86 metres of cheap cable. What could possibly go wrong?

[mansr]

This is 43 metres of cheap audio cable:

 

By connecting the left/right pair in series we get a total 86 metres of horror. What happens if we feed this from an iFi Nano DAC?

 

First test, a 1 kHz sine wave at -6 dBFS.

Blue is a short reference cable, green the monstrosity above. As expected, it picks up a little power line hum. Nothing extreme though.

 

Next, white noise.

At low frequencies, there is some attenuation compared to the reference. Interestingly, the high frequencies drop off more slowly.

 

Now for some scope fun. In the images below, the yellow trace is the DAC output, blue the far end of the cable into a 10 kohm resistor.

 

Nothing interesting at 1 kHz.

 

Still nothing noteworthy at 10 kHz.

 

At 20 kHz, the high-frequency roll-off of the DAC is starting to show.

 

Things start happening at 50 kHz. Here there is a small but distinct phase shift at the far end of the cable.

 

Increasing the frequency to 100 kHz, the DAC starts struggling and the waveform deviates visibly from a perfect sine. The phase shift has doubled, and the amplitude far end is clearly higher.

 

Pushing the DAC to its limits with 200 kHz, the phase shift reaches 20 degrees while the amplitude difference increases further.

 

What does this mean? If nothing else, it shows that differences between cables can be measured at least in some cases.

[mansr]

Here are XY plots of the same setup. I used 16x averaging to make the separation easier to see.

 

 

 

 

 

 

[sandyk]

All that shows is that the DAC has a fairly low Output Z.

Who in their right mind would try to drive 43 metres of cheap screened audio cable terminated by a 10K load ?

Given a DAC with a decent buffer stage at the output, it would be better to use say a series 75 ohm resistor at the output of the DAC, and terminate the other end of the cable with a 75 ohm resistor.

 

Too much spare time perhaps ? (grin)

[alfe]

All that shows is that the DAC has a fairly low Output Z.

Who in their right mind would try to drive 43 metres of cheap screened audio cable terminated by a 10K load ?

Given a DAC with a decent buffer stage at the output, it would be better to use say a series 75 ohm resistor at the output of the DAC, and terminate the other end of the cable with a 75 ohm resistor.

 

Too much spare time perhaps ? (grin)

 

Alex,

Perhaps,may be, I'm not sure, with my limited background, the purpose of the test is to show that difference between cable can be measured.

And hope that the guy that I will happily stick a Regen in his non shining part will give me a break wit his jittery comment in this thread.

[mansr]

Alex,

Perhaps,may be, I'm not sure, with my limited background, the purpose of the test is to show that difference between cable can be measured.

And hope that the guy that I will happily stick a Regen in his non shining part will give me a break wit his jittery comment in this thread.

 

The purpose of the test was to see what happened.

[sandyk]

Alex,

Perhaps,may be, I'm not sure, with my limited background, the purpose of the test is to show that difference between cable can be measured.

And hope that the guy that I will happily stick a Regen in his non shining part will give me a break wit his jittery comment in this thread.

Hi Alfe

 

Really ?

Then get mansr to try and perform the same measurements with more typical interconnect lengths of 1 to 2 metres ,using cables from different manufacturers of similar construction! (perhaps el cheapo types coming with some gear, and Brand Name cables.)

How dare you suggest that this is a good use for a Regen ! (grin)

It is WAY more versatile than most realise. (wink)

 

Regards

Alex

 

P.S.

Please check your email.

[misterspense]

A regen is a USB device, and to my understanding this experiment the cable was used as SPDIF (coax) cable. But linking 40m of USB cables would also be interesting. (And I expect that it wouldn't work)

 

Sent from my MI 2 using Computer Audiophile mobile app

[Speedskater]

I'm confused:

Was this a digital SPDIF signal or an analog signal?

[mansr]

I'm confused:

Was this a digital SPDIF signal or an analog signal?

 

It was analogue.

[Jud]

I'm confused:

Was this a digital SPDIF signal or an analog signal?

 

What would the difference be, for purposes of this test?

[Speedskater]

If it's analog then:

a] The idea of making a 75 Ohm transmission line is totally incorrect.

b] It's more of a test of the output stage of the iFi Nano DAC than the cable.

[Jud]

If it's analog then:

a] The idea of making a 75 Ohm transmission line is totally incorrect.

b] It's more of a test of the output stage of the iFi Nano DAC than the cable.

