Need audio cable advice

[UncD2000]

My new VTF-2 sounds best in a location very close to my Yamaha RX-V1200 receiver, so I am using my HSU 20' audio cable to reach a 2nd subwoofer. I have the receiver's subwoofer output split with a (Monster ILJRY-2 F) y-adapter, and a 3' audio cable (1/8" diameter) running to the VTF-2. I would like to use a 3' length of quad-shield RG-6 with F/RCA adapters, but isn't audio cable supposed to be 50 Ohm? Would the 75-Ohm cable cause any signal loss? Also, does the y-adapter cause significant signal loss or any other problem? I have the subwoofer output on the RX-V1200 at the max (0 dB), and the level control of the VTF-2 on the mark just before the midpoint (at about 10:30), and it seems to be performing well.

[Lwang]

The characteristic impedence of a cable only applies in frequencies much much higher than what's going into your subwoofer. At audio frequencies, are dealing with input impedence of the sub and output impedence of the receiver's sub out. Those are usually in the order of 10k-100kohm and 10-1kohm. Even if the cable actually has a 75 ohm resistance, it would barely alter the impedence as seen by either of the device, relative to its own.

Splitting the output, the input impedence would roughly half (depending on inverse relationship of both subs). It should still be OK unless you have a tube output stage on your receiver/pre-pro.

You would be better off with cables that has dual braided shield, since the foil shield would be irrelevent in low freq application. You want maximum EMI rejection, which the braided shield does.

[UncD2000]

Thanks for the info and prompt reply. I am using a short length of Belden 7916A to reach the VTF-2.

One final question: would there be any advantage to using a 75-Ohm 2-way splitter to divide the receiver's subwoofer output instead of the y-adapter. Thanks.

[Ddavidson]

would there be any advantage to using a 75-Ohm 2-way splitter to divide the receiver's subwoofer output instead of the y-adapter.
No. As a matter of fact its probably neater and a simpler path using a Y adaptor.

The main thing with subwoofer interconnect cables over any sort of distance is to make sure the shielding is first rate to help reject RF and EMI. You soon find over distance that poorly shielded cables act like a big antenna picking up all sorts of grunge. Belden is one of the best and gives you great true specs (unlike the hyperbole fancy a/v cables). Nothing wrong with some of the great 75ohm Belden's for a subwoofer cable, and best of all its well priced.


Ddavidson

[Retread]

What is the mechanism in HSU subs that might make them susceptible to RFI (i.e., Radio Frequency Interference)?

Low frequency EMI (Electro-Magnetic Interference) is mostly from the magnetic component, and a non-ferrous shield doesn't do much for that. Twisted wires are much more resistant to low frequency magnetic coupling than either braided or foil shields, unless they are ferrous.

When one talks about 75-Ohm cable, one is talking about "characteristic impedance." This is the high frequency property of a transmission line. In theory, an infinite length of 75-Ohm cable would look like 75-Ohms to a 75-Ohm source even if the other end were open. This is because the capacitance between the conductors, and the series inductance, gang up to look like 75-Ohms. On a non-infinite length of cable operating at high frequencies, NOT terminating both ends of the cable in loads equal to the characteristic impedance results in reflections at the ends and produces undesirable results. At audio frequencies, the specified characteristic impedance is not a valid parameter and is irrelevant.

At audio frequencies, where the issue is to send voltage between two pieces of electronics, the resistance, capacitance, and inductance are virtually irrelevant. The only parameter of importance (assuming the electronics are properly designed) is the low-frequency EMI rejection properties of the cable.

However, if the object is to transfer power, rather than voltage, then the source and sink need to be matched in resistance (in accordance with the "maximum power transfer theorem"). An 8-Ohm speaker needs to be driven by an 8-Ohm source in order to transfer maximum power into the speaker. Again, the capacitance and inductance of the connecting cable are virtually irrelevant at these frequencies. Also, one doesn't have to be concerned about shielding at these impedance levels. What is important is to have low resistance, since cables with significant resistance delivering power to speakers will dissipate part of the amplifier power before it gets to the speakers, and may also affect the damping of the speakers.