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Topic: Loudspeaker cables - horses For courses? (Read 22481 times) previous topic - next topic
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Loudspeaker cables - horses For courses?

Reply #25 –
Quote
Cable impedence only starts to matter when you find yourself with substantially different voltage at the ends of the cable, and those different voltages are due to time propagation.


For audio cables, this is quite some distance, only the telcos really get into this at audio frequencies under most conditions.
Is this any different for line level audio cables vs speaker cables?

I know from experience that video will have degradation if you try to string a composite or s-video cable too far. But video has a lot more frequency band required. Video also degrades less gracefully. I've occasionally put line audio through twenty or thirty feet of cable and not heard much difference, but I don't think my ears are good enough.

Loudspeaker cables - horses For courses?

Reply #26 –
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Is this any different for line level audio cables vs speaker cables?
[a href="index.php?act=findpost&pid=308254"][{POST_SNAPBACK}][/a]


Well, yes and no. Low-level cables are more sensitive to noise injection, but this has nothing at all to do with cable impedence, rather it has more to do with shielding, balance of twisted pairs, etc.

But in terms of simple propagation, consider what the wavelength of 20kHz is at, say, .5 C, which is a conservative speed of propagation for most any cable. Consider what 1/10 of a wavelength is, and that's roughly where you have to start worrying, for propagation reasons.  It's most possible that other issues, say drive current, driving amp stability, etc, will bite you first.
-----
J. D. (jj) Johnston

Loudspeaker cables - horses For courses?

Reply #27 –
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It's most possible that other issues, say drive current, driving amp stability, etc, will bite you first.
[{POST_SNAPBACK}][/a]
I was about to bring that up. The stability of the amp driving the speaker cables and the output impedence of the amplifier has a large effect on sound. Highly capacitive cables (like Litz wire) can reduce the phase margin of the driving amp and cause it to oscillate or ring on some signals. I personally haven't seen this happen at audio frequencies, but I have seen it happen with 40kHz sonar transponders. Amplifier oscillation is audible - although I can see how people could percieve low amplitude oscillation to be pleasant sounding.

An interesting article on the effects of cable impedence can be found here:
[a href="http://sound.westhost.com/cable-z.htm]http://sound.westhost.com/cable-z.htm[/url]

Loudspeaker cables - horses For courses?

Reply #28 –
Quote
I recall reading somewhere cable impedance is insignificant so long as the cable length does not become significant relative to the wave length of the signal, which means you need a mighty long speaker cable to notice. Is this correct?
[a href="index.php?act=findpost&pid=308186"][{POST_SNAPBACK}][/a]


No. It's the characteristic impedance, that matters only for long distances, not the impedance. The impedance matters even for short distances.
But even if you had a very long speaker cable (more than one kilometer), the characteristic impedance would be difficult to deal with, because it varies greatly across the audio frequencies.

Quote
Cable impedence only starts to matter when you find yourself with substantially different voltage at the ends of the cable,


...the difference in voltage being caused by the cable impedance.

Quote
and those different voltages are due to time propagation.


No, you can have different voltages with permanent DC, because of the resistance of the cable.

Loudspeaker cables - horses For courses?

Reply #29 –
I think that the "cable impedance" Woodinville refers to, is actually cable characteristic impedance, that is the one that begins to have importance  when there are "substantially different voltage at the ends of the cable, and those different voltages are due to time propagation", that is, at very high frequencies.

Of course, regular impedance can have importance at low frequencies and even at DC.

Loudspeaker cables - horses For courses?

Reply #30 –
 Going back to the start of this thread, I would be most interested to hear what happens if you connect a high quality pair of headphones to the Rotel and listen at very low and then at safe(your ears) high levels.

If the headphone circuit is any good, the amplifier should drive moderatly heavily at the higher levels and you can tell if the characterisitic 'sound' of the Rotel changes. If it doesn't, I would be interested to see the results of different speakers.

You want the designed output from your Rotel to reach the speakers unchanged, the only way to achieve this is with a cable large enough that all of it's characteristics can be safely ignored!

Steve.

Loudspeaker cables - horses For courses?

Reply #31 –
Quote
Quote
Cable impedence only starts to matter when you find yourself with substantially different voltage at the ends of the cable,


...the difference in voltage being caused by the cable impedance.[a href="index.php?act=findpost&pid=308316"][{POST_SNAPBACK}][/a]


Well, thank you, but we are in fact talking about characteristic impedence, not lumped-paramater impedence.

