Matching of Interconnect Electrical Characteristics

[Ron Resnick]

How important is it for the measured capacitance and the measured loop inductance of a pair of interconnect cables to be similar if not identical?

Who has measured these electrical characteristics of their interconnect cables to make sure these parameters are similar if not identical?

I can imagine that soundstaging and channel balance might suffer if these electrical characteristics are not very well-matched.

 

[DonH50]

All IMO.

Most interconnects have similar RLC because the wiring and construction does not vary all that much and there are only a few actual cable manufacturers (most others buy from them). Most preamps have low output impedance so a pretty wide mismatch will go unnoticed because the roll-off is still in the hundreds of kHz if not few MHz range. There's a technical thread or two on this (see index linked in my signature).

Tube systems are more sensitive due to their higher impedances (input and output), and if you have really long runs (50' ~ 100' or more) then cable parasitics will matter more. Digital links, including S/PDIF, AES, USB, and HDMI, are more sensitive and require matched impedances (to the source and driver, not each other) due to the much higher transmission rates (MHz to GHz). Phono cartridges are of course much more sensitive to parasitics (I built my own phono cables when I had a TT set up with about half the capacitance of typical coax). And EMI/RFI impacts cable choice -- a foil braid provides 100% shielding, and an extra outer shield is better in a high-RFI environment.

As an example, most consumer coax is in the 20 to 30 pF/foot range so say a 3' or interconnect might exhibit 100 pF of capacitance. Since the system is not impedance-matched like an RF system, all of that capacitance is "seen" by the system. Inductance is usually small enough to not matter. Now if we have a preamp with 100-ohm output impedance (typical SS) and neglect the load then the -3 dB frequency (using a simple lumped model) is about 1/(2*pi*100*100e-12) = 15.9 MHz -- well beyond audibility for most of us. The amp's input adds more capacitance but you get the idea. The input impedance of most components is so high that small or even fairly large differences are still way above the audible range.

I find it more important to match speaker cables but not for the reasons sometimes mentioned like delay (inconsequential) or amplitude loss (usually negligible, though a severe mismatch may impact channel balance). IME/IMO the real reason is mainly the resistance increasing the effective output impedance of the amplifier. Most crossovers are designed with ideal voltage sources, or at least some big amplifier driving them, and so additional resistance causes the frequency response to change from the design goal. And mismatching the lengths causes one speaker to vary slightly differently than the other. The changes are still usually small fractions of a dB unless the difference is huge, but speaker impedances can vary pretty wildly and peaks/valleys are sometimes fairly broad, so fractions of a dB can be audible.

All FWIWFM, IME, IMO, YMMV, my 0.000001 cent (microcent), etc. - Don

 

[Ron Resnick]

Dear Don,

Thank you very much for this detailed reply. I appreciate it!

Would group delay be more of a concern in a bi-amped system in which the line stage preamplifier is feeding a full-range signal to solid-state amps driving woofers and a second preamp output is feeding the same full-range signal to tube amplifiers driving midrange/tweeter panels?

What about phase shift? Can the phase shift of interconnects be measured?

How important is it for interconnects to have similar if not identical phase shift characteristics?

How much more of a concern is group delay and phase shift when solid-state amplifiers are driving woofers and tube amplifiers are driving midrange/tweeter panels?

 

[microstrip]

How important is it for the measured capacitance and the measured loop inductance of a pair of interconnect cables to be similar if not identical?

Who has measured these electrical characteristics of their interconnect cables to make sure these parameters are similar if not identical?

I can imagine that soundstaging and channel balance might suffer if these electrical characteristics are not very well-matched.

I have measured many cables - the best way to check for chinese fakes that sometimes look very similar to originals. Pairs are surely very well matched in terms of capacitance and inductance. A mismatch would indicate a fault somewhere in the manufacture.

Just for fun I have tried using a 1m IC in one channel and a 1.5m in the other (a 50% increase in capacitance and inductance) . The difference was very small, I would say unnoticeable in my system. Cable was Kimber KS1136, a low capacitance cable.

