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Author Topic: Radial JDI Impedance  (Read 5663 times)

burns46824

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Radial JDI Impedance
« on: February 08, 2011, 02:38:22 am »

I recently noticed that my Radial JDI Duplex has an output impedance of 600 ohms.  Most passive direct boxes, however, have an output impedance of 150 ohms.  Anyone know why the JDI has such a high output impedance?

I figure the lower the impedance, the better the signal, right?  And I've heard that, ideally, the output impedance of the DI should be 1/10 the input impedance of the preamp.  Seeing as most preamps have an input impedance of around 1,500 ohms, wouldn't the Radial JDI be a "bad match" for most preamps?
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k jacobson

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Re: Radial JDI Impedance
« Reply #1 on: February 08, 2011, 04:59:41 pm »

I suspect that 600 ohms is actually the minimum load impedance.  From their manual:

http://www.radialeng.com/pdfs/manual-jdi.pdf

the output impedance is given as 150 ohms.  Perhaps you can contact Radial and confirm this.  In any case, I've found that the Radial stuff performs quite well.  

BTW, the "impedance" of a transformer input or output is a more complicated issue.  See http://www.jensen-transformers.com/an/an002.pdf for an explanation.  

burns46824

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Re: Radial JDI Impedance
« Reply #2 on: February 09, 2011, 02:13:43 am »

Oh, much thanks.

So, would it be ok to use a direct box with an output impedance of 150 ohms with a preamp that has input impedance of 1,000 ohms?
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k jacobson

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Re: Radial JDI Impedance
« Reply #3 on: February 09, 2011, 03:58:25 pm »

The 10x rule is a rough guideline, not an absolute minimum. Even if it's a bit lower, it should be fine.  

In reality, the impedance seen by the preamp is determined by the output impedance of the instrument and the windings' turns ratio in the transformer in the passive DI.  Conversely the input impedance seen by the instrument output is determined by the preamp input impedance and the turns ratio.  The Radial DIs use Jensen JT-DB-E transformers, which have a turns ratio of 11.5:1.  If the preamp has an input impedance of 1 k-ohm, then the instrument sees an impedance of 1 k x (11.5)^2 = 132 k.  I haven't taken my Radial DIs apart to see exactly what's in there, so maybe they've got some other resistors inside.  That'll change the story a bit.  

That's good enough for most electric guitar pickups and keyboards.  However, piezo electric pickups in many acoustic instruments have much higher impedance (up to 1 M-ohm ish), so you will need a DI box/preamp with a higher input impedance.  I use an LR Baggs Para-acoustic active DI for upright bass and other acoustic instruments.  There are a few other options too: Fishman, K&K, etc.

burns46824

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Re: Radial JDI Impedance
« Reply #4 on: February 09, 2011, 05:23:59 pm »

Wow, this is all very interesting.  You're a braniac...I love it.

I'm going to compare my Radial with a DW Fearn VT-3 and a DW Fearn PDB to see which sounds best.  (The latter two have 150 ohm output impedances).
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Geoff Emerick de Fake

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Re: Radial JDI Impedance
« Reply #5 on: February 11, 2011, 07:40:31 am »

burns46824 wrote on Tue, 08 February 2011 01:38

I recently noticed that my Radial JDI Duplex has an output impedance of 600 ohms.  Most passive direct boxes, however, have an output impedance of 150 ohms.  Anyone know why the JDI has such a high output impedance?
The JDI is a passive DI box. The output impedance is the reflection to the secondary of the actual source impedance that is presented to it. Just the same, the actual input impedance is the reflection to the primary of the load impedance it is connected to.
The specified input impedance (140k) means that, at even the lowest frequencies (20Hz according to specs), the primary impedance is guaranteed to be higher than 140k. They don't specify if it's with or without output load.
The specified output impedance guarantees that, with a nominal source impedance that they didn't care to specify, the reflected output impedance is never higher than 150R.
Assuming they use a xfmr closely derived from their JT-DB-E, the reflected input impedance of 140k would be for a typical 1-2k load.
Quote:

 I figure the lower the impedance, the better the signal, right?
In a passive DI, reducing the output impedance means also reducing the level. For each halving the impedance the level will decrease by 3dB. There's a trade-on there.
Quote:

 And I've heard that, ideally, the output impedance of the DI should be 1/10 the input impedance of the preamp.  Seeing as most preamps have an input impedance of around 1,500 ohms, wouldn't the Radial JDI be a "bad match" for most preamps?
Actually, the xfmr is probably a 12:1. The ratio of output to source impedance is 1:144, meaning that as long as the source impedance (the actual output impedance of you guitar, or bass, or synth) is lower than about 22k, the 1:10 bridging rule will be fulfilled.
I reckon almost any synth has an output impedance lower than 20k, but most passive gits and basses tend to be more like 100k (Fenders)-200k (Gibsons), so they will "see" a lower than optimum impedance, and the preamp will see a higher than optimum source impedance.
It has been proved times and times again that passive DI boxes, convenient as they are, do not constitue an optimum interface (in terms of electrical performance) between passive instruments and low-Z mic inputs.
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burns46824

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Re: Radial JDI Impedance
« Reply #6 on: February 11, 2011, 11:16:50 am »

Very interesting.  Where on earth did you guys learn the math on all this stuff?  I need to "read up."

Well, I'm only using synthesizers and drum machines for this project (mostly analog), so nothing is passive.  Glad to know I'm going to have no impedance issues.

I'll compare my Radial JDI with a DW Fearn Passive Direct Box and a DW Fearn VT-3 to see what the differences are.
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k jacobson

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Re: Radial JDI Impedance
« Reply #7 on: February 11, 2011, 06:32:23 pm »

Geoff Emerick de Fake wrote on Fri, 11 February 2011 05:40

burns46824 wrote on Tue, 08 February 2011 01:38

I recently noticed that my Radial JDI Duplex has an output impedance of 600 ohms.  Most passive direct boxes, however, have an output impedance of 150 ohms.  Anyone know why the JDI has such a high output impedance?

<snip>....
I reckon almost any synth has an output impedance lower than 20k, but most passive gits and basses tend to be more like 100k (Fenders)-200k (Gibsons), so they will "see" a lower than optimum impedance, and the preamp will see a higher than optimum source impedance.
It has been proved times and times again that passive DI boxes, convenient as they are, do not constitue an optimum interface (in terms of electrical performance) between passive instruments and low-Z mic inputs.



Good point - passive vs active.  I realize that all my electric basses have active electronics, with which I often use a passive JDI.  And usually guitars get plugged into amplifiers and miked.  Guitar amps usually have an input impedance of 1 Mohm or more. Hmm, I notice that Radial's J48 active DI has an input impedance of 220 kohms - so is that not suitable for passive guitars/basses either? I've never used one myself, but I'm sure many people have.  


burns46824 wrote on Fri, 11 February 2011 09:16

Very interesting.  Where on earth did you guys learn the math on all this stuff?  I need to "read up."

...




No one particular place.  But for a start, Jensen has some good information on transformers and other technical things: http://www.jensen-transformers.com/apps_wp.html

Geoff Emerick de Fake

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Re: Radial JDI Impedance
« Reply #8 on: February 12, 2011, 01:09:49 pm »

k jacobson wrote on Fri, 11 February 2011 17:32

 
 Guitar amps usually have an input impedance of 1 Mohm or more. Hmm, I notice that Radial's J48 active DI has an input impedance of 220 kohms - so is that not suitable for passive guitars/basses either?
Although being low-ish, 220k should be acceptable in many cases. However, I don't understand why they haven't gone all the way to the more or less standard 1 Meg.
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