Acoustic Guitar Pickup + Little Gem Amp

[blah2222]

Hello everyone,

This is my first post and I'm hoping that I can get some help from people familiar with the Little Gem amplifier, or pickups in general.

I built my Little Gem amp to spec with no problems, and I found an old computer speaker to use. It is a 3.5" (8.75 cm) in diameter, 4 ohm, and 8 W. I also wanted to make a pickup for my acoustic to use with the amp, so I bought a 1.5" (5 cm) piezo buzzer. I broke it open and got the piezo transducer out. I setup audio jacks from the piezo to the audio cable as well as the audio cable to the input of the amp with no problems.

Once I plugged everything in and turned the volume up I got a pretty solid sound out of the amp but it was rather noisy for the EAD strings. It's more of a muffled sound and any bass in the chords dies out quickly. It has some reverb if you strum out some chords, as well.

After listening to the sound bites on the runoffgroove website I was a unsatisfied with how my amp sounded. I encased the entire system in a wooden box, would metal be better for getting rid of the noise fields or am I thinking too hard?

Another weird thing that I noticed that when I plugged in my old Squire Strat to test the amp, it didn't really make a sound. I plugged my piezo acoustic pickup into my Fender amp and it was pretty darn clean compared to my amp, so that led me to believe that there is something fishy about my amp.

I was also wondering whether this had to do with my speaker not being able to keep up.

Thanks a bunch,
JP

 

[unclejed613]

for the lower strings, you need a bigger speaker. a 6" or 8" speaker will probably sound much better than a 3.5 inch speaker, although part of your lack of low end response is due to the piezo element and possibly the lack of a solid mount for the piezo. a wooden box is better for the sound, but a shielded box for the circuit board would probably be a good idea as well as a shielded cable for the piezo..

i'd also go with an 8 ohm speaker. little chip amps work better with 8 ohm speakers. a 4 ohm load is hard on small chip amps

 

[Nigel Goodwin]

For an acoustic pickup use an electret mike and preamp, the little gem hasn't enough gain (or input impedance) to work directly from a piezo.

 

[blah2222]

Thanks for your replies!

I actually was able to pickup a Ford door speaker that is ovular with a major length of 8" and a minor length of 6". It doesn't say what it's power and resistance specs are though. I tried it with my amp and it cleaned it up a bit, but still kind of fuzzy.

I just made a quick PCB using EagleCAD because I have access to a mill at my school, so that might reduce the noisy connections. Right now my piezo is taped by its outer brass ring, firmly to the body of my guitar, letting the inner crystal circle oscillate freely.

So you guys think that I should encase my piezo as well as my amp in a metal container?

Thanks again,
JP

 

[audioguru]

The piezo disc from a beeper produces no low frequencies because it is designed to resonate and be very sensitive at only 3kHz to 4kHz.
Some people damp the resonance by gluing a piece of foam rubber to the piezo disc or mount it so only part of it can vibrate. Use an electret mic and suitable preamp for it instead.

 

[blah2222]

The piezo disc from a beeper produces no low frequencies because it is designed to resonate and be very sensitive at only 3kHz to 4kHz.
Some people damp the resonance by gluing a piece of foam rubber to the piezo disc or mount it so only part of it can vibrate. Use an electret mic and suitable preamp for it instead.

Any idea on which electret mic specifically works well for this and how would I mount it? I am looking at Digikey and there are quite a few with different specs. Also, what would the preamp setup look like, something like this?

**broken link removed**

Is the preamp just to buffer the signal before it passes through the 386?

Thanks,
JP

 

[audioguru]

You don't need the transistor preamp if you add a capacitor between pin 1 and pin 8 of the LM386 which increases its gain.
Make it like this:

 

[blah2222]

Does the mic attach to the guitar body much like a piezo? I already have the parts for the Little Gem so I was just wondering if I could just replace the piezo with the mic.

 

[audioguru]

The Little Gem MkII has a capacitor as I mentioned to increase the gain but it does not have the bias resistors to power an electret mic.
An ordinary electret mic is a normal microphone that picks up sounds from the air, not a contact one.
Somebody makes a piezo contact mic that is made to be mounted on a guitar. Maybe you can make one from a piezo transducer (speaker) which is different from the piezo in a beeper.

 

[blah2222]

Okay, I am going to head over to the electronics shop to pick-up this electret mic. I had a bunch of piezo saucers lying around so I decided to try putting two on the body of my guitar, (one closer to the bass strings and the other on the treble side). I connected both of their signals together, (as well as reference grounds), and hooked it up to the original audio jack.

It actually sounded a heck of a lot cleaner, and I really noticed less fuzz with the bass being more tolerable.

Stuff seems to be coming together!

 

[unclejed613]

the best place for the transducer is close to the bridge, this is where the string vibration gets coupled to the body of the guitar