Cables
This subject along with some others has to
be at the top of my top ten peeve list. In the past years there has been so much hype written and
sprouted by untrained mouths that it could fit in a 4 inch thick book. When
profits on traditional stereo components started going south, the snake oil and
magic suppliers had to find new avenues for revenue – and they surely found one
in cables. Interconnect cables, speaker cables and power cables. As the saying
goes. “There is always room for one more sucker” and boy there were and are
lots of those suckers running around. But there is hope and another saying goes
like this, “You can fool some of the people some of the time but you cannot
fool all the people all of the time”.
There is no doubt in my mind that there are
bad cables out there. Bit it is like an amplifier designer deliberately
designing a real bad sounding product. Why would a cable manufacturer go out
and make bad cables. Yes they do exist but the average audiophile should never
come into contact with these products.
The following discussions will be for audio
frequencies and so any frequency above 100KHz will be ignored. At last check my
hearing did not go near 100KHz!
Interconnect cables
are the first cables which the signal passes through from signal sources to
preamplifier (in the home stereo) and from head unit to processor or amplifier (in
the car stereo). We shall only discuss cables with RCA plugs at each end as
they are by far the predominant cable type in this category. As in any
interconnection, there are THREE main issues to consider. First the source’s
output impedance, secondly the cable’s electrical impedance and lastly the
receiving gear’s electrical input impedance.
Note: Please read my link on Balanced line as well
The source has a finite
output impedance and in the case of CD players, FM tuners, etc. this is in the
order of 10 ohms to hundreds of ohms. The lower this
source impedance the better. Most of these sources use op-amps as their
final output stage. I shall talk about op-amps often in the various tech talk
subjects and these are typically integrated circuits. They are in essence small
“power” amplifiers with microwatt or milliwatt capability. Most op-amps do not
like capacitive loading. All cables exhibit capacitance between adjacent
conductors and they present a capacitive load on the preamplifier output stage.
The simplest way to cure this is to insert a low value series resistor in the
output of the op-amp which isolates the output stage inside the op-amp from the
capacitor. The value is anywhere from a few ohms to hundreds of ohms, the
latter being more typical. So what we end up with is an output stage with a
series resistor in the “hot” leg and hanging on this is a shielded cable. Let
us examine the drawing below.
The op-amp of the source equipment is at
the left side. It has an output impedance of Zo and
in series we have added a 220 ohm resistor (typical) which isolates this op-amp
from the capacitive loading effects of the cable AND the input capacitance of
the receiving equipment. I have lumped these together
as “Cin”.
The shielded cable also has inductance.
This is determined by the area between the inner and outer cores and the
diameter of each core of the cable and is proportional to this. Typically
inductance in interconnects is very low indeed and can be neglected in our
audio discussions. Of course at very high frequencies it becomes a factor since
inductance (L) is proportional to frequency (F). The inductive reactance is XL
= 6.28 x F x L. (The 6.28 = 2 x Pi where
Pi is 3.14)
The shorter the cable, the lower are its
reactive components being inductance and capacitance. This stands to reason as
cable has xx micro-farads of capacitance per unit
length and yy Henries of inductance per unit length.
If we could make our cables less than say 6.35mm (0.25”) all would be well in
audio land since then the cable would be essentially out of the picture. Of
course this is impossible and so we must deal with practical cables.
Back to our op-amp and let us examine the
effect which the cable has on the performance. The series resistor which we
have manually inserted in the output of the op-amp (The Zo
of the op-amp is added to the 220 ohm we added) forms a low pass filter with
the lumped cable and destination equipment’s input capacitance. Zo depends on the amount of negative feedback which is
applied in the source. Let us assume it is 50 ohms which is typical for most
op-amps. We now have a total series impedance of 270 ohms. Cable capacitance in
either those high end “snake oil” types or the RCA interconnects which come
with the average VCR are about 150-195pF per metre or
50-60pF per foot maximum and typically are far lower. I have some cheap RCA-RCA
cable at the factory which has a capacitance value of only 8pF per foot.
