A layout for a piezo buffer from Jensen

[Gilles C]

I always wanted to try it, so I decided to make a layout for it.

I still have to build it, but if someone is interested to try it before I do, it would be a good test. I won't have time to build it until a couiple of weeks. But I remember some people asked for that kind of circuit before.

You can try it without  the XFMR first, check how it works, and then add the XFMR of your choice if you need to,



Here is the link for a fast check http://jensen-transformers.com/as/as098.pdf

Gilles

[funkbass187]

i don't mean to hijack the thread. but, i'm new at all this and i was wondering what a buffer actually does.

[Gilles C]

No problem there.

First, I will ask you to read a few things. Then you should know what we call a buffer.

http://www.muzique.com/lab/superbuff.htm

http://www.geofex.com/effxfaq/effects_compatibility.htm

So finally, a buffer is something that has a high impedance input so as not to load the source device like a piezo pickup, and a low output impedance so that it can be connected to anything, no matter what the impedance is.

A buffer can also have some gain, or nor at all. This one has a gain of 12 db, which means that the voltage at the output will be 4x the voltage at the input. But you can have buffers that have no gain at all, and only permit you to connect a pickup to an amplifier without loosing any signal. A buffer is more a means of matching impedances than anything else.

I hope that answered your question a bit.

Gilles

[funkbass187]

thanks, that cleared it up a lot.

[no one ever]

odd transistors...



by XMTR, do you mean a voltage regulator?

[Gilles C]

Oops, no. It was talking about the transformers.

And it's a good thing you mentioned it, because my mind didn't work well. XMTR is the acronym for transmitter. I should have used XFMR instead. (Ok, I corrected it in my initial post, thanks...)

What I meant to say is that once the circuit is working and that you're satisfied with it, you can use an isolation transformer like the Jensen JT-11P-1 to isolate the guitar and buffer from the amplifier, as showed in the schematic from Jensen.

A cheaper XFMR could also be used, as the 42TM018 suggested by R.G. in his own hum free splitter application http://www.geofex.com/FX_images/humfree2.gif

The circuit could also be modified for an electric guitar by lowering the 10M input resistor to 1M, and maybe changing the gain of the circuit. That's something that I'll try when I'm done with the project I'm working on right now.

I would like to build an isolated A/B/Y box using that circuit instead of an opamp.

Gilles

[Gilles C]

Btw, the PN4393 (or a 2N4393) is indeed a special FET. It has no direct equivalent. It is a fast switching FETwith a low ON resistance, and it has an ultra low noise perfomance.

I saw it mentioned on a another forum, saying that it had a very high gain when used in an audio preamp application.

So I just ordered some at the store where I buy my stuff because they didn't have any on the shelves. They sell it over $4.00 canadian 

I like to use the original part when I try a new circuit. I could have used a J201, but I'll compare their specs once I have both on hand.

I also remembered about this other application that is for electric guitars.

http://www.jensen-transformers.com/as/as004.pdf

I uses the same kind of circuit, so I'll make a layout for it too. I'll get my 2N4393 next wednesday anyway.

added: I checked in the following application note http://www.ortodoxism.ro/datasheets/vishay/70599.pdf, and it is in the same family of low noise FETs as the J201, except that the J201 is used for low current/low voltage amplifiers, and the 2N4393 for medium current amplifiers.

I also determined that the J113 would be very similar.

Gilles