On the other hand, you would be just as correct to say that the dC/dx and dL/dx of the cable are responsible for propagation speed.  Was that what you meant?
-----
J. D. (jj) Johnston

Loudspeaker cables - horses For courses?

Reply #32 –
Quote
Going back to the start of this thread, I would be most interested to hear what happens if you connect a high quality pair of headphones to the Rotel and listen at very low and then at safe(your ears) high levels.

If the headphone circuit is any good, the amplifier should drive moderatly heavily at the higher levels and you can tell if the characterisitic 'sound' of the Rotel changes. If it doesn't, I would be interested to see the results of different speakers.

You want the designed output from your Rotel to reach the speakers unchanged, the only way to achieve this is with a cable large enough that all of it's characteristics can be safely ignored!

Steve.
[a href="index.php?act=findpost&pid=308367"][{POST_SNAPBACK}][/a]



Well, as it happens, I've been listening to the Rotel via my Senn HD580's quite a lot since I got it, and it's actually acceptable enough that i just sold my Creek OBH-11 headphone amp, (on promising myself I'll get an OBH-21SE as soon as I can afford it).

I've had the cover off the Rotel, and the phone socket is 'standard-issue' as far as I can see - pair of series resistors on the power-amp stage..

The problem with this arrangement is that it creates a 'high-impedence' output. 

This works up to a point, but ironically, despite the abundance of current-capacity, the amp effectively can't deliver current via these resistors that 'follows' the voltage it's trying to swing, and the sound get's muddier as the volume increases. It's quantified as lack of 'damping', I believe (or is it too much damping?).

The actual current flowing is also very low and a  MOSET, or whatever, designed to deliver 30 or more watts isn't going to work properly delivering a fraction of a watt - it won't be 'linear' at these current levels.

I remember often being frustrated when listening via phones plugged into an integrated amp and finding that the 'dynamics' seem to disappear when listening at head-banging levels, and I'm sure many others will have noticed this.

Rega used to make a 'shunt' device with a heatsink for connecting phones to a speaker output to get round this, and I remember a gizmo called "the can-opener" that did something similar.

R.

PS - I think the reason the Rotel's headphone output sounds OK ( for a budget integrated),  is that it works in class-A up to a couple of watts, as i mentioned, and this is also the reason it sounds so good at low volume 'in-room'.

Loudspeaker cables - horses For courses?

Reply #33 –
Quote
Well, as it happens, I've been listening to the Rotel via my Senn HD580's quite a lot since I got it, and it's actually acceptable enough that i just sold my Creek OBH-11 headphone amp, (on promising myself I'll get an OBH-21SE as soon as I can afford it).

I've had the cover off the Rotel, and the phone socket is 'standard-issue' as far as I can see - pair of series resistors on the power-amp stage..

The problem with this arrangement is that it creates a 'high-impedence' output. 

This works up to a point, but ironically, despite the abundance of current-capacity, the amp effectively can't deliver current via these resistors that 'follows' the voltage it's trying to swing, and the sound get's muddier as the volume increases. It's quantified as lack of 'damping', I believe (or is it too much damping?).

The actual current flowing is also very low and a  MOSET, or whatever, designed to deliver 30 or more watts isn't going to work properly delivering a fraction of a watt - it won't be 'linear' at these current levels.

I remember often being frustrated when listening via phones plugged into an integrated amp and finding that the 'dynamics' seem to disappear when listening at head-banging levels, and I'm sure many others will have noticed this.

Rega used to make a 'shunt' device with a heatsink for connecting phones to a speaker output to get round this, and I remember a gizmo called "the can-opener" that did something similar.

R.

PS - I think the reason the Rotel's headphone output sounds OK ( for a budget integrated),  is that it works in class-A up to a couple of watts, as i mentioned, and this is also the reason it sounds so good at low volume 'in-room'.
[a href="index.php?act=findpost&pid=308459"][{POST_SNAPBACK}][/a]


  Rockfan, I think you've nailed it! The audible differences you hear at different levels on your Rotel are caused by the amp, not the cables. I don't really have time for this now (I need to be getting to work) but you are wrong about the output devices not being linear at only low level. Try a different pair of speakers on your amp with decent cable and report back. I for one am fascinated by this thread!

