Tube piezo pickup preamp?

[Mbira]

I was inspired by a post in the Brewery to look at THIS.

I'm curious if there are any freely available schematics for such a thing:

Something that handles the high impedance of a piezo pickup.
a single tube pre that can then plug into something like an old Fender amp.

I'd like to build something for the electric mbiras that I play.

Also, I have three piezos hooked up in series, so I don't know if that changes the impedance on those?

Thanks,
Joel

 

[Gus]

easy build a microphone type circuit,

 

[Mbira]

hmmm...never thought about it that way.

 

[beatpoet]

I've wondered about using piezos in series / parallel before for a keys setup. Anyone tried this?

 

[Curtis]

A direct-coupled triode voltage amp/cathode follower would probably work quite nicely in that application. Dead easy to make too with one 12A_7 tube.

 

[Scodiddly]

Piezos hooked up in series? Never heard of that before, I'd guess that it means you want an even higher impedance load.

The link is interesting... it definitely set off my Audiophool BS alarm just in the first paragraph, though it's not especially close to the worst stuff I've seen. I think that any very high impedance buffer, tube or FET, will help a lot in your situation.

I haven't whored out my usual solution recently so:
http://www.scotthelmke.com/mbb.html

My boss who (as a sideline) has been building and selling a boutique steel guitar amp (he's a pretty well known steel player in the Chicago area) tried my buffer and liked it. Though he normally does use a tube-based steel guitar preamp, at least mine didn't embarrass me any.

 

[Mbira]

heh-even a layout.

I've probably got all that stuff maybe even the mint box) sitting around here somewhere.

Still interested in the "direct-coupled triode voltage amp/cathode follower would probably work quite nicely in that application. Dead easy to make too with one 12A_7 tube.". As someone with mainly guitar amp knowledge, I'm not exactly wsure what I'm looking for without a schematic.

Gus-thinking about the mic circuit-I need to plug this into a guitar amp...

Take care,
Joel

 

[Mbira]

and of course you are right that they are in parallel....still get those mixed up in my head even when I know what I'm doing,. :roll:

 

[Curtis]

As someone with mainly guitar amp knowledge, I'm not exactly wsure what I'm looking for without a schematic.

Do a search for a Marshall JCM800 schematic. The stage driving the tonestack is what I'm talking about. Uses both halves of a 12AX7 - first stage is voltage amplification, second is low(ish) impedance drive.

 

[Mbira]

So that's what confuses me about this-how is that different from just plugging straight into a JCM800 (or Fender, or...)

To get the impedance up, are you just essentially just feeding it through a second preamp gain stage and that's it?

 

[okgb]

take the first section from almost any tube gtr amp ,
and as a cut & paste method the cathode follower
from the effects loop of a gtr amp [ about right ]
slap'em together

one could design it , but if you make the resisters easy to replace
that's an o..k. startting point that may be good enough

 

[Curtis]

So that's what confuses me about this-how is that different from just plugging straight into a JCM800 (or Fender, or...)

Well, it's only one voltage amplification stage, so you can expect gain out of it. It's probably not that much different to a guitar amp, but that's pretty much all a single tube gain stage is anyway, whether it's put inside a box marked "Fender" or a box marked "Manley".

There are lots of different and simple ideas floating around out there - EF86 input stage (triode connected/pentode connected/cathode follower etc) might be another one to play with. The direct coupled CF was just the first thing that popped into my head when I read the product description for that Steel Guitar Black Box.

 

[Sleeper]

just checking this thread out.

Something that handles the high impedance of a piezo pickup.
a single tube pre that can then plug into something like an old Fender amp.

seems redundant. the guitar amp is probably already going straight into a 12asomething something tube. you might need to be sure you are in the Low signal input
just plug it in and see how it sounds first. if you need super high impedence there's some fet circuits ( built the one on jensens web site and it works well)

 

[PRR]

> how is that different from just plugging straight into a JCM800 (or Fender, or...)

Good question.

Another good question: "steel guitar" is NOT the same as piezo.

I don't think piezos have ANY musical purpose. Yes, I know a lot of folks use them and make music, but somehow I always think a wound magnetic pickup would sound better.

Yet a mbira does not seem to be simple to mag-pickup. It could be done same as a Rhodes, except your thumbs have to be where the coils go.

Wound mag-pickups are sensitive to capacitance. A long (capacitive) cable radically affects the high end. You can minimize this with a hi-Z input right AT the pickup (within a foot or three) with a short cable.

Steel Guitar has an added twist. A volume pedal is a standard playing tool. If you put this between pickup and big-amp, it weakens the weak signal and complicates the frequency response. If you tap it into an FX loop on a modern amp there are other issues. The SGBB presumably goes on/near the instrument, buffers the pickup, adds modest gain so that after volume-pedal you still have decent signal level for the main amp.

