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[Steve M Smith]

That is a very common JFET source follower.

I know.  I just didn't want to take credit for the drawing.

Back in the '80s I made a balanced DI using two low noise JFETs that I built into a mic cable (the parts fit inside the XLR shell... I called it a DIC (DI-cord).

I have been thinking about doing that (I'm sure we discussed it in a previous thread). My standard DI circuit just uses a normal transistor in a phase splitter configuration,  Sometimes I add the JFET buffer as already mentioned.  The single transistor circuit will easily fit in an XLR shell.

http://stevesmithphoto.webs.com/DI.jpg


Steve.

[Peter Morris]

Hi everybody,


I'm facing a challenge. Basically I need to mount a Countryman DI (Type 10) to a guitar strap.

We're currently experimenting with Yamahiko elements for Spanish guitars (nylon strings). According to the manufacturer, the combination of the self capacitance of the element and the input impedance of DI or beltpack act like an HPF. The corner frequency is determined by the input impedance.

The Countryman DI's are recommended by the manufacturer given their high input impedance (10 MOhm) and they work fabulous for our application. BUT we need to go wireless.

The input impedance of our AKG PT-4000 beltpacks is 200 kOhm and results, as was to be expected, in way to much LF loss.

The passive Radial Dragster doesn't offer an improvement. Another wireless system (with higher input impedance e.g. Shure 1 MOhm), given our other channels, is not preferred.

So now we're thinking of mounting the Countryman DI's to the guitar straps. Evidently we want to keep the weight down as much as possible, is it wise to attempt to strip the Countryman DI's from all non-essential parts and replace the housing for plastic or should we build something from scratch?

In case of the latter who could/is willing to help me with such an "elementary" design or knows of a better solution?


Sincerely,


Merlijn


Edit: The attached screenshot mentions a piano element but the guitar element's electrical properties are identical

http://leonaudio.com.au/active.htm
FWIW here is one my friend manufactures - 33 meg input impedance ... there is some interesting info on his site

[Merlijn van Veen]

http://leonaudio.com.au/active.htm
FWIW here is one my friend manufactures - 33 meg input impedance ... there is some interesting info on his site

Thanks for the tip Peter. But if I'm not mistaken the unit only runs on phantom power and not a 9V battery?

While trying to obtain the input circuitry of our AKG PT-4000 beltpacks to also pursuit John's suggestion of modifying the beltpack.

Does anyone have experience with either the "Pick-up the World" line driver (JFET) or Schatten Design Mini Pre? The former looks like a very good form factor. Worth the extra buck?


Regards,


Merlijn

[John Roberts {JR}]

I know.  I just didn't want to take credit for the drawing.

I have been thinking about doing that (I'm sure we discussed it in a previous thread). My standard DI circuit just uses a normal transistor in a phase splitter configuration,  Sometimes I add the JFET buffer as already mentioned.  The single transistor circuit will easily fit in an XLR shell.

http://stevesmithphoto.webs.com/DI.jpg


Steve.

Well keep in mind a DI needs to step down the signal and present a balanced output (balanced impedance) to the mic preamp.

Imagine two of your JFET buffers receiving + and - inputs cap coupled and biased up to maybe 1/3 of phantom voltage. A modest resistance between the two JFET sources creates a differential current in the two drains in response to differential signal voltage across the two gates. A smaller resistance say a few hundred ohms connected between the two drains, pads down the output signal before the high gain mic preamps and presents the mic input with a balanced low source impedance.

JR

[Steve M Smith]

Can you do a quick sketch of your circuit for me?

Are you putting the input to one FET and grounding the input to the other and giving them a common source resistor?


Steve.

[John Roberts {JR}]

Can you do a quick sketch of your circuit for me?

Are you putting the input to one FET and grounding the input to the other and giving them a common source resistor?


Steve.

You guys are so needy I should be charging at least $100/hour for this.

The resistor values are just off the top of my head, and you can optimize the several moving parts for different factors.  Use some low noise N channel JFETs, you could probably use film input caps but they may be too big to fit inside a jack barrel.

Enjoy

JR

[Steve M Smith]

Thanks.  I will try that.


Steve.

[Jordan Wolf]

Merlijn,

Try contacting Karl Winkler at Lectrosonics.  He may have a more elegant solution for you, or could at least give you some more insight (not that you haven't had beneficial insight already).

He may also be familiar with the pickup you're trying to implement.

The MI33ARA cable/circuit might be of interest to you.

[Merlijn van Veen]

Hi everybody,

After careful consideration we went with John Roberts' recommendation and successfully modified the beltpacks without a noticeable increase in Johnson noise. AKG was very collaborative with providing us the electronic schematics which we passed on to John. John told us which SMD resistor to replace and we ended up swapping a single 200k resistor for an 3M resistor. Now the beltpacks in combination with the Hi-Z Yamahiko guitar elements provide a flat frequency response all the way down to 80 Hz.

I'd like to thank the people at AKG and John Roberts in particular for the help!

Regards,

Merlijn

Merlijn

[Steve M Smith]

Excellent.  That sounds like a very simple modification.


Steve.

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