Cable Asylum

Posted by Jon Risch on March 02, 2000 at 16:17:09:

In Reply to: newbie help: biwiring cables posted by David on March 02, 2000 at 10:42:51:

Bi-wiring

You might want to try bi-wiring, but it helps to use something better than just zip cord, no matter what the gauge.
If you are running 10-12 foot lengths, a decent single wire set of speaker cables will run around $140 or so, so
double that for bi-wiring the two front mains. I would not worry about bi-wiring the center speaker, or using
anything but 12 gauge zip for the surrounds, as the suround signal is processed through a cheap digital delay
as well as derived from signals buried in the main signal, and hence is not that clean or wide bandwidth.

See this post for a list of decent aftermarket cables that don't cost a fortune, including references to DIY cables:
http://www.audioreview.com/techtalks/tweaks/messages/74992.html

How To Hook-Up Bi-Wiring
See: http://www.soundstage.com/synergize/synergize031998.htm
AND/OR
http://www.davidmannaudio.com/faq/faq3.html
and if that is not detailed enough:
For each speaker, instead of hooking up a single cable to the amp channel, and then connecting it to the
jumpered (+) and (-) at the speaker, you REMOVE the jumper bar from between the (+) and (+) terminals top
and bottom, and from between the (-) and (-) terminals top and bottom. Then you determine which pair
(top or bottom) is the pair for the tweeter. Usually the top pair is for the tweeter, but check your owners manual
to be sure. For the sake of discussion and detailing tthe hookup, let's assume that the top pair IS for the
tweeter. If this is not the case, then modify your instructions accordingly.

Now, you hook up the (+) and the (-) to the amp output terminals for that channel, connecting BOTH speaker
cables at once. Specifically, connect the (+) of the woofer cable AND of the Tweeter cable to the (+) terminal of
the amp. Using spades, this can be done relatively easily by stacking the two spades on top of one another
and tightening the amp post down. It would help to have a helper hold the two spades firmly in place while you
tightened the post. After you are done, do not tug on this connection too much so as to avoid loosening it.
Alternatively, if either cable has a banana plug, then if only one has a
banana, then connect one via spade, and plug the banana in after that. If both have banana plugs, then stack
the banana plugs up into the amp terminal. I prefer dual spades, or a spade and a banana, but two banana's
get's to be a somewhat poor connection, and even if you can place the woofer cable's banana
first, it is still not optimum. Then connect the other polarity (-) of both cables to the (-) of the amp, as appropriate
for the connectors used.

Now connect the woofer cable to the bottom pair (woofer) of speaker terminals, keeping the (+) and (-) correct,
and connect the tweeter cable to the top pair (tweeter) keeping the (+) and (-) correct. Make sure all
connections are tight, and that none of the terminals is shorting to any metal or the other terminal.

You have now hooked your speakers up in bi-wire mode.

I would expect that you would hear smoother bass, a clearer midrange, and more detailed yet smoother and
clearer highs after this hookup compared to the old single cable run.

How Bi-wiring Works
Bi-wiring is accomplished via separate pairs of terminals on the loudspeaker system, typically one pair for the woofer, and one pair for the tweeter or midrange and tweeter. They are completely separated electrically from one another. The normal function of a loudspeaker crossover is to guide the proper frequency's to the proper driver. Lows to the woofer, and highs to the tweeter. This is done in part for protection from the division of labor that has occured with two disparate speakers: tweeters will be damaged or destroyed if exposed to low frequency's and woofers just heat up when exposed to the higher frequency's, as they are too massive to respond at all. The other function that a crossover provides is in allowing the two speakers to blend together, to mesh with one another to become a single apparent sound source. They can also provide some passive EQ of the drive units, as long as there is excess energy to throw away.

The fundamental way a loudspeaker crossover works is to vary the impedance seen by the speaker and by the power amplifier. In the case of the woofer, the crossover network for it has a very low series impedance at low frequency's that gets gradually higher and higher in impedance between the amp and the speaker at higher frequency's. For very low frequency's, there is lots of current flow to the woofer, and for higher frequency's, there is little current flow due to the much higher impedance. In the case of the tweeter, at low frequency's the series impedance is very high and very little current flows, and as the frequency goes higher, the impedance of the crossover network gets lower and lets through more current.

