Simple scalable mic preamp

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dfuruta
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Post by dfuruta » Tue Mar 01, 2016 12:11 pm

actually, unless i'm missing something, don't the LT parts and 317/337 have equal line/load regulation? from the datasheets they look the same...ripple rejection looks the same too?

the lt parts offer greater current capacity, but that wouldn't be necessary here

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Post by ashcat_lt » Tue Mar 08, 2016 2:21 pm

You're going to think I'm fucking nuts, but I've been recording my mics at unity for a while now. My interfaces are generally plenty quiet and any gain I need I just add ITB. It's definitely quieter than tape or any room I might be recording in. An XLRF>TRSM snake into the line inputs of your interface might just be all you really need.

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Post by The Scum » Tue Mar 08, 2016 3:07 pm

I've got a couple of condensor mics that would work with a pad that can supply phantom power, nothing more.

Of course, I'm a rock drummer, so I don't usually tax the noise floor of my gear.
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Post by Jim Williams » Wed Mar 09, 2016 8:26 am

dfuruta wrote:actually, unless i'm missing something, don't the LT parts and 317/337 have equal line/load regulation? from the datasheets they look the same...ripple rejection looks the same too?

the lt parts offer greater current capacity, but that wouldn't be necessary here
LT's specs are very conservative, I get far better performance out of these than common 317/337 regulators. The regulator noise is far less on my scope. With good AC isolation I get -125 db noise on my AP analyzer using those parts.

Discrete designs can do a bit better like the Diddon/Jung designs out of Audio Express magazine but those are far more complex with large parts counts.
Jim Williams
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dfuruta
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Post by dfuruta » Wed Mar 09, 2016 9:13 pm

good to know, thanks. i've never had a problem with noise from 317/337, but i'm not trying to push the envelope.

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Post by honkyjonk » Thu Mar 10, 2016 9:10 am

Sounds like you want to roll your own more than buying a pcb etc. but have you heard of the Green pres over at GroupDiy. Pretty cheap, IC based, supposedly sound pretty good.

I can't find the main thread at the moment, or who exactly is doing the PCB's but here's a couple for sale just to get a visual. The schemo is probably in the technical documents on the home page http://groupdiy.com/index.php?topic=61863.0
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Post by Drone » Thu Mar 10, 2016 11:38 am

ashcat_lt wrote:You're going to think I'm fucking nuts, but I've been recording my mics at unity for a while now. My interfaces are generally plenty quiet and any gain I need I just add ITB. It's definitely quieter than tape or any room I might be recording in. An XLRF>TRSM snake into the line inputs of your interface might just be all you really need.
It's actually not an insane idea. I do gain stage each mic, but my problem is that the drum mics are generally hot, and some of the PAD switches on the Soundcraft are noisy, so I can't use the PAD, I generally just turn the gain all the way down, and hope it doesn't clip.

Not sure what the bottom line gain is of the Soundcraft, from the XLR in to the line out 1/4", maybe I should scope it out and use that. If I get the 1510 it's 0dB without a gain resistor, the 1512 is -6dB so that would save parts.

I had already thought about using plain audio taper pots and wiring them backwards, clockwise for less gain, like I wire my pedals, I could put in a switchable gain resistor, but by the time you mess with that, you might as well use a pot.

Those chips seem so simple, I figured one of those pre-etched PCB's for IC's, use an external power supply, run a couple of rails, add the decoupling caps, input resistors and IC sockets and wire in the sockets (with the caps on the XLR's, and pots if necessary) :mrgreen:
The previous statement is from a guy who records his own, and other projects for fun. No money is made.

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Post by Jim Williams » Thu Mar 10, 2016 1:43 pm

SSM2017 layouts are so easy a breadboard does well. I use one for my PIP test jig here. It's only a few parts and an IC socket. It's all point-to-point wiring without any copper traces. Some "purists" swear by that over using a copper pcb.
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Drone
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Post by Drone » Thu Mar 10, 2016 2:22 pm

http://www.bucek.name/pdf/ssm2017.pdf

Doesn't look like it's made anymore? On eBay I can order 10 from China for $35, dunno how safe that is.

The datasheet doesn't have a specific design, like the THAT does for no phantom power mic usage, but it also doesn't spec a need for decoupling caps on the power lines or input. Just a cap across the two inputs and a resistor on each input to float them.

Is that the minimal design you are using Jim?
The previous statement is from a guy who records his own, and other projects for fun. No money is made.

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Post by The Scum » Thu Mar 10, 2016 9:40 pm

The official datasheet has one more word on it:
http://www.analog.com/media/en/technica ... SM2017.pdf

The huge "obsolote" stamped across every page.

They've been superseded by the SSM2019, which is available, and priced like the THAT parts.

ICs from China are a crapshoot, frequently a learning experience.

Also, you don't need to settle for backwards-turning pots. Mouser and Small Bear both have reverse log taper for a buck or so.
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Post by Jim Williams » Fri Mar 11, 2016 8:06 am

A pair of 10 k input bias resistors, a pair of 47 uf/63 V phantom bloking caps, a 10 ohm gain resistor and a reverse-log 2~5k gain pot does it. Add 4 1N752 zener diodes for surge protection.

I tested all the chip preamps here on my Audio Precison analyzer. The SSM2019 is the best of the bunch, after the PIP of course. One weakness is current drive, none of those chips have much.

Build the layout and you can play with all the contenders.
Jim Williams
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