Onboard Acoustic Preamp

[Aleph Null]

I've always loved the sound of my mahogany-topped dreadnought. It records beautifully, but I've never been able to get an amplified sound I was happy with. Over the years I've tried various arrays of piezo pickups and stereo setups. I finally settled on a magnetic sound hole pickup. It still doesn't sound like the guitar, but it has a cool mutant archtop sound that I also like.



I like playing it into an amp, but I also wanted the option of running it straight through a DI. So I decided to build an on-board preamp to avoid loading down the pickup. The circuit is basically an MXR Micro Amp with a Big Muff style tilt EQ. There is an almost imperceptible scoop in the mid range (welcome in this application) and very robust output level.

The external controls are connected by shielded wire (lug one on both of the controls goes to ground anyway). I was originally planning on mounting the controls inside an enclosure, but I couldn't find pots with long enough threads to get through the enclosure and the side of the guitar.





The gain pot is mounted internally.



I used zip ties for strain relief on the external pots. I went with 3.5mm jacks as opposed to 2.5mm because they are more readily available and a little more robust.



Pickup input at the top. Output to strap button jack on the bottom. The battery is wired externally and sits in its own bag: it's easier to change this way. I wanted the whole thing to be fairly modular. I can swap out the pickup or the preamp without having to solder anything this way.





Drilling those holes was one of the most nerve wracking things I've ever done!

[cajone5]

So cool!  I love seeing folks on this board taking their pedal skills and applying them to other areas.  Really nice work!

[Aleph Null]

Thanks! I've definitely been inspired to be more adventurous after seeing other people's imaginative work on this forum.

[Boba7]

Hey that's really great!! I don't know if I would have dared...

Amazing job, really impressive, hope it sounds as good as you wanted it to!

[JackSkellington]

Nice job!
How did you fix the box inside the guitar?

[gordo]

What a beautiful guitar!!!

[Aleph Null]

Nice job!
How did you fix the box inside the guitar?

I had to affix the enclosure the same way I did the battery box: velcro.

[Marshall Arts]

Nice job!
How did you fix the box inside the guitar?

I had to affix the enclose the same way I did the battery box: velcro.

Would you mind sharing the details about the circuit (schematics/layout) and the pickup as well? I am looking for something similar...

[JackSkellington]

Nice job!
How did you fix the box inside the guitar?

I had to affix the enclose the same way I did the battery box: velcro.

Velcro! It was the only method I thought. Thanks!

[Aleph Null]

Would you mind sharing the details about the circuit (schematics/layout) and the pickup as well? I am looking for something similar...

The vero layout is here. The circuit calls for a TL062, and I would definitely recommend against "upgrading" to a TL072, as the TL062 will have seven or eight times the battery life. I just tacked a master volume (A100k) onto the output and put the gain control inside. The tilt eq is centered around 1kHz, but you could play with the values to voice it to your rig. Were I to do it again, I'd probably move the center up closer to 2kHz.

The pickup is an Artec soundhole pickup. You can find them on Amazon for $40. I chose it because of the adjustable pole pieces. The magnets are ceramic instead of the neodymium ones you get with other designs. You could get something similar (I think it may be a rebranded Artec) here.

If you run the pots outside the enclosure, definitely use shielded stereo cable to connect them. Both controls have a lug that goes to ground, so you could connect the shield to that.

[Aleph Null]

What a beautiful guitar!!!

Thanks! It's been a real work horse for me. And very distinctive on stage.

[Marshall Arts]

Would you mind sharing the details about the circuit (schematics/layout) and the pickup as well? I am looking for something similar...

The vero layout is here. The circuit calls for a TL062, and I would definitely recommend against "upgrading" to a TL072, as the TL062 will have seven or eight times the battery life. I just tacked a master volume (A100k) onto the output and put the gain control inside. The tilt eq is centered around 1kHz, but you could play with the values to voice it to your rig. Were I to do it again, I'd probably move the center up closer to 2kHz.

The pickup is an Artec soundhole pickup. You can find them on Amazon for $40. I chose it because of the adjustable pole pieces. The magnets are ceramic instead of the neodymium ones you get with other designs. You could get something similar (I think it may be a rebranded Artec) here.

If you run the pots outside the enclosure, definitely use shielded stereo cable to connect them. Both controls have a lug that goes to ground, so you could connect the shield to that.

Thanks! I will build that in my travel guitar before summer vacation... recording on the go, here we come!

[p_wats]

That's very cool! Thanks for sharing.

[Marshall Arts]

If you run the pots outside the enclosure, definitely use shielded stereo cable to connect them. Both controls have a lug that goes to ground, so you could connect the shield to that.

I did not find a ground connection on the tone pot on the Vero... Am I missing something?

[Aleph Null]

I connected lug 1 of the "tone" control to the shield of a stereo wire, thinking it was going to ground through the 3n3 capacitor. Looking at the layout again, I think I may have been mistaken...

If that's the case, it would seem that the wires to the tone control don't need to be shielded! I haven't experienced any real noise from the preamp that can't be traced back to the single-coil pickup.