Intro
Contents
Mutec is well known to Alpha-Audio. We have tested various Mutec black boxes such as a USB reclocker and various reference master clocks. The fact that all the editors have purchased one or more Mutec products speaks for itself. During the High End show in Munich, we saw a new series of Mutec digital cables. The coaxial Mutec PSC 75 is on the review bench.
BNC, coax, 50 Ohm, 75 Ohm?
First some basic information about these types of cables.
Coaxial cables were originally made to transport radio signals over long distances. In 1929, Lloyd Espenscheid and Herman Affel (who doesn’t know them?) worked for the American telecom company Bell Labs on a way to realise a 4 MHz signal for long-distance telephony. We quote parts of an article on the Digikey platform here:
“The two researchers analysed the trade-offs between the most important transmission line parameters of attenuation, voltage value and power
Their analysis:
1: Signal loss is largely a function of the dielectric in the cable. In an air-filled coaxial cable, the lowest loss was at around 77 Ω.
2: The voltage maximum is a function of the intensity of the electrical field between the coaxial outer conductor and the inner conductor. For coaxial cables that support RF signals in the TE10 electromagnetic (EM) field waveguide mode, the e-field has its maximum around 60 Ω.
3: The power is determined by the limit value and the impedance (V2/Z). For air-filled coaxial cables that operate below the TE11 frequency, the power transfer is at its maximum at approximately 30 Ω.
As with most technical standards, there is no ‘ideal’ impedance value; the ‘best’ choice consists of weighing up the compromises. The value of 50 Ω is a good compromise for power and voltage, such as the output of a transmitter. On the other hand, 75 Ω is a better choice for situations in which low attenuation is the primary goal, such as for low-level signals from an antenna or an analogue video connection.
Using different impedances realises different objectives in one design and adds another level of complexity. In practice, the difference in loss over a short distance of a few centimetres can be negligible. Furthermore, the voltage standing wave ratio (VSWR) when connecting a 75 Ω cable to a 50 Ω cable is 1.5:1, which can be an acceptable non-unique value (in many low or medium power situations, a VSWR of less than 2:1 is considered acceptable).
Long story short: 75 ohms provides an optimum with the least signal loss, an optimal ratio between power and voltage. The standard impedance for digital audio cables is therefore 75 ohms. The terminals of coaxial cables are equipped with BNC connectors, a type of bayonet fitting that also dates back a long time, to radio equipment used in World War II. This connection method not only guarantees that the cable will remain firmly attached, it is also the best way to feed the copper shielding sheath in the cable into the device. If you have cable internet, you will see that the cable input on the modem is also equipped with a BNC connector.
Why a coaxial cable?
It will not have escaped your attention that the CD has also begun a revival. Especially now that collecting vinyl is becoming an increasingly expensive hobby and CDs can be bought for less than a euro in record stores, at flea markets and in thrift stores. Price is certainly not the only reason. We have listened with amazement and admiration to CD players in different price segments; the ATC CD2 or the Denon DCD A-110. If you have a good DA converter at home, the CD player can be connected to it with a coaxial cable, because most CD players have one or more digital outputs. Your player then becomes a CD drive.
Alternatively, you may have a good CD player from the 90s. Digital to analogue conversion from that era is on a different level than today, so it may be worthwhile to use the player as a drive here as well. We did this in a livestream video about power conditioners with our Rega Planet CD player, connected to the Weiss DAC2 via coax. Listen for yourself to how good and coherent it sounds.
If you work with a master clock or reclocker, you will need coaxial cables. Clock signals are extremely sensitive, so BNC terminals are a must.
Mutec PSC series
All cables are 75 Ohm and are available in three variants: RCA/RCA, RCA/BNC and BNC/BNC. Mutec uses triple shielding and the cable feels very solid and heavy. The workmanship and quality of the terminals is also sublime; Neutrik plugs are used for the RCA side and the BNC side is from Damar & Hagen. The cable is available in various lengths, up to 2 metres.