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?
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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.
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I figure the lower the impedance, the better the signal, right?
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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.
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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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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.