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theMaximus
08-16-2004, 08:07 PM
Are cables that claim to be "made specially for subwoofers" different than regular analog RCA cables or digital coax? Only difference that I can see is that some of the subwoofer cables use solid core as opposed to multiple copper strands used in regular analog or digital interconnects. Has anyone done any experiment with theses cables to see if there is a subwoofer performance gain by using a cable with solid core?

I'm wondering if I can use my digital coax as a subwoofer cable. Please share your opinion with me. Thanks.

Adz
08-16-2004, 09:05 PM
Not sure about digital coaxial cables but I am certain you can use audio analog cables.

aud19
08-16-2004, 09:11 PM
Do sub cables require 75ohm as well... I can't remember... :confused: I think my brains too relaxed from my week off :lol: I'm using the same Belden cable for my IC's including sub cable myself.

Jason

Bam!
08-16-2004, 09:31 PM
Do sub cables require 75ohm as well... I can't remember... :confused: I think my brains too relaxed from my week off :lol: I'm using the same Belden cable for my IC's including sub cable myself.

Jason

uhhhh. NO.

:lol:

theMaximus
08-16-2004, 11:48 PM
I'm sure subwoofers don't require 75ohm cables, but would it hurt if you used a 75ohm cable for a subwoofer?

David Meek
08-17-2004, 09:46 AM
I'm wondering if I can use my digital coax as a subwoofer cable. Please share your opinion with me. Thanks.Yes, you can use a "digital" coax as a sub cable. I've seen it done with plain old RG59 and RG6. Both seemed to pass the signal quite well. In both cases, I didn't have access to an SPL meter or any music/movies I was familiar enough with to identify any loss, so keep that in mind.



I'm sure subwoofers don't require 75ohm cables, but would it hurt if you used a 75ohm cable for a subwoofer?No. It shouldn't cause any problems. That's just an indication that the cable will properly pass a full 75 ohm signal.

mj1856
08-17-2004, 10:36 AM
I always thought subwoofer cables were built to handle the lower frequencies better. But perhaps I just believe the hype too much.

As far as "digital" coax cables - I've always just used regular RCA's. I mean - if it's digital, then it's just a stream of on/off pulses and any quality should work fine over short distances. right?

-Matt

Brandon B
08-17-2004, 11:14 AM
I always thought subwoofer cables were built to handle the lower frequencies better. But perhaps I just believe the hype too much.

As far as "digital" coax cables - I've always just used regular RCA's. I mean - if it's digital, then it's just a stream of on/off pulses and any quality should work fine over short distances. right?

-Matt

If that were true, then you'd want the same construction in any ICs before it is broken out as 5.1/7.1, since all the LF is in those signals too.

A sub cable has the same requirements as a normal IC. The difference is, you typically run a sub cable a longer distance, so you probably want to make sure it is well shielded and has good characteristics over longer runs. Exactly what 75 ohm cables intended for video/digital are supposed to be engineered for.

BB

aud19
08-17-2004, 12:29 PM
Thanks Brandon I knew there was something there re: 75ohm :lol:

Jason

Brandon B
08-17-2004, 04:54 PM
Actually, the 75 ohms is incidental to it all. I don't think there is a characteristic imedance required for an audio IC. So it can be 75 or a little more, or a little less. So many other variables in the chain it doesn't matter. I think. Jon or Lex or someone can certainly correct me.

BB

brucek
08-17-2004, 07:30 PM
...I'm wondering if I can use my digital coax as a subwoofer cable. Please share your opinion with me.

Short answer.....yes

Long answer.....I'll give you my two cents on this issue, for what it's worth. :)

The least consideration should be given to subwoofer cables with regard to response, since they pass the easiest portion of the spectrum.

In that regard though, I have to say that the fixation that some audiophiles seem to have with regard to the impedance of cables for 'audio interconnects' is unfounded. It's a very common engineering practice to ignore characteristic impedance and consider only the DC resistance of a cable (transmission line) when the line is short in comparison with the wavelength of the electrical energy that it conducts.

