Jfet piezo preamp for acoustic guitar - diyAudio
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Old 18th May 2009, 08:24 PM   #1
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Post Jfet piezo preamp for acoustic guitar

Hi guys, I just designed a J201 based piezo preamp for an acoustic guitar, just wanted to know your opinion about it, thanks in advance.


Mark Fricke
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Old 18th May 2009, 10:18 PM   #2
mjf is offline mjf  Austria
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hello.
for what reason do you need the first stage (input buffer)?
if you increase r2 to 5......10meg ohm (input second stage) perhaps you need not the first stage.............
or the other way round: you can build in a drain res into the first stage that gives you voltage amplifikation...........
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Old 18th May 2009, 10:38 PM   #3
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Quote:
Originally posted by mjf
hello.
for what reason do you need the first stage (input buffer)?
if you increase r2 to 5......10meg ohm (input second stage) perhaps you need not the first stage.............
or the other way round: you can build in a drain res into the first stage that gives you voltage amplifikation...........

Hi mjf, I did that first, but I got too much high frequency roll off with a cutoff frequency of about 1.6kHz (probably due to the high impedance of the piezo), and if I buffer the piezo the cutoff frequency is around 10kHz which I think is OK for this application. I was thinking of a cascode to lower the input capacitance but I only have access to a limited number of electronic parts, that's why I decided to use J201's only.
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Old 19th May 2009, 09:52 AM   #4
mjf is offline mjf  Austria
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hello.
1,6khz cutoff frequency..........that is perhaps the simulation and not the amp............
there is a thread
fet based acoustic preamp help
in musical instruments this year(february).
there are some links to fet preamps for guitars (with tranducers...)
greetings............
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Old 19th May 2009, 02:07 PM   #5
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I'm affraid that you will get very high gain with this preamp (not needed for piezo transducer). But since you already have a simulation of the curciut, haven't you check it in the simulation?
High gain will cause distored signal on the output and I think this is not what you need.
Please check the other thread on JFET preamp on this forum. The pramp can be easily adopted to piezo.

Mark
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Old 19th May 2009, 04:05 PM   #6
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Quote:
Originally posted by Markus2006
I'm affraid that you will get very high gain with this preamp (not needed for piezo transducer). But since you already have a simulation of the curciut, haven't you check it in the simulation?
High gain will cause distored signal on the output and I think this is not what you need.
Please check the other thread on JFET preamp on this forum. The pramp can be easily adopted to piezo.

Mark
Hi Mark thanks for your reply,

I've read many threads here regarding on-board preamps for electric guitars, but I want something more simple (and using only J201 because as I mentioned I don't have access to many electronic parts to choose from) as you can see I just added the preamp between the piezo and the original volume and tone control, I don't have many J201s to try out so self-bias is not an option for me that's why I decided to use current sources to bias the two stages.

The circuit has a gain of about 8.5, you are right about that, I'll lower the gain.

Do you have any other suggestion to improve the circuit?
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Old 20th May 2009, 08:31 AM   #7
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The circuit you selected is called "cascode". It has very high gain and I'm not sure whether you can easily make the gain lower.
I suggest that just take the first stage from a preamp discussed on this forum (it's called "Hugsley preamp" - should be easy to find) and adjust it for piezo. It will consist of just one J201 transistor and 3-4 other components - it cannot be simpler. You can decrease its gain (~6dB). And the adjustment for piezo is noting more than a change of gate resistor from 1M to let's say 5-10M (you may use 2-3 resistor in a row). I think you cannot do it simpler.
You may aslo take a look at: http://www.scotthelmke.com/Mint-box-buffer.html but I'm not sure whether components values are correct - especially 220k on the output. You may simulate it in Spice but I would just make the Hugsley preamp - first stage (it was verified).

Mark
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Old 23rd May 2009, 12:25 AM   #8
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I don't think the schematic linked to by the last poster will work well for you with the J201. The J201 doesn't work well as a self-biased buffer in this case because of its low Vgsoff.

This one is a popular one http://www.till.com/articles/GuitarPreamp/index.html but again the J201 is limited by its Vgsoff as to what input voltage it starts distorting at.

The first stage you have would work well running into the tone/vol controls. You could just get rid of the second stage (J1,J2).
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