Hamptone JFET - mic preamp with parts on hand

[Scodiddly]

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.

 

[user 37518]

Nice and fast! what are those transformers next to the Cinemags?

 

[Scodiddly]

Nice and fast! what are those transformers next to the Cinemags?

More Cinemags!  Output transformers.

 

[Matador]

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]

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]

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.

 

[renthomsen]

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.

Ever tried shielding your Wurly?
Ages ago I put tin foil inside the lid, connected it to ground, hum gone.

 

[Scodiddly]

Ever tried shielding your Wurly?
Ages ago I put tin foil inside the lid, connected it to ground, hum gone.

Oh yeah. I've got the Vintage Vibe reed bar shields in mine, but that's not the same thing as getting rid of ground loop hum. But as it happens, back in November VV had a sale and I finally bought their replacement amp - wow! Everything is much quieter now.

 

[Scodiddly]

So now regarding discrete FETs, I see that I've got some J202, some 2SK170, some PF5102. And a whole boatload of 2SK879 for an ongoing product.

Going back to the beginning of this thread for the actual intent. I did build the beast, and the only problem is that it's got an output capable of driving a transatlantic telephone cable. Not really that useful when it needs to go into a prosumer USB audio interface. I built in switchable output pads which help - but the resulting gain structure means that it's a great color preamp, not one that lets me use a ribbon mic on acoustic guitar.

But then I had an idea - what if I just used one gain block instead of two, and assumed the rest of the gain (and the gain adjustment) comes from the audio interface? A quick test with some test leads showed that one Hamptone JFET block works really well for that. I was nearly whispering into a Beyer M500 from 3 feet away and the noise floor was dominated by the humidifer in the next room.

So that's next. Also I found that powering this thing from a Mean Well switching supply, $10, was dead easy and extremely quiet and well behaved.

 

[rock soderstrom]

So that's next. Also I found that powering this thing from a Mean Well switching supply, $10, was dead easy and extremely quiet and well behaved.

Hi Scodiddly, can you please link to that Mean Well switching PSU?

I built in switchable output pads which help - but the resulting gain structure means that it's a great color preamp, not one that lets me use a ribbon mic on acoustic guitar.

I mentioned it before, in my Hamptone JFET Pre I made the source capacitors of the two FETs switchable. This changes the situation drastically, then you can realize a lot of sounds, from clean to fuzz.

This is also a good way to get rid of unneeded gain. Together with the gain pot you really have full control over the sound of the two stages.

 

[Scodiddly]

Mean Well EPS-15 series:
https://www.meanwell.com/productPdf.aspx?i=691#1The connectors are a little weird, I ended up having to buy them from Newark and I had to cut down a 4-pin housing because 3-pin doesn't exist.

I've actually got two in there - 24vdc for the preamp, 48vdc for phantom power. I don't think I'll ever roll my own again.

 

[rock soderstrom]

Thanks!!
Did you work up the voltages a bit more with additional filtering or capacitance multipliers or did you go straight on to the Meanwells?

 

[Scodiddly]

Thanks!!
Did you work up the voltages a bit more with additional filtering or capacitance multipliers or did you go straight on to the Meanwells?

Only the filtering that was called for in the TapeOp article - each section gets its own 470uF filter cap, same with the phantom power.

 

[rock soderstrom]

Cool, thanks for feedback.

 

[Scodiddly]

With only two transistors I started wondering if I could run it from phantom power.

In which case I'm basically just building a Cloud Lifter but with really nice transformers.

Not that there's anything wrong with that.