DIY guitar preamp
Mar 16, 2007 at 1:14 AM Thread Starter Post #1 of 6

star882

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What circuit do you recommend to connect a Squier Telecaster Custom 6 string guitar to a line level input on a PC? I've heard that guitars tend to have poor SNR (so frequency response is much more important than SNR for the preamp) although the Telecaster has shielded pickups and therefore has better SNR than many other guitars of the same price range.
The circuit should be powered by USB (through a DC/DC converter) although that's not absolutely necessary.
 
Mar 16, 2007 at 4:05 AM Post #2 of 6

threepointone

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I was doing some research on this before for a project of my own (unfortunately I never really got around to doing it, so I can't confirm anything) but as I understand, one of the main things you have to be concerned with when working with guitars is that they have a high output impedance and ideally you should have some sort of impedance converter near the guitar, since I don't know what the characteristics of your sound card are. They're also inherently balanced (I think), so you might want to mess around with that ideal of a balanced-to-unbalanced converter right before your sound card input to reduce noise from the cables
 
Mar 16, 2007 at 4:33 AM Post #3 of 6

error401

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Quote:

Originally Posted by threepointone /img/forum/go_quote.gif
I was doing some research on this before for a project of my own (unfortunately I never really got around to doing it, so I can't confirm anything) but as I understand, one of the main things you have to be concerned with when working with guitars is that they have a high output impedance and ideally you should have some sort of impedance converter near the guitar, since I don't know what the characteristics of your sound card are. They're also inherently balanced (I think), so you might want to mess around with that ideal of a balanced-to-unbalanced converter right before your sound card input to reduce noise from the cables


That's exactly what a preamp is intended for, presenting a high input impedance to a high impedance source (like a guitar), and 'converting' it to a low impedance output for driving a line or power amplifier stage. As far as I know, the output is single-ended as well, with the pickups connected to tip and the other side grounded, as with most TRS equipment.

You don't necessarily need an amplifier at all, a simple balun can be enough to do the job.

In the industry, this is similar to a DI box which takes an unbalanced high-impedance source as input and presents a low impedance balanced output. They can be active (include an amplifier) or passive (transformer-based). You can find many schematics online for this that could be adapted to an unbalanced output simply by ignoring the antiphase output. It can be done with a single transistor and a small handful of passives, or using an opamp.

A CMoy might be a good place to start. I would recommend avoiding a volume pot, but you probably will need to tweak the gain so as to not clip your soundcard. I would try measuring the AC voltage on the output of your guitar at maximum output, and then set the gain to make the maximum output around 2Vpp, this is probably appropriate for your soundcard's input.

Or go to a music shop and ask them, they should have a gadget to do this.
 
Mar 17, 2007 at 11:34 PM Post #6 of 6

star882

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Will a RC4580 make a good preamp?
It appears that my guitar needs some form of high frequency compensation. When I tried using an INA121, the thickest string will be loud while the thinnest string is quiet.
Therefore, I plan to use one half of the RC4580 as an amplifier and the other half as a "soft" active high pass filter to boost the high frequencies more than the low frequencies.
Oh, and is there any problem with using a series resistor (something like 10k) and two inverse diodes (1N4148) across the opamp inputs along with diodes to the supply rails for ESD protection?
 

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