However using different brands in each channel, even of similar capacitance, brings significant degradation to the sound quality of the system. All IMHO, YMMV.

 

[GMKF]

Would group delay be more of a concern in a bi-amped system in which the line stage preamplifier is feeding a full-range signal to solid-state amps driving woofers and a second preamp output is feeding the same full-range signal to tube amplifiers driving midrange/tweeter panels?

It will be in the microseconds so I dought its audible.
In my opinion the "rise time" of your tube output stage will be greater than the signal delay caused by the stage in the bass-towers, and I'm not sure if you can hear the "risetime" of your tube output.

What about phase shift? Can the phase shift of interconnects be measured?

Can be done. You hook up an oscilloscope (with a high bandwith/high sample rate) and put one probe on one end of the IC and the other one at the other end of the IC. (Signal has to pass trough it)
Your phase-shift will be visible if inside of the oscilloscopes measurement zone. (If it's not "measurable with a "normal" oscilloscope its small enough that it is irrelevant) .

How important is it for interconnects to have similar if not identical phase shift characteristics?

See quote above. If irrelevant (which is my humble opinion) dont bother. But my mind tells me the shorter the IC the better, so if one can be shorter than the other one do so. I think the added length to get a sonic gain, due to the same phase shift, will be less likely than a possible EMI situation.
(caused by the plus in length to correct the phase-shift).

How much more of a concern is group delay (see below) when solid-state amplifiers are driving woofers and tube amplifiers are driving midrange/tweeter panels?

Will be measurable (due to signal delay of your tube amp). The electrons of the SS amp will reach the woofers earlier because they didnt have to go trough a cable+amp) but I dought it will be audible. (in the microseconds)

"and phase shift"

If the cable does not have a "big" amount of phase shift (which is my opinion) and the amps are properly designed nothing should be audible.

I didn't spell check- sorry for any typos.

Have a great day everyone !

 

[DonH50]

Dear Don,

Thank you very much for this detailed reply. I appreciate it!

Would group delay be more of a concern in a bi-amped system in which the line stage preamplifier is feeding a full-range signal to solid-state amps driving woofers and a second preamp output is feeding the same full-range signal to tube amplifiers driving midrange/tweeter panels?

What about phase shift? Can the phase shift of interconnects be measured?

How important is it for interconnects to have similar if not identical phase shift characteristics?

How much more of a concern is group delay and phase shift when solid-state amplifiers are driving woofers and tube amplifiers are driving midrange/tweeter panels?

Hey Ron,

GMKF answered for you (and me, thanks!)

Group delay is usually in the mud as far as audio is concerned and we are talking about wires. Where delay is a problem is usually larger venues when all the speakers around a concert hall, church, or stadium need to be delayed appropriately so everyone hears a cohesive sound. Signals in the wires travel very fast, from maybe 60% to 80% the speed of light, so that delay is not usually a problem. If the speakers are positioned at different distances, then the speed of sound in air comes into play, at ~1127 feet/second instead of maybe 93,000 miles/second or better (491,040,000 feet/second).

The other place group delay (latency, in this case) is an issue is when processing the signal. When you read about xxx thousand-tap filters, that means the signal going in is delayed by that amount times the sample period. At 44 kS/s, each sample is 1/44,000 = 22.7 us so a 1000-tap filter means a 22.7 ms delay -- roughly the same as moving the speaker about 25 feet! As long as all speakers are delayed the same amount, you will not know, but part of the magic of AVRs is getting the delays electrical and physical to even out so sounds from all speakers arrive at your ears at the same time. Avoids sonic chaos. At least from that source...

Group delay is related to phase shift; in fact, it is the negative of the derivative (slope) of phase with frequency. Constant group delay (just a single number, implying a fixed slope in phase over frequency) is desirable for pulse integrity. How much it matters in audio is debatable (of course) but I've always felt it a good goal. If group delay is not constant over frequency, then some frequencies will arrive at different times, smearing the signal (and potentially the image etc.)

