White follower DI circuit using JFETs

[Voyager10]

I've been playing around with a White follower circuit, inspired by the Tube DI box thread, but using everyday JFETs. Here's what I ended up with, as a phantom-powered circuit:


The idea behind the White follower is that Q1 is a source follower, with Q2 as its load, but R5 and C3 modulate the current through Q2 in opposite phase to the Q1 current. So it stays in Class-A throughout the voltage swing but without drawing excessive supply current, important for a phantom-powered device.

The circuit draws just under 3mA from P48 power, and the supply rail is +31.5V. It delivers 19V pk-pk (6.7V rms, +18.7dBU) into a 3K load before visible clipping. THD, as measured by REW, was 0.0087% at 500mV RMS out, rising to about 0.10% at 5V RMS, at which point my audio interface maxed out.

Input noise is -127dBV, or -129.5 dBV A-weighted, so the total dynamic range is somewhere over 140dB . To be fair, you would do just as well with an OPA1641, but it's quite fun that an 80-year old circuit topology does so well.

 

[MicUlli]

"Alte Besen kehren gut"
This nice example of old school circuits shows us once more that we don't need the newest OpAmps to get ultra high audio performance
Very low component count was always a way i have gone for years..
BR MicUlli

 

[PermO]

Super cool !

I am a bit confused about the ground / case connections...

I would think, J1 connects to ground only, and not case.
XLR 1 connects to case only, and not ground.

Am I missing something ?

 

[Voyager10]

Ah yes, my apologies, this was copied from another schematic.

For a DI box application you would need XLR pin 1 wired to the XLR shield and case as directly as possible.

 

[Wavelength]

But it doesn't create a differential signal on the output.

 

[Mondy]

Sort of an Impedance Balanced output? Wouldn't you need to know the output impedance of the circuit to set the value of R7 to match for this arrangment to be effective?

 

[MicUlli]

Sort of an Impedance Balanced output? Wouldn't you need to know the output impedance of the circuit to set the value of R7 to match for this arrangment to be effective?

No. R7 is only for compensating the source follower output impedance. But it is true that the input impedance of the connected stage has an influence on suppressing unwanted harmonics. Fortuately this is not that critical..

 

[ruffrecords]

Super cool !

I am a bit confused about the ground / case connections...

I would think, J1 connects to ground only, and not case.
XLR 1 connects to case only, and not ground.

Am I missing something ?

No, pin 1 of the XLR should go to the case and also to gnd (because it is phantom powered). The input jack only needs to go the gnd.

Cheers

Ian

 

[ruffrecords]

But it doesn't create a differential signal on the output.

It does not need to. I creates a balanced output.

Cheers

Ian

 

[Voyager10]

I should confess I've not tuned the R7 value for the actual built circuit, it was just a rough value from the simulator.

An "impedance balanced" output with a single driven pin is quite common and used in the Rode NT1, Neumann TLM193, MicUlli's circuit, several OPA164x designs, and more.

 

[merlin]

Don't forget to add a pair of protection diodes to the rails, from the negative side of C5. You don't want to blow your JFET with a voltage spike when you switch the phantom power on.

 

[Voyager10]

Yes, good point.

I guess I'm always careful with phantom power switching but it seems common in some places to use a jack patchbay for mic signals, where plugging in or out with the power on causes all sorts of momentary shorts.