Phono leads - Maximum length and best make?

Cooperman69

Novice Member
Hi all,
I'm new on here, an AV novice & newbie and this will be my first post,

Firstly want to say great site with lots of interesting posts and knowledge.

So my question is just how long can a phono to phono lead be before it loses quality?

The reason is I would like to connect my Panny G10 Plasma to my hifi amp to run it via the room speakers, but the amp is the other side of the room causing the lead length to be about 7 metres (under woodern floor).

I'm looking to use a quality cable for screening and signal strength, been looking at QED Performance but only made to 5 metres, but don't mind making my own if they are not available off the peg. Can anyone recommend a good cable and phono plug type either ready made or diy?

Also I will be keeping the lead away from mains cabling were possible but just how close would be too close when running the phono lead under the flooring along with the mains? It is quite a big void but wondered if their was a figure to keep the 2 seperated to avoid crossover?

Cheers,

Colin.
 

MarkyMark70

Active Member
Hi Cooperman,

I can't talk with any expertise on lengths of cable and whether there will be any problem with the cables near a power source. From what I've read on the subject, it would seem that the length of the cable should be fine from the TV to amp. I would imaging folk with more experience can pitch in here.

As for buying vs making cables - my vote goes to making the cables. If you're useful with a soldering iron (and it's only two points per plug to solder), then it works out remarkably cheap to make up cables (There's an excellent thread on "how to" in the sticky's here).

Click on my name and check out my thread for an example and some plugs/cables. In the meantime, for the fun of it I've just ordered some Belden 1505A cable - which by some accounts should be a better cable than my previous. It'll be interesting to see if there's any difference. To give an idea of cost - a 31m reel has set me back £30. So far each interconnect has cost between £5-£7 depending on if I add braiding or heat shrink.

Hope this helps. Cheers, Mark
 

Alan Mac

Active Member
The maximum useable length of cable is determined by the capacitance of the cable and the output impedance of the driving amplifier and its current driving capability.

75 ohm coaxial cable has a capacitance in the order of 60 pF / m

So, should you decide to make your own lead, your 7 m length would have a capacitance of about:

(60 pF / m) * 7 m = 420 pF

which (I would think) should be easily driven by the phono output amplifier


Alan
 

Cooperman69

Novice Member
Thanks for the replies guys, really helpful.

I'm pretty handy with a soldering iron, use to work on surface mount pcb's, so I'll take a look at the DIY route I think. Will take a look at the Belden 1505A cable you recommend Mark.

Alan, how would you work out what would be too much capacitance for the amp your using?
 

Alan Mac

Active Member
Alan, how would you work out what would be too much capacitance for the amp your using?

At audio frequencies (below 20 kHz) a coaxial cable of less than a few hundred metres in length looks to the driving amplifier like a capacitor. For Belden 1505A cable that capacitance is 53.5 pF / m.


The driving amplifier output resistance and the cable capacitance form a simple low-pass filter with a -3 dB angular frequency of:

1 / [(output resistance)(cable capacitance)] radians / second


For example, to answer the question:

How long is the Belden 1505A cable if:
the driving amplifier output resistance is 100 ohms
and the -3 dB frequency is 100 kHz (= 200,000 Pi radians / second)


200,000 Pi rad/s = 1 / (100 ohms)(cable capacitance)

so the cable capacitance = 15.915 nF

So, for a -3 dB frequency of 100 kHz the cable length of Belden 1505A is:

15.9 nF / (53.5 pF/m)

= 297.5 m

The output resistance of the amplifier driving the cable is unlikely to be much greater than 100 ohms. So, as Joe says, 7 m of cable should pose no problem. You could measure the output resistance but if you have the test equipment to do that you could measure the system frequency response directly.

For very long lengths and hence large capacitances like the 15.9 nF we calculated you would have to consider the current capability of the drive amplifier, but that should not be a problem with a 7 m cable.


Alan
 

MarkyMark70

Active Member
Thanks Alan, really useful post... I've learned something new today.

Cheers,Mark
 

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