Awesome, thanks guys.. I'll give that a try tonight!

Hi.  I'm having fun building these "Ruby" practice amps from runoffgroove.com.  I've wired it with a switching DC jack that cuts off the battery when I plug in a 9v wall transformer.  The only problem is that the transformer introduces hum into the circuit.  I've used shielded wire for all connections, and have even wrapped the circuit itself in copper foil tape with conductive adhesive (touching ground).  I'm stumped as to what else I can do to get rid of the transformer hum.  Any ideas?

Here is the circuit:
http://www.runoffgroove.com/ruby.html

And this writeup shows how the DC jack is wired in:
http://www.generalguitargadgets.com/pdf/ggg_ruby.pdf

Thanks for any advice you can offer,
-Scott

Thanks Phil.  I'm ok with the volume pot controlling both the guitar and the media player since the media player has its own volume control.  I'll give it a whirl...thanks for your time and advice.

I realize this isn't the most practical project. This is just one more in a long line of impractical instruments.  I'm satisfied with the volume and clarity of the amp, both with the media player and with the guitar.  I just need a way of mixing the two.  I know it isn't practical, but is it possible?  It doesn't have to be the ruby schematic (although that is already built), it just has to be fairly small and be battery powered.

Phil, thank you so much!!  I'm going to give your buffer a try this weekend.  Question: Does using the pre-amp bring the piezo's signal up to "line level"?  I found a couple schematics for mixing line-level inputs - would you mind taking a look and giving me your opinion on which you think would be best?

FET Audio Mixer: http://www.aaroncake.net/circuits/mixer1.asp

2-Input Audio Mixer: http://www.geocities.com/tomzi.geo/mixer/mixer.htm

Would either of those be a good choice?

Thanks again for your help!  I really appreciate your time.
-Scott

pattonBa, thanks for the schematic.  Have you tried it yet?

Phil,

the piezo PU is passive.  However, i have built JFET based preamps in the past for my passive piezo creations.  The schematic i used is from diyguitarist.com:

http://www.diyguitarist.com/PDF_Files/FET-PiezoPreamp.pdf

Should i build another one of those and put it between the piezo and the ruby amp?

So i built this travel guitar with a built-in practice amp.  The amp circuit i built is The Ruby.  The guitar is amplified using a piezo element built inside the bridge.  I also included a 1/8" input jack so that you can use it to play music with your media player.  The idea was that you can jam along with songs or prerecorded accompaniment, or switch off the guitar part and just use it to play your media files.  There is also a 1/4" jack that allows you to output to an external amp.  The jack is switched and wired in such a way that the little practice amp gets switched off when you plug into the 1/4" jack.

Well, the guitar and the media player work fine independently, but not so well when both are active.  I foolishly thought that simply merging the wires would allow the signals to mix... like it does if you merge a left and right signal into a mono application.  Apparently this isn't the case.  I thought maybe i needed to put a simple JFET buffer between the piezo and where the signals merge, but the little Ruby amp has a JFET already so I'm not sure what effect that would have.  Can anyone tell me what needs to be done to get the two signals to play nicely together?

Thanks for all your help!!
-Scott

Phil, thank you so much for taking the time to type all that.  Very helpful and informative!!!  Thank you!!

Hello everyone.  I've just finished building my second Ruby circuit (http://runoffgroove.com/ruby.html).  On my first amp i used a little 8ohm 5" speaker out of an old stereo.  It works fine, but for my second project i need to balance function with  size.  I know almost nothing about speakers, so i was hoping someone here could make a recommendation.  When selecting a speaker what sort of things should I look out for? (other than a total impedance of 8ohms)  Is the size of the magnet something to look at?  Do certain speakers respond better to guitar frequencies?  What would you say the smallest diameter speaker i could use and still produce a strong sound & good tone?

Thanks for your time,
-Scott

So i saw this Heiniken keg amp that someone built using a kit from Canakit.com.  That is SO up my alley and decided to build something similar.  My thing is building guitars (and other instruments) out of recycled materials, and a little keg amp would be the perfect companion.  I've built simple FET based preamps for some of my piezo powered instruments, but have never attempted a solid state amp.

I've been looking around the net for a good schematic, but the trouble is that i wouldn't recognize a "good schematic" because i have no experience.  It doesn't need to be complex... a simple volume control & on-off switch is enough.  I would like to power it from a 12v rechargeable battery supply so that it's portable.  I also want to be able to do the project on perf-board (i don't want to make a circuit board).

Can anyone recommend a good schematic for this project?  Also, i really like recycling old commercial products and using the parts in my projects.  If you have a recommendation for a common product that i can use (i'm thinking old stereo receiver amps or something?) for this project please let me know.

Thanks for your time!
-Jack

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