For all their other sins, Piezos don't mind capacitive loads. A mile of cable will drop response overall, but will not droop or bump the treble. The usual trouble is that resistive impedance loads choke the bass. Gus is correct that a condenser mike capsule amp is a good model, at least if you wanna drive mike-inputs or know how to adapt to Fender/Marshall inputs.

The dynamic range of guitar is not as high as percussion generally. The guitar's range from loud to LOUD can nicely fit a tube's range from "colored" to "fuZZZZ"; percussion's dynamic range may be too wide for easy player control of tube colorations. The mbira is of course not a stick-beaten instrument so I may be off-track.

A Steel Guitar is a large instrument, and a tube-box is a small accessory. Most mbiras are small, such that an onboard tube-box is just awkward. There is however the performance deze which might (like acoustic versus solid guitar) be adapted from a passive resonator to an electric interface/handle.

"Als je luistert naar een stukje mbira muziek zal je opmerken dat de vingers in een hels tempo over de lamellen bewegen en dat elk register een eigen melodie en ritme volgt."


---------------------------------------
I didn't realize "our" Scott Robinson had gone on to be a leading mbira (and other world percussion) scholar. 20 years back he was a fixture on campus.

 

[pstamler]

I know the guy who builds these, and have heard them in action. Everything he says about what it does with the sound is true.

The most spectacular results I heard were on a piezo pickup attached to an oud (played by my rabbi). Without the Black Box, operating into a smallish acoustic amp (yes, I know that's an oxymoron), it sounded like a piezo pickup. With the box, it sounded like a real instrument. The difference was sufficiently unsubtle that one of the congregation, a visual artist with zero technical knowledge, remarked that it sounded real, whereas it hadn't previously.

I'm not persuaded that his explanation for how the box does what it does holds water; my feeling is that it sounds so good because it lacks the level-dependent nonlinear input capacitance of a FET, while still providing a high-impedance load to the piezo (or steel pickup). But it does work...and it seems to be a simple 12AU7 circuit. More than that I don't know.

Oh, one more thing. The guy is a dynamite steel player, and he's become one of the most popular mastering engineers in town. A man of many talents.

Peace,
Paul

 

[Gus]

I would not use a 12ax7 or 12at7 might be too much gain for the input. measure how hot the signal is into just a scope probe, Volts peak to peak.

If you make the first stage a plate out one you will need to know the input level into say a 10meg load so you can design the stage for the input level it might "see". If a 12a-7 maybe underheat to 5.8VDC with a very clean fil supply for lower grid leakage current.

If you go to a tube amp you should not need a CF after the plate out

EDIT looking at 12au7 curves
-4VDC grid at .5ma and 300VDC B+, fil maybe at 5.8VDC
about 510K plate resistor
and a 8.2K cathode resistor
Now the plate should sit at about 60VDC looking at the curves
try 10meg to 22meg grid to grounds

 

[Salvatore]

On a side note.
The EH 2ube is a cheap alternative.
It has 2 channels hi voltage tube circuitry with 10M input imp. on the instr. setting for around 200 bugs.
I like how it sounds.

 

[Mbira]

This is all great. Thanks for the great knowledge. I will be finishing up the keys to the specific mbira I am working on today or tomorrow. THey are diatonic, so I need a different everytime I want to play in a different key. The piezos really fascinate me. Generally I use a stick on oyster. The great thing about the oyster is that I can move the placement of the pickup to find the sweet spot.

The great thing about the installed piezos is that I can just plug a guitar amp in and go. I'll post some pics of the one I'm building with three piezos

Take care,
Joel

 

[Mbira]

Hi PRR,
Yeah, Scott is a recent addition to our mbira communty. We have communicated several times over email when he has had questions about various tuning schemes of some more obscure mbira types. I look forward to meeting him in person at the PASIC conference this year.

Take care,
Joel

 

[tv]

>>> If you go to a tube amp you should not need a CF after the plate out

what about the interconnects' parasitics etc.? alternatively a mosfet follower would make it a much ruffer driver.


OTOH are you a tube purist? I have seen (somewhere ... , can't remember from top of my head) a "lownoise" circuit that cascoded a lownoise jfet into a triode. Gut feeling says this could be the ticket to "supersonic" performance. Just a suggestion, no schematic I'm afraid.

On the other hand - why not seperate the preamp into two parts: a low-consumption super-lownoise-superclean jfet onboard buffer that would match impendances/provide clean "startpoint" signal (maybe something with a 2sk389/lsk389..) - and a seperate "box" tube unit that would add "oomph"?

... again, just a suggestion.