If you have two amplifiers with significantly different delay (or phase shift) then yes you may need to compensate. I did that using analog circuits when I had a tube mid/upper amp and SS lower amp driving my Maggies. One other thing to watch is polarity inversion -- in my case, one amp inverted the signal with respect to the input, and the other did not. I found that out when I was trying to set everything up, and worked up a pretty nice all-pass circuit that would provide a full 360 degrees of phase shift without changing amplitude of frequency response. One of those engineer things; it was a fun challenge. I had it done and was testing it out when I realized I could have just flipped the speaker cables. As it turns out, I needed it anyway, because the bass and tweeter inputs had a common ground and one of the amplifiers had a floating output that would not have worked if I flipped the connections and shorted the "wrong" side to ground through the other amp. At least I did realize that before I smoked an amp (maybe two, and a speaker or two).

Crossovers also introduce significant phase shift and is why it is most important to align say subs and mains at the crossover point so the signals from them add instead of cancel (subtract). As you move away from the crossover, one speaker contributes less and the problem diminishes. But, with typical AVR crossovers, there is still a lot of signal (about half-loudness in terms of SPL) an octave either side of the crossover, one reason I prefer to keep my sub's crossover about an octave above the -3 dB point of my main speakers. Phase shift typically gets larger below the -3 dB point, especially for ported designs.

Yes, phase shift is (and many other things are) very easy to measure in an interconnect. You can use an oscilloscope as mentioned above, but I use a vector network analyzer (VNA) that plots amplitude and phase for you. Most modern VNAs will also generate group delay and many will do an inverse FFT to give you the time-domain response (pulse or step) as well. A time-domain reflectometer (TDR) can also be used to measure the impedance along a cable and provide the delay by looking at when a pulse sent down the cable returns.

Most cables are well-matched unless something is very wrong, at least as far as audio is concerned. If the delay is different it means the length, resistance, capacitance, inductance, conductance, and/or all of the above are different. But again in the real world you'd have to have a very large difference in cables to hear anything. At work I have to match delay (skew) among cables to a picosecond or less, and sometimes well below, but that is multi-GHz RF stuff and not audio.

I am a skeptic when it comes to interconnect and power cables affecting the sound but there are cases where it happens, and usually the reason is rational and measurable. Shielding, or lack of it, can affect things. How ground is handled is significant. Some cables include passive networks and those can also affect phase as well as the effective cable resistance. Those intentionally modify the signal so are outside the scope of my discussion (babbling). I've always been curious why Teflon-type dielectrics are sometimes touted by the same companies that advocate lower sensitivity to charge traps and sensitivity to motion, both of which tend to be worse with PTFE (Teflon). Etc.

Lot to digest, I'll stop there for now.

HTH - Don

 

[Folsom]

To put it shortly, none of these things are a concern. That is barring any really goofy things like interconnects where the signal and ground run far away from each other, or junky gear with bad passive filters in them.

 

[Ron Resnick]

Thank you Max and thank you Don for those very detailed and comprehensive replies. I will have to read these posts a few times to make sure I understand them. I appreciate it!

Don, what sonic anomalies did you hear as evidence of the group delay/phase shift problem between the solid-state amp driving the woofer section and the tube amp driving the midrange/tweeter section?

 

[Folsom]

Ron, your bass towers have adjustable phase. And the minor amount of phase difference between the two amps compared to phase changes both the bass drivers and panel towers naturally have is like throwing lit matches at a bonfire. The problems between the different amps isn't going to be group delay or phase shift. Don mentioned inversion, which can be an issue since some amplifiers are inverting. That shouldn't be an issue with your bass amps that are adjustable. Finding the right phase adjustment on bass amps usually is never easily done by flipping the 180* switch, but FR may need the flip depending on some factors.

Here's an example of how much phase can change on a woofer from one frequency to another.

 

[DonH50]

Interesting, I figured after the "dreamworld" power cord comment Folsom would tell me I was out to lunch on this as well...

