Scodiddly

Re: Hamptone JFET - mic preamp with parts on hand
« Reply #40 on: October 04, 2020, 03:14:18 PM »
Rectangular eyeball switches from a PM1000 rack job a few years ago, had to dig into my junk box for a few odd things on this build.  Left button is transformer tap, right button is pad.  Don't know what I'll do if I end up getting around to adding phantom power.


Re: Hamptone JFET - mic preamp with parts on hand
« Reply #41 on: October 05, 2020, 08:13:04 AM »
Nice and fast! what are those transformers next to the Cinemags?

Scodiddly

Re: Hamptone JFET - mic preamp with parts on hand
« Reply #42 on: October 05, 2020, 08:14:18 AM »
Nice and fast! what are those transformers next to the Cinemags?

More Cinemags!  Output transformers.

Matador

Re: Hamptone JFET - mic preamp with parts on hand
« Reply #43 on: October 06, 2020, 12:53:10 PM »
Forgive my ignorance, but where's the output?
It basically looks like this:  the source resistor is replaced with a CCS, set to roughly 1mA.  The CCS is bypassed with a cap so that the drain can move up and down.

This circuit should allow any reasonable JFET to drop in, and the CCS will adjust to the specific VTH of the FET to keep the drain at half the supply for maximum signal swing.

Scodiddly

Re: Hamptone JFET - mic preamp with parts on hand
« Reply #44 on: October 06, 2020, 08:35:20 PM »
It basically looks like this:  the source resistor is replaced with a CCS, set to roughly 1mA.  The CCS is bypassed with a cap so that the drain can move up and down.

This circuit should allow any reasonable JFET to drop in, and the CCS will adjust to the specific VTH of the FET to keep the drain at half the supply for maximum signal swing.

Ah, OK.  That makes sense.  Though I ended up in a pretty decent place just by matching my resistors to the device, a more automatic solution would certainly be the manufacturing way to go.

Scodiddly

Re: Hamptone JFET - mic preamp with parts on hand
« Reply #45 on: October 06, 2020, 08:37:52 PM »
So anyways, this thing works pretty good!  Seems to like my Beyer M260-80 mic, which never got along with my audio interface (old Sound Devices USBPre). 

But the real miracle thing is that it works perfectly with my Wurlitzer 200 piano - no hum at all into the USBPre.  Maybe because it's powered from a wall wart?  I dunno, but I'm super happy about that.  The 200 is one of those things where you always need an AC ground lift handy because a DI still doesn't manage to get rid of the ground hum.


 

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