ruby amp compatible with piezo pickups?

[rich gallagher]

built a ruby on perfboard from the schematic at ROG - works fine with my strat (tho some hum and maybe not as much volume as it should put out) plug in my cigar box guitar with disc piezo pickup and nothing but an air horn sound when i turn up the volume - cigar box guitar works fine with store bought amps - it has been suggested that gain is the problem, but i don't know how to solve that - i know nothing about electronics - thanks in advance for some help that  newbie can follow

[earthtonesaudio]

Probably an impedance mismatch.  Piezos like high impedance inputs, and the lm386 that's in the Ruby is a tad on the low side.  Put a buffer between the piezo and the ruby and you should be good.

[Top Top]

Have you tried it with any other instruments?

Might be worth checking with something else, because the most common problem people have in general with the ruby is oscillation problems. I have had that type of problem when my ground wires weren't thick enough gauge or were too long. Also if pins are accidentally bridged on the 386 chip.

[rich gallagher]

earthtones - what would be a buffer - remember i'm a newbie

[earthtonesaudio]

Hang on... did you use this schematic: http://runoffgroove.com/ruby.html ?

If so, it already has a high impedance buffer, so I was wrong, and that's not the problem.  The only other thing that jumps out at me is that the Ruby is DC-coupled, so you might try adding a capacitor in series with the input.

[rich gallagher]

ok - now i built another ruby - this one with no gain pot and put a 2k2 resistor betw pins 1&8 on the 386 and a 220pF cap betw lugs 2&3 on the vol pot and replaced the .047 cap from the source pin of the FET with a 0.1uF - SAME PROBLEM  works great with my strat, but air horn sound as soon as i turn up the vol!!  if this is a gain problem, how can i tone down the gain - again i know nothing about electronics

[earthtonesaudio]

You can omit the resistor between pins 1 and 8 (they can be left floating) to reduce gain.  If it still oscillates there's almost certainly a wiring problem, possibly a grounding problem.

[rich gallagher]

earthtones - i omitted the resistor as you suggested - still same problem - earlier you mentioned putting a cap in series with the input - what size cap would you suggest

[jimbeaux]

I have an acoustic guitar with both a magnetic pickup & a cheap $2 piezo  & my Ruby works fine with either pickup.

[Top Top]

The pot across pins 1&8 is what adjusts the gain.

Cap in series with the input, you could start with one of the normally used input caps like .1uF. If it helps with your problem, you can adjust up or down to get more or less bass out of it.

My guess though is still that it is a ground issue. Is your piezo pickup home made? What type of cabling comes off of it?

[rich gallagher]

top top - piezo is homemade - i've tried 2 different cbgs - one has a single 6.30KHZ piezo disc and the other has 2 1500-3000HZ piezo discs wired in series - both hot glued to the bottom side of the cigar box top - these are piezos used in buzzers and are commonly used by cigar box guitar makers for pickups - both work fine with store bought amps - have built 2 different types of ruby : 1 has a .047uf cap and two have a 0.1uf cap from leg#2 to the vol pot - have also tried with a gain pot, without a gain pot and a 2k2 resistor betw legs1&8 and with legs 1&8 unconnected to anything - all same result - they all work great with my strat