DIY Microphone preamp?

[MikeH]

My friend is getting his studio going full speed now, but he's all but tapped out in funds and could use another mic pre or two.  So I've been looking around at some DIY solutions, I figured maybe I could make something that sounds good enough for, I don't know- something, he just needs more pres, so I figured I'd take a stab.  I found this one searching:

http://sound.westhost.com/project66.htm

Which looks pretty good, I figured it's pretty close to typical stompboxology; low voltage, solid state, easily acquired parts (I have most of them already).  However unlike most mic preamps (or at least what he uses) it's a balanced input that gets combined into a single ubalanced(?) output.  XLR input and 1/4" mono output, if you will.  If I want a balanced output as well, can I just omit the opamp stage and take the outputs from what would be the - and + opamp inputs?  Or would the output be too low?

And if not, anyone have any suggestions for other DIY mic preamps that would be more like what I'm looking for?

[earthtonesaudio]

The op-amp is used as a buffer, which is probably a good idea.  If you want balanced outs, just use 2 buffers, one for each side.  A TL072 takes up just as much space as a TL071...

[MikeH]

So would I then send the hot (+) side of the balance signal into the positive or negative input for the opamp buffer?  And likewise for the cold (-) side?  Or vice versa?

[earthtonesaudio]

Yeah, two sides, two buffers.  Just make sure the buffers are identical (i.e. don't make one inverting and the other non).

[MikeH]

What I meant was, does the positive (from the circuit) go into the positive input (on the buffer) and the negative (from the circuit) into the negative input (on the buffer), but it sounds like you say no, they should be identical, right?

[earthtonesaudio]

Sorry if I was unclear before.  Sounds like you want to make sure that your (+) and (-) don't get switched.  In order to be sure of that, you just need to keep track of which signal is inverted and which is not, compared to the source going into the microphone.

If you follow the signal in the link you posted, you'll notice that the (+) input and (-) inputs are inverted once before going into the op-amp.  Thus it makes sense to put the inverted (-) signal into the non-inverting input on the op-amp, because one inversion means it is now in-phase with the source signal.

The mic pre in the schem is, I believe, intended to go in a mixing board, therefore it's not necessary to keep the lines balanced.  Unless you're driving really long lines I would just use the schem as-is.  But, you asked about how to make the output balanced, so here goes.

...I changed my mind from my earlier suggestion.  The op-amp provides gain, so it's not just a buffer.  Scrap whatever I said before, and do this instead:
Take the output of the circuit, don't change anything, and make that your non-inverting output.
Tap off the non-inverting output, and feed it into an inverting, unity gain buffer (seems logical to use a TL072) and call it IC1b.
Take the output of the inverting buffer and call that your inverting output.
Attach the inverting and non-inverting outputs to an XLR jack, TRS jack, or two mono jacks.


I think that should work.  If you can find a copy of Craig Anderton's "Electronics projects for musicians," a few of the circuits use this method of creating balanced outputs.

[MikeH]

Awesome thanks.  One more question:

So in the schematic above, "OUT" will be my non inverting output.  And I tap that and send it to an inverting buffer such as:



Where Ra (left) and Rb (right) - both labeled R, but we can use our imagination - are equal (gain of -1?) and the output becomes the inverted output.  My question is, does R16 (100ohm) from the original schematic serve as Ra in this instance?  So Rb is 100ohms as well?  Or do I need to tap Ra from the junction of R16 and "OUT" and make both say, 10K or 100K, or whatever?

Sorry, I don't think I'm asking this question so well...

[earthtonesaudio]

That's the basic idea, yeah.  A picture makes it a lot simpler.  I should have thought of that!

Well, in theory you could make the feedback resistor any value (such as 100 ohms) as long as it's equal to the input resistor, and it should come out fine.  In practice, it's better to use a larger value resistor that provides some decent impedance to your signal (just speculation, I forget the actual math).  10k would probably be fine, but I'd use 100k or more for both resistors.  You can leave in the 100 ohm resistor or take it out, doesn't really matter.  Leaving it in will make it slightly less than unity, but it's less than 5% tolerance on a 100k resistor, so no big whoop.

Speaking of pictures, here ya go:

Just pretend the +3V is your input, and also that everything is AC coupled.


Source: http://www.play-hookey.com/analog/experiments/ref_plusmn_10.html

[Gus]

Google there is site that has a number of DIY microphone preamps.

[Mark Hammer]

The Rolls website has manuals with schematics for a whole bunch of their various "utility" boxes, many of which have simple balanced mic preamps.  I adapted this one - http://www.rolls.com/data/mx34man.pdf - with a fixed gain and a mono output with a volue pot and it worked great.  Used an NE5534 for the op-amp.

[flo]

I was planning to try this Microphone preamp with a INA217 (good) or INA103 (even better):

- Giant-killing $5 mic preamp - its secrets revealed (with INA217 or INA103)
http://www.record-producer.com/learn.cfm?a=3080
- Microphone preamplifiers - can *YOU* hear the difference? (with audio)
http://www.record-producer.com/learn.cfm?a=3062
- Well, could *you* hear the difference? Could you??
http://www.record-producer.com/learn.cfm?a=3071

INA217:
THD+N is 0,004% (measured with a gain of 40dB).
Low noise inputstage 1,3 nV/Hz.
High badwidth of 800 kHz (measured with a gain of 40dB).

------------------

Futurlec:
- INA103 Low Noise Instrumentation Amplifier
INA103
$9.95

Mouser:
- INA103
595-INA103KP
$8.45
- INA217
595-INA217AIP
$6.25

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Another good and very easy one to try would be:
THAT1510 or THAT 1512 - Low Noise, High Performance Microphone Preamplifier - compatible with SSM2017

http://www.profusionplc.com/pro/gex/psrchrslts#
THAT1510P08-U
$5.548

http://www.profusionplc.com/pro/gex/psrchrslts#
THAT1512P08-U
$6.659