Mic Pre-amp Project

[NoiseBoy]

Hi Guys,

My first post in this forum so hope i got the right section!

Firstly, bit about me, I'm a final year Sound Technology degree student at the Liverpool Institute of Performing Arts in the UK. The course is mainly how to use the gear as opposed to how to build it so i'm going out on my own a bit here and building a Microphone Pre-amp for this project. Thought i would start a thread on here and tap the knowledge and hopefully help a few other people along the way as the project progresses.

So I plan to build a fairly high end pre-amp in a 'prototype' arrangement as i cant afford to print PCB's so its veroboard all the way. Im basing it around a Burr Brown OPA627 (would like to hear peoples thoughts/suggestions on this IC) and hope to see what sort of performance i can extract from an op-amp based setup (ill pop 1 chip if i have to, just to see what it can do).

Anyway, so far the plan is to use This core circuit design and add the transformer isolated 'Go-between Plus" and D.I input module from JLM Audio. Powered by a JLM AC/DC Ver 2. which gives +-2v-37v so i can test to see where the chip is happiest.

I have access to a Neutrik A2 Test Unit for objective testing too (which is a major part of the assesment)

Think that'll do for now, as you can probably tell im in way over my head! but thats how learning happens right. Ill try to keep you all updated on here in return for some comments and advice!

-Paul

P.s. Im talking to Protocase about making me a very nice rackmount enclosure for the finished product too.

[Steve Conner]

Have you seen this? :-)
http://www.record-producer.com/learn.cfm?a=3080

[Enzo]

This can range from very simple to as complex as you want. I suggest you explore the world of existing mic pre circuits to see what sort of things they do. They anticipate certain situations, certain needs, and certain potential problems that you might not have thought of. Phantom power included for example? Ground lift switchable? Isolation between inputs and outputs?

I am not up to date, but PMI used to make a line of SSM ICs, among them the SSM2015. The chip was designed as a mic pre IC. I don't know if they are still made or if something has taken its place, but this data sheet might be informative even if you wouldn't be using it.

http://www.ka-electronics.com/images/SSM/SSM2015.pdf


PMI was bought by Analog Devices a long time ago, so you might also check them out for various application notes and IC specs.

www.analog.com

And there is always pplenty to learn over at National Semiconductor

www.national.com/analog

enter mic preamp in the search for starters, but just in general a great web site. Also on that home page is a feature called WEBENCH Designer. Select audio there and microphone amps is a section that pops up. DOn't know how much applies, but it might be informative.

ALso enter Analog University in their search to get a number of tutorial offerings.

[Enzo]

Or just build Steve's project...

[Robert M. Martinelli]

Talking 'bout Analog Devices, how about their AD745? Its performance compared to the B-B OPA627 seems much better ( IMHO )

[Steve Conner]

Enzo, I remember those SSM2015 chips too, but I don't think they are made any more. The chip in the project I posted is pretty much the same thing, I think.

An op-amp is not the same thing as these chips, which are dedicated mic pres-on-a-chip. To use an op-amp as a mic pre, you need additional circuitry to convert from unbalanced to balanced and allow gain adjustment.

I think old pro-audio gear used a step-up transformer to do the balanced-unbalanced conversion, in front of a textbook op-amp circuit, so maybe that's what you meant by the JLM Audio go-between thing.

However, 99% of mixing desks now use a circuit similar to this one:
http://sound.westhost.com/project66.htm

I never mentioned this because it probably performs much the same as the INA217 thing, but it's more complicated. It uses some transistors and a cheap op-amp, instead of one expensive chip.

PS, with the INA217 or Rod's Project 66, you don't need the input transformer, since they're already electronically balanced. The only reason to use it is if you think the transformer adds mojo somehow.

[NoiseBoy]

Originally Posted by Enzo

Phantom power included for example? Ground lift switchable? Isolation between inputs and outputs?

the JLM PSU has a +48V rail and there will indeed be a switch.

not too concerned about a ground lift switch, never come across a commercial pre that has one, and im using treated power.

will take a look at some of the suggested IC's. would save me faffing about with transformers on the output if i can keep it balanced right through.

JLM Clicky

@Enzo, building an already made design isnt really an option as there needs to be original thought. 'tis a degree after all.

[Enzo]

You needn't steal a design, but looking over existing ones sometimes will point out potential pitfalls to avoid, and may make you aware of particular products that had heretofore escaped your notice. Also sometimes reinventing the wheel as opposed to making wheels better is effective. Most research includes a review of literature.

Ground lift was just a potential example, could be anything, phase reversal, pads, low cut, even metering. Proposing a list of features wanted and not wanted.

[NoiseBoy]

Already got phase reversal and a 20db pad as part of the JLM Go-between.

metering is something ill be looking into, most probably LED's as a VU will stretch the budget.

thanks for the suggestions

[jrfrond]

I'll take transformer balancing in a mic pre any day of the week. Copper and iron is pricey, which is why you see so many circuit designs dispensing with transformers. Transformers will also yield a lower noise floor.

[NoiseBoy]

Ok so almost finished drawing up my stripboard/veroboard layout. looks a bit mickey mouse though but bright colours are easier. will post up later.

Need a bit of advice, how do i go about working out the resistors needed on the gain pot. the pot i have is a 50k log. is it just a trial and error thing to find the gain range that works best? also am i right that pins 1 and 8 on an INA217 are where i connect that up. schematic is a bit vague there.

thanks in advance
-Paul