Steve 

Loudspeaker cables - horses For courses?

Reply #34 –
Quote
Quote
Going back to the start of this thread, I would be most interested to hear what happens if you connect a high quality pair of headphones to the Rotel and listen at very low and then at safe(your ears) high levels.

If the headphone circuit is any good, the amplifier should drive moderatly heavily at the higher levels and you can tell if the characterisitic 'sound' of the Rotel changes. If it doesn't, I would be interested to see the results of different speakers.

You want the designed output from your Rotel to reach the speakers unchanged, the only way to achieve this is with a cable large enough that all of it's characteristics can be safely ignored!

Steve.
[a href="index.php?act=findpost&pid=308367"][{POST_SNAPBACK}][/a]



Well, as it happens, I've been listening to the Rotel via my Senn HD580's quite a lot since I got it, and it's actually acceptable enough that i just sold my Creek OBH-11 headphone amp, (on promising myself I'll get an OBH-21SE as soon as I can afford it).

I've had the cover off the Rotel, and the phone socket is 'standard-issue' as far as I can see - pair of series resistors on the power-amp stage..

The problem with this arrangement is that it creates a 'high-impedence' output. 

This works up to a point, but ironically, despite the abundance of current-capacity, the amp effectively can't deliver current via these resistors that 'follows' the voltage it's trying to swing, and the sound get's muddier as the volume increases. It's quantified as lack of 'damping', I believe (or is it too much damping?).

The actual current flowing is also very low and a  MOSET, or whatever, designed to deliver 30 or more watts isn't going to work properly delivering a fraction of a watt - it won't be 'linear' at these current levels.

I remember often being frustrated when listening via phones plugged into an integrated amp and finding that the 'dynamics' seem to disappear when listening at head-banging levels, and I'm sure many others will have noticed this.

Rega used to make a 'shunt' device with a heatsink for connecting phones to a speaker output to get round this, and I remember a gizmo called "the can-opener" that did something similar.

R.

PS - I think the reason the Rotel's headphone output sounds OK ( for a budget integrated),  is that it works in class-A up to a couple of watts, as i mentioned, and this is also the reason it sounds so good at low volume 'in-room'.
[a href="index.php?act=findpost&pid=308459"][{POST_SNAPBACK}][/a]


The whole point of that "pair of resistors" is to limit the amp's current, otheriwse either your headphones or your ears will blow! It doesn't neccessarily create a high impedance either, just high resistance. Consider that any headphone or loudspeaker is essentially current-operated anyway - it actually creates the ideal current-driver stage by using those resistors (ok, not quite true, but it's nearer to the ideal if anything).

Wrong again about the MOSFETs. The amp uses something called "bias" to keep a certain amount of current flowing, enough to keep them off the "knee" of the characteristic and hence keep them in the linear range. You statement is like saying a 160mph sports car can only be driven at that speed - wrong!

As for the original bit - interconnect is designed for just that, interconnects. In the crazy world of "hi-fi" where people pay riduculous amounts of money for things that provide no provable improvements, do you not think that someone would market interconnect marked as speaker cable for low power if it was even vaguely sensible? Sensible cable rated appropriately is more than adequate for low power as well as high. It simply isn't true that a high-power cable can't handle low powers as well - analogy as for the MOSFETs.

Loudspeaker cables - horses For courses?

Reply #35 –
Quote
The whole point of that "pair of resistors" is to limit the amp's current, otheriwse either your headphones or your ears will blow! It doesn't neccessarily create a high impedance either, just high resistance. Consider that any headphone or loudspeaker is essentially current-operated anyway - it actually creates the ideal current-driver stage by using those resistors (ok, not quite true, but it's nearer to the ideal if anything).

Wrong again about the MOSFETs. The amp uses something called "bias" to keep a certain amount of current flowing, enough to keep them off the "knee" of the characteristic and hence keep them in the linear range. You statement is like saying a 160mph sports car can only be driven at that speed - wrong!


[a href="index.php?act=findpost&pid=308538"][{POST_SNAPBACK}][/a]


I have to disagree, impedence across the audio band is a figure which includes resistivity, and  those resistors add substantially to it on a power-amp stage. 

