Very High Impedance Direct Box
« on: November 27, 2008, 06:54:03 AM »
Hello all !

I'am new and happy to find a board like this ,very interesting !

I'm currently searching for a Very High Impedance Direct Box Schematic for a Upright Bass Piezzo pick-up wich work very bad with a standard Active DI like BBS AR133 i used in my last recording session.
I noticed harmonics distortions around 100 to 400 hz  so all the interesting frequencies.

Due to the input impedance i think...

somebody talk me about tube DI or Jfet DI

some example about stuff i'm searching for :

Leon audio Active DI With 33Mohm Z input !
Radial eng JDV DI with variable Z input 3,9Mohm
Countryman Type 85 with 10Mohm in pickup mode

or tube DI :

groovetube DI with 3 Mohm

so you will understand i would make my own  Direct inject. Box for reccord and the stage
could you help me by giving some advices, project link or schematics

Thanks a lot

excuse me for my english in french ...



Re: Very High Impedance Direct Box
« Reply #1 on: November 27, 2008, 08:16:22 AM »

For sale (might still be US only...) or easy to DIY, all info made available.

BTW, as you might know already, note that 'Ohms' don't tell the full story w.r.t. input impedance.
An input that has a 100MOhm input resistance but happens to have 20pF input capacitance
results in an input impedance magnitude of under 8 Mohm (at 1kHz). Still high, but you'll get the picture.

W.r.t. capacitance, cable from bass-PU to preamp-input will obviously add to this.




Re: Very High Impedance Direct Box
« Reply #2 on: November 27, 2008, 08:33:09 AM »
As simple as it gets - 10M input impedance:

Here's a very simple, yet well-functioning instrument splitter:

We have several of these in the studio, and they get used all the time...

Jakob E.
..note to self: don't let Harman run your company..

Bo Hansen

Re: Very High Impedance Direct Box
« Reply #3 on: November 27, 2008, 03:24:06 PM »

You say Piezo pickup, yes it will see a verry high impedance on the amp input, but what is manufacture recommendation impedance for this pickup ?

As Peter mention here before, it is not only the load impedance that can influence the pickup, it is also the load capacitance from bothe cable and amp input.

Do a experiment to find out the impedance that sounds good on this pickup.

Build this very simple simple FET impedance converter,
and use a good JFET as 2SK-170 (Y, GR or BL)

You can build it in a small metal "mint box" or similar, and use a very short cable (some decimeter) direct to your piezo mic connector, and from the FET amp output you can have some meter cable to your usual DI-box or amp input.

Test and try-out a suitable gate-resistor (1 Mohm in orginal) that get the sound you like, and it is ok to test resistors from 100 Kohm to 100 Mohm.

« Last Edit: November 27, 2008, 03:28:27 PM by Bo Hansén »
I met a man with a dollar, we exchanged dollars, I still had a dollar.
I met a man with an idea, we exchanged ideas, now we each had two ideas.


Re: Very High Impedance Direct Box
« Reply #4 on: November 27, 2008, 03:57:35 PM »
If for some reason you need to have maximum Ohms and minimum pFs
then have a look at the dual-bootstrapping circuit in the old National FET-collection:

see top of page two: > 100Mohm, < 250fF

Dunno if it sounds pleasant, but if that's the case then it might be a very good 'sniffer' (tapping a signal
for say a DI'd version of your bass-to-amp chain) without disturbing anything.




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