Audio - what can I plug into what (with sufficient adapters)

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Overview of what (not) to do

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

⚠ This is early stages, don't trust it yet

Such an overview is hard to balance between succinct and complete, because some things come in more real-world flavours than you'ld think.

(...especially 6.35mm (1/4") sockets, but also 3.5mm and others)

	TRS stereo in (consumer, unbalanced )	TRS balanced in	TS line in	TS Inst/Hi-Z in	TS pedal batt in	XLR in	3.5mm / RCA in
TRS balanced out (mono)	could be fine if wired mono(verify)	same / intended	?			same / intended
TS guitar out	(passes sound, left only, but noisy)(verify)	(passes sound, but noisy)(verify)	intended	intended	fine	(passes sound, noisy, specific adapter, can be confusing(verify))
TS effect out	(passes sound, left only)(verify)	(will make it unbalanced, possibly noisy, but fine)(verify)	(often fine?)(verify)	(passes sound, may distort)(verify)	fine	(will make it unbalanced, possibly noisy, but fine; (phantom??))(verify)
3.5mm TS/TRS mic (plug-in powered)
TS mic (self-powered)
TS mic (old Hi-Z, 6/35mm)
XLR out	weird	same / intended	quiet if mic, fine/loud if XLR interconnect output(verify)			same	can be fine if wired mono, quiet if mic, loud if an output(verify)
3.5mm / RCA out	quiet but fine	weird unless specifically wired mono	passes sound, but noisy	passes sound, but noisy		may harm output (when someone turns on phantom)	same / intended
consumer TRS stereo out	same / intended	weird (difference between the channels), fin left-only if TS cable?(verify)	left only, fine(verify)	left only, fine(verify)		weird if stereo cable, (can be fine if wired mono)

Notes:

• "TS pedal batt" refers to an effect pedal that is switched on by having a TS cable plugged into it

this is done in a specific way that is clever but a little hacky - so introduces some edge cases

Potential variation:

• In consumer 3.5mm mics, TS/TRS is sometimes the same, sometimes not

the mics plugs are often TS, but if TRS they often put the same signal on T and R

the mic input sockets are

frequently mono (in which case either of the just mention versions act the same)

sometimes stereo (in which case they could be read out separately)

• 6.35mm consumer mics mics are from before strong standards. Your mileage will vary.

• 3.5mm consumer mics are also not really standard, and there is some variation, but less.

Edge cases:

• many TS mics to XLR

impedance difference - this is what a DI box is for

• TS instrument to XLR:

impedance should be fine, voltage may be fine

DI box would still helps isolate against ground issues (but depending on case may not be necessary)

• Plugging line level output into instrument level input usually so much stronger that it distorts

there is a very small (but non-zero) chance of some damage

• plugging TRS outputs into battery powered pedals is potentially harmful ...to the TRS output you plug into the socket (an actual instrument tends not to care)

specifically when effects are switched on by plugging it in

they do this by actually using a TRS socket, and completing the power circuit through the plug itself -- by having the TS's sleeve short Ring to sleeve

but when you use an actual TRS plug, it puts 9V between ring and sleeve (cold and drain on balanced, right and ground on unbalanced) which its driver may not like

• 3.5mm TRS consumer stereo into 3.5mm TS-only input will lose the right channel

because it shorts ring (right) to ground (sleeve)

• 3.5mm TRS consumer stereo into 6.35mm TRS balanced - depends a little on the case

if just a straight cable, then it will sound quiet and weird, because it effectively presents left minus right channel as the signal

• TRS balanced into a unbalanced-only TS input

the TS plug itself already shorts cold and shield of the TRS socket

You can assume shield is probably grounded somewhere (probably the device itself, really).

If it's grounded in the output, you have just grounded half of that balanced pair

you've essentially converted balanced to unbalanced

and possibly added ~3dB attentuation

if the TS went to an unbalanced input, adding a ground is something you actually need to do.

but doing it this way also means defeating much of the noise immunity. If you're going to do this, better keep that wire short. And preferably shielded.

