The Cable Termination Isn’t The Signal

The connector on the end of a cable doesn’t necessarily indicate what kind of signal is present.

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The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

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Just recently, I ran into a musician who decided to solve a problem with a cable.

The problem was that he couldn’t get his instrument pickup to work with direct boxes. He had heard that the signal from the pickup was “mic level,” and so he did a bit of thinking. Pro-audio microphones that connect directly to general-purpose preamps (whether the preamps are outboard or contained within consoles) have XLR connectors. His instrument pickup has a 1/4″ phone jack. It seemed reasonable, then, that a TRS phone plug wired to a male XLR would help.

One the one hand, this is rational. Although his pickup is almost certainly an unbalanced output on a 1/4″ TS connector, the TRS cable has a good probability of working. The likelihood is that the tip and sleeve portions will mate with the jack, while the ring simply floats. At the other end, the XLR connector can’t be mistakenly mated with the input side of a direct box, which would increase the likelihood of the instrument being connected to a mic pre. Purely as a question of physical connectivity, the cable solution is okay.

However, the basic, physical connectivity probably isn’t his issue. My guess (which ended up appearing to be correct) was that what he really had was an impedance problem. He has probably been running into audio humans who assume that his pickup will play nicely with basic DI boxes. Basic, passive DI boxes usually have input impedances that are too low to get proper voltage transfer from pickups with high-impedance outputs. (For more, you can read this article I wrote for Schwilly Family Musicians. You’ll have to scroll down a bit.) When we connected his instrument pickup to an active DI via a bog-standard TS cable, everything worked beautifully.

I should also mention that, if his custom cable had been mated to a jack with phantom power applied, he might have ended up with a very dead pickup. Some things these days are built to tolerate having 48 volts DC applied. Some things simply “release their magic smoke,” and that’s that.

Now, I can’t say that I know everything that was going on the player’s head. It’s entirely possible that his solution was just a “shorthand,” and that he’s entirely aware of the separation between cable connectors and the signals on the cable.

Some people aren’t aware of that, though, and that’s why this is worth talking about. If you’re new to audio, here’s what you need to remember:

The termination used on a cable does not guarantee any aspect of the signal flowing on that cable. The termination only represents an upper-limit to the functionality of signals flowing on the cable.

Let’s flesh that out a bit.

Voltage Level Uncertainty

Let’s say I hand you one end of a cable. The end is terminated with a male XLR connector. You don’t know anything about the other end. If you complete a circuit by mating that male XLR with another device, what will the RMS voltage across the connection be?

Millivolts? (Common microphones subjected to SPL levels in the 90 dB range – “mic” level.)

Volts? (“Line” level devices, like mixers and pro-audio signal processors.)

Tens of volts? (“Speaker” level. Twenty volts RMS across an 8-ohm load is 50 watts continuous power.)

Well? Which one is it?

You don’t know. That XLR connector doesn’t guarantee that some particular, overall voltage level can be expected. The other end of the cable might be joined up to a microphone. Or a signal processor. Or even a power amplifier. Yes, it’s not likely that the output of a power amp would be on a cable terminated with XLR, but it’s entirely possible. It has been done.

All you can really guess at is the upper-limit of the XLR connector’s functionality, and that’s not even all that useful in this context. Assuming that anything larger than 16 AWG would be too hard to stuff into the connector, the upper amperage limit of what’s practical on a common XLR connector is something like 3.7 amps. In theory, you could use a specially-built cable to successfully supply power to some models of 120 V lightbulb via an XLR connector. (DO NOT ATTEMPT THIS. You may electrocute yourself, burn yourself, or end up setting fire to something.)

The point is that the presence of XLR connectors does not mean mic-level audio. Not necessarily. You can have a similar range of voltages on TS and TRS-terminated cables. To make an educated guess, you need to know what’s connected to the send-end of the cable…and that’s at a bare minimum. To be 100% sure, you need a reliable meter.

The Unknown Balance

Let’s continue the thought experiment above. Is the signal on the cable balanced?

Again, you don’t know. Cables terminated with XLR and TRS connectors can support balanced signals, but they don’t guarantee balanced signals. It’s quite common to use TRS for unbalanced stereo. It’s also possible (although I’ve never run into it) to use XLR for unbalanced stereo. From an electrical connectivity standpoint, TRS and 3-pin XLR connectors are the same thing – three terminals. What’s done with those terminals is up to equipment manufacturers, not the connectors.

It’s entirely possible to connect an unbalanced output to a connector that supports balanced signals. The reason is because of what I said above. The connector only indicates the upper functionality limit. If one of the signal terminals is left unconnected, or just isn’t supplied with any voltage, the signal on the cable is unbalanced. The connector doesn’t care.

Because of the connector imposing an upper functionality limit, you CAN sometimes determine if the signal on a cable is unbalanced. If you’re handed a cable end that’s terminated with a connector that has only two “poles,” like a TS cable, then you can’t have balanced audio on that line. Balanced audio requires three poles: Two for actual signal, and one for ground. If a connector doesn’t have the required number of terminals, it can’t handle balanced signals.

But a connector can certainly be capable of handling a balanced line, and yet not be handling balanced audio at that particular moment.

Looking at the ends of a cable isn’t enough to know what’s going on. You have to dig a little deeper, because the cable termination isn’t the signal.