JFET DI question.

[babyhead]

Hello DIYers-

I recently finished a mic amp based on Forssell's schematic for the 992. As of now, I have the DI wired on a TRS jack with the ring in contact with the tip. The 6.1k termination for the input transformer is before the jack, so there is no termination between the opamp and the DI source.

I then came across this:

http://www.muzique.com/lab/buffers.htm


This is an interesting link/read.

The basic jfet buffer shown in the last paragraph may be improved upon by connecting the gate resistor to a bias voltage instead of to ground. This allows the gate voltage to set the bias at the source to a higher vlaue which increases the headroom and allows a large signal input before clipping. The Vr point on this circuit connects to a bias voltage source as shown in the first paragraph. Input impedance is again approximately the value of R1. It could easily be increased to 10M or more for a cleaner sound with high impedance signal sources such as high-output humbuckers or piezo sensors with only a slight incerase in the thermal noise contributed by the higher value of R1

Is this something I should consider to increase performance?

Can I bias to the + dc rail?

I am thinking about a low-none-high switch between the TRS and the opamp. Opinions? Schooling?

Thank you!

:guinness:

 

[tv]

You can bias to V+/2 - Vgs

 

[babyhead]

[quote author="tv"]You can bias to V+/2 - Vgs[/quote]

I am apologize for my ignorance, what does Vgs represent?

V+ is 22v/2 not 44v/2, correct? The +rail, not the full swing of the psu... ( I lowered the voltage because I felt like the opamps were running hot.)

Thanks again..

 

[clintrubber]

Vgs is the gate-source-voltage.

Question: you want to dive in & learn & play here or want a nice proven design ? If it's the latter, then I'd say go for the JFET + PNP-stage as for instance by Jensen & Great River / Dan K.

http://www.jensentransformers.com/as/as004.pdf
http://www.jensentransformers.com/as/as098.pdf

The 'Dan K.' is around here as well and is of similar-but-not-identical toplogy.

There's also a single JFET-circuit out there, by one of the kit-companies, it wasn't 7th Circle but perhaps the French one ? (the name escapes me for now)

Bye,

Peter

 

[Samuel Groner]

If you use a JFET-992 as opamp I see little point in using an extra DI input--why not route the instrument directly to the opamp?

Samuel

 

[babyhead]

[quote author="Samuel Groner"]If you use a JFET-992 as opamp I see little point in using an extra DI input--why not route the instrument directly to the opamp?

Samuel[/quote]

That is what I am doing. What I am curious about is if a loading resistor or bias for hiZ sources would improve the performance of the 992 as an instrument amplifier. Piezo's especially.

:guinness:

 

[Samuel Groner]

If I'm not mistaken the JFET-992 has built-in 1 Meg resistors.

Samuel

 

[tv]

http://www.colorado.edu/physics/phys3330/phys3330_sp08/pdfdocs/AN102FETbiasing.pdf

Hey! Here's the appnotes on fet biasing.

If you want a buffer, use the circuit marked as (d). (You can omit the Rs1)

 

[babyhead]

[quote author="Samuel Groner"]If I'm not mistaken the JFET-992 has built-in 1 Meg resistors.

Samuel[/quote]

Looking at the earlier JFET opamp on fred's page, that makes complete sense.

Thank you!

 

[beatpoet]

IMHO, If you want sonic inspiration to build more complex designs, a single-stage buffer for inductive coil pickups is a good launching point.

 

[AMZ-FX]

I'm not sure if the 992 has a built-in 1M (I tend to think not) but anyway, you don't need a buffer between the transformer and the 992 input.

Also, the simple buffer shown in the first post is perfectly fine in guitar pedals which have limited frequency response and small amounts of distortion, but I would not use it in a quality mic preamp.

regards, Jack