This isn't something I just made up, it's received wisdom, and is the whole reason for using a dedicated headpone amp, or one of those shunt gizmos on a speaker output.

The effect, as I said, is quite commonly heard in the inabilty of such headaphone outputs (ironically) to go loud convicingly,  ie. deliver the current necessary (even if it results in no more than a few 100 millwatts), while keeping dynamics intact,  to 'follow' the output device's voltage faithfully.

I also don't understand what you mean about maintaining 'linearity' in a high-current output device when flowing tiny fractions of that current to headphone tranducers. I just can see how, for an extreme example, a 2000 watt Krell can be 'optimised' in any sense of the word  to drive tiny headphone transducers.

And 'supercars' like Ferrari's have long been known for being absolute pigs to drive around town! Until the advent of electronic fuel and ingnition management, you had to put up with chores like taking the plugs out every other day to get the soot off them! And they still don't exactly 'work' for commuting and shopping.

To me, the 'horses for course' axiom seems applicable again.

R.

PS - one thing to realize is that those headphone resistors primary function is to make the volume control usable - without them you's barely be able to get it 'off the stop', and given the lack of accuracy of most pots, levels would probably be heavily skewed to left or right. Of course there's the added benefit of preventing people instantaneously blowing their headphones and/or eardrums out, which is prbably a good idea!

Loudspeaker cables - horses For courses?

Reply #36 –
Quote
This isn't something I just made up, it's received wisdom,

Well, OK then.
Quote
The effect, as I said, is quite commonly heard in the inabilty of such headaphone outputs (ironically) to go loud convicingly,  ie. deliver the current necessary (even if it results in no more than a few 100 millwatts), while keeping dynamics intact,  to 'follow' the output device's voltage faithfully.


Just hipshotting here, but I think the point is that (as long as neither amp nor phones are starting to compress) the resistive network may be detrimental, but should be equally so at low levels.

Loudspeaker cables - horses For courses?

Reply #37 –
Quote
Quote
The whole point of that "pair of resistors" is to limit the amp's current, otheriwse either your headphones or your ears will blow! It doesn't neccessarily create a high impedance either, just high resistance. Consider that any headphone or loudspeaker is essentially current-operated anyway - it actually creates the ideal current-driver stage by using those resistors (ok, not quite true, but it's nearer to the ideal if anything).

Wrong again about the MOSFETs. The amp uses something called "bias" to keep a certain amount of current flowing, enough to keep them off the "knee" of the characteristic and hence keep them in the linear range. You statement is like saying a 160mph sports car can only be driven at that speed - wrong!


[a href="index.php?act=findpost&pid=308538"][{POST_SNAPBACK}][/a]


I have to disagree, impedence across the audio band is a figure which includes resistivity, and  those resistors add substantially to it on a power-amp stage. 

This isn't something I just made up, it's received wisdom, and is the whole reason for using a dedicated headpone amp, or one of those shunt gizmos on a speaker output.

The effect, as I said, is quite commonly heard in the inabilty of such headaphone outputs (ironically) to go loud convicingly,  ie. deliver the current necessary (even if it results in no more than a few 100 millwatts), while keeping dynamics intact,  to 'follow' the output device's voltage faithfully.

I also don't understand what you mean about maintaining 'linearity' in a high-current output device when flowing tiny fractions of that current to headphone tranducers. I just can see how, for an extreme example, a 2000 watt Krell can be 'optimised' in any sense of the word  to drive tiny headphone transducers.

And 'supercars' like Ferrari's have long been known for being absolute pigs to drive around town! Until the advent of electronic fuel and ingnition management, you had to put up with chores like taking the plugs out every other day to get the soot off them! And they still don't exactly 'work' for commuting and shopping.

To me, the 'horses for course' axiom seems applicable again.

R.

PS - one thing to realize is that those headphone resistors primary function is to make the volume control usable - without them you's barely be able to get it 'off the stop', and given the lack of accuracy of most pots, levels would probably be heavily skewed to left or right. Of course there's the added benefit of preventing people instantaneously blowing their headphones and/or eardrums out, which is prbably a good idea!
[a href="index.php?act=findpost&pid=308564"][{POST_SNAPBACK}][/a]


The point of the resistors is to limit the amp's output current to a safe value - headphones never require anywhere near the current a speaker does. If the output doesn't go "convincingly loud", try reducing them in value. The primary function of the resistors is certainly NOT to make the volume control usable, though you're quite right that they have that rather beneficial side-effect. Omit them and you'll get not convincing volume, but convincing smoke from your headphones!

