Author Topic: Simple microphone preamp?  (Read 4286 times)

Beros

Simple microphone preamp?
« on: September 22, 2007, 05:41:01 PM »
Hi!

I know this is not a stompbox project, but anyway I'm looking for a simple 2in/2out preamp do drive and set the mic levels to my soundcard. Balanced XLR inputs and RCA outputs.  Basically it just needs to be a good sounding buffer times two. So does anyone have a schematic or any ideas? The microphone used is a Shure sm57

Thanks,
Beros
Man ska inte ligga med lik

boogietube

Re: Simple microphone preamp?
« Reply #1 on: September 22, 2007, 05:44:13 PM »
Cheaper to buy one than to build one.

http://www.behringer.com/FCA202/index.cfm?lang=eng
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dr.benway

Re: Simple microphone preamp?
« Reply #2 on: September 22, 2007, 05:57:46 PM »
from what I understood about the fca202 it had no preamps.

boogietube

Re: Simple microphone preamp?
« Reply #3 on: September 22, 2007, 06:49:58 PM »
Pedals Built- Morley ABC Box, Fultone A/B Box, DIY Stompboxes True Bypass box, GGG Drop in Wah, AMZ Mosfet Boost, ROG Flipster, ROG Tonemender, Tonepad Big Muff Pi.
On the bench:  Rebote 2.5,  Dr Boogie, TS808

widdly

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Re: Simple microphone preamp?
« Reply #4 on: September 24, 2007, 04:00:22 AM »
If you want to DIY then this looks pretty easy.  No need for phantom power on the sm57 either.

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

Mark Hammer

Re: Simple microphone preamp?
« Reply #5 on: September 24, 2007, 02:10:50 PM »
Though not exactly the highest quality, you can build surprisingly decent mic preamps from standard decent-quality op-amps.  I think one of the tricks is to aim for several stages and use modest gain in each stage, rather than try to squeeze tons of gain from a single stage.  What this does is to optimize the gain-bandwidth product.  Remember that bandwidth decreases with greater gain.  By keeping gain quite modest, you end up with a gain stage that has no problem faithfully replicating stuff way out at the perimeter of the mic's high end.  Even though the final overall output may be quite substantial, from each stage's perspective, it is only amplifying a little bit.  The linked-to preamp at ESP also divvys up the overall gain between the op-amp and discrete stages for that very reason.

A pair of NE5532 or LM833-based op-amps, with a gain of x10 each yields a combined gain of x100, which is a decent amount of boost for an SM57 and likely more than enough for your soundcard to deliver decent sampling.  The first stage can be built as a instrumentation amp with differential inputs (i.e., balanced but with gain).  Make a point of using 1% resistors.  The nice thing about using op-amps is that its a whole lot easier to match resistors than it is to match transistors.

Like I say, not necessarily the finest quality you can get, but surprisingly good.


Minion

Re: Simple microphone preamp?
« Reply #6 on: September 24, 2007, 07:15:47 PM »
You might also want to look at the INA217 or THAT1510/1512 mic pre-amp chips.....They are low noise High gain (2000 X or 66db) ballanced mic preamp chips and can be implemented with very few external components if you want to make a basic version....

these chips are pretty cheap and are used in some fairly good pro equipment, and the datasheets have schematics for how to use them....I use several of these preamps in my studio and they sound great.....


Go to bed with itchy Bum , wake up with stinky finger !!

jlullo

Re: Simple microphone preamp?
« Reply #7 on: September 26, 2007, 02:58:48 AM »
i can't find the link to save my life right now, but someone posted something a while back along the lines of the $5 mic pre, or the $15 mic pre?  i can't remember the price exactly, but it looked really cool and really simple...

it was the one where they were doing a side by side audio comparison with a $1000 pre as well....

brett

Re: Simple microphone preamp?
« Reply #8 on: September 26, 2007, 08:14:03 AM »
Hi
I've built simple discrete preamps using 2 x 2N5088 or 89 stages.  (see the output stage of the big muff pi for an example of this).  For the first stage, the collector to base bias resistor could be 10k and the base to ground 1k.  The collector resistor should be 15k and the emitter resistor 1k.  That should make the collector bias up to around 5 (+/- 1) volts.  The second stage should have resistors of 470k, 47k, 100k and 10k, respectively. Again, the collector should sit at about 5V.  First stage gain is 15 and second is 10, for a total of 150, which is about right. Input impedance is a bit less than 1k.

