R/E/P > Fletcher
Radial JDI Impedance
burns46824:
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?
k jacobson:
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:
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?
k jacobson:
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:
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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