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Old 13th December 2011, 10:59 PM   #11
jez is offline jez  United Kingdom
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Quote:
Originally Posted by DF96 View Post
RG59 is 75ohms characteristic impedance at RF and somewhat lower frequencies. Its distributed shunt capacitance will be around 21pF/ft whatever termination it has - the capacitance is a property of the cable, not the termination. Similarly, it also has a fixed distributed series inductance.

It will not be 75ohms at most audio frequencies, because at low frequencies the series resistance dominates over the inductive reactance so the characteristic impedance rises and become reactive (capacitive, I think). Fortunately this does not matter unless the cable is very long - much longer than a typical interconnect. All that matters is that the total capacitance does not load the source impedance too much. The terminating resistance can be quite high. Regarding an audio cable as though it is an RF cable is a common mistake made by audio people.

So I guess the quick answer is that RG59 is not a 75ohm cable at audio frequencies, but that does not matter as impedance matching is not required.
Yes! I couldn't be bothered to say all that but you have done it most succinctly It is not a transmission line at audio frequencies
Mind you, I was only stirring up the "cables wound on a full moon with blessed silk insulation" mob in the first place
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Old 13th December 2011, 11:52 PM   #12
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One more time to make clear:

Quote:
I stand against that and i base my argument on that RG 59 cables meet specification to applications that are terminated at 75 Ohms generally. If cable is terminated to 22 to 100k input imp. which can be the input of one amplifier is way out of specification and behavior of the cable is expected to be unpredictable
This is not true, at all, at audio frequencies, in fact at any frequency where cable length is less than 1/10th wavelength even by the most utterly conservative measures. (eg 30m length at 1Mhz, though 200m would be more normal)
And even then.. the results can be calculated , precisely.

Meanwhile the total lumped capacitance presented by the cable only matters (creates a passive roll-off) when compared with the output impedance of the preamp/line driver driving it. The imput impedance of the supplied equipment e.g. '22k to 100k input' etc simply does not matter, or figure in any roll-off calculation.

Last edited by martin clark; 13th December 2011 at 11:56 PM.
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Old 13th December 2011, 11:59 PM   #13
ilimzn is offline ilimzn  Croatia
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@Sakis,

Your specific use of the cable actually points to the reason why it was inadequate. A guitar pickup is altogether different compared to a standard source component, let alone something like a preamp or CDP. As an example, remember how MM cartridges need to see a specific value of parallel capacitance to preform properly - a pickup behaves, in some manner, like a very large MM cartridge, so the capacitance of the cable makes most of it's effect be felt much lower in the audio band (on a MM cartridge, it would be up in the top octave, or perhaps octave and a half). Also, the input of a guitar amp is not at all a typical audio amplifier or preamplifier input, regarding impedance and especially gain.

RF cable capacitance may be an issue, even in low impedance source situations, in fact, in some cases the low impedance may be the main culprit of 'unexpected' performance problems - think driving relatively high capacitances directly with an OPamp (especially a fast one). It's not too uncommon for people to 'forget' a series resistor on the output of a very low impedance amplifier with NFB, let alone a complete zobel/boucherot (sp?) cell, which puts some 200pF of capacitance from 3m of such RF cable directly to the output of the amp. Inductance also comes as standard, and you have an undamped resonant circuit. Even worse if your amp output starts looking like an inductor at HF.
Another possibility are tube circuits with relatively high output impedances, where the cable will form a low pass filter. That being said, someone might well find such a cable a blessing because it will filter out stuff at the end of the audio band, normally associated with listening fatigue.