Note: (0.001mfd = 1000pF)
In a home stereo environment interconnects
are relatively short, mostly less than 1 metre in
length and in automobiles the longest is maybe 5 metres.
Let’s do some calculations based on these two cases and also taking into
account the input capacitance of the destination equipment. These examples only
take the series output resistors of the source into account.
In the case of the 1 metre
cable, C-lump is 50pF + Cin (say 220pF) = 270pF
C-lump forms a potential divider with the
270 ohm resistors. The capacitive reactance of C-lump (C) is Xc = 1/6.28 x F x C.
At 1KHz Xc =
589,761 ohms and at 20KHz it is 29,488 ohms and at 100KHz it is 5,897 ohms.
So now using our formula to calculate Vout we arrive at the following. (Vout
= Xc/Xc+R)
1KHz
: Vout = 589761/589761+270 =
0.999 of the input which is 0.999 volt, a drop of 0.0086dB hardly audible I
would imagine.
20KHz:
Vout = 29488/29488+270 = 0.9909 of the input which is
0.9909 volt, a drop of 0.079dB again not audible unless you are bat!
100KHz:
Vout = 5897/5897+270 = 0.9562 of the input which is
0.388dB which is barely audible EXCEPT that our hearing does not go anywhere
close to 100KHz! (Unless you are one of those cable manufacturers who claim
that we can hear several octaves above 20KHz but then again they may be
relatives of the bat)
In the case of the 5 metre
cable, C-lump is 250pF + Cin (say 220pF) = 470pF
At 1KHz Xc =
338,799 ohms and at 20KHz it is 19,439 ohms and at 100KHz it is 3,388 ohms.
So now using our formula to calculate Vout we arrive at the following. (Vout
= Xc/Xc+R)
1KHz
: Vout = 338799/338799+270 =
0.999 of the input which is 0.999 volt, a drop of 0.0086dB hardly audible I
would imagine.
20KHz:
Vout = 19439/19439+270 = 0.986 of the input which is
0.986 volt, a drop of 0.112dB again not audible.
100KHz:
Vout = 3388/3388+270 = 0.926 of the input which is
0.66dB which is audible EXCEPT that our hearing does not go anywhere close to
100KHz! (Unless you are one of those cable manufacturers who claim that we can
hear several octaves above 20KHz but then again they may be relatives of
the bat).
If we lower the value of the resistor we
insert in the output lead of the preamplifier to say 50 ohms, you may redo all
the above calculations and the differences are not worth discussing. Also
reducing the value of the destination equipment’s input capacitor ** will also
not change the numbers by any significant amount. Do the mathematics for yourself.
Even if cable manufacturers eliminated nearly all capacitance, the input
capacitance of the destination equipment will be dominant. There are cables out
there that have very low capacitance of around 26pF per metre
(8pF per foot) and the results with these cables would be mathematically
different but not audible.
So what have we learned from the above
calculations. Yes we must use low capacitance cable. We must know what the
input capacitance of the equipment is and we must also know the total output impedance
of the source equipment. Knowing these parameters we can easily calculate the
dB drop at any frequency. Of course at lower than say 15KHz the drop is of
ABSOLUTELY no consequence.
Another point to consider is that the low
output impedance of the source will tend to discharge the cable/equipment
capacitance very quickly so this notion of the charge held by the cable is
nonsense.
Almost all manufacturers DO NOT specify the
input impedance of their equipment in a 20Hz-20KHz
bandwidth. If they did it would be relatively simple to work out the
capacitance at the input. The amplifiers which Zed Audio produces have low pass
6dB/octave filters at the RCA input. The 3dB point of these filters is set at
339KHz. The reason for these filters is to prevent very high frequencies from
entering the input stage of the preamplifier.
The DC resistance of these cables is very
low and even if it was 1 Ohm per metre this 1 Ohm is
not even a factor compared to the series output impedance of the final stage of
a preamplifier and then any added resistance completely swamps any cable
resistance. This is easily confirmed by anyone with access to an Ohmmeter. Just
measure the resistance from RCA tip to RCA tip of your favourite
5 metre cable.
