• Y-cables, looping audio signals through

    This article will explain loop-though connections and "y-cables" for analog audio signals.

    To obtain one or more copies of a signal (for example, to distribute the signal from a mixer to various self-powered speakers or power amplifiers) we use parallel connections. To do this we simply connect each terminal (pin) of the connectors in parallel. That is, 1 to 1, 2 to 2 and 3 to 3 (or tip to tip, ring to ring and sleeve to sleeve in a TRS connector). When we split a signal in two in this way, we refer to a "Y-cable" or "Y" connection, since the division of a signal in two looks like letter "y". Contrary to what it may seem, a y-cable is not a technically incorrect solution, but a correct way of splitting the audio signal. In fact, when a self-powered loudspeaker system or one channel of a power amplifier has an input connector and a loop-though connector, both are simply paralleled in the inside.

    In the following illustration we can see a schematic "Y" connection from a balanced XLR output to two balanced XLR inputs:



    In the following illustration we can see a connection of what would be a "Y-cable" with the previous connection:


    In the same way we can do it from a 1/4" connector balanced (TRS, tip-ring-sleeve) by wiring each of the terminals in parallel:


    This can be done with both balanced and unbalanced connectors. We continue to simply connect pin to pin. Below we can see what it would be like with a 1/4" unbalanced and RCA (Cinch, Phono) connector:



    To split (divide) the signal into more than one copy, we can use a "pigtail" in which we sequentially connect each input connector in parallel by wiring pin to pin to the next using a short piece of cable:


    Limits

    It is not possible to make an infinite number of parallel connections. The maximum number of times a signal can be divided will be determined by the output impedance of the mixer (or other device from which the signal is output) and the input impedance of the power amplifier (or other device to which the copies of the signals arrive). The most common recommendation to avoid frequency response degradation is that the total input impedance (Zin.total) should be kept at least ten times greater than the output impedance.

    Zin.total > 10*Zout

    The total input impedance is simply that of an input (Zin) divided by the number of copies we have taken of the signal (following the parallel impedance calculation, since we are doing nothing else than wiring inputs in parallel):

    Zin
    Zin.total = -----
    N

    And therefore the maximum number of divisions (Nmax) of signal is given by

    Zin
    Nmax = -----
    10*Zout

    For example, for a power amplifier with a 20k (20000) ohm input and a mixer with an output impedance of 150 ohms, the maximum number of signal splits would be:

    20000
    Nmax = ----- = 13'333
    10*150

    Therefore in this case we could split the signal up to 13 times. Mixers and other high-end devices usually have lower output impedances, so we can usually split the signal more times when using more professional equipment.

    Other parallels. Amplifiers and self-powered loudspeakers

    These rule of thumb for the number of copies that can be made of a signal is applicable to both Y-cables and pigtails, but also to signal splits that take place within the loop-through connectors of the equipment.

    In general, self-powered boxes have an input connector and an output connector (which can be labelled in various ways such as "output", "loop-through", "thru" or "link"), which are in most cases the same connection (though you might find a few exceptions where the output is active), as they are connected pin to pin. Therefore, when connecting (looping-through) one loudspeaker to another, what we are doing is connecting several inputs in parallel. In this case Nmax would correspond to the number of powered speakers.

    Although it is common for power amplifiers to have a "parallel" switch that internally sets a pair of inputs in parallel (multi-channel amps will have one switch for each pair of inputs), if this functionality is not available or we want to pass the signal to another unit, paralleling can also used with amplifiers. For example:


    • In a low-end stereo amplifier with only one input connector per channel, we can use a Y-cable if we want the two channels to share the same signal.
    • In a mid-end amplifier it is common to find one input connector and one output connector for each channel, or two input connectors with different connectors (for example an XLR and a 1/4" jack). In this case we will use a short cable to pass the signal from one channel to another. In the higher end amplifiers there is usually a "parallel" switch to split the input signal to the two channels of the amplifier, so an external cable is not needed.
    • We can also use a cable to link the signal from one power amplifier to another, in which case Nmax is the number of amplifiers. Similarly, amplifier racks usually have connection panels with loop-through connectors to pass the signal from one rack to another. In this case, the total number of amplifier channels in all racks should be taken into account, and this number is our Nmax.

    A warning should be made regarding splitting the signal with parallel connections. If a short circuit occurs in one connection, it will affect all the equipment connected in parallel. Therefore, although the probability of a short circuit is low, an alternative to linking many systems in parallel would be the use of a distribution amplifier.

    What about the other way around? Passive mixer

    The reverse of a Y-cable should not be used to mix two signals together, as there is an impedance mismatch at the output. If you need to combine several signals passively, the signal terminals must use a high valued load resistor (such as 1k Ohm, although the optimum value will depend on the specific equipment used), although it must be taken into account that as it is a passive sum, so level is lost.


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