How long can audio pre-amp cables be run?

[dsteinschneider]

Does anyone know how long low level RCA terminated audio cables can be assuming no nearby interference if they are coming out of an Audio Authority Universal Distribution Amplifier?

 

Thanks

[Allan F]

Does anyone know how long low level RCA terminated audio cables can be assuming no nearby interference if they are coming out of an Audio Authority Universal Distribution Amplifier?

 

Thanks

 

While it does not specifically answer your question, FWIW the following appears on The Cable Company's home page:

 

Is it better to use shorter speaker cables and longer interconnects or vice versa?

 

This is another area that has no hard and fast rule. As a guideline, with XLR interconnects, performance would almost always be better using longer interconnects and shorter speaker cables. With RCA interconnects, the guideline is to try and keep the maximum interconnect length to 3M (10 feet), and use longer speaker cables if necessary. There are some specialty cables, like the Cyberlight cables from Harmonic Technology that allow the use of almost unlimited length of RCA interconnects with no adverse effects. (emphasis added)

[One and a half]

If the pre-out source has an earthed mains supply as well as the receiver, keep the RCA cable as short as possible max, 3m, but would feel more comfortable with 1m or less to avoid the maximum amount of shield voltage difference between the two points.

The main problem in the above scenaraio is hum, the other reason to avoid RF injection from various sources with longer leads.

 

XLR can go to 100m, lovely stuff.

[dsteinschneider]

Thanks for the quick answer. I should probably solve this problem with something like airplay or sonos. I'm looking to feed a bunch of power speakers around the house from an Android Tablet.

[sandyk]

If the preamplifier is capable of driving a 75 ohm load, and has series output resistors of say 68 to 100 ohms, you can directly drive longer 75 ohm (approx.) coaxial cables if you terminate the inputs of the amplifier with 75 ohms (approx.) resistors. The output level will of course then be divided by approx. 2.

[Speedskater]

I'm not aware of any audio pre-amps designed to drive 75 Ohm loads. Nor of any components with 75 Ohm analog inputs.

 

But getting back to common modern audio equipment. A pre-amp might have an output impedance of from about 75 Ohms to maybe 200 Ohms. A typical input impedance is about 10 kOhms. Under these conditions, a real good coax like the Blue Jeans LC-1 should be good for 50 feet.

[sandyk]

I'm not aware of any audio pre-amps designed to drive 75 Ohm loads. Nor of any components with 75 Ohm analog inputs.

 

But getting back to common modern audio equipment. A pre-amp might have an output impedance of from about 75 Ohms to maybe 200 Ohms. A typical input impedance is about 10 kOhms. Under these conditions, a real good coax like the Blue Jeans LC-1 should be good for 50 feet.

 

I said if YOU terminate the amplifier input with75 ohm resistors. This could be done directly across their RCA sockets. Many non opamp based (discrete) preamps will be capable of driving what is in effect a 150 ohm load if the series output resistors are 75 ohms or thereabouts.

[Speedskater]

Almost no (99.99%) pre-amp line outputs can drive into a 75 Ohm load. A output that can drive into that 75 Ohm load is a headphone output.

 

You might have your output & input impedance's upside-down.

[sandyk]

You might have your output & input impedance's upside-down.

 

I feel sure that quite a few commercial Preamps are capable of doing what the DIY Class A preamp in the photo, which uses + and -20V supply rails, and has a bias of 100mA can do.

 

[Panelhead]

I feel sure that quite a few commercial Preamps are capable of doing what the DIY Class A preamp in the photo, which uses + and -20V supply rails, and has a bias of 100mA can do.

 

 

 

I try to use shaft extenders to keep the signal path from input selector and volume pots extremely short. Total of six inches is possible.

Not sure if it sounds better. But definately is not worse, and it feels better keeping the internal wiring very short and away from the power supply.

[Speedskater]

It's just hopeless.

[sandyk]

I try to use shaft extenders to keep the signal path from input selector and volume pots extremely short. Total of six inches is possible.

Not sure if it sounds better. But definately is not worse, and it feels better keeping the internal wiring very short and away from the power supply.

Panelhead

Agreed. That is an area that I would like to improve on, but the ceramic rotary switch is relatively flimsy, and switches both active and earth returns of each input.The input wiring is well separated from the PSU wiring.

Regards

Alex

P.S.

Quite a few of these were constructed both in HA and Preamp versions, by members of a U.K. based forum after I posted this as a project. Even the PCBs for the main PCBs were designed by a U.K. member of the forum specially for this project.

[mayhem13]

M

If the pre-out source has an earthed mains supply as well as the receiver, keep the RCA cable as short as possible max, 3m, but would feel more comfortable with 1m or less to avoid the maximum amount of shield voltage difference between the two points.

The main problem in the above scenaraio is hum, the other reason to avoid RF injection from various sources with longer leads.

 

XLR can go to 100m, lovely stuff.

 

Pretty much sums it up.

[davethewave]

Dude,just run RCA.I've got about 50 to 60 ft under my house. Computer out to Home DVD

input then out under house to living room system.With the Amp cranked WAY up, sure there's some hiss,but that could just be from the cheap mixer I'm using right now.