Building a super simple / small recording preamp (for noobs)

[lossfound]

Long story short, I want to build a little brick for mobile recording with the phone / fast demo and arrangement use. Said brick needs to output line level (heading into the line input of one of those $5 Cmedia USB audio dongles), will contain both an internal electret mic (or maybe two) *and* some kinda preamp for direct connection to passive guitar and bass, with gain trimpots for both options.

I will not need preamplification for both mic(s) and guitar at same time. Guessing I can add a selector switch of some kind, but whatever.

Above all, the whole shebang needs to be small, very simple, 100% self-contained, and run off a 9V or a few AAs. Sound quality doesn't need to be amazing, I just want it to be quick-gain-adjustable, simple and small (smaller than just about any equivalent commercial USB interface save maybe the UCA202 etc.).

I can solder and read / assemble a simple circuit from a schematic but am a complete n00b with theory and circuit design. Having a hard time wrapping my brain around what might be needed for the guitar circuit. I'm guessing I can more easily find some simple cookbook circuit for the mic-capsule preamp but I'd appreciate any solid pointers there too.

Heck, if anyone knows a good choice of el cheapo mic capsule for this project I'd be vastly appreciative too

[Jazz P Bass]

Is this post a joke?

[Steve Conner]

I don't think it's funny enough to be a joke.

I've never seen schematics for a gadget with that exact combination of features, although now you mention it, it seems like a handy combination. You will have to figure out how to combine the two subsystems yourself.

You can get some fairly cheap stereo mics from Chinese brands like Yoga, that have two cardioid electret capsules inside and battery power. I'd buy one of these and cannibalise it.

For the guitar preamp part, check out the FET circuits at runoffgroove.com, they specialise in 9v powered circuits that are supposed to be inspired by classic tube amps. Or get a Vox Amplug and add that to the mix.

[lossfound]

Is this post a joke?

Um, nope.

I could get into why I need one of these things and/or why the various commercial solutions are not currently acceptable, but it's ultimately irrelevant. I suspect it's more than possible to find a DIY solution to "I need a crappy electret mic and a no-frills guitar to line level output in a single little box."

[Steve Conner]

When you say "guitar to line level output", do you just mean a simple buffer like what is inside the IRig?

[lossfound]

You will have to figure out how to combine the two subsystems yourself.

Yeah, they don't even need to be combined, strictly speaking, aside from sharing the same housing. I'd be happy with a ridiculous jerry-rigged solution that just lets me use one at a time while keeping it all bundled together. I can see a switch that toggles the power to each respective circuit simultaneously with the output, for instance (although visualizing the lug-out of said switch might be "fun" for my muddled mind).

You can get some fairly cheap stereo mics from Chinese brands like Yoga, that have two cardioid electret capsules inside and battery power. I'd buy one of these and cannibalise it.

For the guitar preamp part, check out the FET circuits at runoffgroove.com, they specialise in 9v powered circuits that are supposed to be inspired by classic tube amps. Or get a Vox Amplug and add that to the mix.

Those are (potentially) really great tips. Thank you. I must admit I'm confused about what it will take to get something like a guitar to consumer line-level. The runoffgroove circuits seem to be meant mostly for plugging into the instrument input of an amp. I've been doing a lot of thread-reading about buffers and stuff like that and don't know if that rules such circuits out for my purposes or not. Another thing I'm considering is the SDD3000 preamp circuit. I don't need tuuuubelike etc. or even necessarily distortion / crunch, although I'll take it if it can still fit in the box.

[lossfound]

When you say "guitar to line level output", do you just mean a simple buffer like what is inside the IRig?

Along those lines, yes, although I don't know what the input expectation of an iPhone etc. at the TRS terminal really is. I'd assumed the iRig type stuff actually aimed for an output signal more like a plug-in headset mic (since that's the input they usually plug into). I will be using either a USB audio dongle with what I assume is a consumer level line in, or the aforementioned UCA202 that DEFINITELY has consumer-line RCA inputs.

[J M Fahey]

Long story short, I want to build a little brick for mobile recording with the phone / fast demo and arrangement use. Said brick needs to output line level (heading into the line input of one of those $5 Cmedia USB audio dongles), will contain both an internal electret mic (or maybe two) *and* some kinda preamp for direct connection to passive guitar and bass, with gain trimpots for both options.

I will not need preamplification for both mic(s) and guitar at same time. Guessing I can add a selector switch of some kind, but whatever.

Nice product, somebody should make it.

Above all, the whole shebang needs to be small, very simple, 100% self-contained, and run off a 9V or a few AAs.

