to mark that resistor change on the schematic (I'll reupload). I currently have it at 1k, but will try 10k per your advice.

Shorting to +8.3v was designed to emulate the advice found on this page, in order to power the electret microphone.

9. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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Why not refer to the 'sticky' I posted earlier?.