 

Regarding (a) - You're saying that the 75 ohm spec is for SPDIF, and what the cable's connected between doesn't necessarily meet/expect that spec?

[mansr]

Regarding (a) - You're saying that the 75 ohm spec is for SPDIF, and what the cable's connected between doesn't necessarily meet/expect that spec?

 

What (pre)amp has a 75 ohm input impedance?

[Jud]

What (pre)amp has a 75 ohm input impedance?

 

Just trying to understand what Speedskater's objection is.

[Speedskater]

The SPDIF digital interconnect system is optimized for a 75 Ohm transmission line system.

That's:

75 Ohm output stage > 75 Ohm coax cable > 75 Ohm input stage.

 

None of this has anything to do with an analog interconnect system.

 

*******************************

Afterthought:

This is in reference to post # 3.

 

Given a DAC with a decent buffer stage at the output, it would be better to use say a series 75 ohm resistor at the output of the DAC, and terminate the other end of the cable with a 75 ohm resistor.

[esldude]

Some very nice test results you are posting lately.

 

Next time someone asks about USB extenders, maybe this would encourage them to just use some good long analog cables and not worry about it.

[sandyk]

The SPDIF digital interconnect system is optimized for a 75 Ohm transmission line system.

That's:

75 Ohm output stage > 75 Ohm coax cable > 75 Ohm input stage.

 

None of this has anything to do with an analog interconnect system.

 

*******************************

Afterthought:

This is in reference to post # 3.

 

Given a DAC with a decent buffer stage at the output, it would be better to use say a series 75 ohm resistor at the output of the DAC, and terminate the other end of the cable with a 75 ohm resistor.

 

Speedskater

Given a Preamp/DAC with a low output impedance buffer stage, how would YOU drive a very long interconnect without degradation ? This method used to be used for Video distribution between rooms etc. in buildings, even for S-VHS ,and should work every bit as well with Audio over long distances, although the level will be reduced by half.

 

Alex

[sandyk]

Some very nice test results you are posting lately.

 

Next time someone asks about USB extenders, maybe this would encourage them to just use some good long analog cables and not worry about it.

 

Stupid is, as Stupid does !

Dennis

Why must you keep posting such inane suggestions when some of us know that you are capable of far better than this, and could be very helpful if you put your mind to it?

 

Alex

[mansr]

Some very nice test results you are posting lately.

 

Next time someone asks about USB extenders, maybe this would encourage them to just use some good long analog cables and not worry about it.

 

USB needs an analogue bandwidth of at least a gigahertz, and the cables should have an impedance of 90 ohms. Audio cables are not a good idea there.

[sandyk]

What (pre)amp has a 75 ohm input impedance?

 

There is nothing stopping you from fitting 75 ohm terminating resistors across the input connections of a Preamp/Power Amplifier and driving it from a 75 ohm source over a typical 65 to 75 ohm long shielded audio cable as far as I can see..

[esldude]

USB needs an analogue bandwidth of at least a gigahertz, and the cables should have an impedance of 90 ohms. Audio cables are not a good idea there.

No you misunderstood me. My idea was keep the USB short. Then if needed use long analog cables for the analog out of the DAC.

 

Sent from my Nexus 6P using Computer Audiophile mobile app

[mansr]

No you misunderstood me. My idea was keep the USB short. Then if needed use long analog cables for the analog out of the DAC.

 

That makes a lot more sense.

[Speedskater]

Speedskater

Given a Preamp/DAC with a low output impedance buffer stage, how would YOU drive a very long interconnect without degradation ? This method used to be used for Video distribution between rooms etc. in buildings, even for S-VHS ,and should work every bit as well with Audio over long distances, although the level will be reduced by half.

Alex

Analog output or digital output?

The two very different circuits seem to have gotten co-mingled in this thread.

[sandyk]

Analog output or digital output?

The two very different circuits seem to have gotten co-mingled in this thread.

 

Either Analogue Video or Analogue Audio.

Mansr is discussing stereo Analogue Output in this thread.

[Speedskater]

Analog Video operates in the radio frequency range and often does use a 75 Ohm transmission line interconnect system.

 

Analog Audio operates at a much lower frequency and has a low impedance output stage to a high impedance input stage.

A cheap 86 meter coax will have a ton of capacitance and will make many analog output stages very unhappy.

 

For long analog cables and I would consider 20 meters long, only use coax cables with low capacitance.

What's low? Lets say well under 20pF per foot (60pF per meter).