The analogy with a "supercar" was maybe not the best, but the fact is that it's possible to drive them slowly.

If you fail to understand the bit about bias, go do some reading. Bias means running the output devices at some minimum current to keep them in their linear region. That current appears as waste heat, essentially. A 2kw amp needs no "optimisation" to run headphones, but it sure as hell needs the resistive chain in the output to limit current!
As I said before, headphones and most speakers are current-driven devices. The ideal driver (amp) for them is one whose current is independent of the load impedance. Adding the resistors to the amp's output takes it nearer to the ideal, not further away.

Loudspeaker cables - horses For courses?

Reply #38 –
The impedance curve of headphones is not flat. If they are driven through resistors, their frequency response will differ from the one that they have if they are voltage-driven.

Loudspeaker cables - horses For courses?

Reply #39 –
Does this mean that the headphone jack of an amp is basicaly a source of current, rather than one of voltage? Which in turn means your headphones output more power at frequencies with high impedance, and less power at frequencies with lower impedance, right?

How big is this effect, and how are headphones supposed to be driven?
Veni Vidi Vorbis.

Loudspeaker cables - horses For courses?

Reply #40 –
Quote
Does this mean that the headphone jack of an amp is basicaly a source of current, rather than one of voltage? Which in turn means your headphones output more power at frequencies with high impedance, and less power at frequencies with lower impedance, right?

How big is this effect, and how are headphones supposed to be driven?
[a href="index.php?act=findpost&pid=309845"][{POST_SNAPBACK}][/a]


Yes up to a point - they are a source of current, but it's limited. The end result depends on the amp's output voltage capabilities, the headphones' impedance and efficiency, the value of the resistors and so on, so it's not a simple picture. I'm simply making the point that conventional-design (i.e. moving-coil) speakers and headphones are current-driven. The current passing through the coil at any given instant determines the force on the diaphragm. In reality of course that's a simplification since the suspension, inertial mass of the diphragm and a whole heap of other factors affect the diaphragm too. Ideally, the amp would supply a current that was independent of impedance and frequency. Arranging that isn't quite so easy if you want 100% accuracy, but considering the distortion inherent in the actual speaker/headphone design I can't see it being a major factor.

It doesn't follow that your headphones output more power at frequencies where they have higher impedance. You're equating higher impedance with higher voltage for a given current and, therefore, higher power. That's only true for a resistive load, which headphones aren't. The acoustic efficiency might be lower as well (I've no idea if that's true in real designs though). Remember, the whole process is horribly inefficient and non-linear anyway. I'd be quite interested to know, if an amp was driven at low level directly into headphones, how it would compare to the output-limited setup at the same listening level. Noise would probably be worse, but I wonder how the quality would be if you ignore that.

I've often wondered why no-one designs an amplifier to operate on the current-source idea, maybe I'm missing something here?

Loudspeaker cables - horses For courses?

Reply #41 –
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The impedance curve of headphones is not flat. If they are driven through resistors, their frequency response will differ from the one that they have if they are voltage-driven.
[a href="index.php?act=findpost&pid=309766"][{POST_SNAPBACK}][/a]


Can't argue with that :-) Which response is "correct" though? Neither, at a guess!

Loudspeaker cables - horses For courses?

Reply #42 –
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I've often wondered why no-one designs an amplifier to operate on the current-source idea, maybe I'm missing something here?
[a href="index.php?act=findpost&pid=309878"][{POST_SNAPBACK}][/a]


Most amps, at least solid state ones, are designed to be as much like a voltage source as possible, and most speakers are designed to be driven by a voltage source.

Using resistors to drop the output for headphones is the problem.  Since the resistors are outside the amplifier's feedback loop, they add to the output impedance, making the output to the phones less like a voltage source and more like a current source (but still not close to being either).  If the phones have an impedance that varies with frequency, then the percentage of the amplifier's output voltage  that reaches the phones will vary with frequency.  If the impedance across the board is >> the resistors, then it won't make much difference.  If it is comparable or less, then sound will suffer.

Loudspeaker cables - horses For courses?

