Guitar preamp to LINE IN

[pecamuzicar]

Hi all!

I have Behringer UCA 200 which I got with some their midi keyboard. It's USB sound interface for playing or record some VST musical instruments on the PC with it midi keyboard, or any electric musical instrument or audio device over UCA 200 - LINE IN. So I tried to connect my Stratocaster directly in UCA 200 - LINE IN, with NI Guitar Rig or some different VST effects together, and It was amazing how fast is UCA 200 with Behringer original ASIO driver (I tried it long ago, and latency was even below i think 4ms), but sound is something fuzzy (too fat and undefined) when I use some overdrive or distortion VST guitar effects, or just too fat, when I play clean sound - without clean middle guitar sound. Also I used some VST equalizers in front of other effects, than it was better, but still something messy. My conclusion is obvious - it was because of LINE IN impedance and sensitivity characteristic where I directly connected my guitar. So, lower frequencies are present too, and in general also, sound is something weak. First, I though that modify something this LINE IN stage, but there are looking triple layer PCB although it is small, with SMD parts, and I was not sure for result in sound.

So, I need some simpler schematic of guitar preamplifier using opamp IC, but with appropriate high input impedance for guitar (over 250Kohms), and with appropriate output impedance to connect him on UCA 200 - LINE IN.

UCA 200 - LINE IN characteristics given in manual:
LINE IN
Connectors RCA, unbalanced
Input impedance approx. 27 kΩ
Max. input level 2 dBV

Thanks in forward! :-)

[Nigel Goodwin]

You just need a high impedance buffer - I've used a dead simple one many times over the years - usually for feeding ceramic cartridges it to guitar amps.

Just an FET (2N3819 or similar), a 1K resistor from source to 0V, a 2.2M resistor from gate to 0V, and +9V to the drain. Input to the gate, and output from the source - via a coupling capacitor if needed (it's often not).

It's a simple source follower, gain of 1, with 2.2M input impedance.

EDIT: Don't forget a decoupling capacitor from drain to 0V.

[nightanole]

GuitarNutz 2 - JFET Buffer and buffer cable



I think this is a visual of what nigel is talking about.

http://beavisaudio.com/techpages/Buf...Transistor.gif

[Nigel Goodwin]

Originally Posted by nightanole

GuitarNutz 2 - JFET Buffer and buffer cable



I think this is a visual of what nigel is talking about.

http://beavisaudio.com/techpages/Buf...Transistor.gif

Yep, I didn't have time to draw it

It's VERY non-critical.

[pecamuzicar]

It looking more than fine. Thanks!
Before some time I found such circuit with some J-fet which is hard to find here, but I was not sure about it - whether output impedance is appropriate to UCA 200 LINE IN - 27Kohms. Do you think it's ok?

[pecamuzicar]

MPF102, 2N5457, 2N3819, J201 are not available here, so I will try with BF245, BF244 or J108, J310 ... I hope it will be ok, otherwise - only OP. Thanks!

[Nigel Goodwin]

Originally Posted by pecamuzicar

It looking more than fine. Thanks!
Before some time I found such circuit with some J-fet which is hard to find here, but I was not sure about it - whether output impedance is appropriate to UCA 200 LINE IN - 27Kohms. Do you think it's ok?

Perfectly so, you should feed a low output impedance into a considerably higher input impedance (at least five times higher, and preferably more).

The output impedance of the simple FET preamp is similar to standard line outputs, so it's absolutely perfect.

Try different FET's, like I said it's not critical, you may have to change the value of the source resistor for best performance though - essentially the value sets the bias of the FET, so it will affect the level it clips at.

If you have a scope, fit a preset resistor, and adjust it for symmetrical clipping - but personally I've always just chiucked a 1K or 2.2K in place.

Bear in mind FET's have very wide tolerances, so really you should select the value for every single one you build.

[pecamuzicar]

I made preamp with schematic from Beavis Audio Research. Somewhere I found that BF245C can substitute MPS102. I put it there and I got some gain, sound is cleaner, but not so much. I doubt on some resistor attenuator in front of input of OP in LINE IN circuit of UCA 200, with 22k and 27k. It's not looking like standard part of circuit in (for) guitars preamp's. Usually such attenuator lose some high, but middle frequencies for harder guitar sound also, which I like. I like lower half of the middle band something exposed for good guitar sound. There is easy to modify this input stage parts, but do you have maybe some nice idea how and what.

Also I have idea to put between JFET buffer and LINE IN, some guitar tone blend for bass, middle, treble to fine tune the sound.

Really I am not lazy to do such things to get best guitar sound, so if you have idea about this things I mentioned, please tell me.

Peter

[pecamuzicar]

I made JFET buffer cable, very nice.