The situation is such that when the full range musical signal is applied to the terminals of a full-range speaker system, the woofer only gets sent low frequency signals, and the tweeter only gets sent high frequency signals. Once the crossover networks have been electrically separated, they still continue to function in the same manner, having a low impedance in their passband of application. This means that if separate speaker cables are hooked up for the woofer and it's portion of the network, and the tweeter, and it's portion of the network, not only have the speakers and the frequency's directed and divided for them, but the two separate speaker cables will now also carry different signals, the woofer cable mostly the lows, and the tweeter cable mostly the highs.

Once the highs and lows have been separated in this fashion, the strong current pulses and surges that a woofer demands when reproducing bass or drums will not interact with the delicate sounds of a flute or cymbal. The magnetic field of the low frequency signals cannot modulate or interfere with the highs, and to a lesser extent, the reverse is true.

Now that the low and high frequency signals have been divided among not only the speaker drivers, but the speaker cables, these cables can be more specialized for their intended purpose. The woofer cable can concentrate on low DCR, and not have any big concern for extremely low inductance, the tweeter cable can be designed for very low inductance, and not as concerned about total DCR.

Using one much larger unsophisticated cable to achieve the same thing as bi-wiring is just not possible, the separation of work has not occured, and the ability to optimize each separate run is not available. Additionally, as the gauge of a wire decreases (wire gets fatter) and the spacing between the pair of wires that constitute a speaker cable gets greater, the inductance tends to go up. Using one larger unsophisticated cable actually makes things worse for the tweeter, as even though the DCR has gone down and the woofer gets more energy compared to the thinner single cable, the tweeter now gets less energy in the extreme highs. The net result is a shift in the tonal balance that can even exceed the criteria held dear by the ABXers.

The current path's I describe can easily be plotted, measured and verified by any speaker or cable engineer. There is absolutely no doubt whatsoever about their existence or validity. In point of fact, properly implemented bi-wiring has benefits that can not be achieved by a single unsophisticated cable or even a single exotic cable.

Jon Risch

Bi-Wiring 101

In order to explain how bi-wiring works, it is necessary to explain a bit about how crossovers work. It will also be necessary to contemplate more than the usual voltage output of the crossover sections, so do not assume that if you know the basics for crossovers, that you will know what this will be all about.

Let's look at a simple two-way system with a first order crossover, the simplest crossover and system we can examine. It will be relevant to other more complex systems, so once you understand this one, the others will fall into place. We will not address the issues of tweeter level padding, response EQ, etc., just the basic crossover function itself.

In a simple first order crossover, there is an inductor in series with the woofer, and a capacitor in series with the tweeter. These two components comprise the crossover system. Normally, these two components are connected to the same input terminals on the speaker, in parallel. Hence this type of crossover topology is called a parallel type crossover.

A full range voltage signal is sent down a speaker cable, and appears at the single pair of input terminals. A current is drawn based on the input impedance of the speaker system as a whole, which in most cases, will have a relatively flat impedance curve once we get above the bass resonance region, where the impedance will be dominated by the cabinet design resonance's. If we say that (for purposes of this discussion) the overall impedance of the speaker in the midrange and on up is relatively flat, then a consistent amount of current will flow through the single speaker cable all across the audio band.

So there are several elements to the total circuit formed by the amp output terminals, the speaker cable, and the speaker system and crossover network. A signal appears at the amp terminals, represented by a voltage, the impedance of the speaker system causes it to draw an amount of current proportional to it's impedance for a given drive voltage, and this current flows through the speaker cable.

Now in order to examine what happens when we bi-wire, it will be necessary to go into some of the detail as to how a crossover "crosses over". If we look at just the woofer, and it's series inductor, the inductor provides little impediment to low frequencies traveling through the inductor, and a high amount of impediment to the higher frequencies. Looked at another way, the inductor impedes the highs but not the lows. If we examine an impedance curve of just the woofer with its series inductor, we would see that the impedance was pretty much just that of the woofer in the low frequencies, and would rise with frequency as the inductor impeded more and more of the highs. For this situation with just the woofer, for a given voltage drive level, a certain amount of current would be drawn at low frequencies, and this amount would decrease as the frequency went up, due to the rising impedance.

If all that was hooked up to the amp was the woofer and it's associated inductor, then the current flow in the single speaker cable would follow the impedance curve, a certain amount of current flow at low frequencies, tapering off at higher frequencies. Perhaps a glimmer of the true situation with bi-wiring is beginning to appear.

Now let's just look at the tweeter, and it's associated capacitor in series. At low frequencies, a capacitor tends to impede the flow of current, and at high frequencies, it provides little impediment. Hence, when we hook up just the tweeter and it's capacitor to the amp terminals through the single speaker cable, there is little current flow at low frequencies, and an increasing amount as the frequency goes up. At some higher frequency, the current draw is determined by the impedance of the tweeter alone.

