Cigar box guitar preamp / booster / buffer

[chi_boy]

Does anyone here have any experience with cigar box guitars?

My guitar teacher had one at lessons and now I'm hooked.  His is electrified, but he says the electric tone is pretty bad.  Has no bass and too much treble and too little output.

It has the Rat-Shack piezo pickup that is common for these things and has the output running right to the output jack with a volume knob thrown in.

It didn't take a lot of research to figure out that some form of high impedance preamp was needed.  I found one page that was referred to by several other pages that talk about electric CBG's.

This is the link: http://scotthelmke.com/Mint-box-buffer.html

The schematic there looks a lot like some of the buffers or simple boosters I've seen here.

So my thought was that I could build him a simple booster like circuit that would do a few things:

1)   Have high input impedance to play nice with the piezo
2)   Provide a little more output
3)   Have a simple tone control  (2 or 3 band would be a plus)

And an added bonus would be:

4)   be able to add a little grit to the signal for a little more of the dirty blues tone.


I think 1 and 2 can be done with one of the simple boosters that are all over the forum.  My main question in going that way would be the high impedance requirement.  What would I need to do to something like an EH-LPB to make it high impedance?  Or is it already?  I see several high value resistors going to ground in the mint can schematic.  Could it be as simple as using similar values in the LPB?

I would also appreciate any input that might steer me towards a simple circuit that would also accomplish 3 and 4 if anyone has any ideas.

Any constructive help and feedback is appreciated.

Thanks,
George

[9 volts]

There is a pedal called the eighteen which uses jfets. Has a tone control, gets a little dirty when the gain is up, I think it would sound great on the cigar box. Check 'run off groove' website, from what I remember jfets make a nice preamp for this type of pick up.

[Gus]

For piezo pickups you will need a high input resistance stage after the piezo(s) As you already posted

So a jfet or jfet input opamp or tube stage with a >3meg input resistance and small miller capacitance.


Piezos can generate a bit of voltage when unloaded and sounding better.   Note the use of a input voltage divider in the Scott H schematic in the link.

If you want to use 9VDC.  What might be the first thing to try is build the linked circuit (It is a jfet source follower(SF), gain less than 1 with a voltage divider on the input) and use it before other circuits that you like for the extra dirt you might want to us a volume control after the buffer and before the next gain stage(s) tone control etc.

Look for the current Tillman thread the stock circuit might be something you can use

What are the constraints size, power supply voltage, batteries or wall powered, cost?

EDIT about the voltage part think about a gas grill with the push button igniter.
http://rimstar.org/materials/piezo/ignitor1.htm
http://en.wikipedia.org/wiki/Piezoelectric

[chi_boy]

There is a pedal called the eighteen which uses jfets. Has a tone control, gets a little dirty when the gain is up, I think it would sound great on the cigar box. Check 'run off groove' website, from what I remember jfets make a nice preamp for this type of pick up.

This is a great idea!  Since the cigar box guitars are a bluesy type sound, I would probably lean toward a Professor Tweed.

[chi_boy]

For piezo pickups you will need a high input resistance stage after the piezo(s) As you already posted

So a jfet or jfet input opamp or tube stage with a >3meg input resistance and small miller capacitance.

So if I focus on adapting a Professor Tweed http://runoffgroove.com/professor.html can I take this to mean that if I change the first 1M after the input to >3M, I will increase the input resistance / impedance?  And if I change to >3, would the selectable voltage divider be worth the effort?

If you want to use 9VDC.  What might be the first thing to try is build the linked circuit (It is a jfet source follower(SF), gain less than 1 with a voltage divider on the input) and use it before other circuits that you like for the extra dirt you might want to us a volume control after the buffer and before the next gain stage(s) tone control etc.

I had considered this, but wanted to see if I could accomplish the same thing with one of the more versatile circuits for stomp boxes.  This option isn't off the table, but then I would have questions about linking the two individual circuits and the powere supply.

Look for the current Tillman thread the stock circuit might be something you can use.

I'll do this this evening.

What are the constraints size, power supply voltage, batteries or wall powered, cost?

Main contraint is that it be 9v.  As long as it fits in a cigar box, size isn't a problem.  Cost is relative.  For DIY project the cost of the components between a booster and the Proffers Tweed is small and well worth it if the end goal is achieved.

[petemoore]

  Just make sure you have a DC blocking cap in the signal path between stages, and the supply voltage can be 'shared'..use the same 9vdc supply to power all the various types of stages, buffer, booster...etc.
  Take a look at runoffgroove, Fetzer and other schematics to see what a Fetzer [which is similar to other gain stages in many ways] and multiple stage builds [which have fetzer or other gain stages in them] which demonstrate what multiple stages look like [sharing PS, and DC blocking between actives in the signal path].

[chi_boy]

Well, I checked out the Tillman, and I think that is the way to go.  At least for now.

Step one is to get the piezo to work as best they can, considering the application.  It is a cigar box guitar, not a Martin after all.  I think I am going to start with a 10M for R1 and see what happens. 

I found the mod for the volume control, but I'm stil confused aboout it.  Does the 51k stay or go if the 100k pot is paralled to it?  I'm just not seeing how that works right now.

[sean k]

I've just been thinking about piezo preamps and was going to start a topic about one that has only a few discrete components and lo and behold you've found it. I've got one for use with TL072's and suchlike but I really prefer something easy these days. If I was doing what you're doing I'd add the easyvibe.

You could have a volume off the end of the piezo pre. Just remove the 10k resistor and put a 100kA pot in there. The easy vibes neat 'cause it dirties up nice by just upping the input signal, from a volume pedal or pot, without getting too much louder.

Theres something else out there, I've got a drawing, where you use one half of a tiny signal transformer from Mouser to create the roll offs of a guitar pickup. I'll post it tomorrow if you want.
I say this because I've already got a pedal with the guitar pickup 'emulation' followed by an easy vibe and it's a sweet thing.

By the way, whats the consensus on mounting piezos? silicon? doubled sided tape? anything rigid like a resin glue won't work as it stops vibration. What do you think? 

[petemoore]

anything rigid like a resin glue won't work as it stops vibration.
  Rigid I think would be good for HF / vibration transfer, gouie [like Hot Melt] would have more damping effect.
  Stiff and lightweight are two top priorities for tone-transfer IME.
  Whether you ever want to be able to remove it might have something to say about adjesive choice.
  You can mix up ultra lightweight epoxies...but wouldn't worry about weight if the amount used can be kept relatively small, all these glues, in small amounts [= small amount of weight] are 'nominal' as far as damping effect.