Now, I would certainly consider a one or two meter cable 'short' in comparison to a 10 kilometer wavelength (using an example of a worst case highest audio frequency of 20Khz that has a wavelength of about 15 kilometers in air. Considering the velocity factor of the wire, then a rule of thumb for wire transmission would be about 10 kilometers).
So, as far as standing waves on a 1 meter piece of coax, when we're dealing with a 10 kilometer wavelength, well, the 1 meter portion appears as DC. No reflections possible, it won't happen.
In fact, in very short cables, in relation to the wavelength of the signal, the resistance of the line is considered completely insignificant and the energy transferred is considered lossless. This would definitely be the case for an audio interconnect with regard to impedance matching.

Characteristic impedance is a culmination of the 4 standard distributed electrical constants that a transmission line will possess of (capacitance between conductors), (inductance along its length), (resistance of the wire) and (low level leakage conductance between the conductors). Actually the L and C are so dominant in the infinite line used to calculate the impedance value, that the formula for characteristic impedance derives down to the simple square root of L divided by C......

When a coaxial cable, for example, is specified as 75 ohms, this is not a DC resistance, but an AC impedance that states if I terminate this cable in its characteristic impedance with a 75 ohm load, then that impedance will be reflected at its input. The line will theoretically then appear as infinite, and will exhibit no standing waves or reflections from the load, with its ratio of voltage to current being constant over the entire length of the line. Well, so much for the technical stuff. Kinda boring, and it also doesn't matter a darn when it comes to analog frequencies.

The analog interface between a audio preamp and a power amp (and this is also the case for subwoofer interfaces) is considered a line level, high impedance connection where the input impedance of the power amp will usually be in the order of 10Kohms-100Kohms with the output impedance of the preamp being in the order of 100 ohms or less. This is a voltage bridge type high impedance connection where the preamp acts as a voltage source with a very low source impedance and almost no current is drawn. We are not interested in power transfer in this interface and we sure don't care much about the characteristic impedance of the cable used as explained in the paragraph above.

Certainly, since this is a high impedance connection where we are considering an input impedance to the power amp of usually greater than 10Kohms, you have to be aware of the small shunt effect that capacitive reactance of a long interconnect can have on higher audio frequencies (subwoofer need not apply). This reactance will create a small low pass filter effect. Low capacitance per foot is far more important in audio interconnects than any impedance concern. On a side note: This is especially important when using a passive preamp. With active preamps, when using short interconnects though, it's statistically insignificant, but since there is anecdotal evidence that various cables sound different, you have to consider it. I'll give you an example of the significance of capacitive reactance. A good typical audio interconnect might realize about 25picofarads of capacitance per foot. For a three foot cable that would be about 75 picofarads. The shunt impedance at 20Khz worst case would be about 106Kohms. Not much of a low pass filter is it?

It's the job of an audio interconnect to degrade the signal as little as possible. In that regard, since you want to present a relatively low voltage to the power amplifier input with as low a noise floor as possible, a properly shielded cable is important. Coaxial type cable certainly fits this bill. Wouldn't matter if it was 50 ohms or 100 ohms characteristic impedance. The characteristic impedance of an audio cable is of little concern here. There is just no possible transmission line theory that would support any type of reflection on a piece of coaxial cable at audio frequencies in any length less than many, many kilometers. In fact the characteristic impedance specification of a piece of coax is only valid at higher frequencies - audio doesn't qualify... digital and video connections are another issue.....
What makes a good audio interconnect? Properly terminated in quality connectors, well shielded, low capacitance..... and if they look good, that's a bonus ...... :)

brucek

theMaximus
08-17-2004, 10:05 PM
Wow! I'm blown away by the vastness of your knowledge. Thanks for the detailed info.

