Onboard JFET Instrument Low-Z Preamp Help? - diyAudio
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Old 22nd April 2014, 03:27 AM   #1
diyAudio Member
Join Date: Jan 2006
Location: Dallas-Fort Worth area, TX
Default Onboard JFET Instrument Low-Z Preamp Help?


It would be great if some of you could comment on the onboard low-Z JFET instrument preamp I have come up with and simulated.

The major requirements are about 30 dB of gain, and overall 750 microamps or less draw (less than 500 microamps ideally) from a 9 volt battery, and low noise.

The other requirement is to be able to drive an instrument cable of about 5 to 10 meters - so about 650 pF capacitance into a 1 Meg amp input impedance.

This preamp is not designed for vanishingly low distortion - but for use with low impedance (about 200 ohms Z at 1Khz, 30 mH inductance, about 50 ohms DC resistance) magnetic string pickups, as in the ones I am making for guitar or bass.

I have used a 50K:1K transformer with a simple JFET common source preamp and like the results, but want to go transformerless.

Yeah, there are some op amps that can do this, but I want to conjure with the JFET mojo for this go-round, and like the specs of the LSK170A and LSK389A for this job.

Any vetting or suggestions for circuit topology, alternate or better/cheaper parts, etc. would be greatly appreciated.

This version combines a bootstrap load common source JFET front end, with a Erno Borbely JFET version of a White cathode follower for output. It simulates really nicely in CircuitLab, with around 630 microamps static current draw.

The pickup is simulated by the voltage source into the inductor, and the shunt capacitor and resistor in the input gives a compensated load to flatten the response out to 20 KHz.

Thanks for everyone's time and comments welcomed.

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File Type: jpg Bootstrap Load Preamp with Borbely Follower.JPG (88.0 KB, 65 views)
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