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Computer to Stereo Interface

04/28/2010 9:05 PM

I want to connect my computer's audio out to my stereo amplifier. Using cables with a 3.5mm 2-circuit plug on the computer end to 2 RCA jacks on the amp, what is the maximum distance I can run the cable without a lot of loss? The cable length is about 20-25 feet.

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#1

Re: Computer to Stereo Interface

04/28/2010 10:06 PM

I do that without a problem.

For that run you should use a good quality audio cable with a foil shield (if the cable remains stationary).

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#2

Re: Computer to Stereo Interface

04/28/2010 11:03 PM

I looked around a little and found that computer sound card outputs are mostly high impedance (Z). High Z signal loss is not an issue until you exceed twenty feet with standard shielded cable. I do not know the actual loss in dbm, I do know from experience that losses in audio signal will be most noticeable in high frequencies as you begin to exceed twenty feet.

The input on your stereo is also high Z (RCA jacks are high Z), or you would notice a loss of gain immediately. If you need to exceed twenty feet (don't) there may be some high performance shielded cable that will compensate to a point (5 feet maybe without loss of gain).

If you are an audiophile and have to have the best, move you puter. I would just hook it up and see if I like the way it sounds.

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#5
In reply to #2

Re: Computer to Stereo Interface

04/29/2010 8:02 AM

Well, it is just a computer sound card. How much high quality can we be talking about?

That is why there really is no issue with that length or even longer. The highest frequency is limited to 22 kHz (nyquist point).

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#7
In reply to #2

Re: Computer to Stereo Interface

04/29/2010 11:12 PM

No, you are incorrect,, output impedances are typically under 4 ohms,, these cards are intended to drive low impedance headphones. Your typical headphone will have and impedance of less than 50 ohms, if its a quality headphone more like 8 or 16 ohms. High impedance outputs would preclude driving such a load.

The input impedance of your stereo amp will normally be 47K, in this case, unlike with microwave work, you are not worried about power transfer, so driving a high impedance input with a low impedance output it just fine, voltage transfer is the only important thing.

In short, you can drive hundreds of feet of reasonable quality shielded cable.

Cheers

Martin

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#10
In reply to #7

Re: Computer to Stereo Interface

04/30/2010 7:09 AM

Many cards have a line-out and a headphone out. My Mac has both.

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#12
In reply to #10

Re: Computer to Stereo Interface

04/30/2010 9:44 AM

If we list signal levels from high to low:
Loudspeaker Level: Output voltage is several volts (28 Volts for 100W output)
Headphone Level : Maximum output capacity varies device to device. Let's say it's about 0.5 Volts.
Line Level : It's about 150~300 mV (0.150~0.300 V)

So to drop to line-level from headphone-level you need about 1/3~1/2 voltage attenuation.
To do that you use attenuator resistors. You use two resistors per channel, one series, the other parallel. Their value is not critical. The sum of both resistors should have a value about what the headphone output expects (or slightly higher), lets say 300 Ohms.
You may try a Series Resistor=200 Ohms and a Parallel Resistor=100 Ohm for a start. If the signal is still loud you may increase the series resistor a bit. Power value of the resistors is not important, they can be 1/4 W each. %5 tolerance types are preferable. The resistors can be soldered inside an RCA jack. A friendly electrician may produce such an adapter cable for you.

Or opt for a simple DAC

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#13
In reply to #12

Re: Computer to Stereo Interface

04/30/2010 10:55 AM

A typical CD player's line level output is about 1.0 to 1.5 AC Volts.

Bottom line is, no attenuation network should be needed to do what the original poster wanted.

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#14
In reply to #13

Re: Computer to Stereo Interface

04/30/2010 11:28 AM

Indeed. The only reason for an attenuation network would be to match input levels so that when switching from one input to another, the volume remains relatively constant. Probably isn't an issue in this case.

Cheers

Martin

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#15
In reply to #7

Re: Computer to Stereo Interface

04/30/2010 12:34 PM

You are right.. I was wrong, but not for the reason you stated. He/She wanted to go from "audio out." I think that's different from a headphone jack. I also looked a specs for several different sound cards that had line out impedances of 900 to 1600 ohms. Because I'm use it seeing low impedance at 50 to 300 ohms I called it high impedance when it is neither high or low. Turns out that audio out of a computer sound card is designed to hook up to home stereo components with up to 50 meters of cable (see link). I guess I shot from the hip a little.

Headphone and speaker level outputs are rated by power at a certain load (e.g. 5 watts @ 4 ohms). It is not their output impedance. The driven impedance depends on the speaker or headphone and affects the output level.

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#16
In reply to #15

Re: Computer to Stereo Interface

04/30/2010 1:36 PM

Good job on the link!

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#17
In reply to #15

Re: Computer to Stereo Interface

04/30/2010 2:36 PM

Correction to previous post: per the link I referenced, the cable length should have been "up to 10 meters."

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#3

Re: Computer to Stereo Interface

04/29/2010 12:40 AM

You can run the signal the entire legnth of the cable (20 to 25' in this case) If you try to run it a greater distance than the legnth of the cable you will have a lot of loss.

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#4

Re: Computer to Stereo Interface

04/29/2010 1:46 AM

If you run between the headset output and one of your amps line or aux inputs you can run hundreds of feet long without signal loss and without using shielded cable. A twisted pair (times 2) will do the job. If the card is ok, hiss will be minimal.

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#9
In reply to #4

Re: Computer to Stereo Interface

04/30/2010 5:45 AM

I fully agree with dvmdsc. As the out put impedance of the audio card is quite low and the line-in of any other system is more than 47K, with just good quality twin twisted cable (like telephone cable) can be used, for the use within the room, ensure that they are not routed along the power line. if you have to use the signal at far location, please use good quality two core screened cable and take care of the cable routing.

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#6

Re: Computer to Stereo Interface

04/29/2010 8:11 AM

Thank you all. There is a tremendous amount of good music available over the internet; more than from local music sources and all without commercials.

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#8

Re: Computer to Stereo Interface

04/30/2010 1:33 AM

No worries, minimal signal loss is experienced out to about 300' but I've gone to about 800' without problems. Give it a whirl... If you get some weird buzzy background sounds the word clock on the sound card needs adjustment.

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#11

Re: Computer to Stereo Interface

04/30/2010 9:38 AM

A lot of computer speakers have a built in amplifier to the first speaker then for that speaker the a wire runs over to the second speaker.

So run your wireing from the computer to an amplifire then to your speakers and you shouldn't have a problem.

Data transfer is a different issue, however you can run 50 feet before needing a router to continue on further without data loss.

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