As I stated above, the inductance of
interconnect cables is very low. A reasonable quality interconnect should have
inductance of about 0.48 micro Henry per metre (0.15
micro Henry per foot) What does this inductance do to our sound? Let’s examine
the following diagram.
What we have is the lumped inductance of
the 5 metre cable shown as “Lx” and it’s value is 2.4 micro Henry. Typical cable inductance is about 0.48 micro
Henry per metre (0.15 micro Henry per foot). The
inductive reactance of a 2.4 micro Henry inductor is XL = 6.28 x F x L
As we see from the formula, the higher the
frequency, the higher the value of XL. This value of XL will add to the value
of the two resistances, Zo and Rx. Three of these
will form a low pass filter with C-lump. So the total series impedance is Zo +Rx + Lx. Lets us calculate this at three frequencies
again.
1KHz:
XL = 6.28 x 1000 x 2.4/1,000,000 = 0.015 ohms. Not worth considering since Zo and Rx are 270 ohms!
20KHz:
XL = 6.28 x 20000 x 2.4/1,000,000 = 0.3 ohms. Not worth considering since Zo and Rx are 270 ohms!
100KHz:
XL = 6.28 x 100000 x 2.4/1,000,000 = 1.5 ohms. Not worth considering since Zo and Rx are 270 ohms!
So the inductance of a 5 metre cable is totally negligible and certainly on a 1 metre cable it is even less of a concern.
The material of the cable is some what
important. The cable should be flexible, so multi-strand cable is generally
used. The insulation should be of good quality and the RCA jacks at each end
should be durable and not clinch the female RCA sockets on the equipment. All
the mumbo jumbo about OFC wire, silver wire, ten
thousand stranded wire is nonsense. The reactive properties of these cables are
affected to such a small degree by the wire material used for their
construction that it is hard to believe that they contribute anything. I am not
saying for one moment that one should use interconnects made from slivers of
iron but simply that if reasonable care is taken in the manufacture of cables
they will not affect the sound. There has never been any proof that the
dielectric properties of the insulating materials affect the sound in any way.
Note: A very
easy way to compare two RCA cables is to use a high quality signal source such
as a CD player (A high quality turntable with an outboard phono
preamplifier is even better). Use a Y-adaptor and connect the two sample cables
to the Y-adaptor and then one cable to any high level input on your home
preamplifier and the other to the tape monitor loop. The Y-adaptor cannot
impart any change to the sound – I do not believe it. Also the tape monitor
switch contacts are in circuit whether the switch is in “source” or “monitor”
positions. Now sit in your favourite armchair with
your favourite music playing and ask someone to
alternately flip the tape monitor switch to either position at your command.
You of course cannot know which position it is in at any given time. There will
be NO difference in the sound with any pair of test cables you choose. I have
tried this with cheap VCR cable and $900/foot cable – no difference whatsoever.
Rather use acceptably good cables and use your hard earned money for something
else. Nearly all who buy useless expensive additions to their sound systems (be
it car or home) have to “hear” a difference in order to justify their expensive
toy.
Skin effect is often talked about. The
electrons tend to flow on the outside surface of the wire as the frequency
increases. The following equation approximates the difference between the DC
resistance and the AC resistance of a strand of cable.
Res
AC = Res DC x n x square root of freq (MHz) and n is
the wire gauge factor. For typical interconnects it is about 7.
So at a frequency of 1MHz, Resistance at AC = 10 x 7 x 1 = 70
ohms. Assume that we have a high DC resistance cable of 10 ohms. The 70 ohms
added to all our above equations does not change the answers significantly
especially that I have just done this at 1MHz. This equation is not 100%
accurate for lower frequencies but is a good enough indicator. Plug in say
50KHz to the above equation and we get 3.5 ohms of AC resistance. This is of
course lower than the DC resistance but what is clear is that at all audio
frequencies, skin effect has NO effect on the sound.