Re microphony, it's unlikely to be piezoelectric in nature, and more likely to be there because of capacitance modulation due to geometry changes when the cable is tapped. (Try it with large coupling capacitors... espacially in tube circuits!). This is especially common when there is DC voltage on the cable. In the steady state there is a finite charge stored in the cable capacitance. Since the geometry of the cable changes when it's tapped, the capacitance changes, but the impedances at both ends of the cable prevent the stored charge from being changed instantly, and because V=C/Q, if C changes due to mechanical issues, and Q is constant, the voltage will change. Normally, this will be a very small change because the impedances at the ends of the cable form a very short time constant with the cable capacitance, but if the gain of the amp where the cable connects is high, you will hear a form of microphony. In fact, exactly the same form used by capacitance cell microphones. This can actuallybe a serious problem in cases where you need to get signals from high impedance sensors that need DC voltage through a long cable.
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Old 14th December 2011, 08:18 AM   #14
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ok ... I thought so that the effect on the Guitar for some imp.matching reasons was working like a mike ....still though i believe that this type of unpredictable behavior will apply also to line level signal cables but probably in lower magnitude....

Cavel is a company that manufacture mostly RG cables ...here is what they think ....

Dear Sir,

first of all, we have no experience in audio cables or audio applications
of our cables. Our customers are mainly TV installers which use standard 75
Ohms cables for high frequency applications (470-860 MHz DTTV or 950-2150
satellite TV or 200-300 MHz CATV). Anyway, you can use a TV cable to carry
low frequency signals but, as you tell right, the problem is in wrong
fitting between the high impedance of the cable (at 50 kHz the real
impedance of a coaxial is also higher than 75 Ohm, it is around 85/90 Ohm)
and the low impedance of audio devices. At both ends of the cable, in
proximity of the connections, you will experiment a signal loss due to this
unfitting. The higher the impedance gap, the higher the signal drop (the
signal will be reflected back due to differences in impedance). The
reflected back can cause damages to final tubes or transistors of
amplifiers. This is what happens, for example, in RF transmitters in
presence of a maladjustment.

Furthermore I think RG cables are in any case not the best choice as the
screening against outside interfering signals is very poor (RG have only a
braid of copper wires, no tape to ensure 100% screening coverage). Our
RP913B or DG113 are by far more performing (compare the screening
attenuation graphs in our web site). I think that also in audio application
the protection against RF pollution is important, especially in today's life
which is full of electronic devices mutually interfering.

Feel free to contact me for further info.

Best wishes for an Happy Christmas and a Better New Year.

Cordiali saluti/Best regards

dott. Alberto Scardovi
CAVEL - Laboratory & Quality Manager
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Old 14th December 2011, 08:38 AM   #15
qusp is offline qusp  Australia
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surely the added foil in the higher performance cable he mentions would make the capacitance higher? actually coax of high quality low capacitance tends to make a pretty damn good analogue interconnect and just because its line level analogue does not mean that termination is of no benefit, just look at Erno Borbely's designs, particularly preamps; they often use BNC and terminated lines

you'll find that they are generally much lower in capacitance than the average RCA or phono cable
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Old 14th December 2011, 08:53 AM   #16
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Quote:
Originally Posted by qusp View Post
you'll find that they are generally much lower in capacitance than the average RCA or phono cable
Exactly. And at the end of the day, that's what matters - along with the typically better/more effective shielding and build quality these cables feature (since they're built for more demanding applications).
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Old 14th December 2011, 09:34 AM   #17
euro21 is offline euro21  Hungary
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Belden 89259 (RG59, teflon -foam FEP- insulation and outer FEP jacket) has only 17pF/feet (~57pf/m) capacitance.
http://www.belden.com/techdatas/metric/89259.pdf

Ideal for first time DIY-ers:
Belden 89259 DIY RCA Interconnect Cables

Jon Risch's twisted core interconnects (Belden 89259/89248, or 1506a cores) are better (~6-7pf/feet).
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Old 14th December 2011, 10:22 AM   #18
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Also BlueJeanCables traditionally recommend coaxial cable for line level interconnections.

