Need Opamp Mic Preamp Circuit

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#2
Hello JojoD,
Eric Wallin has a mic pre amp on his page. It uses four op-amps: one to buffer the voltage reference, one to do the amplifing and two to show clipping, all running off a single rail. It can be found here:
http://www.gti.net/wallin/audio/preamp2/preamp2.html

I've got a pre-amp on my webpage but the write-up isn't finished. It's a single op-amp running from a split rail. It can be found here (you want the circuit in the lower left corner):
http://www.apcl43.dsl.pipex.com/SW_jig.htm

R12 and C6 form a switchable gain stage and can be omitted if not needed.

Nice one,
David.
 
#9
more confusion

hi, i'm a bit confused...:scratch:

I will be using a "dynamic microphone" for the preamp(schematic) i am asking everyone about. the question is this, a dynamic mic = unbalanced right? can I use this for a mic preamp that has a balanced input design? such as those used by rane?

i see that there are 3 pins from the bottom of the mic and 3 pins from one end of the wire that came with the mic. but the 1/4" phone plug at the other end of the wire only has 2 connections. tip and sleeve only, NOT tip-ring-sleeve.

this means that pin #3 from the mic is "ground", then if I plug this to a balanced input preamp, then the "negative" input of that preamp will be grounded right? is that the way it's done? :scratch:

thanks in advance 🙂

JojoD
 
#10
Re: more confusion

JojoD818 said:

i see that there are 3 pins from the bottom of the mic and 3 pins from one end of the wire that came with the mic. but the 1/4" phone plug at the other end of the wire only has 2 connections. tip and sleeve only, NOT tip-ring-sleeve.
Click to expand...

The 3-pin connector on the bottom of the mic is a balanced XLR connector. Ideally, you would use an XLR <-> XLR cable, and plug it into a balanced mic input. The cable you have is converting the balanced connection to unbalanced, by connecting one of the balanced lines to ground, as you suggested. This will work, but will be much more likely to pick up noise and interference, especially with long cables.

If you're building your own preamp, you would be much better off building a balanced design and replacing the cable. You could probably just replace the 1/4" connector with an XLR, as long as the cable has two conductors plus ground.

You might also want to look at this thread.
 
#11
i tried using a multitester to test the pins at the bottom of the mic.

when i test pins 1 and 2, i measure around 620 ohms.

when i test pins 1 and 3 i get a short.

when i measure pins 2 and 3, i measure around 620 ohms.

i think the connectors at the bottom of the mic (XLR) is used for an unbalanced connection. is that assumption correct? :scratch:

or is it a balanced connection mic?

JojoD
 
#12
The normal (balanced) pinout for an XLR connector is:

pin 1: ground
pin 2: +
pin 3: -

Sounds like pin 1 and 3 are tied together in your mic, meaning it is unbalanced. So your cable is fine. It will work with a balanced input, though not ideally.
 
#15
JojoD818 said:
so if i'll build my own preamp, i can just ground the "negative" input of the balanced preamp right?

jojod
Click to expand...

Yes. If you build the preamp balanced, with a 1/4" TRS jack, then simply pluging in the unbalanced 1/4" cable that came with your mic will ground the negative input, running the preamp unbalanced.
 
#16
Check Jenson Transformer website for some good quality a.k.a "Professional" quality circuit schematics. Not sure of URL, use google search. As far as what op-amp to use? I would go with the Analog Devices AD-797, as long as it's used nowhere near unity gain (no mic preamp would be) it is excellent.
 
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