Home made guitar preamp?

[jaywalker]

So, i have this little volume controller circuit board from an old "build it yourself" am/fm radio kit. I was curious to know, if i install it in my guitar, will it act effectively as a preamp? i dont know how it will affect tone, because i've never used a preamp before. is it even possible/safe to do this?

Here are a few pictures of the circuit:





it has wires for signal in/out and the battery wires. it originally used the 6v supplied by the 4x AA batteries in the radio, but since then I have lost the battery holders.... so would it be safe to use a 9v battery?

basically my questions are.... would it work, is it safe, and what should i expect for results?

[olddawg]

If it is a LM4558 opamp or such it won't care. I would make sure that any electrolytic caps are rated at more than 9 volts. If they are 6v caps they might pop. I can't see that using it as a preamp will hurt anything. It might just sound like crap. The input and output loads may not be matched for a guitar and amp. An easy way to bring the power supply voltage down without a regulator if you need to is to put rectifier diodes in series on the B+ side. Each one will bring it down .6v or so. 4 foreward biased in series would bring your 9v closer to 6v. Or you could just replace the caps. BTW, there are some really good guitar preamp kits out there that do the same thing right that don't cost much. General Guitar Gadgets comes to mind but there are lots of others. They have schematics on line though which might be usefull in modding what you have.

[jaywalker]

well, 2/5 of them are rated for 16v, and the other 3 are 50v, so i think im good as far as voltage goes.

the chip on the PCB says:

386D
JRC
9116D

(dont know if any of that is useful... but i figured id post it just incase)

as far as the input/output loads for guitar/amp like you said... if i checked it with a multimeter what values should i look for to make sure its safe, so i wont blow my amp up?

and in the end, if all that checks out, the only thing i have to worry about is it might sound like poop right?

edit:

did a little looking around for the chip number and came up with some interesting results. apparently it is pretty much intended for this purpose, it can be used to make a sort of pocket-amp like this one:





the circuit for which, is using an almost identical chip, and a few less capacitors.


this might just work afterall

[Robert M. Martinelli]

LM386?No thanks ( at least as a preamp )

Hi,
the 386 is designed to work as a small amp to deliver 1 W max directly into a loudspeaker, so it is not suitable for preamp designs; other than 4558s, i tend to use NE5532s ( in case I need a double opamp ) or NE5534s ( single opamp ) - their equivalent input noise is less than 4 nV/Sqrroot(Hz), their preformance as audio preamplifiers is outstanding, they' re internally compensated for gains higher than three, and with very few passive components around them you can get excellent results, especially if the thing has to be installed inside your guitar, an excellent noise figure is mandatory if you use a lot of effects, otherwise you could end with a "tornado" hiss coming out your amp....so you could get some metal film resistors as well, as their equivalent noise figure is lower than carbon ones.

[jaywalker]

alright well ive decided to build one of those mini amps instead, seems a little easier and more practical (given the materials i have).

I just have a question about the schematic:



what i understand as the "ground" label (the ones that look like downward arrows with lines through em), i need to know what they connect to. do they all connect together ie: on one common ground point? what exactly do they connect to?

[jaywalker]

well i put the amp together today, heres a video if anyone wants to see how it turned out:

http://youtube.com/watch?v=kaojZVLmPCM

[bixkitboy]

I just saw a very small chip that evidently glues on the back of your volume know and does the preamp thing. Think I saw it on ebay. Looked cool.