Topic: DIY / homebrew preamp questions

[ipl]

Hello all,

This is my first post here, so please be gentle. I think I put it in the right place....

Short version:
I want to design and build a preamp to go between a couple of electret mics and a line-input. How much gain do I need, and where should the bass rolloff start?

Long version:
I'm just dipping a toe into the idea of taping at the moment, and don't really want to spend any money. I've tried recording a couple of shows with just my mp3 player's built in mic, but it was completely overwhelmed by the bass. To be honest, that was more or less what I was expecting, but hey, it was zero cost or effort to try it 

It occurred to me that I could use the line-in on the mp3 player instead, but obviously I would need a pre-amp. I'm sure I can design and build one with a little effort (I'm an electronic engineer by profession, though most of what I do for a job is digital, rather than analogue), but I don't really know how much gain to put in, or where to start the bass rolloff. So answers to those questions would be very much appreciated.

I've knocked up a quick first-cut schematic, which is (hopefully) attached to this post as pdf & jpeg, so if anyone wants to look and comment, I'd appreciate that too. There aren't many component values on it yet.... If anyone does look, the two op-amps on the right are supposed to be an envelope detector / schmitt trigger to act as an 'overload' indicator, as my mp3 player doesn't have any way of monitoring the recording level (I know this makes it a poor choice for recording :-) This bit of the circuit was a late addition, so might be completely wrong! Without it, I would have to use fixed gain (which seems bad), but with it, I should be able to put variable gain in the preamp, by changing the feedback resistors to variable resistors.

I hope this makes some vague kind of sense. I've probably forgotten half-a-dozen crucial things....

Cheers,

Ian.

[SmokinJoe]

Short version:
I want to design and build a preamp to go between a couple of electret mics and a line-input. How much gain do I need, and where should the bass rolloff start?

It depends on the mics (some have more output than others) and how loud the source is.  With my AT-853 mics at a typical "rock concert" I can run a battery box (no preamp gain), line in, and get great results.

Designing and building your own preamp is a pretty aggressive project for most of us around here, newbie or veteran.  Most of us use purchased gear.

[ipl]

Ah, that's interesting. So what's in a battery box? Just some power / bias for the mics? I guess you'd need to AC couple, which would automagically give you some bass rolloff too..... Though I expect the line-input would be AC coupled anyway. Either way, that would certainly be a lot easier to build.

The question of where to start the bass rolloff still applies though. Anyone got a suggestion for that? Or a schematic for a battery box?

Cheers,

Ian.

[SmokinJoe]

Yes, a battery box provides bias voltage, and I believe some coupling caps on the output are standard.

With cardioid caps you don't need bass rolloff generally.  Store-bought battery boxes will have a variety of options... for instance some have switches allowing you a 16hz to 1000hz rolloff (I think that frequency generally refers to the point were you are down 3db).

google search on "microphone battery box"

[fivefishdiy]

Try out this $10 kit.  2 electret mics, line out (or headphone).  Just mod it to add your filter circuit.  Considering it's price, it's actually pretty good. Check out the sound sample I did.

http://www.diy-central.com/HowToBuildYourOwnStereoMicrophoneForLessThan10.aspx

[ipl]

Ok, thanks for all the advice. I think I'll go with a simple battery box for now, and see how I get on.

Cheers,

Ian.