GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Horn-Loaded Speakers
In the Thread: Macondo’s Midbass Project – the grown up time.

Post Subject: How to drive a long cable.Posted by Romy the Cat on: 6/21/2010

Romy the Cat wrote:
If I succeed to make my midbass horn at attic then I will face another problem that I do not know how to address at this point. The horns will be powered from Melquiades bass channels – it means it will use a good 70 feet speaker cable. If I go with Copper wire then it will have per foot:

4 ga - .000292
6 ga - .000465
8 ga - .000739
10 ga - .00118
12 ga - .00187
14 ga - .00297

Let presume that I will go for 12ga cable it means that I will have: 0. 00187 * 70 = 0.139R. This permanent impedance is a bit bothers me. It is not the impedance distributed in the secondary of the output transformer but it I impedance that voice coil get increased. This perm impedance only adds compression to the sound of driver. It is very possible that for my 16R driver the .1R will be negligible but who knows how much negligible is negligible. I am a bit afraid to go with thicker wire as it usually comes with less interesting upper bass – the sound feels like become “hollow”. Sure, it possible to get involved in the cabling wars then, and I am sure I will. What are the alternatives? To run a long line level interconnect and to put the amps on attic? To fire-dangers in my view… I was thinking to do it with fire sensor above the amps in attic and close circuit TV monitor but I kind of getting more complicated then I would like it to be…

As I mention above the issue with long 70 feet speaker cable is something that makes me to think. If I run a double or triple run of cable (to keep impedance lower) then even if I go with domestic 8Ga solid core copper wire from Home Depot then we are talking about a few hundred dollars. A few hundred dollars might not be a big price to pay for speaker cable but it is ONLY of I use it. In my case I do not know how 70 feet of wire will sound and I feel to spend few hundred dollars (multiple times) just to try it would be too expensive. My leading idea is to go got a twisted two pairs of magnetic wire, the one that transformers are made from. If I go for 8G double wire per run then it might be OK as it will have the lowest isolation and therefore the lowest capacitance. Then I ask myself – why shall I worry about capacitance if my channel will care only up to 180hz?

The digest dilemma that I am thinking not and have no answer is how to powers this long 70 feet speaker cable. We know that a step up transformers with very low current in primary do not do well sonically to drive long cables. The Melquiades’ output transformer is step down transformer – it trades voltage for current. So, presumably it would not be a big deal for the Melquiades’ top see another 0.2R in voice coil of the load. However, this 0.2R is not active impedance distributed in the magnetic gap but a static DC resistance that does not participate in what driver does. I’m afraid that it might create thermal compression. It is not to mention that a relatively low voltage will be sending across the 70 feet speaker cable.

So, I wonder if it worth for me to use the same techniques with my midbass cheval as I use with my tweeter and to put an amp’s output transformer on the speaker side. The presumed benefits are that higher AC voltage will be send across the long 70 feet speaker cable, the cable will be biased with DC from output tube’s anode and the most important is that DCR of the 70 feet speaker will be added not to OPT’s secondary (that has own DCR of fraction of Ohm) but to primary (that has own DCR of 20R). BTW, in case of use the OPT on the speaker side I would not need to go for a crazy Ga number for the 70 feet speaker cable and I will be juts my B+ long cable. The negative think is that the transformer primary will have loooooooooooong path to ground and I am not sure that it is a good think.

I do not have a decision yet and keep thinking about it.
The caT

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