What effect does a splitting an audio signal using a y-cable have on that audio signal?

Question

Given a clean instrument-level audio signal of reasonable amplitude passing from point A to point B via a TR/unbalanced cable, what effect would replacing that cable with a TR to dual TR cable running from point A to points B and C have on the signal reaching point B? Would it result in a degraded signal, either in terms of amplitude or signal quality? The purpose of the cable is to take a mono signal and duplicate it into two identical mono signals.

Assume both cables are of the same length.

For a more concrete version, assume that A is a guitar and that B and C are unbalanced stereo input jacks on a guitar pedal with these specifications of which I believe the important one is:

• Input Impedance: 2.2 M ohms

Answer 1

For a more concrete version, assume that A is a guitar and that B and C are unbalanced stereo input jacks on a guitar pedal. Input Impedance: 2.2 M ohms

Guitar pick-ups are problematic: -

Image from here.

With an inductance of maybe up to 8 henries, at 10 kHz, that's an impedance of just over 502 kΩ and, this is why we use several MΩ input impedances on guitar pre-amp circuits. We do this to ensure that the high frequencies do not get attenuated: -

So, as you should be able to see we aim to design a guitar pre-amp with an input impedance of at least 1 MΩ and, in many cases on better equipment this is around 10 MΩ. There ain't a guitarist I know of that particularly wants a dull sound to their axe.

But, cable capacitance can also be a problem: -

With a higher input impedance (3 MΩ) there is a distinct peaking effect seen that, to some guitarists is desirable. The above mimics a 3 metre long cable having a capacitance of 50 pF per metre. Personally, as a guitarist, I aim for short cables but, that only works when I'm sat down right next to my mixer and audio capture circuits; it doesn't work on-stage of course!!

_{I added 100 kΩ series resistance to the pick-up coil to avoid massive and non-realistic peaking. This isn't a cheat because the 1 MΩ pots used in a guitar (for volume and tone) will inevitably have some output resistance of that sort of order.}

So, a 2.2 MΩ input impedance is good but, depending on how long the cables are, you might get a less-than-ideal-or-undesirable response.

Answer 2

Provided that the source impedance is low (should typically be less than 100 ohms) and the destination impedances are high (typically > 10k), no problem.

To be precise, check the three impedances, calculate the effective detination impedance (parallel equivalent of the two destinations). Rule of thumb is that the parallel imepdance should be greater than 10x the source.

If you are lower than this, you typically lose a bit of amplitude, but no big problem. In extreme cases you might cause the source amplifier to start clipping or do other odd things.

This applies to the situation when the source has already been through a preamp, and we are talking about the output impedance of the preamp. You cannot generally split something like a dynamic microphone or an electric guitar directly. The results will not be good.

Answer 3

The guitar will see double the load, as two inputs are used, and additional cabling will add capacitive load. Both inputs will then receive identical signal which has smaller amplitude and somewhat different frequency characteristics, because both the load resistance has gone down to half, but capacitance also up by few percent.

So, yes, it will degrade amplitude and quality. But how much it will affect? Well, with the provided info, it's all hand-waving, because for that we would need to know cable capacitance and guitar source impedance.

The effect may also be completely unnoticeable.