The "very simple" requirement throws everything else overboard.
Not simple at all.

Sound quality doesn't need to be amazing,

Sound quality is the *least* of your problems, that product is an exercise in advanced design and packaging.

I just want it to be quick-gain-adjustable, simple and small (smaller than just about any equivalent commercial USB interface save maybe the UCA202 etc.).

Mmmmmmhhhhhh.................

I can solder and read / assemble a simple circuit from a schematic but am a complete n00b with theory and circuit design. Having a hard time wrapping my brain around what might be needed for the guitar circuit. I'm guessing I can more easily find some simple cookbook circuit for the mic-capsule preamp but I'd appreciate any solid pointers there too.

Fine, but IMHO somebody who could not only design such a product but debug, layout and package it into a commercially acceptable way would hardly do it for free, to go Public on a Forum.
So maybe you get some "basic useful ideas", what you'd call "drawn on the back of a napkin", but a ready to print PCB, which includes its own layout, plus wiring layout (pots/jacks/switches), custom designed to fit some commercial box you can buy, is something I don't find way too feasible.

It often happens that somebody carries on such a project for his own pleasure and then publishes it on the Net, happened 1000 times, such as, for example, Albert Kreuzer's *excellent* Bas Preamp, but it grew along the *years* from a simple schematic and Proto Board to proper PCB to beautiful Commercial versions ... but it took years.

That said, I think you must split your needs in two, maybe each block can be found on its own.

1) there are many Guitar preamps or Amp Emulators, such as suggested above, try one(or more) of them.

2) search for an electret mic preamp, Rodd Elliott, as well as others, have a lot of projects, including the all important: how to modify the capsule to avoid overloading and harsh clipping when placed close to a fully driven Guitar speaker.

And now you will understand when somebody thinks "is it a joke?" when reading the "spec sheet"
No offense meant by anybody, of course

Heck, if anyone knows a good choice of el cheapo mic capsule for this project I'd be vastly appreciative too

Practically all Electret capsules are very good.

[Steve Conner]

I believe the output voltage of guitar pickups is not that different to "consumer line level". They are both nominally a few hundred millivolts.

The main difference between a guitar input and a line input is the impedance. A line level input will have an impedance of maybe 10k ohms, but a guitar wants to see more like 1M. So your guitar input circuit could be just a simple buffer: Basic Buffers

I think the headset mic input on the iPhone is designed for an electret mic capsule. The iRig probably needs to attenuate the guitar signal quite a bit. An electret capsule typically wants to see a bias voltage of 1.5 to 3v fed through a resistance of about 1k, which is also the input impedance.

If you put an op-amp in your interface to buffer the guitar, you could add a switch to bump the op-amp's gain up from 1 to about 10, which would suit electrets.

Electret capsules are amazing value for money. However, most of them are omnidirectional, which makes them pretty useless for constructing stereo mics. I have a couple of cardioid ones, bigger than usual with holes in the rear plate. I had to dismantle some Yoga mics to get them. I mounted them as a 90 degree coincident pair inside the windshield of an old broken SM58 clone, and this turned out to be an excellent stereo mic.

[lossfound]

OK, thanks for all the input, appreciated.

Just so it's clear, I'm not looking for commercial production or nice clean PCB layouts or anything. When I said sound quality doesn't matter much, I mean it (is fundamental pitch still identifiable at the output? OK, fine, needs 100% met).

I do not need ANY kind of guitar signal processing -ANY - aside from just bringing up the passive input to feed a line level input. If it ends up being a bit bigger, say, the size of a 1590BB and I have to have the 1/4" input jack in a sleeve / permahanging by a wire out the side of the box to keep the size down, that's fine.

Based on various circuits I have built and half-studied, I don't think this should be THAT big of a deal for an amateur to perfboard or even dead-bug together without using SMDs or blah blah blah. I'm mostly looking for hints at established "cookbook" recipes that are built around a bog standard op-amp and a few extra components.

Really, I'm kind of at a loss as to why something like this DOESN'T yet exist commercially. The evidence suggests that it would be very easy for Them to do, and profitably / nicely so. Conspiracy!

I'll look into that bass preamp for sure. Sounds great.

[lossfound]

That is all SUPER helpful info and I do very much like the idea of using one op-amp, er, stone to kill both birds. I hadn't even thought about the omni nature of electret capsules - I was sort of leaning toward "just mono anyway for now" but yeah, that's an excellent point too. Gonna look to see how much those Yogas might run as an academic exercise at minimum. Thank you so much.