David Meek
08-18-2004, 09:26 AM
Now, I would certainly consider a one or two meter cable 'short' in comparison to a 10 kilometer wavelength (using an example of a worst case highest audio frequency of 20Khz that has a wavelength of about 15 kilometers in air. Considering the velocity factor of the wire, then a rule of thumb for wire transmission would be about 10 kilometers).
So, as far as standing waves on a 1 meter piece of coax, when we're dealing with a 10 kilometer wavelength, well, the 1 meter portion appears as DC. No reflections possible, it won't happen. 8O 8O

Bruce, you've really upset my "personal knowledge" applecart here. :p: I've been operating under the impression that the further DOWN the spectrum you go the longer the wavelengths get. And secondly, that audio wavelengths for say a 30-40 hz tone were measured in feet - not kilometers/miles. Help! Have I just been off the reservation for all these years or is there something basic I'm missing here? Or both? :banghead:

Brandon B
08-18-2004, 09:33 AM
David, you're thinking acoustic waves in air, he is talking electrical signals in air and in wire.

So you were fine, you just missed that he meant signal wavelength, not what comes out of the woofer cone.

BB

Brandon B
08-18-2004, 09:37 AM
And you really really have to appreciate a board with people with this kind of knowledge and the willingness to take the time to write a post like that.

Hat off to you (and Jon and the others who do so on a regular basis too).

BB

David Meek
08-18-2004, 09:49 AM
And you really really have to appreciate a board with people with this kind of knowledge and the willingness to take the time to write a post like that.
:agree: ABSOLUTELY! Bruce, Jon, Thomas, everyone of the "poobahs" - thank you.

David Meek
08-18-2004, 09:50 AM
David, you're thinking acoustic waves in air, he is talking electrical signals in air and in wire.

So you were fine, you just missed that he meant signal wavelength, not what comes out of the woofer cone.

D'oh. :oops: :p: Gotcha. Thanks B.

George Bellefontaine
08-18-2004, 10:14 AM
D'oh to most of everything to me when it comes to audio. Man, Bruce lost me after the first few words of the first paragraph. But like everyone else at The Guide, I do appreciate all the knowledgable folk like Bruce, Jon, Dean and what have you.

Aussie Geoff
08-21-2004, 01:16 AM
TheMaximums,

Are cables that claim to be "made specially for subwoofers" different than regular analogue RCA cables or digital coax? Only difference that I can see is that some of the subwoofer cables use solid core as opposed to multiple copper strands used in regular analogue or digital interconnects. Has anyone done any experiment with theses cables to see if there is a subwoofer performance gain by using a cable with solid core?
The short answer is "Yes, Yes"....

Key differences of good sub cables tend to be:
1) Directionally shielded (no grounding on the sub end) this takes away any EMF etc from the sub.
2) Tight connection to the sub (avoid any momentary contact changes from heavy vibration in the sub)
3) Low capacitance, induction and impedance per metre to allow for the long length.
(Forum sponsor Lex at CatCables may be able to add to this since he makes specialist SUB cables)

In terms of "do they make a difference". I was a doubter for several years on principle (low frequency signal - what could go wrong with it). Then a friend (prompted by a loan from a store) upgraded his cable from a generic one to a Monster Bass 201 and I could clearly hear the tighter (seemingly deeper) bass... So I bought a MonsterBass 400 and was amazed - tighter (seemingly deeper) bass...

With my second sub - (with the aid of a friendly store) I tried a few sub cables and found that some worked and others didn't - and ended up with the matching MonsterBass 400 to get a good tonal match... I've since then mothballed or given away all my other Monster cables but have kept the sub cable since I never found anything better for sensible $... However from listening, there seems to be several good sub cables out there (they seem nothing like as "finicky" as standard analogue cables) - certainly if I was buying again I'd try CatCables as I have since had great results with some of their other cables and the price is hard to beat for the quality...

Bottom line - find a dealer who will let you borrow a quality subwoofer cable overnight. Try it on your system and see if you hear a difference - if you don't then save your money - if you do then consider selecting from a range of options one that suits your ears / pocket.

PS - You will note that major Internet Sub retailers like SVS now also resell maching high quality sub cables at discount prices to make sure that people get the best out of their subs...

Geoff