Helmke Circuit Stumpage

[Toobme]

Hey folks,
Referencing the circuit here, I’m trying to breadboard the Helmke/Schoeps Circuit with a 2-wire electret, and am getting no signal.
Power is supplied by a Sound Devices 702 P48 via xlr.

I reworked the layout a couple times and once got some decent radio signals - like pretty darn clear - obviously not wanted!

I’ve attached a pic of my layout here so far.
What is confusing me is the bridge between the 6.8k load resistor and the 2 100k bias resistors. I’m confused on the junction point, among other things.
Can anyone see the issue here?

I’m using the part J-113 by OnSemi (an N-JFET) in place of the 2N4416.
What the heck am I doing wrong here?
Mic capsule goes to ground next to the 1G resistor, and + goes into hole J3 to hit the Gate of the FET.

Using a JLI-1813 capsule - no internal FET.

Any suggestions would be very much appreciated.

Thanks!

 

[Ike Zimbel]

It's a long shot, but just about every one of those pre-made alligator clip jumper cables that I have ever owned has failed at some point...sometimes from new. They're very poorly made, I find. I would take a few minutes and verify each one with a DMM before going any further.

 

[Toobme]

Thanks, @Ike Zimbel !
I just checked and the first one I grabbed was open!
That might explain some other head-scratchers, no doubt.
Going to replace those now with proper wire and recheck.
Thanks

 

[Ike Zimbel]

Thanks, @Ike Zimbel !
I just checked and the first one I grabbed was open!
That might explain some other head-scratchers, no doubt.
Going to replace those now with proper wire and recheck.
Thanks

If you open them up and solder them properly, they're fine. They are all crimped, shitilly, at the #16 Defective Cable Jumper Factory in you know where.

 

[Toobme]

Replaced with solid wire, now just fullscale buzzing.
Back to the breadboard - I must have something reversed.
Anyone use the J-113 in a similar way before?
Or know of decently inexpensive source for 2N4416’s?

 

[Migs 31]

If you open them up and solder them properly, they're fine. They are all crimped, shitilly, at the #16 Defective Cable Jumper Factory in you know where.

"#16 Defective Cable Jumper Factory" in..... China.
It's ok you can say it here, not PC, but hey! You GET what you PAID FOR.
I heard that #19 Factory, is better, but more expensive too.

For real. @Toobme I have not used J-113 in that position.
But have used 2n3819 (see schematic attached) and even a BF256b in a similar mic.
It should not be a the problem. Lots of good FET's (J305) will work in those mics.
Just check you got the correct pin out - in your layout.

The one thing that comes to mind is: That circuit is Hi Z.
And was not designed to be in the open air just like that.
If its not covered with a metal mic tube (or Faraday cage) it will hum.
Maybe " just fullscale buzzing" is due to that ? Maybe?
It happened to me before.

My 2 cents.
Good luck with your build.
M

 

[Toobme]

Thanks @Migs 31 - I will study your circuit now as well. Gives me plenty to think about! Wow!

 

[Migs 31]

Morning @Toobme,

Naaa it's not my circuit. It's a "pimped up" version of the circuit your are working on.
It was posted on the [email protected] by ... a Scott Helmke, it says there ?!?!
Don't study it too much, stick to your simpler version, looks much much easier.
My point was just that this circuit can be built with various versions of FET's.
2n3819 in that particular case & many more I'm sure.
Good luck & good DIY.

M

 

[Toobme]

I’ve tried again, with another set of parts and still just get 60hz hum at full scale (0dbfs) with almost no gain.
How hard is it to zap a FET - I’m worried I’ve ruined both now. Scared to try again, even.

 

[Hubbub]

I'm not sure you are using a 1G resistor. Is it actually striped brown-black-grey? When dealing with super-high impedance points it is always best to make the connection in mid-air. Otherwise the stray capacitances and leakage may kill your circuit's performance. And finger oil can cause the leakage, so maybe an isopropyl alcohol wipedown is needed for the 1G resistor. As for gain, the Schoeps circuit basically has unity gain to each output (+ and -), so it doesn't really increase the signal by that much at a whopping 2X. FWIW using a crummy breadboard (they all are!) in an unshielded configuration is asking for a load of hum. Put all the parts on a very clean small perfboard, inside a grounded metal can/tube of any sort, and see what that gets you.

 

[Toobme]

Bonus points and shout out to @Hubbub for noticing the error in value of the 1G Ohm resistor. It’s crazy because I ordered Gigohm resistors and Mouser sent me this pack of 10 Megohm resistors. My first time seeing an error in values sent. I usually check values by looking/metering but for whatever reason, went with the 1.06M I got from the reading thinking “yep that’s nice and close.” So off by enough to make the circuit not work and me pull hair for a week or so.

Here’s the bag they came in . . .


Thanks very much to everyone for helping out - and especially Hubbub!
I’ll order some ACTUAL Gigohms and report back later.

 

[Toobme]

Chatted to Mouser today and they cleared it all up - sending me a batch of actual MEGohms! So a few days to wait. Meantime it’s research time while I wait for the resistors. Thanks again for the help - I’ll post something once I get the thing running. Cheers!