Cheap ratshack cable for subwoofer signal?

[tomes]

I need about 35 feet of cable to go from my receiver to the sub, which will be a vtf3mk2, and I am running out of money (i.e. I blew the budget bigtime, already...)

I was asking on AVS, and some people are telling me that I only n eed a cable like this from Radioshack:
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=42-3502

While Coax wouldn't be much more expensive, I have the added problem that the wife tells me not to show any cables. This means I have to conceal them by the floor, having several 90 degree breaks, that I fear could damage a Coax cable...

What do you guys think? While I don't want to loose quality, by saving 20 bucks, I still seem to need a flexible and fairly thin solution...

[tomes]

I should probably say that the AVS' guys reasoning was that they considered sub cables to be less susceptible to noise and audible signal degradation than other components in the system. They figured the cable is sufficiently shielded for this purpose.

If anyone has an alternative in the 30-40 dollar range, at least 40 feet long, I'm listening :)

[Retread]

The best kind of cable for this application is shielded twisted pair. Radio Shack calls it "Audio Cable."

The connection from the receiver to the sub is low frequency audio. The shield on a coax cable doesn't do much for low frequency pickup, because it is mostly magnetic, and the typical non-ferrous shield doesn't block magnetic.

The best approach to preventing low frequency pickup is twisting the wires. This results in both wires picking up the same signal (in reverse phase) and cancelling out the pickup. Wrap the pair of wires in a shield, and you are covered.

Radio Shack sells shielded twisted pair (Audio Cable) in 50' spools for about $12. You'll need a pair of RCA (or whatever) connectors to solder on each end. Choose your color (e.g., blue) and solder it to the center electrode on both ends. Twist the other color (e.g., white) and the shield together ON ONE END ONLY and solder them to the shell. Cut the shield back slightly on the other end and solder the white to the shell. Make sure the shield doesn't make contact with the shell. Otherwise you may get loops.

Be sure to use rosin core solder. Acid core will corrode the metal, and no-core will give you "cold joints," which are essentially open circuits. Also, make sure you don't jar the connection while the solder is solidifying -- that will also give you a "cold joint." Otherwise, it ain't rocket science.

Speaking of loops, I've been having all kinds of trouble with the unbalanced sub connection. My TV cable was throwing hum into my sub. I solved that with an RF isolation transformer between my entrance TV cable and everything else. I bought a RANE parametric equalizer to put between the receiver and the sub (VTF-3R), but it uses balanced connections, and causes motorboating when in the path, so I'm not using it yet. I bought a calibrated microphone and mic-amp to use in running spectrum scans, but it seems to cause the same kind of problem. Today, having reached the attitude of the vulture on the limb in the desert (forget patience, I'm going to kill something!) I ordered four professional line-level audio isolation transformers -- Sescom IL-19 -- at $49 each. They use XLR connectors, so I'll have more cables to make or buy.

[BeFree]

Try this one...


http://www.a2zcables.com/a2zcables.storefront/4009caa104d56f7c2719424d361d0644/Product/View/VC194&2D35

[tdekany]

or this one - I got my cables/wires from these guys.

http://www.ramelectronics.net/html/audio_video_cables.html

[tomes]

Retread,thanks for the great advice, but unfortunately, I lack the tools and know how to do this! (I'm pretty much all thumbs..)

Tdekany, Befree, some great links! I also found another one, that seem good: bluejeanscables.com

[tomes]

I should also mention that the cable type that seems pretty good is the Belden 1505F.

-Tom

[tdekany]

Originally posted by tomes
Retread,thanks for the great advice, but unfortunately, I lack the tools and know how to do this! (I'm pretty much all thumbs..)

Tdekany, Befree, some great links! I also found another one, that seem good: bluejeanscables.com

Yes but $28.00 for 35' is way cheaper.

[shannonv]

How about Belden 1694a? Is there anything like this that I can pick up at Home Depot / Lowe's? I need to complete this project this weekend.

Thanks.
-shan

[pingpongdave]

Try NAPA Auto Parts, there sell belden wires, aleast they use to when I work there years ago. ppd

[BeFree]

Here's something to keep in mind when shopping for cables-- (excerpt from www.a2zcables.com):


Audio Cabling Reality

Note: The article that follows discusses standard analog audio cables, not “digital” audio cables such as SPDIF 75-Ohm, 110-Ohm XLR or Fiber Optical.

