guitar -> SS line in

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A simple option would be a buffered guitar pedal, maybe switched off. But if you can get hold of a Bad Monkey by Digitech, then it has a few features that may serve well. It is buffered, it has a cab-simulated output option, bass and treble controls and a range of mild to medium crunch tones. They are well built and cheap.

For a little more, a basic guitar processor could be had, eg a Zoom G1 or G2, or several others. Plenty second hand are available.
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op amp as pre?

could this be adapted, not worked with sand much before. but hav inherited a stack of 741's and the 12 volt input is good for me

Resistor (all ¼-watt, ± 5% Carbon) R1 – R12 = 10 KΩ
R13 = 1.2 KΩ
R14 = 47 KΩ
VR1 – VR6 = 10 KΩ
VR7 = 250 KΩ
Capacitors C1, C2 = 0.39 µF
C3, C4 = 0.1 µF
C5, C6 = 0.025 µF
C7, C8 = 0.018 µF
C9, C10 = 3300 pF
C11, C12 = 1600 pF
C13, C14 = 5 µF/25V
Semiconductors IC1, IC2 = 741


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could this be adapted,
IMO that's not a great way to go. Not only are 741's ridiculously noisy, they are not great at providing the very high input impedance you want for a guitar (1M or even higher).

The specific circuit you showed has far too low an input impedance, and also, it's unnecessarily complex.

You certainly can build a buffer around an opamp (a cheap TLO72 or TLO82 will be a far better choice than a 741). But I too think the simplest way is to use an off-the-shelf guitar pedal to provide buffering and gain.

If your friend plays mostly using "clean" tones, I'd suggest a Danelectro Fish-n-Chips pedal. They're cheap, and you get clean gain, a buffered input, and a very nice graphic EQ as a bonus.

If distortion/overdrive is also wanted, or a variety of effects like reverb, echo, etc, tones, then I'd echo the earlier suggestion of looking for a cheap, maybe previously-owned, multiFX pedal by Zoom, etc. to act as a front-end for the amp you have. If you can find a used Zoom G3 affordably, it provides a *lot* of bang for the buck.

Save those 741 to build a classic MXR Distortion+ or same era pedals ;)

You can build a simple 2 channel preamp/mixer with just a TL072.

9 to 12V powered, 2 high impedance / high gain or both inputs, 2 RCA outputs (your EBay amp seems to be stereo) , so he can accomodate a Piezo powered Guitar, and a conventional electric Guitar pickup *or* a non balanced (no need for it) low impedance microphone input or an active microphone input so he can use a cheap PC type earphone/microphone combination so he can sing and play and not need a microphone stand.

I have commercially made hundreds of such setups (and thousands of conventional Guitar amps).

Your preamp/mixer does not even need volume or tone controls if you use those in the EBay amp, although 2 volume pots are handy for individual adjustment ... but if you want to keep it micro mini you know it can be done.

No need for PCB, you can build it on Vero or perfboard and mount it in any small case, from a dedicated Hammond type pedal enclosure to a plastic box to a tuna or sardine can.
No kidding, there's people selling just such pedals.
VERY popular in Brazil:
Including a mic input is a really good idea for busking. :up:

It's easy to make a simple op-amp buffer. The trouble is, an electric guitar through a 100% clean op-amp buffer into a clean (hi-fi) solid-state amp usually doesn't sound that good. Too clean, too sterile.

It really helps (guitar sounds much better) to get a few percent of 2nd harmonic distortion into the signal somehow. If you have any FETs, using one of these is an easy way to make a simple guitar buffer which also adds a little 2nd harmonic distortion.

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