Shure SM 57 Battery Mic Preamplifier

[RuudNL]

I don't think the AC126 is the best transistor for a microphone preamp...

 

[Liutmod]

it intrigued me because it seems to be the schematic of an old low impedance commercial mic preamp

 

[RuudNL]

An AC126 isn't exactly what we would call 'low noise' today!

 

[Bo Deadly]

No. That has no gain.

The one posted by Strawtles is good. Try that one.

 

[RuudNL]

Well, in fact it has gain. And noise. And distortion...
The Strawtles one is much better. And with modern components.

 

[Liutmod]

In the Strawtles circuit, what should be the value of Rf?

 

[Strawtles]

Sorry I don't know. I have the schematic but I never built this preamplifier
I am sure someone can help you with this value 

 

[radardoug]

Change Rf to change the gain. Put a pot in there and see what happens. And you still haven't told us why you cant use the mike preamp built into your device.

 

[Bo Deadly]

Well, in fact it has gain. And noise. And distortion...

Right sorry. The old upside-down PNP tricked me.

 

[Tubetec]

What about a coin cell powered pre board that solders inside the body of the mic itself ,  could fit on a postage stamp  ,unscrew the mic body to change the batteries ,of course two 3 volt cells will limit output voltage , but if your well upto line level into 10 or 20 kohms it wouldnt matter much . Ive seen very high quality coin cell pre's made for acoustic guitars ,Fishman ones they work great ,tons of output level . Requirements for dynamic mic pre with balanced output are slightly different ,but might be do-able easily enough  . 

 

[PRR]

The 200k 100k is an essential part of the DC stabilization.

(EDIT- fumble fingers)

 

[Liutmod]

What value of potentiometer can I use?

It can be better to increase the value of input capacitor (C1) to 1uF ?

May help to reduce the R3 Value?

Consider I will use this preamp with a SM57

Thank you


radardoug:  the preamp in my device (M-Audio Microtrack) is noisy 

 

[RuudNL]

What value of potentiometer can I use?
It can be better to increase the value of input capacitor (C1) to 1uF ?

The input impedance (R3 in parallel with the impedance on the base of the first transistor) is pretty high, so I suppose 0.1 uF is more than enough for a good frequency response. Changing R3 would also have some influence on the gain of the circuit.
I would start with a 100 K.ohm pot for Rf and see what happens.

 

[Strawtles]

Is it possible the 200k mentioned by PRR is for R3 value?

 

[Liutmod]

No other suggestions to improve this circuit?

 

[Khron]

Prototype it (on a breadboard, or even point-to-point if you're brave / adventurous), test it, THEN maybe see if it needs any tweaks.

 

[RuudNL]

I get the impression that forum member Liutmod would like us to deliver him a finished and optimized design, and preferably build it for him...

But this is GroupDIY! 

 

[Khron]

No offense, but this forum is called "GroupDIY", not "Group-have-someone-else-do-it-for-you"...

I get the impression that forum member Liutmod would like us to deliver him a finished and optimized design, and preferably build it for him...

But this is GroupDIY! 

 

[mjrippe]

Just buy an RTS 1400.  Transformer in/out, gain control, switchable limiter, runs on 2x 9V.  On ebay for about $50 used.  Search this site for more info.

 

[Liutmod]

Beacuse at the momentI have not the time to try the many circuits on the net, in my first post I have written that I am searching for a battery preamplifier schematic optimized for Shure SM57
This means I need a schematic tested not that I want it realized for me