Onboard JFET Instrument Low-Z Quality Preamp Help?

[charrich56]

(This message is cross-posted to DIYAudio.)
Everyone,

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.

-Charlie

 

[okgb]

the guitar grps have a few versions of single fet boosters if you have checked them out yet
start with the SHO

 

[My3gger]

Those 10u caps can be tantalum instead of film. They are cheaper, smaller and some people prefer their sound over film types. There are old threads here with comments about them, check for Hamptone HJFP (or forum's version named Fetboy) preamps.

 

[ricardo]

Your Drive capability is dominated by R26, your 50k pot.

All you good circuit performance is wasted unless it is either at MAX or at ZERO.

Do you need a pot at this end?  If so, you need to put it between an input FET & your output circuit.  Which means more current draw  :'(