Now just to make sure that it is understood, it is the current flow through a dynamic driver (one with a magnet and a voice coil) that causes it to move. A voltage applied that had no current capability would not cause any movement. This means that in order for the voltage at the amp terminals to cause a speaker to move, it must have a relatively low source impedance, so that when a given voltage appears at the amp output terminals, a given amount of current can flow into the load's impedance. That is why when the crossover components impede the current flow, they cause the output of the driver to drop off, hence the crossover function is achieved.

Note that the woofer and it's associated inductor, and the tweeter and it's associated capacitor will function independently, they roll-off the frequencies out of the driver's operating band without regard to whether or not the other half of the crossover is present or not. When both sections of the crossover are present, and connected in parallel, the overall impedance curve looks relatively flat, as when the tweeter section has it's impedance going up in the low frequencies, the woofer has it's impedance going down. At the crossover point they are more or less equal, and this is the point in frequency at which the impedance's of the two sections in parallel equal approximately half that of either section alone. This is how two 8 ohm drivers can be connected together through a crossover, and not equal a total load of 4 ohms.

By now, you should be getting the idea about bi-wiring. Instead of one speaker cable, or just one of the drivers and it's associated crossover component being connected to the amp's output terminals, two separate speaker cables are connected to the same amp output terminals and run to the now separated crossover sections. With different impedance's being presented across the audio band, each cable carries a different signal than a single speaker cable. The separate cable for the woofer carries mostly the LF currents, and the separate cable for the tweeter carries mostly the HF currents. This is due to the differing impedance's we discussed above.

Now if all you think of is the voltage at the amp terminals, and how the two cables are carrying the same voltage to the woofer and the tweeter sections, then it still may seem that the same signal is being delivered to the drivers as through one speaker cable. IF the speaker cables were perfect, and had zero impedance, infinite mass, and no digressions from ideal LCR behavior (DA, DF, hysterisis, etc.), then it may be that this would be the case. Since the cables we have available to us are not perfect, there are losses in the cables.

The $64,000,000 question is, how much does the real world speaker cable compromise the performance of a speaker compared to bi-wiring?

To make this easy to figure out, we will ignore the effects of inductance and secondary effects, and focus strictly on DCR effects. Let us assume that a cable sufficiently large enough to keep speaker system impedance variations from affecting the amplitude response by more than 0.1 dB was used, meeting the Krueger criteria. In many cases, this is a very large cable, usually at least a 14 gauge, and often 12 gauge OR LARGER.
For a copy of the Krueger criteria see: http://x42.deja.com/[ST_rn=ps]/getdoc.xp?AN=450322078&CONTEXT=927059192.1901920287&hitnum=6
(I should warn that I do not agree entirely with Arny's criteria, it completely ignores inductance, which typically gets worse as a ratio of DCR to HF impedance of the cable as the gauge gets smaller, or larger zip cords)

How quick do the gauge requirements add up? If you have only 10 foot cables, and a speaker with a minimum Z of 6 ohms, then a 14 gauge wire is necessary to prevent any more than a 0.1 dB amplitude variation due to the cable DCR. If the speaker Z minimum hits 3.7 ohms, you are now up to 12 gauge. Anything longer in terms of the speaker cable, or lower in terms of the Z, will require larger than 12 gauge to reduce the amplitude variations due to voltage drops to less than 0.1 dB.

For a single speaker cable, lets look at a simple signal containing only two frequencies: 100 Hz, and 6 kHz. (As a point of information, let's say that the crossover point is 3 kHz, a common crossover point for a 2-way system) In the single speaker cable that meets the 0.1 dB criteria, a lot of current will be drawn at 100 Hz to feed the woofers demands. This will cause a voltage drop due to the finite amount of resistance in the cable. While the loss of 0.1 dB of woofer output may not seem to be a problem, the current draw at 100 Hz will tend to modulate the 6 kHz signal. How much distortion will this generate? To cause a 0.1 dB change, the signal is being affected at levels of approximately -40 dB down from nominal. If the 6 kHz signal is modulated by the 100 Hz signal by that amount, the IM distortion would be on the order of 1%. These are ball park numbers, not intended to be absolute.

This is a distortion that would be reduced by the amount of LF current reduction in the tweeter cable, typically 26 to 36 dB lower in level. This would reduce the distortion from borderline audibility to very likely not audible.

If we were to look at the simple change in DCR from merely doubling up on the cable, then distortion would only go down 6 dB, from halving the DCR and nothing more.