When it comes to audio cables there is an enormous gap between audio reality and marketing hype used to justify grossly overpriced cables. In order to convince consumers that their overpriced cables are necessary, many companies attempt to confuse cable shoppers using complicated technical terms or scare them using scary-sounding scenarios in which "other" cables will be unable to defend themselves from elements that somehow effect sound quality. The fact is that most of the provided information about "electrical" phenomena has nothing to do with sound quality, and this is why such companies offer little or no evidence or testing information beyond someone's personal opinion. So, in an effort to persuade consumers many companies will use terms such as "crisp, clear, harmonic, smooth and true" and contrast them with terms like "noisy, grainy, muffled and harsh" when describing a cable’s so-called “performance”. Again, no proof or verifiable test results are offered. The boring truth is that cables should not alter the audio signal in any way. So, the ideal cable would transmit audio signals between your components without any changes to the signal whatsoever because creating and altering audio signals is the job of your system components, NOT the job of your cables. In order to minimize changes to the audio signal a cable should both preserve signal integrity and protect it from interference. That's it. No magic and no "high performance." Yet some companies actually go to the extent of claiming their cables are “directional” (working better in one direction than the other), an idea that is totally absurd. Other companies might emphasize their fancy-looking or “jet engine” style connectors (as if such connectors improve sound in some way) while others will decorate ("hide") their cables and connectors with braid wrap and and heat shrink tubing. These companies are probably selling you more marketing hype rather than audio reality.

Let’s return to reality. To preserve and protect the audio signal there are generally four factors that effect audio cable construction. These factors include shielding, capacitance (described below), connector design, and connector termination.

Shielding protects audio signals from EMI (Electromagnetic Interference) and RFI (Radio Frequency Interference) which come from sources outside the cable and can produce unwanted noise. Note that most imported, mass-produced, molded audio cables are constructed using unshielded audio cable.

Capacitance is measured in pF (picofarads) and reveals the ability of a dialectric material (nonconducting insulation) to store energy between conductors when a difference in potential energy exists between the conductors. Generally, as long as a cable is below 100 pF per foot and made by a quality manufacturer, capacitance will not impact sound quality.

Connectors are often highly decorated to improve their marketability, but a focus on decoration can lead to problems in connectivity. Fancy-looking connectors with tapered contacts can reduce contact area or exert too much force on a jack. This can damage the jack’s plating and surface area and produce noise. The best connectors feature precision machined, cylindrical walls. Such connectors ensure a firm, smooth, even pressure on a jack and minimize damage to contact plating. Gold plating has both advantages & disadvantages. Although sound is not improved with gold plating, it does minimize tarnishing. However, because gold is a softer metal it will sometimes wear off or come off in flakes on many of the fancy-looking connectors. Again, beware of connectors covered by heat shrink tubing.

Termination of the connectors onto the cable is one of the most important factors in ensuring audio signal integrity. The solder points where the connector meets the cable should be done following professional soldering guidelines to ensure high conductivity and prevent the solder from growing brittle under temperature changes or cracking due to stress. Solder that contains silver rather than standard tin & lead is preferred to increase end-to-end signal transmission.

Consumers should also beware of highly marketed cables that look thick and/or heavy, as often the weight of these cables will put too much strain on your jacks and weaken the solder points both within your components (because jacks are soldered internally to a circuit board) and within the cable’s connector. Another trick used to sell overpriced audio cables is the use of "locking" connectors. Would you prefer that your audio component fly off the shelf when someone kicks the cable or just have the connector pull out? Think about this.

Again, keep in mind that braid wrap and heat shrink tubing are often used to decorate or hide a cable and its connectors while making them appear expensive. Many of the companies will tell you they manufacture their cables, but what they really mean is that they attach connectors to cable, test them (hopefully), and often "decorate" them with stylish connectors and tube wrap. Although the spools of cable used in making their cables might come from a reputable cable manufacterer, many companies use cheap, imported cable built with a thick plastic jacket to give their cables an expensive feel and "high performance" appearance.

[shannonv]

What about parts express? Check out 100-280.

It seems like there is conflicting advice here- one is to go with coax (belden 1505 / 1694) the other is to go with shielded twisted pair.

