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Author Topic: Question about impedance and inputs...  (Read 2044 times)

faintwhitenoise

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Question about impedance and inputs...
« on: April 19, 2004, 01:34:22 am »

I have a pair of KRK V4's that I am just basically using as computer speakers. I am just using a typical Sound Blaster card and coming out using an 1/8" TRS to two 1/4" TS cable into the Neutrik combo connector. I want to add the KRK S8 subwoofer to this setup, but have a question about the input connectors. The S8 only has XLR ins, no combo connector. Would it benefit me to buy a 2 channel direct box to run my Soundcard into first and then into the sub? Would a cable that is 1/8" TRS to 2 XLR's (provided they were wired correctly) work just fine? Which is better. Are inputs that are XLR like that only designed to recieve a low impedance signal? Thanks...
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Tim Padrick

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Re: Question about impedance and inputs...
« Reply #1 on: April 19, 2004, 02:15:22 am »

It appears that the sub has a variable low pass, and a fixed high pass.  As such, it is designed to accept a full range signal, and pass 80Hz and above on to the satellites.  So I think this is what you need to do:

Change the TS connectors to XLR Male, with the shield connected to pins 1 and 3, and the hot connected to pin 2.  This will allow the sound card to feed the sub.  Then run a pair of XLR leads from the sub to the Sats.

(A direct box is not required, and indeed would not be suitable - you would want a 1 to 1 transformer instead of a direct box.  See http://www.padrick.net/LiveSound/Interfacing.htm where I'm sure I've made such things clear as mud.)

faintwhitenoise

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Re: Question about impedance and inputs...
« Reply #2 on: April 19, 2004, 04:10:50 am »

So the direct box thing is a definite no go, right? I guess I'm confused a little. XLR ins like those on the sub are considered to be low impedance right? I have a high impedance signal coming from the sound card right? A direct box converts from high to low. What is it about the transformer that is bad for that? Are low impedance and balanced the same thing? Thanks for you help.

(btw, I'm not meaning to seem suspicious of your advice if it comes off that way- I'm just trying to work this all out in my head)
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Andy Peters

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Re: Question about impedance and inputs...
« Reply #3 on: April 19, 2004, 04:58:41 am »

faintwhitenoise wrote on Mon, 19 April 2004 01:10

So the direct box thing is a definite no go, right?


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.

Quote:

 I guess I'm confused a little. XLR ins like those on the sub are considered to be low impedance right?


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."

Quote:

 I have a high impedance signal coming from the sound card right?


(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.

Quote:

A direct box converts from high to low.


It converts impedance from high to low.  It also balances the output.  These are two separate functions.

Quote:

What is it about the transformer that is bad for that?


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.

Quote:

Are low impedance and balanced the same thing?


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
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faintwhitenoise

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Re: Question about impedance and inputs...
« Reply #4 on: April 19, 2004, 01:37:27 pm »

Sweet. This is all great info. Thanks guys. So it sounds like what I should do is just have a cable made that is 1/8" TRS to two XLR's, make sure it's wired like you guys specified and I'm good to go... Thanks again.
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