Universal Preamplifier from 5V Single Supply

[Ruebezahl]

Hey everyone,

i am trying to design a Preamplifier that is kind of universal, eg it is suitable for contact mics, guitar pickups and regular microphones (not the ones that need phantom power or plug in power probably, as that would mess with the other input options, right?)

I want to runt it from USB, so 5V single supply, and to keep it relatively simple, affordable and with not too hard to find parts. Of course i hope the sound is as good as possible, considering the other limitations, but it is not meant to be super HIFI, not going to do field recordings, studio recordings or something with it.

The use case will be in a bigger circuit that is mostly for education and playing around. giving workshops, playing around with different ways to amplify. there will also be some oscillators to modulate that signals, or being modulated by those signals. i might do a seperate track about it, but this is mostly me being curious about this particularly question.

I already breadboarded something with the LM358, and can confirm it works, at least for the contact mic, and the guitar pickup, but i would be happy if anyone has some ideas how to improve it, make it more universal, or less noise (at the moment i find it okay, but who knows what kind of different environments, power supplies, pickups this will have to deal with in the future, especially it will be patchable. so if there is room for improvement, i am all for it!)

I did find the MCP602 as a possible alternative, which seems to have lower noise, higher output voltage swing and also a higher Input Impedance than the LM358. Its easily available and while a bit more expensive, its still quite affordable.

What do you think? Any other ideas? Or any improvements to my circuit?

Thank you

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[Cynosure]

I have used the LM358 a few times, and I always find it to be noisy.

If it's a pre-amp, then you should be able to use any amplifier circuit since it's just boosting up to line level. If it's a power amp, then you need something that can drive speakers.

You can get little diy amp kits for a decent price. I'd look into that option.

[PHOBoS]

Although it might not be very noticeable you're currently only amplifying half the waveform when using a
single supply, the other half is going to be clipped off. You could add another 1M resistor from pin3 to +5V
and add a capacitor (1uF..10uF) between R9 and GND. Ideally you'd use a scope to look at the output
waveform while adding a sine or triangle signal to the input.

A single supply, low voltage preamp can be a bit tricky without "specialized" opamps. It does depend a bit on
how large you want the output amplitude to be. A rail-to-rail high impedance opamp, which the MCP602 seems
to be, would definitely be an improvement.

There is also the option to not use an opamp at all and go for a transistor preamp. Those do tend to be low noise
but the design is a bit less straight forward.

Also keep in mind that a (piezo)contact-mic can produce some fairly high voltage spikes, however the current is
very low and if you have a 100K input impedance it probably won't do any harm to the circuit. I haven't used guitar
pickups myself but I think that, since you aren't going for hifi, a mic and guitar are pretty much interchangeable.

[Sven]

Hi,

first thought here: I would choose an opamp that can run at 3V.

Good luck with your project.

Sven

[Ruebezahl]

[Ruebezahl]

[Sven]

Hi,

I mean using an opamp like the MCP602 which can operate down to a 3V single supply because you get 1V more "headroom" than with the LM358. The 602 has also 4 times the gainbandwith and a bit less noise.

[Ruebezahl]

[Ruebezahl]

[Cynosure]

[Ruebezahl]

Thank you for the suggestions, but i am not trying to build a modular synth, it's a small circuit board meant for use in workshops with kids and grown ups to have some fun with basic electronics and sound experiments. I figured USB is the most accessible way to power it.

I won't add too much more, just two oscillators with cmos chip and a modulator with a vactrol (and a buffer for that.) The biggest current draw will be the on board amp and speaker. It will be pretty Lo-Fi but i already breadboarded it and it works kind of fine (even with the LM358 it did).

But again, i appreciate the suggestions, and i understand i should post some more info about the circuit, in order to explain myself better, just have to clean up the schematic a bit.

[PHOBoS]