Of course, once we start using real music, with more than just two frequencies, and real world cable situations that might have even more DCR, and the inductance differences between a single zip cord and two high performance speaker cables, the amount of distortions in a single speaker cable go up considerably, and the amount of reduction in distortion is increased for the bi-wire comparison. This means that we might be into 2% IM or more, and with multiple frequencies, which make it even worse sounding.

All of the above totally ignores any potential magnetic field interactions, many of which would be time delayed and would smear out transients and large signals. The magnetic field distortion reduction would come from the separation of the LF currents and the HF currents.
The time delayed and resonance associated signature would tend to make these distortions even more noticeable than the self-IM of the cable due to voltage drop.

I think it is easy to see that a multidriver system with higher order crossovers will react similarly to this very simple first order two-way system that has been analyzed.

It is interesting to note that higher order crossovers tend to have a similar input impedance for each section as a first order, and it is the output signal of the various sections of the crossover that are made to roll off steeper. In essence, the reductions in current for each cable in a bi-wire pair will be at a 6 dB/octave slope almost regardless of the crossover order.

Jon Risch

Last ditch explanation for those who still don't get it.

Assuming that you have read the above information, and have the beginings of an
understanding of how a crossover works, and how it divides the frequencies to the speakers, lets try this:

Traditionally, a crossover cirucit for the woofer, and a crossover circuit for the tweeter are hooked up together inside the speaker (wired in parallel, hence the term parallel crossover is used for this type of network) and one set of terminals are present on the outside of the speaker box. In a bi-wire capable speaker, these two crossover sections are electrically separated, and a separate pair of terminals made available on the back of the speaker, one pair for the woofer, and one pair for the tweeter.

At each one of these separate pair of terminals, the LOAD seen by the amp is
different: for the woofer, the majority of the LOAD is in the bass, with little loading in the treble for that separate circuit and driver. If you hooked up just one cable to the woofer terminals, then the current draw from the amp would be almost all in the bass region, with little or no current draw in the treble region. The woofer crossover is high impedance on the input to the woofer at high frequencies, and hence does not draw very much current. Compared to the traditional speaker system, with it's two crossover sections in parallel, this single cable to just the woofer section would only tend to draw current from the amp at low frequencies, while the full range crossover would draw current all across the audio band.

If we were to hook up a speaker cable to just the tweeter section on a bi-wired
speaker, then this connection would draw very little current in the bass, as the
tweeter crossover would be high impedance in the lows, while in the highs, where
the tweeter provides output, there is some current being drawn from the amp. Again, just the tweeter section of the crossover and the tweeter only tend to draw significant current in the high frequencies, and very little in the bass.

Now, if we hook the two sections back together AT THE SPEAKER, we have
essentially provided the traditional speaker/crossover hookup, and the single
speaker cable will once again carry current at all frequencies, not just the bass, or not just the treble.

HOWEVER, if we run a separate speaker wire to each of the bi-wire terminal pairs
at the speaker, each cable will now carry a different signal than a single cable, as the load at each pair of terminals, the woofer pair, and the tweeter pair, is different. The cable from the amp to the woofer will carry a lot of current in the bass, but hardly any in the treble region. This is a direct consequence of the way the crossover functions, and the fact that a dynamic speaker needs current to work.

Note that even if the cables were zero resistance, and zero inductance, etc., they would still carry different signals, due to the differing current draws vs. frequency.

This ties in with the diagrams at:
http://members.xoom.com/Jon_Risch/page9.htm

Which shows first the current draw of a single speaker cable, and then the current draw through a set of bi-wire cables connected to the same speaker, only bi-wired.

Because real world speaker cables do have some resistance, and do have some
inductance, they will exhibit voltage drops bassed on this current flow. The voltage differences would be quite small for low DCR/low inductance cables, but still present nonetheless. The variations in the voltage portion of the signal may be -40 db or -50 dB or more down, but this is not as low as one might think, and since the IM distortion that would arise due to bi-wiring is not at any single frequency, but will be occuring at many different frequencies. This raises the potential audibility of the total amount of IM that occurs, and makes it more likely to intrude into the musical presentation.

Again, see the web pages I reference.

There is no doubt that differing currents are flowing in the two cables of a bi-wire set-up, and since current flow sets up a magnetic field, and magnetic fields interact with current flows, the oportunity for IM and other deleterious interactions is present.

The IM part has been measured, and I present this on my web site.

Start at :
http://members.xoom.com/Jon_Risch/page7.htm

Perhaps this will all make more sense to you know.

Jon Risch