For
some useful information on cables go to this link
http://www.audioholics.com/techtips/buyingguides/interconnects/cable_budget.php
One cable manufacturer says
the following and I quote
·
it has to be high quality, solid core silver
·
diameter must not exceed 0.5mm (skin effect and its
implications)
·
insulation has to be as thin as possible (dielectric
absorption, electrostatic micro-discharges)
·
insulation has to be as natural as possible
(unbleached cotton)
·
connectors have to use as small as possible quantities
of metal
·
sensitivity to vibrations
What a lot of rubbish if ever I have heard.
Another respected home
amplifier manufacturer states the following and I concur 100%
The main implications are that the cable used should have
reasonably low values for its capacitance and
When the load impedance is much higher than the impedance of the source and the
characteristic impedance of the cable the signal current is likely to be
relatively small. Since this is true in most domestic systems it seems
reasonable to expect that effects due to interconnect co-ax inductance, series
resistance, and internal impedance should be very small, and it is questionable
whether they are audible. Given its shielding properties co-axial cable seems a
good choice for interconnect provided that we follow the general implications
drawn above.
Speaker Cables
are the next on our list. They are different from interconnects because of the
source impedance and the load impedance. The same situation exists as far as
cable resistance, capacitance and inductance is concerned. In the case of
speaker cables driving conventional dynamic speakers the capacitance of the
cable is of very little concern** but the DC resistance and inductance are.
**Some power amplifiers behave adversely
when loaded with significant capacitance.
The DC resistance of the cable should be an
order of magnitude lower than the load impedance simply because we do not want
any volt drop across the cable. How do we obtain low DCR? Simply use heavy
gauge wire. The table below shows the resistance of #10 wire
using various metals.
Material Resistance in Ohms/metre
Silver 0.003068
Copper 0.003246
Gold 0.003705
Aluminium 0.00533
Brass 0.013
Iron 0.0188
Platinum 0.0188
Lead 0.0412
The table shows that silver and copper are
so close to each other that the difference is negligible but the cost of silver
is a lot higher than copper. In a car system the longest speaker cable length
is maybe 5 metres. So if we use #10 copper wire the resistance is 0.03246 ohms per leg for a total
resistance of 0.0649 ohms. Let as assume we are using a 500 watt amplifier to
drive a 4 ohm speaker. The RMS current is 11.18 amps and the peak is 15.8 amps.
Using Ohm’s Law the volt drop over the length of this cable is 0.0649 x 11.18 =
0.71552 volts RMS or 1.02 volts peak. Assuming the amplifier was playing at
maximum power (an impossibility of course since we cannot average 500 watts
from a 500 watt amplifier but let us be ridiculous and say we can) then the
volt drop of about 1 volt as compared to 44.72v (500w with 4 ohms) is a drop of
13 watts or 1.22dB. Of course this does not occur in practice and a realistic
number due to a crest factor of 10dB is an average of 50 watts per channel.
The RMS current is 3.5 amps and peak is 5
amps. So the volt drop is 5 x 0.0649 = 0.32 volts. Not a number to get excited
about!
The capacitance of speaker cable is
normally low and can be ignored BUT there are some of these fancy cables that
have high capacitance and this can cause problems with amplifiers which are on
the borderline of stability. Typically using say #8 wire
for speaker wire lengths of less than 6 metres (20
feet) will not cause any problems for 99% of amplifiers.
The inductance of speaker cable can play a
part in affecting the sound. Again common sense should rule. Keeping the
inductance low is not a problem in car systems as the speaker runs are very
short. Home systems are a little more complex. Let’s look at a few examples.
The inductance of twin speaker cable is
given by this formula
L= 0.913 x log (w/r) micro Henries per metre or L= 0.281 x log (w/r) micro Henries per foot where
“w” is spacing between cable centers and “r” is the radius of each conductor.
