Bass Ukulele / Amp combination

[bikemusic]

I was looking for the ultimate portable bass solution for the acoustic jam or campfire experience. The goal was an URB-type sound for folk and bluegrass music, and loud enough to keep up with a couple of acoustic guitars, some singers, or maybe bongos. So I bought a cheap Harley Benton bass ukulele and mounted a 2x30 watt Class-D amp inside, together with a pair of Dayton Audio long-excursion drivers. My original plan was to seal the body and mount a passive radiator in the back to simulate the approximate specs of a PJB Double 4, but I ran out of space and rigidity, in spite of added internal bracing. Too much body vibration made for nasty harmonics and feedback.

So I ended up just putting a grille in the back, and closed off the soundhole with a 3D printed cover/thumb-rest. I put a stripped down Behringer EQ pedal inside to boost the lower frequencies, lower impedance, and squelch feedback. I power it with a 12v lithium laptop power bank which is good for about 10 hours. Speaker grilles were also 3D printed. I left the output jack connected so I can also play through an amp, or even plug a guitar into the bass and play both instruments through the internal speakers.

It has a surprising amount of bottom end, although it rolls off below the open A string.

I performed once with a 20-voice youth choir, along with piano, acoustic guitar and Cajon drum, and the audience said they could hear the bass in the mix. (Nobody had additional amplification). It's not quite as loud as an URB, but there's always a PA in a larger venue... It has plenty of volume for acoustic living room jams. Attached is a picture next to my old arch-top to show how small it really is. Total length is 32", and weight is about 8 lbs. Total cost for the project was about $350 including shipping the HB Bass from Germany.

If I did it again I might start with a larger guitar body and make a rigid box to fit inside, so I wouldn't be fighting the thin plywood's resonance.

 

[lug]

we need soundbites!!!!

 

[Raf Seibert]

An interesting project! In similar circumstances (several players/singers, no one amped), I’ve done pretty with a Martin BM model acoustic bass (basically all lam). That is, everyone could hear me, but I was far from dominating the sound.

 

[beans-on-toast]

I've seen that done with solid body guitars in the 60's. A neat idea.

You can try bracing the inside to dampen the resonance. Even stuffing the inside with a towel.

 

[bikemusic]

I've seen that done with solid body guitars in the 60's. A neat idea.

You can try bracing the inside to dampen the resonance. Even stuffing the inside with a towel.

I have a maple u-shaped brace connecting the back at the widest point to the underside of the top, right under the bridge. I've also tried polyester fill - a little bit helps but too much makes a tight, dead sound. The only feedback remaining is right around 220Hz (A) when I play an octave below that (G string 2nd fret) and I've damped that by cutting the 200Hz level on the equalizer about 6db lower than the 100 and 400. Now it won't feed back at anything but the highest volumes, and even then I can palm-mute the strings. But it took a struggle to get here.

 

[beans-on-toast]

I have a maple u-shaped brace connecting the back at the widest point to the underside of the top, right under the bridge. I've also tried polyester fill - a little bit helps but too much makes a tight, dead sound. The only feedback remaining is right around 220Hz (A) when I play an octave below that (G string 2nd fret) and I've damped that by cutting the 200Hz level on the equalizer about 6db lower than the 100 and 400. Now it won't feed back at anything but the highest volumes, and even then I can palm-mute the strings. But it took a struggle to get here.

Sorry if I missed it, are you using a piezo pickup? If yes, I'm wondering if a dense damping material such as dynamat under the bridge would help prevent waves from getting to the bottom of the pickup. With some magnetic pickups, I find that candle wax on the bottom of the metal case is enough to dampen a metal case and prevent it from vibrating.

 

[bikemusic]

Yes, there's a piezo pickup under the saddle. It seems to be glued in because I couldn't pry the saddle out of the bridge after I took the strings off. Otherwise I would have put some felt or leather under there. I also found that tuning up the E string pinched the saddle against its slot, and it was very weak until I lifted it off a couple times.
Attached is a sound file of sorts. Apologies for the poor recording (and playing).

 

[beans-on-toast]

Yes, there's a piezo pickup under the saddle. It seems to be glued in because I couldn't pry the saddle out of the bridge after I took the strings off. Otherwise I would have put some felt or leather under there. I also found that tuning up the E string pinched the saddle against its slot, and it was very weak until I lifted it off a couple times.
Attached is a sound file of sorts. Apologies for the poor recording (and playing).

I hear what you are referring to. These sort of issues are difficult to resolve as there are many possibilities of something vibrating. If you put your ear to the body, test if you hear it when the amplifier is off. See if applying pressure to the printed sound hole cover or the speaker covers makes any difference.

It could be a badly installed piezo, the bed in the bridge is not cut properly. If you tested it befor the modification that would rule that out.

The idea with the damping is to install it inside the body under the area where the bridge sits around the brace to extend the damped surface area. This is a bit of a long shot.

 

[bikemusic]

I rebuilt the bass by putting the speakers in sealed enclosures, about 1 liter in size, made of PVC pipe caps. So they are now acoustically isolated from the body (held in with velcro) and I can get much more volume without feedback to the bridge. I played it yesterday in a mix with a miked guitar, accordian, two violins and a drum set, and I had volume to spare. The only weakness is the roll-off at the bottom end which makes the E string quieter than the others. An upright has a similar roll-off so it's not so bad. I can partially offset it by boosting the 50Hz equalizer channel and leaving the rest flat. Overall I'm pleased with it - as loud as an URB and much more portable.