Making a Mic Preamp?

[owenjames]

I found this link:

http://www.audiomasterclass.com/arc.cfm?a=giant-killing-$5-mic-preamp-its-secrets-revealed

It may be my untrained ears but I actually got the order of expense correct, but the difference really isn't anywhere noticable to matter to the tune of $1495.

I was wondering, as I am going to make a mic preamp of my own, do i really need the +/- 18V output Swing that this amp is theoretically capable of. I am recording into a soundcard, which i think only expects a +/- 1 V swing, correct?

Not really knowing much about mics and preamps could someone tell me any possible pitfalls of building a preamp with a TL074 in the standard way, or even building a transistor amp, maybe three cascaded gain of 10 stages thats a 60dB boost?

[Gus]

That has been around for some time.  Build a preamp put it in a box get UL and CE buy insurance add the costs of running a business and paying workers with heath care add a power supply add a balanced output circuit and on and on.
   The different preamps can have a sound.  Transformer or transformerless input to a preamp and a transformerless microphone or a microphone with a transformer can sound different because of input and output loading and other interactions.

[Mark Hammer]

I was hoping to score one of those INA217 op-amps ( http://focus.ti.com/lit/ds/symlink/ina217.pdf ) from Mouser in a current order, but they're something like 9 weeks back ordered, so I'll try out a THAT 1510 instead ( http://www.thatcorp.com/datashts/1500data.pdf ), which seems to be good enough for these faltering ears.  About the same price.

[Gus]

Good preamps have been built with the ICs.  Also google the green preamp.  There are a lot of preamp circuits on the web solid state and tube transformer and transformerless.  Google some name preamps and schematic you should find links

[tempus]

I've got a 4 channel DIY mic pre here. 2 of the channels use SSM2017's and the other 2 use THAT1510's. I can't hear any difference at all. DO NOT use a TL074, unless you're going for that recorded in a windstorm sound. The main problem you're going to run into with building a discrete micpre is trying to get the noise down to acceptable levels. Here' it depends on how much you want to screw around, but I think you'll have a hard time coming close to the noise figures of the THAT or 1NA designs. Since I record a lot of classical guitar, low noise is a huge concern of mine. Also, these ICs are not opamps - they're micpres on a chip, so they're designed to sound good, and this makes a difference too. I also have it on good authority that the THAT design is essentially what is used in $100K mixing consoles.
The THAT data sheet gives a pretty much complete schematic of a micpre in it. Also, check this out:

http://sound.westhost.com/p-list.htm
http://sound.westhost.com/project66.htm

[Minion]

Ive built several INA217 preamps and they sound great and are easy to build... Overall harder than a pedal but still easier than say a guitar amp ....

The datasheet has a good schematic but it does have a few features that aren"t that necessary (DC Offset Servo) and is missing some features that sorta are (Ballanced output) .....

You can build it with a DC servo (to get rid of DC offset) or use an output cap , if you use a good quality output cap there is no perceptable differance in sound compared to the DC Servo version .....

If your PCB designing skills are even moderately decent you can make a PCB really small , I fit 2 channels inside an old CD Drive with lots of inside room to spare (just didn"t have enough front pannel room for more XLR"s and knobs ....

Cheers

PS: You can create an impedance ballanced output with just a cap, resistor and ferrite bead(optional) ....

[ashcat_lt]

+1 to the question.

My thanks to everybody who's replied.

PS: You can create an impedance ballanced output with just a cap, resistor and ferrite bead(optional) ....

You can't just drop that on us and walk away.  Like, how?

[StephenGiles]

Look over at

Prodigy-pro.com/diy/index.php

There is a mound of info on mic preamps there.

[owenjames]

thanks alot guys, all good info. Any answer to if I need +/- 18V when i am recording into a +/-1V sound card?

[snap]

(OT) heavy springtime ka-BLOOM 



(NOT) pricetag for lower outputvoltage swing IS lower.

[tempus]

Any answer to if I need +/- 18V when i am recording into a +/-1V sound card?

I'm running mine off of that supply, and it goes directly into my soundcard. You're not so much worried about what the output voltage will be but what the voltage swing needs to be to ensure that your signal doesn't get distorted. You can probably get away with +/-15v, maybe even +/-9v, but if you try to do +/-1v you're going to distort the signal for sure.
I think you're looking at this the wrong way. Suppose you start with a signal from your mic that's 20mV. The THAT chip that you ordered is capable of gains up to 1000. This means that when the preamp is cranked, the output will be 20mV x 1000 = 20V. You're probably not going to crank the preamp all the way up if you've got a 20mV signal, but you get the idea. You want to have a fairly wide power supply range to allow for any peaks that may be (considerably) higher than the general signal level. Having a power supply that has an overall voltage swing of 30v allows for this, even though the next stage (the soundcard input) may not need an output level that high. Also, it seems kind of pointless to build a mic pre that you know will distort under most conditions because the PS wasn't properly designed.

Here's some good reading:

http://www.rane.com/note148.html

[Minion]

+1 to the question.

My thanks to everybody who's replied.

PS: You can create an impedance ballanced output with just a cap, resistor and ferrite bead(optional) ....

You can't just drop that on us and walk away.  Like, how?

Yea, sorry, I should have explained myself ...

here , Look at the output of this mic preamp schematic... It shows a impedance ballanced output ....

http://www.jensen-transformers.com/as/as017.pdf


Cheers