Signal Mixing

[Jack.Straw]

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

[Dylan]

 I have the same problem with you.
i found a schematic of 2 line mixer.
http://www.freeinfosociety.com/electronics/schemview.php?id=1060

[phatt]

Hi Jack,
          A Q before I go on.
Is the peizo PU passive? (ie, no onboard preamp)
If so then even a mix buffer won't help you much.
Cheers Phil.

[Jack.Straw]

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?

[Jack.Straw]

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

[phatt]

Hello Jack,
              Yes you will need a preamp for peizo.
Media player has line level output and you can't just join that with a PU
(of any type really).  The Ruby has a 1m5 gate R but that ain't enough for hiZ PU.
you need more like 2meg or even 10meg.   Better to make the peizo *Active*then you can plug into just about any line level which makes it versitile.

I get "Forbidden" on that link for peizo, (I have no clue with computers?) but not to worry there are quite a few and they are likely to be similar. The Fet based ones that have bootstraping will likely be the go for pz PU's as they multiply the input imp.
Anyway here is one I whipped up for a chap on the cheap.

My peizo buffer is low output but don't let that fool you as it has very high input imp and any good guitar amp will deliver the rest.
This was made to replace a painful onboard unit that suffered from way too much hi freq content which makes Acoustic guitars sound stupidly harsh and very unnatural (though some may want that)
and of course the manditory feedback issues that tend to go with acoustic Electronics.
R1 can be as low as 500k but the bootstraping of C2 will make it much larger.
Use metal film for R1 if you want to keep noise low as possible.
You need a very high imp input when using peizo's otherwise you will loose a lot of the low freq content. Again a balancing act, to much and they tend to suffer feedback.
make C1 smaller if you wish to wipe off more bass.
The optional cap across the peizo element may work on some units, depends on the peizo used.
Don't ask me to explain just try it and see what you think.
Play with C4 if you want, to cut the top higher use a smaller cap.
Try from 10nF up to 47nF and see what you prefer, If you want ALL the high freq just delete C4.
Oh yes The 9volt battery will last a long time.
A chap by the name of "Don Tillman" (I Think??) has a whole page dedicated to simple fet preamps, his ideas may give much more info.

The mixer pattonBa refers to is passive and there will be overall level loss.
If your ruby has enough grunt it may not be a problem but it will be of limited use on other equipment so may I suggest a *dual input* on the ruby Amp and be done with it.
there should be enough schematics around with 2 inputs mixed into one to give you
an idea of how to achieve that. I don't use fet circuits much but I'm sure someone here will chime in here and offer something useful in the way of mixer circuit ideas using Fets.
Have fun with it.
Cheers Phil.

[Jack.Straw]

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

[Dylan]

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

sorry,no. 

[phatt]

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

No my circuit is down in output level but you WILL Get better sound quality over direct passive.
A 1meg Resistor for R1 in the circuit shown will give you 50meg (ish) input imp,,,plenty enough for hi Z input.

Arron cake circuit is going to work but the input imp is quite low so don't expect it to work well as magnetic guitar PU. It's just a basic line mixer.
The other one is more complex but the mic input is not really any good for guitar.

Forgive me but I'm wondering at the wisdom here?
Surely you have a small audio amp that can play music and then just play along with your ruby?
Or did everyone thow out their old ghetto blasters from the 80's
Heck it seems like a lot of trouble for such a small rig as the ruby is only of limited power and has a very small speaker?

Might be better to start a whole new project with two inputs.
With just a couple of opamps you could build a really versitile preamp rig which you could still plug into the ruby if you so wish but you gain the advantage of being able to plug into other powered equipment.
Just my thoughts.
Cheers Phil.

[J M Fahey]

Heck it seems like a lot of trouble for such a small rig as the ruby is only of limited power and has a very small speaker?

Just my thoughts.
Cheers Phil.

Hey, Phil hit the nail right on the head.
I was vaguely wondering that there was something wrong here, didn´t exactly realize what.
Now I see it: the Ruby barely has any clean power for a guitar, definitely not enough to also amplify some clean backing music.
And the slightest distortion from the guitar will kill the music.
*Maybe* building two Rubys, each with its own speaker but sharing supply and case coud fill the bill.
Test it "outboard" before comitting it to your guitar.
Bye.

[Jack.Straw]

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.

[phatt]

That's kool,
then just use that very simple passive mixer that "pattonBa" suggested.
nice to chat, see ya. Phil.

[phatt]

Just had another idea,,
                              With the onboard preamp installed just try mixing the two signals again and see if that works as there maybe enough swing from the guitar preamp.
It maybe all you need, Though turning down the guitar level pot will also turn down the line input.
Phil.

[Jack.Straw]

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.