[drewd]

Well, Dr. Gilmore aside, there should not be any square wave components in the music, especially on a compact disc - as you alluded, the CD format doesn't really have the resolution to capture a square wave.

Even at 44KHz, that's not sufficiently high to generate reflections in a three foot cable...the wavelength is over 30 times that!

So getting back to Kevin's post, I don't doubt that there is a difference in the sound. Consider that you are taking a wire (because at the frequencies we're dealing with, that coaxial cable really is a wire) and providing a path to ground through a 75 ohm resistor. So, sure, you've added 75 ohms in parallel with whatever the input impedance of the amplifier (assuming that it's DC coupled), meaning that you've gone from what is probably a very high input impedance to a very low one. So it's certainly going to change the character of the sound!

I guess it could sound better, but it certainly could sound worse. There's a synergy between components of an audio system that can't be overlooked. I guess what I'm ultimately getting at is that the classic idea of terminating a transmission line is applicable only when the frequencies and the length of the path combine to cause transmission line effects to occur. In the case of audio, it just doesn't happen.

By the way, I really am enjoying this thread. I work with high frequency digital signals all day and nobody at my office has much of a clue about low frequency analog stuff. This is really refreshing!

-Drew

 

[Garbz]

In which case if the low impednace load makes it sound "better" then why wouldn't other equipment manufactures put this directly in their amps. Then regardless of cable the source would see a 75ohm load.

On that thought, doesn't the quality of a signal improve as the load gets HIGHER? e.g. rmaa scans of amps come out much better with a 300ohm load then a 30ohm load. And wasn't that a major arguement for the case of an amplifier anyway? So that the source wouldn't see the straight 30-300ohm load of the amp but something significantly higher?

There seems to be conflicting opinions here. I love it

 

[endia]

Garbz, i think, this article has certain answer for your first post

http://www.borbelyaudio.com/kit_upgrades_term.asp

 

[Voodoochile]

I like Canare LV61S and L5CFB 75 ohm cable. Even if the termination issues are set aside, they make very nice cables. Of course with proper termination, they are excellent.

The L5CFB in particular, terminated with the true 75 ohm Canare crimp RCAs or BNCs makes a truly outstanding digital IC for pretty short money.

 

[Pars]

There was a very interesting thread on rec.audio.high-end about a year ago started by Bruno Putzeys, one of Philips chief digital engineers, regarding cable termination or input/output impedence specs (thread title: Some serious cable measurements with interesting results. ). It appeared to be going down much the same line of thought as Dr. Gilmore WRT the relationship of cable "sound" and the impedence issues of both the source and input. Basic conclusion I believe was that cable differences disappeared when the source and input impedences were optimized.

 

[13405]

[deleted]

 

[drewd]

Quote:

Originally Posted by endia Garbz, i think, this article has certain answer for your first post http://www.borbelyaudio.com/kit_upgrades_term.asp

There's still a problem with the basic premise of the article and that is that at the frequencies that we're concerned with, there just aren't any reflections to damp.

I did some simulations with Cadence's Signal Xplorer here at work and posted the results on my web page. I certainly don't consider it to be the last word, though. Clearly, there is an audible difference when you put resistors on a cable, maybe even a beneficial one, but it isn't because there are reflections on a transmission line.

And my apologies in advance for the slow-loading, large graphics on the page.

-Drew

 

[13405]

[deleted]

 

[drewd]

Quote:

Originally Posted by Oli I guess it also comes down to what was mentioned earlier, the 'synergy' of a system. Its looking more and more like my ultimate system will have to be designed from the ground up, with each component of the system being optimised specifically for the associated components.

Now that sounds like an excellent idea! It might be kind of hard and a little expensive, but a great idea, nonetheless!

-Drew

 

[J.Y.]

I just had to dig this thread up; does the final parallel resistor form a high pass filter when used with a source that has a relatively small output capacitor?

It sort of makes sense to me that the prosinked cables exist as an load parallel to the resistor setting the input impedance of the following stage. If this be the case, will the nitrogens not be pulling the input impedance down so much that a high-pass filter is formed?

Sample Calculations:

Source:
4uF Output Capacitor, 0.000004F

Cable:
50 ohm coaxial
50 ohm in series at the input side of cable
50 ohm parallel at output side of cable

Amp:
100k input resistor, Zi=100k

Since the 100k and the 50R resistors are in parallel, this yields a new input impedance of 49.975R at the input of the amp.

Calculating using formula 1/(2piRC) yields 796hz, which sounds terribly wrong to me. Perhaps the 50R in series also comes into play when affecting the final impedance of this circuit?

If I am reading the borbely article correctly, the 50 ohm in series should also be added to the input impedance, resulting in 99.975R. This seems to lower the cut-off by half, but is still well beyond 20hz.

Perhaps having a inverted opamp at the 2nd stage which uses a series resistor to set its input impedance would get around this issue? As in like the signal only sees the HPF on the cables, but sees a 2nd load @ the amp.(something in the back of my mind tells me signals can't do that)

 

[Garbz]

I could be wrong here but most times when there's an electrolytic cap on the output there's usually a bleeder resistor to charge the thing at poweron. At least there is in my integrated, and my computer's soundcard which about coveres my experience in hifi gear.

That would probably effect the equation quite a bit.

Also i'll just add that since I started this thread last year i've learnt a LOT about interconnects and hifi in general. I've come to the conclusion that most of this means extremely little if the termination on the amps is changed. Without balancing the signal (that only happened recently) I decided to use XLR's with only 1 hot and 2 ground connected. All of a sudden I could no longer distinguish between cables (at least as much). So now I have a cupboard full of bling and a hifi which i'm enjoying exponentially more with just microphone cables.

 

[J.Y.]

Assuming that the input impedance of an inverted amp is viewed the same way as a non-inverted amp, which probably is. Unless I purposefully set the Zi on the amp to some really low value so that it affects the total impedance, the general Zi seen by the source wouldn't be much affected.

It would look somewhat odd however, if I set Ri(which sets Zi on the amp) on a inverted amp to 50 ohms, making the actual input impedance of the amp 50 ohms, and have another 50 ohms in series with it at the source end to raise the output impedance from the source to 50 ohms.

This results in 2x50 ohms resistors in series, and it seems like we might as well put just a 100 ohms resistor there.

I would assume that the "bleeder resistor" will just be another resistor in parallel with the 50R and the Ri, and it's purpose be discharge the output cap on the source power off. From my experience the value of the resistor should usually be large enough to be out of the consideration.

The electrolytic output cap, depending on how large it might be, could lower the cut-off. My faithful calculator just told me that in order to get a barely acceptable cut-off freq(~15hz) using a 50ohm cable, I need a 100uF cap at the source output. This leads me to think that maybe sources that are having trouble with resistor terminated cables are actually having trouble with low freq cut-off from the output cap.

Somebody please correct me if I am going down the wrong end of the road.