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Measurements of speaker cables in frequency and time domain

scott wurcer

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I confess I can't recall ever doing this in 50 years of being a "hi-fi bug".

I was at a party where the host was playing the music a little loud through some old valve amps (interfering with folks "chatting up" each other). So someone just pulled the cable off, instant magic smoke.
 

Chrispy

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Guys with goofy gear should perhaps be careful if allowing either Costanza or Seinfeld to control the stereo.....
 

scott wurcer

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I made an attempt to measure input impedance of 5m zipcord speaker cable with open end (no termination), with a point-by-point measurement at discrete frequencies and Excel sheet. The result looks quite realistic. There is a dip near 10MHz.

Appreciate the effort, but as I have always said wire threads have only down to go. From the totally inappropriate use of Heaviside's telegrapher's equation to the knee jerk "speaker cables are not transmission lines".
 
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pma

pma

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Finally the last one. Measured input impedance of 10m coaxial speaker cable (not terminated) and corresponding oscillations of one amplifier to which this cable was connected. No magic smoke, only very hot heatsink due to cross-conduction in the output stage. This time, the amplifier was saved by its large heatsink.

10mcoax_open_impedance.png


oscill_amp_10m_coax.JPG

Power amplifier oscillations with the measured cable. The oscillations go away if the cable is terminated by a resistor of some 47 - 200 ohm.

Edit: as my measurement method of cable impedance is simple with not enough resolution, I am adding a simulation of the 10m measured cable. Please note the enormous capacitance at the 1st impedance dip. This is the killer we speak about.

10mcoax_open_impedance_sim.png

Simulation of 10m coax cable input impedance
 
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restorer-john

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I confess I can't recall ever doing this in 50 years of being a "hi-fi bug".

Parties. Tripping over long speaker cables and pulling them out at high volume. Tube amps go pop. Some transistor amps do too.
 

Thomas savage

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Parties. Tripping over long speaker cables and pulling them out at high volume. Tube amps go pop. Some transistor amps do too.
Wow audiophiles with actual friends and a social life , the good old days , or maybe all just a dream .

Dreaming about parties where someone not only turns up the precious volume knob but the room is filled with people enjoying music and dancing rather than sitting in respectful silence in a trance like state consumed by awe for the equipment and revelling in the safety of solitude.., THEN the wires get ripped out by these ravers and the amps go bang!

Thats like the ' teeth falling out ' dream and the ' walking around school with no trousers ' dream all rolled into one .

Good job @pma , its a shame how the whole ' gotcha ' ' no you didn't ' vibe gets in the way so often.
 

KSTR

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Finally the last one. Measured input impedance of 10m coaxial speaker cable (not terminated) and corresponding oscillations of one amplifier to which this cable was connected. No magic smoke, only very hot heatsink due to cross-conduction in the output stage. This time, the amplifier was saved by its large heatsink.

View attachment 127863

View attachment 127864
Power amplifier oscillations with the measured cable. The oscillations go away if the cable is terminated by a resistor of some 47 - 200 ohm.

Edit: as my measurement method of cable impedance is simple with not enough resolution, I am adding a simulation of the 10m measured cable. Please note the enormous capacitance at the 1st impedance dip. This is the killer we speak about.

View attachment 127870
Simulation of 10m coax cable input impedance
Nice to see a documented real-life example here.

Another take-away we should not forget:
Preamplifier or Headphone amplifiers can and will have the exact same issues. Headphone outputs which have been "optimized" for lowest output resistance by practically omitting any series resistance are especially prone if they don't have a series decoupler for RF frequencies installed -- which one should do anway to avoid RF sneaking into the amp from the output side
 
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scott wurcer

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Thats like the ' teeth falling out ' dream and the ' walking around school with no trousers ' dream all rolled into one .

The number one dream is falling short on credits to graduate. Had the teeth falling out one last night.
 
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pma

pma

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Another take-away we should not forget:
Preamplifier or Headphone amplifiers can and will have the exact same issues. Headphone outputs which have been "optimized" for lowest output resistance by practically omitting any series resistance are especially prone if they don't have a series decoupler for RF frequencies installed -- which one should do anway to avoid RF sneaking into the amp from the output side

Hi Klaus, I tried my headphone amplifier with 0.06 ohm output impedance when loaded with 10m of Tasker C121 cable once with no termination (open end) and secondly with the far end terminated by 50 ohm. I was measuring the 200 kHz square response at the headphone amp output, i.e. the cable input. There was nothing special happening, only that in case of the open cable there is a predictable reflection on the rising edge and some other well damped reflections. The amp has no special anti-RF elements at its output, only it was thoroughly tested with any imaginable load to ensure stability. Yes we would see reflections (in case of the open end cable) with higher amplitude if the amp was even faster.

10mcoaxC121_openend_headampdrive.png

Tasker C121 10m, open end, cable input. Cable driven from headamp with 0.06 output impedance.

10mcoaxC121_50Rend_headampdrive.png

Tasker C121 10m, 50 ohm terminated end, cable input. Cable driven from headamp with 0.06 output impedance.
 

BDWoody

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I was at a party where the host was playing the music a little loud through some old valve amps (interfering with folks "chatting up" each other). So someone just pulled the cable off, instant magic smoke.

How was the rest of the party? I bet it got very quiet very fast...I hope the host had a sense of humor about it.
I am thinking this would make for a good SNL skit.
 

KSTR

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Hi Klaus, I tried my headphone amplifier with 0.06 ohm output impedance when loaded with 10m of Tasker C121 cable once with no termination (open end) and secondly with the far end terminated by 50 ohm. I was measuring the 200 kHz square response at the headphone amp output, i.e. the cable input. There was nothing special happening, only that in case of the open cable there is a predictable reflection on the rising edge and some other well damped reflections. The amp has no special anti-RF elements at its output, only it was thoroughly tested with any imaginable load to ensure stability. Yes we would see reflections (in case of the open end cable) with higher amplitude if the amp was even faster.

View attachment 128280
Tasker C121 10m, open end, cable input. Cable driven from headamp with 0.06 output impedance.

View attachment 128281
Tasker C121 10m, 50 ohm terminated end, cable input. Cable driven from headamp with 0.06 output impedance.
Pavel, sure there are designs that handle any amount of real or reflection-generated capacitive load without sustained oscillation, even without any R//L isolation cell.
One just cannot rely it that small signal low current amplifiers are automatically any safer than speaker level amplifiers.
 
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