Ron, the main thing was a blip (null) in the frequency response around the crossover point. If not in phase at the MLP (main listening position) then the signal from the upper and lower drivers will not combine constructively (add) and so you get a "valley" in the frequency response. This was with my old Magnepan MG-IIIa's so the crossover was around 300~500 Hz. The passive crossover was asymmetric which complicates the phase response. I used a symmetric active crossover after playing around and decided I liked it better but had to add the aforementioned phase-adjusting circuit to better align the phase and eliminate the crossover notch. I liked to say I could hear the transient smearing but frankly, though it was easily measured, it was hard to hear with regular source material. The notch was quite audible when you knew it was there and where to listen for it as upper bass and low vocals sounded "weak". Sometimes I thought I could tell on toms and such that the attacks were smeared but DBT's indicated I was mostly guessing (IIRC I was around 60% accurate, just enough to be tantalizing).

Multiple subs and speakers not equidistant or in the same environment (i.e. distance from walls and such, furniture around them, etc.) can induce peaks and valleys in the frequency response from either out-of-phase direct sound or different reflected sound arrival times. Those sorts of things should swamp most if not all (audible) differences in a pair of equal-length interconnects, or even a pair mismatched by a few inches or few feet. My subs have continuous phase adjust so I can measure and ial in the response. A simple polarity (180 degree) switch is usually not enough; if that's all ya' got, you probably need to move the subs, MLP, and/or mains to get everything aligned and in phase.

HTH - Don

Edit: I was typing when Folsom posted. I agree with his post, though probably would have not stated the interaction in terms quite so strong...

 

[Folsom]

Don, his bass towers are 250hz down. They are made to match and highly adjustable. The only way he can really screw it up is by placing the bass towers well over 4.5ft from the panel towers. At that point a notch would be reallly hard to get rid of, and things would sound disconjointed.

 

[Ron Resnick]

Ron, your bass towers have adjustable phase. And the minor amount of phase difference between the two amps compared to phase changes both the bass drivers and panel towers naturally have is like throwing lit matches at a bonfire. The problems between the different amps isn't going to be group delay or phase shift. Don mentioned inversion, which can be an issue since some amplifiers are inverting. That shouldn't be an issue with your bass amps that are adjustable. Finding the right phase adjustment on bass amps usually is never easily done by flipping the 180* switch, but FR may need the flip depending on some factors.

Here's an example of how much phase can change on a woofer from one frequency to another.

Folsom, Thank you for assisting here. But there is no phase adjustment on the Gryphon based tower electronics.

 

[Folsom]

Perhaps I am mistaken but I thought the crossover (including phase) was programmable through the data port?

 

[Ron Resnick]

No, phase is not programmable.

 

[DaveC]

To put it shortly, none of these things are a concern. That is barring any really goofy things like interconnects where the signal and ground run far away from each other, or junky gear with bad passive filters in them.

I agree.

Also, I think I've said the same exact thing to Ron in Don H's 1st post after he mentioned he was choosing his IC cables based on capacitance measurements.

Ron, it seems like you're doing anything you can to avoid having to actually try different interconnect cables. It's a lost cause. The way a cable sounds isn't going to be defined by measurements unless there's a really big problem, especially in home systems with relatively short runs of cable. Even your proposed long run isn't that long relative to what's done in pro audio. The explanation for this has been given more than once and the wording in this thread by Don is correct imo, and easy to follow.

What matters most is the material the conductor is made out of, including the plug. Then the cable's insulation, then geometry/shielding, and then a bunch of lesser factors. To think of cable capacitance as having a specific sound doesn't work. Why? Capacitance can be caused by different things. Capacitance caused by shielding sounds different vs capacitance between wires in a star quad. Why? Probably because it does different things to the EM field produced by the cable. The truth is, this isn't simple... cables may be simple and look simple, but what's actually going on and what causes cables to sound different from one another is not simple.

 

[Ron Resnick]

Thank you, Dave. You are correct!

I am trying to avoid buying four, six, eight, ten pairs of expensive speaker cables, and expensive custom-made 50’ or 55’ long XLR terminated interconnects, and then comparing them laboriously myself. So I am trying to find rational technical reasons to narrow down the list of potential candidates.