-shan

[Retread]

There's really no contest between coax and shielded twisted pair at sub frequencies. Shielded twisted pair is the better cable. Coax is for high frequencies.

[Dudley]

The best kind of cable for this application is shielded twisted pair. Radio Shack calls it "Audio Cable."

So is the Radio Shack cable in the first post the best after all - it is called "audio cable"

[Lwang]

The ratshack cable is probably a mini-coax design. I've opened enough el-cheapo cables to know that they are pretty much all coax with a cheap spiral shield instead of being braided.

I could be wrong, but those cables seem to be too thin to hold a twisted pair and a shield.

[Retread]

Originally posted by Lwang
The ratshack cable is probably a mini-coax design. I've opened enough el-cheapo cables to know that they are pretty much all coax with a cheap spiral shield instead of being braided.

I could be wrong, but those cables seem to be too thin to hold a twisted pair and a shield.

I haven't a clue what's inside those prefab cables. I've been using Radio Shack part #278-513, labeled 24-Guage Audio Cable (subtitled "Two conductors plus shield"). It tends to be somewhat flatish, since it's two conductors twisted under a shield under a sheath. Dimensions are 1/8" by 3/16".

Also, my memory failed me. I paid $8 for a 50' roll, not $12.

[shannonv]

So if I buy the RS cable but I need to run longer than 50 feet (80 feet in my case) will soldering two segments together cause a noticeable loss in signal or increase in hum? Any idea where I can order cable like the RS stuff but longer?

-shan

[Retread]

Originally posted by shannonv
So if I buy the RS cable but I need to run longer than 50 feet (80 feet in my case) will soldering two segments together cause a noticeable loss in signal or increase in hum? Any idea where I can order cable like the RS stuff but longer?

-shan

Could do that. I used an RCA barrel connector to join two 50' segments. No increase in hum. At subwoofer frequencies, and at high resistances, there's not a noticable loss in signal.

Be sure the shield has continuity across the connection. The shield needs to be lifted on one-end only. Since I already had a length with the shield lifted on one end, I connected the shield on both ends of the second segment.

Radio Shack had a 100' spool on their web site when I first started looking, but I wanted some more and couldn't find the 100' spool. I still need it and will be looking some more. If I find it, I'll post it.

[Retread]

Originally posted by shannonv
So if I buy the RS cable but I need to run longer than 50 feet (80 feet in my case) will soldering two segments together cause a noticeable loss in signal or increase in hum? Any idea where I can order cable like the RS stuff but longer?

-shan

Further thoughts on potential losses in the subwoofer circuit.

Think of the subwoofer circuit as consisting of a series chain of receiver signal, receiver source resistance, cable resistance, cable inductive reactance, and subwoofer input resistance. Across this series circuit is the parallel cable capacitative reactance.

My receiver has a source resistance of about 2.2K Ohms. My Hsu VTF-3R has an input resistance (I believe) of about 10K Ohms. If the receiver were connected directly to the Hsu, the resistance divider rule would result in the terminal voltage at the Hsu being only 82% of the receiver open circuit voltage. However, being resistive, there would be no change in the frequency response.


A 24GA "audio cable" has resistance equal to 2.5 Ohms per 100 feet. With a 100' subwoofer cable, the resistance would be 5 Ohms (200' of wire counting round trip). In a series circuit with other components adding to 12.2K Ohms, this is completely negligible.

Inductive reactance goes up with frequency in accordance with the formula XL= 2PiFL. At 100 Hz, XL= 628L. In order to be even 5 Ohms, cable inductance would have to be 8mH, which is a lot of inductance. Cable inductive reactance can be ignored at subwoofer frequencies.

The Capacitance between the conductors and between the conductors and the shield act to short-circuit the signal, according to the formula XC = 1/2PiFC. A typical 24GA shielded pair has wire-wire capacitance of about 25 picofarads and wire to shield capacitance of about 50 picofarads. Call it a net 75 picofarads per foot, or 7500 picofarads for a 100 foot cable. At 100 Hz, the capacitive reactance would be about 200,000 Ohms. Higher at lower frequencies. This parameter must be viewed as a drain across the receiver output terminals. Its effect is to lower the receiver's terminal voltage in accordance with the divider rule. Since the capacitive reactance is only about 1% of the source resistance of the receiver, the loss is only about 1%, a small fraction of a dB. Since capacitive reactance goes down with frequency, the loss goes down with frequency, with the worst case being at 100 Hz. Frequency tilt is a very small fraction of a dB.