If you want to use different values, or check what I mean, use this calculator and select "Voltage divider" from the pull-down menu. (http://www.daycounter.com/Calculators/Transistor-Bias/NPN-Transistor-Bias-Calculator.phtml)

Alterantively, the NE5532 op-amp is very quiet, easy to get and eminently suitable.
Here's a schematic showing it as a mic amp.  A bit fancier than what I'd do, but if you want quality I suppose a few extra resistors and caps will only cost a couple of $.
cheers
http://www.nomad.ee/micros/micamp1.gif
Brett Robinson
Let a hundred flowers bloom, let a hundred schools of thought contend. (Mao Zedong)

Paul Perry (Frostwave)

Re: Simple microphone preamp?
« Reply #9 on: September 27, 2007, 08:06:18 AM »
http://www.nomad.ee/micros/micamp1.gif

That's an interesting way to get +17v - stacking  +12 and +5 regs - I presume it works, seeing as the late John Simonton was a pretty hot (and unusually creative) analog designer. But it makes my head hurt to try to work out exactly WHY it works.. I guess each reg does its best to maintain the regulated voltage between its ground & output pin. And so long as there is enough voltage between the input pin & the ground pin to cover this plus the required few volts headroom, all will be well.

StephenGiles

Re: Simple microphone preamp?
« Reply #10 on: September 27, 2007, 11:07:07 AM »
Though not exactly the highest quality, you can build surprisingly decent mic preamps from standard decent-quality op-amps.  I think one of the tricks is to aim for several stages and use modest gain in each stage, rather than try to squeeze tons of gain from a single stage.  What this does is to optimize the gain-bandwidth product.  Remember that bandwidth decreases with greater gain.  By keeping gain quite modest, you end up with a gain stage that has no problem faithfully replicating stuff way out at the perimeter of the mic's high end.  Even though the final overall output may be quite substantial, from each stage's perspective, it is only amplifying a little bit.  The linked-to preamp at ESP also divvys up the overall gain between the op-amp and discrete stages for that very reason.

A pair of NE5532 or LM833-based op-amps, with a gain of x10 each yields a combined gain of x100, which is a decent amount of boost for an SM57 and likely more than enough for your soundcard to deliver decent sampling.  The first stage can be built as a instrumentation amp with differential inputs (i.e., balanced but with gain).  Make a point of using 1% resistors.  The nice thing about using op-amps is that its a whole lot easier to match resistors than it is to match transistors.

Like I say, not necessarily the finest quality you can get, but surprisingly good.



I hope you don't mind if I interrupt here, but what, I wonder, would be the effect of using 2 or more intermediate stages in a Rat distortion, where the total gain is configured to be the same as a Rat, instead of whacking the huge amount of gain into just one LM308 or similar?
"I want my meat burned, like St Joan. Bring me pickles and vicious mustards to pierce the tongue like Cardigan's Lancers.".

Mark Hammer

Re: Simple microphone preamp?
« Reply #11 on: September 27, 2007, 11:09:29 AM »
More treble and less "wooooooooooooo".  The whole point of using a 308 in a Rat is that the gain attempted exceeds the gain/bandwidth product using that compensation cap and op-amp.

StephenGiles

Re: Simple microphone preamp?
« Reply #12 on: September 27, 2007, 01:05:57 PM »
 :icon_biggrin:
"I want my meat burned, like St Joan. Bring me pickles and vicious mustards to pierce the tongue like Cardigan's Lancers.".

Aren

Re: Simple microphone preamp?
« Reply #13 on: September 28, 2007, 03:41:09 PM »
Hi
I've built simple discrete preamps using 2 x 2N5088 or 89 stages.  (see the output stage of the big muff pi for an example of this).  For the first stage, the collector to base bias resistor could be 10k and the base to ground 1k.  The collector resistor should be 15k and the emitter resistor 1k.  That should make the collector bias up to around 5 (+/- 1) volts.  The second stage should have resistors of 470k, 47k, 100k and 10k, respectively. Again, the collector should sit at about 5V.  First stage gain is 15 and second is 10, for a total of 150, which is about right. Input impedance is a bit less than 1k.

Interesting. Did you bother with balanced inputs at all (transformer, maybe?)?

the_random_hero

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Re: Simple microphone preamp?
« Reply #14 on: October 03, 2007, 05:35:40 AM »
http://www.nomad.ee/micros/micamp1.gif

That's an interesting way to get +17v - stacking  +12 and +5 regs - I presume it works, seeing as the late John Simonton was a pretty hot (and unusually creative) analog designer. But it makes my head hurt to try to work out exactly WHY it works.. I guess each reg does its best to maintain the regulated voltage between its ground & output pin. And so long as there is enough voltage between the input pin & the ground pin to cover this plus the required few volts headroom, all will be well.

I thought that the regulators output is 'regulated' through what voltages are seen on the ground pin - referenced to ground, the output voltage is whatever the regulator was designed for. In the same way that placing a 15V Zener diode on the base will raise the output voltage by 15V or by placing a resistor will raise the voltage, referencing the regulator to +12V will raise the designed output voltage by 12V.
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