Stereo Cables at Blue Jeans Cable

They even make their own, LC1 (12.2 pF/ft):

LC-1 Audio Cable Design Notes -- Blue Jeans Cable

Quote:
These two attributes, shielding and capacitance, are the most important factors in unbalanced audio cable quality. Shielding is important, of course, because it keeps out externally-induced noise, and because unbalanced audio, unlike balanced audio, can't take advantage of common-mode noise rejection. Capacitance contributes to high-frequency rolloff, so the lower the capacitance of the cable, the flatter the frequency response in any given application (how flat will depend on the device impedances as well as the capacitance, so it's not possible to generate a one-size-fits-all frequency response chart; but in every case, the lower the capacitance, the flatter that curve will be). Capacitance, like many cable attributes, is a per-foot characteristic; while high capacitance won't ordinarily make a significant difference in short runs, it becomes an increasing problem with longer runs.

When we entered the cable business, the best cables we could find for unbalanced audio use were cables which had actually been designed and built for video. Why? Well, that's because video cables are coaxial, which is the right geometry for an unbalanced audio cable; because video cables are typically well shielded; and because video cables, being designed to a 75 ohm characteristic impedance, are relatively low in capacitance, ranging from about 16 pF/ft for an HDPE-foamed dielectric precision video cable (e.g. Belden 1694A) up to about 21 pF/ft for a solid PE dielectric cable (e.g. Belden 8281 or Canare LV-77S).
So does Van Den Hul, who's range of line level cables has many coaxials (50-60-75 Ohm ones).. An example of 75Ohm coaxials sold for line or line+digital applications:

Van den Hul: The D - 300 III HYBRID (Halogen Free)
Van den Hul: D - 310 Mk II (Halogen Free)

Van den Hul: MC - SILVER IT Mk II 75 Ohm (PVC Free)
Quote:
We therefore have decided to introduce our MC - SILVER IT Mk II 75 Ohm:
This cable is specially adapted to maximum quality 75 Ohm signal transport as found in SPDIF digital audio (CD to DAC) and radio / TV / video (including home theater) applications. However, since the design of this cable only slightly differs from our (now discontinued) MC - SILVER IT Mk II Coaxial and employs the same high grade materials, the MC - SILVER IT Mk II 75 Ohm is equally well suited to be used in high quality audio applications as a single-ended (unbalanced) interlink as our MC - SILVER IT Mk II Coaxial is.
Also available with RCA type connectors, The MC - SILVER IT Mk II 75 Ohm now fully replaces our discontinued MC - SILVER IT Mk II Coaxial in high quality audio applications as a single-ended (unbalanced) interlink.
On the same subject:
Coaxial cable - a 75 Ohm example
Co-axial cable - effect of characteristic impedance
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Old 14th December 2011, 10:35 AM   #19
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by sakis View Post
hmmm are you sure about that?

Rg 59 presents a capacitance of 21pf per feet ...would you like to know what exactly will do a capacitor of 100pf in the input of on amplifier that features impedance of 100K ?
a cable has no sound of it's own.
The effect that a cable can have on the source is predictable.
The effect a cable can have on the load is predictable.

Both the source and the load can and will misbehave when they are terminated by inappropriate cables. The misbehaving electronics either side of the cable do have a sound of their own.
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Last edited by AndrewT; 14th December 2011 at 10:46 AM.
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Old 14th December 2011, 11:08 AM   #20
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Default Audio Co-ax

Standard RG59 meets an RF impedance requirement but is not completely screened against RF like RG6, as often specified for digital broadcast reception. Capacitance is determined by diameter as much as screening cover, so it is a matter of size and dielectric, once you have selected the quality and RG59 is the cheapest, with some poor grades only having stranded shielding with 50% or less coverage.

This quality or Ethernet RG61 may have low capacitance and look cool with BNC or N connectors, but it will be poor as audio screened cable. If specific capacitance is your concern, cut the length to your required value. It is not so difficult if you have a convenient spacing of equipment.

By comparison, pro audio leads like Belden are designed to have maximum flexiblity and cover, so the screening has to be very special and noise-free as it is dragged around in use. Simply using RF cables because they look big and are cheap and available everywhere is not really a good idea IMO.

I don't say RF cables can't work well, just that they are not best suited to reliable audio applications. Anyone playing electric guitar will tell you what are the best types of cable for that application.
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