This Works

I had a couple of OP275's sitting around for a few years and I decided to use one or two, so I made this mic preamp.
I really like this pre, I tend to use it more often than I thought I would.
No single ended conversion taking place, I have done it before with the usual IC balanced in, unbalanced out, but to my ears this sounds much better.

Analag

that's pretty interesting.. I might have to build a couple..

what are you using for the input BJTs? 560's like the current sources?

BC560 all the way.

It looks to me as if Q1/Q2 were adding much noise at low gain settings. Did you do any measurements?

What about joining R14/R16 instead of grounding them? Might improve CMRR significantly and saves one part.

Linearity cannot be expected to fulfill more than basic expectations with the choosen topology...

Samuel

Linearity is overrated :grin:

Sound quality is all that matters to me and the people I deal with.
No noise measurements taken yet. It's very quiet, it sounds rich and I been running the hell out of it. I use it especially for microphone mod testing, which I have been doing a lot of lately.

Analag

Looks good.

what Voltage are you running it?

+/-18V

Looks like you wanted to use four 9V batteries. :wink: :green:

Actually mine runs at +/-21V

how interested are you in SMD parts? I might lay this out for surface mount but no one seems to make the bc560 in SMD. any other choices Analag?

[quote author="Svart"]how interested are you in SMD parts? I might lay this out for surface mount but no one seems to make the bc560 in SMD. any other choices Analag?[/quote]

Not sure (at all), but wasn't the BC860 the SMD-version of the BC560 ?

let me spec it to see..

EDIT:

they are identical! awesome clint!

http://www.fairchildsemi.com/ds/BC/BC858.pdf

You will find the data sheet here.

yeah i found it and edited the post about 2 seconds before you typed yours!

give me a few days to find some time to lay this out.

what color diode(LED)? rather, what current?

edited. :guinness:

[quote author="Svart"]let me spec it to see..

EDIT:

they are identical! awesome clint![/quote]

Glad to be of help. I was just reminded to all this recently when I ran into a design that called for a BF845 IIRIC. Turns out to be not unlike the good old BF245 JFET, so it seemed wise to remember that prefix '8' :wink:

yeah i use the bc856 a lot in some video designs but i was not aware of the different part numbers for different footprints!

Any interest in smd versions? I could pack a bunch of these in a small pcb.. and i need a bunch of good diff ins and outs on my preamps.

also, would you mind terribly if i were to use one LT1002 instead of two LT1001?

EDIT, ah i see that in your post you used the 275 instead.. So should i just lay this out for a dual opamp and someone could put whatever they wanted in there or leave it dual single opamps?

The LED is red, and I'm using one op275 at the moment. Also c1 and c2 are polyprop.

It sounds rich.

Click to expand...

That's surely a good way to describe the sound of this design!

Trying not to be too annoying, but did someone thougth about my suggestion of joining R14 and R16? 20 dB more CMRR is always handy, especially if it even saves one part, improves DC offset and shouldn't have any sonic side effects.

Talking about offset: what about some output coupling caps? I'd be prepared for some serious offset at the output.

Samuel