 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
74.138.144.88
I have a cable design as follows: 8 runs of a twisted pair of 28AWG, solid core wire. Each twisted pair contains the + and - leg. Each of these + and - legs are then soldered together and terminated at the ends, making the cable.
The problem is that some SS amplifiers run hot with this cable. Tube amplifiers are ok.
The problem goes away if I make the cable as follows: 4 seperate runs for the + and 4 seperate runs for the -. These are not twisted but bundled together. SS amplifiers run ok with this.
Is the problem inductance with the twisted pair version? Perhaps amplifier oscillations?
Is there a fix for this that will allow me to use the 1st version, the twisted pairs, with the + and - leggs twisted together?
Follow Ups:
Yes, you are getting too much C by soldering together the '+' legs of the 8 pairs, and the '-' legs of the pairs. SS amps generally don't like C ... I believe tube amps are generally immune.
I was interested in doing something similar, a few years ago so I conducted an experiment. I was building a house and wanted to run some 40-50' speaker cables in the wall (to the study). Teflon coated Cat5 makes a superb - and relatively inexpensive - speaker cable for long runs.
Belden 1585a Cat5 consists of 8 strands of 24g solid-core, teflon-coated wire ... arranged in 4 twisted pairs inside a jacket.
Using 2 'jackets', there were 3 ways I could make up a run of speaker cable:
1. 1 complete jacket (8 wires) for '+' and 1 jacket (8 wires) for '-'.
2. Like you, one wire in each twisted pair as '+' and the other as '-' (ie. 8 wires in all, each).
3. 2 pairs in each jacket for '+' and 2 for '-' (again, 8 wires in all, each).
#2 had a capacitance of about 5x the capacitance of #1. #3 was in between.
Due to the long lengths I needed to run - and hence the high overall C for the cable - I went for #1; your scenario is my #2.
Regards,
Andy
I am considering trying three runs of Cat5 in order to make some 25' speaker cables. I ran across this article and it seems like a fairly cheap way to do it.
Just wondered what your or anyone else’s thoughts were.
Regards,
Todd
yup, that is pretty similar to the infamous "Cobra Cables" from the '70s that would regularly blow up certain solid state amps. I've replaced output transistors in a lot of PhaseLinear 400's because of that kind of cable.What was made is a very low-inductance cable -- but lowering inductance in ways like that increases capacitance. Basically, you'll never get 1/sqrt(L'C') to be higher than 3E8[m/s] (the speed of light), blame Dr. Einstein for that.
L' and C', by the way, are inductance per meter and capacitance per meter (not just the lumped inductor and capacitor values, though those won't be far off at audio freqencies for a cable 1meter long). The only way to approach the speed of light limit is to use insulators of the lowest dielectric constant (though you won't make any large changes in that velocity even with that). The balance between the inductance and capacitance is the "characteristic impedance" sqrt(L'/C'). The net result is that if you try to make a cable with impedance down near 8 ohms (which people for some reason want to do), it will have very large capacitance (because of the speed of light thing again). Which would be fine if a speaker really had an 8 ohm constant impedance; but most speakers actually go inductive at high frequencies so the amp ends up seeing the capacitance only at high freq which can make them unstable.
You might be able to get it to behave by adding, across the speaker terminals, a 10 ohm resistor in series with a capacitor of about 0.1uF to 0.47uF. That would cut the speakers inductive impedance at high frequencies, though the exact result might depend on the particular speaker impedance.
Howdy
You might try propheads (if you haven’t already) or directly email inmate jneutron ( http://cgi.audioasylum.com/cgi/mail.mpl?user_ID=15149 ). He’s great with the math.
My gut guess is that you are running into an amp like some Naims which can’t deal with too much capacitance without problems. Some amps (notoriously early SS amps) use a lot of negative feedback weren't designed well enough when the phase angle goes too far towards the capacitive instead of the inductive. (I.e. when the freq reaches the point that thru a fixed delay (the feedback loop time) that signal is in phase rather than out of phase…) For a more accurate description you might want to read http://en.wikipedia.org/wiki/Negative_feedback_amplifier and in particular http://en.wikipedia.org/wiki/Bode_plot#Gain_margin_and_phase_margin
Put more simply, I believe your first recipe is has too much capacitance for some amps (tho I guess I'm surprised that you are running into amps that are this sensitive.)
There are also posts around the place that talk about the trade offs between capacitance and inductance in various cable geometries.
-Ted
Sounds like an older solid state amp that doesn't like a capacitive load.
Keeping the cables as short as possible might help. Otherwise I'd measure the cables, making sure there are no shorts, etc, and find out the actual capacitance/inductance and conntact the maker of the amp. Could be the amp has a problem that hasn't been noticed so far.
Russ
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