Typically the ratio “w/r” is about 3 with
average #10 or #12 wire. Solving for “L” in the above
formula we get 0.435 micro Henries per metre or 0.134
micro Henries per foot. OK this looks good and now we can apply this to say a 5
metre (15’) run of cable. The answer is 2.175 micro Henry. What does this mean to the sound and how
can we measure it? The circuit below shows what happens.
The inductance is in
series with the speaker, and let us assume it is a 4 ohm speaker which
remains at 4 ohms from 20-20KHz (not true as the impedance will rise near
20KHz). The inductive reactance is XL = 6.28 x F x L where F is frequency and L
is in Henries so 6.28 x 20000 x 0.000002175 = 0.273 ohms. So what we have at
20KHz is a series impedance of 0.273 ohms with our 4 ohm speaker and the
attenuation is -0.573dB. Vout is 9.36 volts. This
0.573dB loss is not typical of the real world since the impedance of the
tweeter will rise to almost 10 Ohms at 20KHz and so the attenuation falls to
-0.23dB. At 20KHz we would be hard pressed to hear a 0.5dB drop. If we move our
head off axis from the tweeter by a say 20-30 degrees the dB drop is far
greater. I challenge anyone to drive their car and keep their head still and on
axis with the front tweeters. (Do not attempt this as it is dangerous)
The ratio of “w/r” should be kept as small
as possible to keep the inductance of the cable short. At lower frequencies the
inductance has less effect since the inductive reactance reduces with lower
frequencies. At 1KHz it would be a twentieth of the above at 0.013 ohms.
Dielectric absorbtion
is also not worth worrying about because at ALL audio frequencies the worst
dielectric which is PVC has shunt impedances of many millions of ohms. This
parallel impedance cannot affect a 4-8 ohm speaker.
Skin Effect is caused by the self
inductance of the wire. This causes the inductive reactance to rise at higher frequencies
and electrons are forced to the surface. The circumference of the wire is
therefore preferred at these higher frequencies and so the net resistance of
the conductor is increased. The center core of the wire is not used. At 20KHz
the losses with a 4 ohm speaker are less than 0.01dB with a 3 metre (10’) cable. I hardly think that this is worth
worrying about. I quote from a renowned source.
“Some so called "exotic" Cable
Companies enjoy spreading the fallacy that Skin Effect can cause deleterious
effects on your audio performance. While Skin Effect is a real world problem in
high frequency applications such as RF Power and Transmission, it is negligible
at audio frequencies as I will demonstrate in this article based on fundamental
engineering and scientific principles. “
EMI and RFI are not a problem with speaker
cables since the impedances are so low that it is very difficult for these
types of signals to enter the cable. I of course assume that your speaker run
is not 3Km (1.8 miles) long which may act as an antenna.
One of the cutest products I
have seen are cable lifters. Yes you read this
correctly – cable lifters. These are little stands which are used at maybe 1 metre intervals to lift the speaker cables a few cm off the
floor. So, several must be used on average length speaker runs. They claim that
by lifting the cable off the floor affects the sound. They are about $20-$30
each! Of course they do not affect the sound BUT after spending a few hundred
bucks on this junk you better hear some difference @#$%. You could of course
just use small pieces of 2x4 wood blocks and save your money.
Power cables both
for 12 volt and 120/230v 50/60Hz. Don’t you love
those 12 volt #2 gauge power cables with the little arrows printed on the
insulation which indicate that the cable must be connected with a certain
“polarity”. What is amazing that there are some out these who ACTUALLY BELIEVE
THIS JUNK! The cable which delivers current to your amplifiers has one mission
in life, to deliver the current without losses. It has NO knowledge of which
way it is connected (Electrons are dumb and do not care either) This means one
thing and one thing only. Use the largest gauge wire possible. This cable
carries DC and so there is no skin effect, dielectric absorption, inductance
issues, capacitance issues. In fact if the cable is inductive (which it is in
fact because every piece of wire no matter how short is inductive) it is
advantageous since the inductance helps to reject alternator noise.