So the conclusion that shielded twisted pair "audio cable" at 100' in a subwoofer circuit doesn't experience significant loss is correct.

[shannonv]

I ended up finding some carol 22-2 copper twisted pair at home depot that is foil shielded with some third wire (aluminum? tin?) that wraps around the two signal wires.

I don't have my sub yet so I can't comment on hum/signal. Let's hope this works.

[Retread]

[QUOTE]Originally posted by shannonv
[B]I ended up finding some carol 22-2 copper twisted pair at home depot that is foil shielded with some third wire (aluminum? tin?) that wraps around the two signal wires.

They call that third wire a "drain." It runs through the cable in contact with the foil and "drains" the interfering signals off. Treat it the same way you would a shield.

[Gemini]

Nice discussion...

I have a question:

I have two 10' cables, can I combine them using a barrel (a small metalic connector/adaptor that combines two cables to make one long cable) to make a 20' cable?
I hear that such cable cacscading is much worse than having one 20' cable.. is this true?
And why? Does it increase the impdeance of the cable, thus distorts the signal? or just because it may wear overtime and contact will be less..?

Thanks in advance...

[Gemini]

Originally posted by Retread
My Hsu VTF-3R has an input resistance (I believe) of about 10K Ohms.

Hmmm.. I thought that subs have around only 4ohm of impedance.. Is not this what they say on the spec sheet?

Maybe I take that back, it is the 'woofer' that has only about 4ohms of impdeance (hence to generate high current to get high power from a moderate voltage level), but the active sub will probably be a few kohms I guess, since it is an amplifier, and it generate that power through the DC power supply builtin...


Can somebody answer my question in the previous post? I guess this is directly related to my question: if the sub input impedance is very very high, then adding a barrel to connect two cables should not be a big deal, I guess, is this true :) ?

Though impdeance is a function of frequency, but it will still be very high impdeance across the whole band..

Any clarification?

Thanks in advance...

[Retread]

Originally posted by Gemini
Nice discussion...

I have a question:

I have two 10' cables, can I combine them using a barrel (a small metalic connector/adaptor that combines two cables to make one long cable) to make a 20' cable?
I hear that such cable cacscading is much worse than having one 20' cable.. is this true?
And why? Does it increase the impdeance of the cable, thus distorts the signal? or just because it may wear overtime and contact will be less..?

Thanks in advance...

It depends on the frequencies and properties of the signal.

At high frequencies, there is a property called the "characteristic impedance" of the cable. This property is compounded of the series resistance/inductance and parallel capacitance of the cable, and is theoretically the impedance that would be measured at the end of an infinitely long cable, whether terminated at the other end or not. TV cable has a characteristic impedance of 75 Ohms. Apparently, a lot of subwoofer cables also use 75 Ohm coax. At frequencies where characteristic impedance matters, it is essential to terminate finite-length cables in their characteristic impedances. If you don't, you get reflections at the ends that mess up the signal. Also, every connector you put in the path causes some discontinuity and causes both loss and reflections. Hence, at higher frequencies, adding a barrel between two segments is not as good as having a single length of cable.

At audio frequencies, and particularly at subwoofer frequencies, characteristic impedance is totally irrelevant, and there is no "reflections" issue. Putting a barrel between two segments should not produce any significant deterioration. However, shield integrity is important. If you are using coax, the barrel should provide shield integrity. If you are using shielded twisted pair, you need to have shield continuity through the mid-point connector, have the shield connected to the low side on one end, and the shield lifted on the other end.

[Sorny]

Hmmm.. I thought that subs have around only 4ohm of impedance.. Is not this what they say on the spec sheet?


This 10k refers to the input impedance of the subwoofer amplifier, not the actual subwoofer driver impedance, which is approximately 4 Ohms.

Sorny

[Gemini]

Originally posted by Sorny
This 10k refers to the input impedance of the subwoofer amplifier, not the actual subwoofer driver impedance, which is approximately 4 Ohms.

Sorny

I guess I said the same thing in the same post you quoted :)

I have the same habit of not reading long posts all the way :)