faintwhitenoise wrote on Mon, 19 April 2004 01:10 |
So the direct box thing is a definite no go, right?
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Yes. A direct box is designed to interface a high-impedance unbalanced source, like a guitar pickup, to a lower-impedance balanced input, like a mic preamp. (Notice that I say "lower," and not "low.") That's not what you're doing here. You're connecting a low-impedance output to a higher-impedance input.
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I guess I'm confused a little. XLR ins like those on the sub are considered to be low impedance right?
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No, the XLR connector is hooked to a line-input circuit that probably has an input impedance of about 20k ohms. This is called the "bridging impedance."
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I have a high impedance signal coming from the sound card right?
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(This all oughta be in the FAQ.)
Most all OUTPUTS (with the exception of passive guitar pickups and rare other things) are very low impedance. (This includes power amps!) The circuit designers actually set the output impedance with a resistor, which may range from, oh, 20 ohms (for pro gear) up to 1k ohms for consumer things like Discmen. Figure your soundcard probably has a 300-ish ohm output impedance. (The higher output impedance prevents destruction of the output drivers if the output is shorted to ground. This protection comes at the cost of reduced output swing, which usually isn't missed.)
Gear I/O sections are designed such that an input "bridges" an output. (This is NOT the same as bridged poweramp outputs.) This means that a device's input impedance is designed so that it doesn't load down a driver's output. The Rule Of Thumb is that the input impedance of a typical device oughta be at least 10 times the output impedance of a driving device. In practice, that pro-gear 20k input impedance is about 400 times the typical 50-ohm pro-gear output impedance. It's about 66 times the 300-ohm soundcard output.
In comparison, a guitar amp typically has an input impedance of at least 1 Megohm. This is because the typical guitar pickup's impedance may be anything from 5k (for real hot pickups) to 20k or more. A direct box also has a similarly high input impedance. Again, this is to keep the input from loading the pickup.
Summary: the soundcard has no problem driving the line-level input presented by your speakers. However, you have to wire things properly because the soundcard has an unbalanced output and the speaker's amp has a balanced input. That's what Tim's wiring recommendation handles.
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A direct box converts from high to low.
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It converts
impedance from high to low. It also balances the output. These are two separate functions.
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What is it about the transformer that is bad for that?
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Nothing; the transformer does both the balancing and the balancing. A consequence of the impedance transformation is that the output voltage is less than the input voltage. The problem with the direct box, then, is that the monitors expect a line-level (1.4V nominal) input signal, and a sound card's output is wimpy. Futher attenuation by the direct box just makes things worse; you'll probably have to crank the gain on the speakers to get them up to a reasonable volume, and you'll end up increasing the noise, too.
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Are low impedance and balanced the same thing?
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No, not at all. Balanced simply means that you have two signal wires (like a mic cable), as opposed to one (like a guitar cable). Nothing to do with impedance.
--a