The connections at each end should be of
high quality and the connectors should be of robust construction. Contact
resistance is important to avoid losses and again common sense should prevail
here. Zed Audio does not recommend the use of power distribution blocks when
using multiple amplifiers. The reason is that when an amplifier draws current
it causes voltage spikes to be induced in the power lead. These can interfere
with other amplifiers. The capacitors used in every amplifier across the power
input are not large enough to dampen these spikes. The battery being an
enormous battery has the capacitance to do this. We advocate the use of
separate power cable for each amplifier.
AC power cables for the home stereo have to
be the biggest con which cable manufacturers and suppliers have pulled on the
unsuspecting public. The claims of how “the noise floor of your home system is
reduced” and “the sound seems to come out of total blackness” is their favourite. “The mids and highs
seem to be so much more transparent than before once I substituted my power
cables with cable XXX”. Only a fool can believe such silly comments.
I have a catalog in front of me and I see
real hot specials. How does $175 for a power cord sound? Or even $649? No way
those are cheap… let us look at one for $1,995.00 yeah nearly two grand for a
dumb power cable that this company claims will almost cure the common cold if
asked to do so! If $1,995 is too much they have a slightly less expensive model
at $998.00 – wow what a bargain. There
is another company offering what they call the “Magic Power Cord” at a steal of
a price of $1,499.99 for 1.8 metres (5.85 feet). The
reporter who “tested” the cable said “The first thing I notice is the
presentation becomes BIG, stage width, depth and the space between instruments
all seem larger, images are more solidly located in space and don’t wonder
about” I think this dude has been
wondering about himself. It performs magic on your wallet and that is about all
it does. It lifts the Dollar bills right out of your back pocket without you
being aware.
A long with the power cable rip off artists
are the guys who sell AC outlets. They paint them a new colour
and claim all wild and wonderful things for “their” scientifically designed AC
outlet. Or what about those hospital grade types which cost almost $50 per
pair! The same company sells a power strip for $199 for 6 outlets,
that is $33 per outlet!
I think that maybe these power cables at
two grand and the hospital grade outlets with nice colours
and silkscreened logos would do better if we could
rewire our home with solid 99% pure silver wire, replace the circuit breaker
board with gold plated contactors and fuses and then the ultimate demand of
your city that they run the same silver wire from YOUR home to the substation
and then from the substation to the main distribution grid. Wow what a deal and
then the music will jump right out at you. PS I just woke up from my dream.
One simple question one should ask. The
power to your home comes from a transformer some many kilometers away in some
cases. It enters your home and a distribution board with many circuit breakers
distributes the power throughout your home. NOW HOW CAN THE ADDITION OF MAYBE 2
METERES (6’) OF CABLE AFFECT THE SOUND? The electricity has traveled a
torturous route to get to you and now it encounters a magical piece of cable
and all of a sudden all the ills of your stereo are cured. I say, you must
believe this if you have spent a stupid amount of money on your new power cable
because how would you justify this expense to your family and friends! Reading
the hype which these snake oil merchants put out makes me sick, but if these
kinds of things ring the bells of some people, be my guest.
I am a firm believer in direct A-B blind
listening tests. The brain cannot remember the differences (if any) between two
pieces of gear when the time between the listening experiences is more than a
few seconds and even then this may be too long. Immediate comparison is the
ONLY way to find out if one can detect any difference. Those who say “I took
out the standard power cord and replaced it with brand XXX and wow the sound
was transformed” are simply lying. The time interval to substitute one power
cord, interconnect or speaker cable with another is just too long to allow the
test to have any meaning whatsoever. To compare cables is a tedious process. A
multi-pole changeover switch is required or a switch with multi-pole relays. I
did this once with the power cable to my preamplifier. I borrowed a “snake oil”
type IEC power cord and compared it to a standard off the shelf #18 version. I
had a friend do the switching and I could not detect any difference. He
listened and he could tell no difference.
The brains of those who spend
money on these magical wires, cable, etc. have to “hear” a difference because
the shame attached to not hearing any would be too much too bare.
It is sort of similar to those claims made
by the companies who advocate a 600 pound block of exotic rock taken from the
frozen wastes of
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