50M input impedance buffer?

[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.

 

[Nico Ras]

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

 

[Terry Demol]

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

 

[Speedskater]

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.

 

[Bo Hansén]

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

 

[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.

 

[Bo Hansén]

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.

--Bo

 

[mlloyd1]

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

 

[Terry Demol]

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

 

[takodabut]

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.

 

[unclejed613]

if you use preamps with "stratospheric" input impedances like 10M and 50M, you will not want any substantial amounts of cable capacitance between the pickup and the preamp. just 10pf with a 10M input impedance converts your preamp into an active low pass filter with a corner frequency of about 1.6khz.

 

[PMA]

Has anyone done design on very high impedance preamp buffers?

Yeah, 200M. Enough? For measurement of HV potential near HV line pilots.

 

[wg_ski]

if you use preamps with "stratospheric" input impedances like 10M and 50M, you will not want any substantial amounts of cable capacitance between the pickup and the preamp. just 10pf with a 10M input impedance converts your preamp into an active low pass filter with a corner frequency of about 1.6khz.

The impedance of piezoelectric transducers decreases with increasing frequency. It may be 1M at few hundred hertz, but it'll drop at 6dB per octave so the filtering effect of a few tens of pF of stray capacitance isn't that catastrophic. Just keep the strays to something reasonable, and use a 10M gate bias resistor for the JFET buffer and all should be well.