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Old 4th December 2008, 08:36 PM   #1
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Default 50M input impedance buffer?

Has anyone done design on very high impedance preamp buffers? I'm need to create a 50M input impedance buffer for a guitar pickup preamp. I haven't found a FET that is a good fit, and the chip options have been either way to low or extremely high impedance.

Much thanks.
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Old 4th December 2008, 08:54 PM   #2
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Default Re: 50M input impedance buffer?

Quote:
Originally posted by takodabut
Has anyone done design on very high impedance preamp buffers? I'm need to create a 50M input impedance buffer for a guitar pickup preamp. I haven't found a FET that is a good fit, and the chip options have been either way to low or extremely high impedance.

Much thanks.
The best method is to put the pre-amp at the pick-up and power it over the long lead, then the whole system is low impedance and not prone to noise.

Nico
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Old 4th December 2008, 08:55 PM   #3
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Default Re: 50M input impedance buffer?

Quote:
Originally posted by takodabut
Has anyone done design on very high impedance preamp buffers? I'm need to create a 50M input impedance buffer for a guitar pickup preamp. I haven't found a FET that is a good fit, and the chip options have been either way to low or extremely high impedance.

Much thanks.
What is the specific application? Most hi Z gtr pickups like 1M.

cheers

T
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Old 4th December 2008, 11:49 PM   #4
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It's hard to build a 50Meg input amplifier. Leakage resistance everywhere. You need special wire, p.c. board material and every thing needs to be cleaned with alcohol then de-ionized water.
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Old 5th December 2008, 01:18 AM   #5
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Takodabut,


Do a experiment and build this very simple FET impedance converter, http://web.telia.com/~u31617586/
and use a good JFET as 2SK170 Y, GR or BL.
(there are other good JFET as 2SK30, 2SK68, 2SK163 or the old workhorse 2N3819)

Change the orginal 1 megohm gate bias resistor to a 50 megohm, and I think it will work for you.

You can build it in a small metal "mint box" or similar, and use a very short cable (some decimeter) direct to your guitar mic connector, and from the FET amp output you can have some meters cable to your usual DI-box or amp input.

Of course there are more complex and better hi-ohm circuits, with a input FET connected in a cascode or bootstrap configuration.
(can bee find in capacitor microphones when the capsule front end amp impedance must be 500 megohms and up)


--Bo
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Old 5th December 2008, 01:29 AM   #6
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I am building a preamp/EQ for a B-Band piezo undersaddle guitar pickup. This particular type of pickup has very high output impedance.
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Old 5th December 2008, 01:48 AM   #7
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Takodabut,

I think 10 megohms is normal input impedance for a piezo microphone, but try with different "gate bias resistors" (10 to 100 megohm) on my JFET buffer, and check if you can here any different.

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Old 5th December 2008, 02:18 AM   #8
mlloyd1 is offline mlloyd1  United States
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i was going to suggest a bootstrapped jfet follower , but it looks like bo has already mentioned that.
are you really sure you want that high of an input z?

mlloyd1
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Old 5th December 2008, 02:39 AM   #9
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Quote:
Originally posted by takodabut
I am building a preamp/EQ for a B-Band piezo undersaddle guitar pickup. This particular type of pickup has very high output impedance.
B-Band already make pre amps for their pickups don't they?

T
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Old 5th December 2008, 01:58 PM   #10
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Thank you to all who responded. Bo, thank you in particular for your analysis and circuit. I will try your JFET buffer.

Yes, B-Band does have a preamp available for their pickups. I have a goal of building as much of my gigging gear as possible. This is what I meant by "I need to". By building gear for myself and friends, I'm learning about construction that needs to work in rough environments. I find it satisfying to perform using gear that I've built myself.
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