Reply #43 –
Would it be a good idea to connect two resistors, say two and six ohms, in series to the amplifier's speaker output, and then connect the headphones parallel to the two ohms resistor? It'd be terribly wasteful of power, but it would turn a big amp into more or less a voltage source at headphone levels, right? And has the benefit of loading amp more so it may behave more linear...
Veni Vidi Vorbis.
 

Loudspeaker cables - horses For courses?

Reply #44 –
Something like a 20 Ohm and a 2 Ohm resistors in series with amp speaker terminals, with the headphones in parallel with the 2 Ohm resistor, will drive any headphone so that the frequency response deviation is negligible (voltage source-like), providing a 20 dB attenuation. The 20 Ohm resistor must be rated several watts.

Loudspeaker cables - horses For courses?

Reply #45 –
Quote
Something like a 20 Ohm and a 2 Ohm resistors in series with amp speaker terminals, with the headphones in parallel with the 2 Ohm resistor, will drive any headphone so that the frequency response deviation is negligible (voltage source-like), providing a 20 dB attenuation. The 20 Ohm resistor must be rated several watts.
[a href="index.php?act=findpost&pid=310062"][{POST_SNAPBACK}][/a]


Only thing is it raises the damping factor considerably. Series resistors would do that too, to an even greater extent though. The 20 Ohm resisors would need to be quite a high-wattage rating, roughly 1/5 of the amp's output rating (into 8 Ohms) if you're going to wind the amp up. The 2 Ohm ones need to be about a tenth of the 20 Ohm ones, power-wise. It'll work though.

I'm curious how you calculated the 20dB attenuation - that'd be a voltage attenuation, ignoring the headphone impedance?

Loudspeaker cables - horses For courses?

Reply #46 –
I have a couple of questions. These may be stupid, as the discussion went beyond my electrical knowledge about the time people started talking about voltage source vs current source.

First, if a resistor in series with the phones is a negative presence, what about volume control? That's just a variable resistor. Wouldn't it have the same effect?

Second, are the headphone jacks made this way because they have to deal with two totally different designs of headphone? (ie, cheap ones and audiophile ones, which tend to have totally different impedance and response characteristics)

It seems to me like the best thing to have, if one tended to listen to both speakers and headphones equally and only wanted a single device, would be a receiver that had two amp circuits in it. The standard high power speaker driving amp, and a little headphone level amp for when the phones are plugged in. Much more efficient too. I guess until someone makes such a device (hah! fat chance) we will have to just use headphone amps on the line in.

Loudspeaker cables - horses For courses?

Reply #47 –
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First, if a resistor in series with the phones is a negative presence, what about volume control? That's just a variable resistor. Wouldn't it have the same effect?[a href="index.php?act=findpost&pid=310643"][{POST_SNAPBACK}][/a]


No, because the volume control is located before the power amplifier. It changes the voltage of the input, and the output impedance remains the same.

Loudspeaker cables - horses For courses?

Reply #48 –
Quote
No, because the volume control is located before the power amplifier. It changes the voltage of the input, and the output impedance remains the same.

A durrrr. I'm an idiot. I should have knew that, there aren't many variable resistors that can handle tens, much less hundreds, of watts.

Loudspeaker cables - horses For courses?

Reply #49 –
Quote
Would it be a good idea to connect two resistors, say two and six ohms, in series to the amplifier's speaker output, and then connect the headphones parallel to the two ohms resistor? It'd be terribly wasteful of power, but it would turn a big amp into more or less a voltage source at headphone levels, right? And has the benefit of loading amp more so it may behave more linear...
[{POST_SNAPBACK}][/a]


I assume that's what the 'can-opener' and Rega gizmos did -  basically a simple voltage-divider/shunt attenuator.

Both had substantial heatsinks to dispose of (presumably) several watts of power in 'pretending' to be loudspeakers.

I think you could probably go for a total of at least 16 ohms, maybe even 30 or so, which might be useful if the amp was a powerful one.

I did think seriously about DIYing someting along these lines after the Rega and can-opener both disappeared, but bought a Creek headphone amp in the end.

R.

PS - found this;

[a href="http://cgim.audiogon.com/i/vs/i/f/1082826750.jpg]http://cgim.audiogon.com/i/vs/i/f/1082826750.jpg[/url]