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Very long cables

M Gregg

Disabled Account
2010-06-28 11:04 pm

The capacitance of the cable is one issue.
The other issue is that the further away the speakers are from the amp the less control the amp will have over the damping of the speaker cone. Also if you have a speaker of 8 Ohms and the cable is 1 or 2 Ohms due to the length, the transfer of power will be compromised because the load is now not 8 Ohms. You will also get voltage drop across the cable.
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50M of 12AWG wire would be 100M total length per amp/speaker *1.8 Ohm/300M => 0.59 Ohms resistance for the total run of cable per amp/speaker. this works out to 4 ohm speakers (roughly 10%), 8 Ohm (5%) or 16 ohm (2.5%) of your load impedance.

What kind of performance are you looking for?

Why the distance between the two?

I would recommend a differential line driver / receiver between the Mixer/Amplifier and the power amp/speakers as a solution.

I built one such system in college with 2N2222A transistors and achieved over 200KHz BW. I eventually built a system in my previous house that allowed me to place power amps and speakers in every room with the main amp in the den driving all of them. Unfortunatly the documentation and system were lost in my last move.

No reason a tube based system could not be made to perform the function.
You could use a low impedance line driver to pipe the audio signal thru the 50 meter long cables. Use 50 or 75 ohm coax, and build a cathode follower with an impedance equal or close to the impedance of the coax. A pair for stereo. Similar to sending a video signal, audio is similar to video from 10 to 20KHz (both type of signals use the same spectra here). This gets rid of the capacitance rolloff issue.

Or else use long speaker cables, but use heavy wire, the kind intended for house wiring of electricity. 10 or 12 AWG. For speakers, the only parameter worth worrying about is the resistance of the wire, capacitance and inductance effects over 50 meters is not significant. If you were going over several kilometers it becomes significant, but only the phone company deals with that.
Hi ,
I would prefer 2. Because the power loss on the speaker cable take a lot of your pricely bought output power. And that means you have to use cable with a large size to minimize the loss. An 8 Ohm circut need at least 6mm² cable a that length.

Another problem ist that your ruin your damping factor of the amp. Most speaker need to be damped by the amp, otherwise they sound poor. So its better to keep the speaker wiring as short as possible. And this means to place them close to the speaker set. If you have professional equipment, take the advantage of their balanced line in- and outputs. These are nomally fitted with XLR sockets. These wiring can be several hundreds meters long without any audible loss of quality.

Think about how a radio program is fed from the innercity studio to the transmitter on the hilltop . They using balanced lines hired from the local telecom.

A cheap shielded twisted pair cable will do the job perfect. e.g microphone- or computer network cable.

If your equipment is fitted with RCA jacks , you shoud run them with 50 or 75 ohm coax leads like RG-58, 59 or the green Video cable. Feed the Mixer and the amp from the same power outlet to minimize hum loops.

regards from Hamburg
The pro world uses signal cables longer than speaker cables every time, first of all because 50 m speaker cables sound bad (high ESL damping out high frequencies, noticeable even on 15 m cables), second because 50 m of signal cable weighs less than the equivalent length speaker cable, and on tour that counts on trucks and people.

You also want to run low impedance balanced to the power amps for noise & hum suppression.
tube amp remote speakers

Bar bands always do two, because fat speaker wire is so expensive and so difficult to roll up and put in the truck after 4 beers.
Hammond organ faced exactly this problem with big churchs, and did 2, with the differential drive arrangement mentioned. They had a phase splitter tube, then they used two 1/2's of a 12at7 to create differential 2-8v outputs, then connected one side to ground out in their Tone Cabinets which had a power amp and speakers in it. They ran the 120 VAC in the tone cabinet cable, to avoid having the church organist plug the tone cabinet on a wall plug that was the opposite side (220 VAC) from the circuit the organ was on.
See captain-foldback.com Hammond H schematic sheet 41 lower right corner. See the console/tone cabinet switch to disable the output tube. Also his PR10 tone cabinet schematic.
Hammond put the 120 vAC and the 8vpp signal in the same 100' cable, and got away with it.
Both approaches are valid.
In pro audio usually for approach 1 were used 100V transformers,
for second approach were used 600 Ohm transformers.
When I use powered mixer I put it directly on or near the stage,
when I need to use console farm from stage, or in control room, I prefer the second approach, and keep power amps close to speakers.
What is the source? if its CD or any other digital format then fibre is the way surely with the amps as close to the speakers as poss.

If not then balanced is the way to be going. You can do this in loads of ways. If valves/tubes is your poison (like us all) then study all the forms of phase splitter, choose the one you like;) and maybe add some cathode followers.

Certainly great big long speaker cables are not the way to go.

Cheers Matt.
Thank you to all for the praecious suggestions.
I enclose the general schematic of the mixer (for Pro audio: instruments + microphones).[IMGDEAD]http://agora.lesina.org/files/General_schematic_0.jpg[/IMGDEAD]

It seems clear that transporting the signal (for a 50 mt distance) from the mixer to the amplifier is better than from the amplifier to the speakers.

Now the problems that remain are only two:

1) Is the balance conversione necessary or could the signal be conduced in a non-balanced way by means of a two pole cable (i.e. RG58) without significant degradation by external interference?

2) Can an Aikido stage (SRPP with cathode output) adequately drive a so long cable?