I would rather attempt to employ reason or logic to narrow the list of candidates rather than haphazardly or anecdotally narrow the list by selecting brands and models based merely on what friends happen to use.

A well-known loudspeaker designer kindly sensitized me to the issue of group delay between solid-state amps in the woofer towers and tube amps for the ribbon panels so I was wondering if the electrical characteristics (fast signal transmission and lowest phase shift) of certain speaker cables might help to ameliorate that issue.

 

[DaveC]

Thank you, Dave. You are correct!

I am trying to avoid buying four, six, eight, ten pairs of expensive speaker cables, and expensive custom-made 50’ or 55’ long XLR terminated interconnects, and then comparing them laboriously myself. So I am trying to find rational technical reasons to narrow down the list of potential candidates.

I would rather attempt to employ reason or logic to narrow the list of candidates rather than haphazardly or anecdotally narrow the list by selecting brands and models based merely on what friends happen to use.

A well-known loudspeaker designer kindly sensitized me to the issue of group delay between solid-state amps in the woofer towers and tube amps for the ribbon panels so I was wondering if the electrical characteristics (fast signal transmission and lowest phase shift) of certain speaker cables might help to ameliorate that issue.

I totally understand, and I wish it were easier! It would make my life easier too!

I don't think length matters that much. I'd test out some normal length IC cables and I think it'll be obvious to you what kind of sound you prefer.

 

[Folsom]

Ah, weird... The Pendragons are a little hard to pull up information on now. The good news is that they would have worked phase out to be within the envelope of distance that you have from tower to tower (4.5ft center to center or less of drivers) from Gryphon. Phase has a little more margin of error at low frequencies anyways.

Group Delay with SS and Tubes... Whomever this speaker designer is, I would say probably found something that could be an issue but it isn't group delay. It especially isn't likely to be group delay with the Pendragons, seeming as how they don't use DSP programmed through the data port? What they find audible is likely something rather different than group delay. You'd have a hard time getting anyone to identify group delay problems by ear; and near impossible at lower frequencies. There are a lot of "problem" things that happen in a stereo, and to this day many will have no fix. There's no piece of gear out there that couldn't be modeled/measured in some way to produce a graph to scare end users.

 

[DonH50]

I'm working like 70~80 hours/week lately so have not followed the entire thread and what Ron's system is like. Apologies in advance for being a "pigeon poster".

Dave, I don't know if you meant I was off-base, but for the record I do not choose just based on capacitance but for interconnects often find that and shielding are the major variables. Those go together most of the time, natch... Two things driving low-C cable selection in my primordial past were when I had my TT further from my preamp than desirable (space constraints), and when I was using a tube preamp over a fairly long run (maybe 20') to a tube power amp. The cathode follower output was OK, but one of the outputs was unbuffered (don't ask me why, somebody decided a follower was too degrading I guess, but such a high output impedance was sort of nuts). Eventually I built a custom follower for the second output.

I am using 50' XLR cables to my subs but just went with Pro-Co as I have used them for many years in professional installs. As I may have mentioned elsewhere, I also use Mogami, and when I need better isolation/noise rejection Canare quad cables. These are for runs that are usually no more than maybe 300' and often closer to 100'. If you have a long run I would pick a good well-shielded cable and call it a day but again I am a skeptic when it comes to high end cables. Dave and Folsom would be better folk to ask. Make sure they are CL-2 or CL-3 for in-wall if needed.

Ron, cables are extremely unlikely to ameliorate differences in group delays among amplifiers. Signals propagate at about 1 ns in 9" so even allowing for the 50% velocity reduction in a cable means you'd need a LOT of cable to balance even a microsecond of delay. That's why there are circuits to do that. If you need to time-align (or phase-align) the amps, chances are very good that it will take more than cables to do so. I think that is what we are all saying. Again, sorry if I have misstated anybody's opinion but (or including) my own.

IME/IMO - Don

 

[DaveC]

Dave, I don't know if you meant I was off-base...

Nope, sorry for the misunderstanding, I was agreeing with your post.