Simple op amp based mic preamp?

[TimWaldvogel]

I am looking for a simple mic preamp design I can build for my home studio, something that can be powered by a simple 9-18volt guitar pedal power supply, something low noise preferably. However, my question I also have is how to you hook up an xlr to it with balanced lines

[Mark Hammer]

http://www.rolls.com/pdf/IS2_PM50sOB.pdf

This'll do ya.

[MarcoMike]

just out of curiosity... why does one channel use 2 opamps instead of just one as the other channel does (and I would expect it to do)?

they just had one spare in a 4x package and needed to get rid of it somehow? why not using it for Vref?

[TimWaldvogel]

Is that switch a power switch for on/off? And just curious I didn't notice at a glance what op amp is prefered? Is there a site that shows schematics of old mixing board pres?

[moosapotamus]

Go google up the datasheet for the INA217. Very simple, single chip mic preamp with balanced input. If you also need a balanced output, there are a number of "balanced line driver" chips available, too. I can't think of any specific part numbers ATM, but you can probably search them up, too.

~ Charlie

[Mark Hammer]

The part you're interested in is everything around U1B up to the 100k pot.  Pin 1 on the XLR goes to ground, and pins 2 and 3 provide the two parallel inputs needed for balancing and noise cancellation.

The schematic shows a gain of around 50x, and a low-end rolloff around 32hz for a mic input.  You can use a pair of back-to-back 10uf caps for each input line, or you could decide that you're comfortable with a gain of something less and a slightly higher rolloff.  For example, replacing the 10uf caps with back-to-back 2u2 caps and replacing the 1k resistors with 2k2, would get you a rolloff around 66hz and a gain of 22x.

U2A and U2B are paralleled to provide more drive current to feed a headphone.  If all you want to do is feed a mixer, you don't need to do that, and would be just fine with  everything up to and including P1, but with a 10uf cap between the pin 7 of U1B and the pot.  The ground lug of the pot should go to actual ground, rather than Vbias.  But that's all you'd need to have a balanced-in-unbalanced-out mic preamp.  I made one like that to use with my cheap Behringer mic and I'm really impressed by how quiet it is.  I used an NE5534.

[TimWaldvogel]

can i use any of these for the op amp and still be relatively quiet?
i have a bunch of dual op amps and a few single maybe i can make a two channel preamp?

tl082
jrc 4558d
lm741
tl071
i have a lm324 but its just 4  741s prolly noisey huh?
idk i  have a few others.

and here is another question. when your dealing with kinda of two in one circuits that are lets say 9volts a piece, like lets say one box but two tube screamers or two mics pres in this case.
is the voltage for the dc input still 9 volts just with a high amperage (mill-amperage?) or does the voltage needed change?

[Ripthorn]

voltage drop is the same, you just double the current because they are in parallel.

[TimWaldvogel]

So yes it's still 9volts?.... I am a noon with terminology lol

[John Lyons]

Yes, same voltage.
As long as you have a decent Power supply it will be fine.
300ma should be ok.

[TimWaldvogel]

So I think I will build a two channel preamp, but how do you make it balance out too? How necessary is a balanced output really?

[PRR]

> Simple op amp based mic preamp?
> simple 9-18volt guitar pedal power supply
> low noise preferably
> how to you hook up an xlr

There is no "simple" answer.

And most of the "simpler" hacks will soon prove disappointing. Capturing audio without injury is the key to a clear interesting sound.

I know, I have used, modded, or designed dozens of them.

The simplest requires a transformer which you can't get at RadioShed or the DIYstompboxes Store.

The one-opamp plan ROLLS used has a high hiss level and does not supply the Phantom Power which many modern mikes demand. It works as an on-stage headphone feed, where Phantom is supplied by the main board, and when a little hiss does no harm. I have used a PA system which had that same plan, and it was awful hissy even in a bad rehearsal room.

Most (probably all) general-purpose chip opamps have much higher hiss than a dynamic mike. They may not add much hiss to a large condenser mike, but LDC mikes mostly need 48V Phantom power.

You usually need a proper GAIN control (not a post-gain volume pad like ROLLS uses) to handle the extremes of mikes and performers.

> How necessary is a balanced output really?

In small-studio work: IMHO, balanced is often NOT needed even for microphones. Yet that's the only way you can buy mikes anymore. While dynamics can be worked unbalanced, their low output makes hiss challenging. The popular-price condensers, even when internally unbalanced, really do need a balanced receiver so their power scheme works, but their high output makes hiss un-fussy. 

You can't re-engineer the mike, but leave your preamp output unbalanced until you have lines over 30 feet or must work under a radio tower.

> I didn't notice at a glance what op amp

At a glance, Rolls used 4560. This is well-used in commercial audio. It seems to be a NJR design, rooted in 741 topology, with much lower input hiss and higher output current, and pretentions of NE5532 performance. Good chip. Might not be easy to source just a few.

LM324 is not "4 741s". As an audio chip, it is worse.

All your chips are fine guitar chips. But microphone impedance is much lower and microphone noise voltage level is often 10 times lower than a guitar. Also guitar transients are actually limited by string-mass and pickup parasitics, acoustic microphones can spit much faster transients.

My advice may be blasphemy on this forum. Get a MIDIman/M-Audio AudioBuddy $99 preamp, or a Bee-ringer 2-input mixer which can be found for $59. They have Phantom. They have proper gain control. They give reasonably low hiss. They let you make Music instead of snorting flux fumes.

If you just can't stay away from the solder: the INA217 is the $5 answer, killah chip, but will need a special gain-pot/switch and really wants a triple power supply +15V/-15V/+48V.

[TELEFUNKON]

how about this one?  http://www.profusionplc.com/pro/gex/pcatdtl0?ipartno=NJM4580D

[TimWaldvogel]

wEll a friend and I are starting a local studio and me being th diy guy I am, it's something I would like to say I have under my belt and  use it inthe studio

[petemoore]

wEll a friend and I are starting a local studio and me being th diy guy I am, it's something I would like to say I have under my belt and  use it inthe studio
  Here's the suggestions:
 The suggestion to blasphemy is a very sound bit of advice.
 I would add that a second channel will be desires as soon as the first channels 'light-my-signal-up' capability is realized, re-re-patching is a drag, sometimes I'd like have 2 running AtsTime.
 Ploppin' 99$ for a pre-channel with a tube in it is a whompin' bargain anyway, and a high mark that'll be one tuff nut to crack when attempting to beat it for performance/features and price.
 So they make a few bukks...gib deal, they also stamp out or even mold the parts, whip all the peices together...Tube-Pre = beautiful, less human suffering than a DIY unit demands...kind of modern looking to be sure, put some weave on it or wrap it in wood...

[TimWaldvogel]

Ok, well I already have a nice behringer tube pre, just lookin to expand my knowledge and skills lol

[Nasse]

I believe if rock/pop music is played in your studio there are some loud sound sources too that need recording. Not every mic is low noise either. Perhaps such two stage opamp thing might be useful, say, recording kick drum or snare toms or gtr bass etc. I would go higher dual voltage for headroom and low noise opamps, and the second stagemight be made adjustable gain for least noise.