• TS output (often instrument) into balanced-only TRS input

the plug will short between sleeve and ring, i.e. tie one of the signal lines to shield.

which on the 'cheaty' single-ended balanced driver design might be fine(verify),

on a halfway proper one can cause damage

while particularly fancy ones may be protected against this case (but don't assume it)

might introduce common mode from the mixer itself. (verify)

Further edge cases, introduced by more creative adapters

• XLR-to-RCA to connect devices, or
• XLR-to-3.5mm to connect devices

you will want an isolation transformer to avoid connecting the grounds

no noise rejection, so keep cables short

you will want to set the output volume low (because of the ~12dB difference in signal levels)

• XLR-to-RCA or XLR-to-3.5mm to connect a (dynamic) mic

works because mic floats

won't work for mics that need phantom

...so only works when you use a dynamic mic on the XLR side and actually have a bunch of amplification, which... isn't common

no noise rejection, so keep cables short

Note that some of the "XLR to 3.5mm" cables are only good for this and not for devices - say, the last cable like this I cut open just had

1-strand shielded cable

T and R = core = XLR pin 2

S = sleeve = XLR pin 1 and 3

...which won't work for phantom mics, and will be problematic or not work to interconnect XLR

• XLR devices with the pin 1 problem (of tying shield to PCB signal common)

...which invites in common-mode noise, in a few different ways

Since you were spending money and time to avoid them so far, why would you do that thing?

Things that won't work / things to avoid

• amplified TS out to instrument TS in

plugging speaker out to mixer in / powered speaker may fry that input (sometimes but not always protected against that)

plugging instrument into a non-powered speaker does nothing

• mixing TS-speaker cable and instrument cable

instrument cable is thinner, and often shielded; speaker cable is thicker, and often unshielded

using speaker cable for instrument will work, though easily somewhat noisier due to lack of shield

using instrument cable to connect speakers may possible probably burn that cable (if/when the wiring is too thin)

• Consumer 3.5mm mic to 6.35mm input (via adapter)

most mics are electret, which require plug-in power, so so won't work

a few sort-of exceptions, like battery'd lav mics

• 6.35mm TRS (output) to XLR male (input)

if used to plug balanced gear into a mic in, this works (attenuates the signal though)

trying to use it for instrument will probably work too.

can't think of other cases

• XLR female (output) to 6.35mm input

to unbalanced (TS): does not convert

to balanced (TRS):

common enough for microphones.

Mixing TS and TRS

Top: TRS in TS socket (ring floats)
Bottom: TRS in TRS

See the image on the right, which is a TRS plug plugged into a mono TS socket (top) and into a TRS socket (bottom).

TS in TRS socket (ring shorted to sleeve)

In TS sockets, inserting a TRS plug will

often leave ring floating.

In TRS sockets, inserting a TS plug will

often short ring to sleeve.

Either can be a problem.

But sometimes it's intentional. For examples, 'bal/unbal' inputs on mixers may be implemented (it's not the only way to do this) by having differential inputs that become single-sided/unbalanced by your mono TS shorting one side to ground .

Given a TRS socket, plugging in a TS plug will short that socket's ring and sleeve.

in a balanced input, that shorts cold to shield, which invites in noise

💤 Engineers should also consider that in a TRS socket, there might be brief periods where

• the socket's T and R are connected (possibly longer, half-plugging can do this)
• the socket's T and S are connected
• the plug's R and S are connected

The stuff you can get away with - but that has annoying footnotes

Mixer-insert tricks

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

For context: mixers with an insert socket's tip and ring are output and input (respectively). When you insert a TRS plug, this is expected to 'contain' a mono effect.

That socket is very likely normaled, meaning that when nothing is plugged in, a spring rests on an additional contact that means its input is shorted to its output. Bypassed, if you prefer.

Half plugged TRS: still normaled (see bottom), but also connecting to one contact (see top)

For context: fully plugged (no longer normaled)

Half-plugging

Consider the first image on the right: While not in its intended place (see the second image), the plug's tip is held in place by the ring's spring.

At this point the socket is still normaled (no plug is fully pushed in; in these images, the extra bottom leaf still connects, but the details can vary a little per socket type).

What does that do?

If indeed the socket's tip and ring are still directly connected, they they are electrically equivalent and carry the signal in this channel at this point in the circuit (after gain and before most other things)

So the net effect is that

to the mixer this is still bypassed

to the half-inserted plug, you get to listen to it

If your mixer does not have channel outputs (or group output you can abuse), this gives you an extra line output / send from that specific channel.

However, this doesn't always work -- because depending on the exact shape of the plug tip, and the socket design, it may

mute the channel

not make any contact on sleeve/ground - you may get a floating ground so poor or nosignal through that wire

sit too loosely to keep making good contact - it may crackle at the smallest movement

Notes:

• It doesn't really matter that (as in the image) the plug tip may or may not touch the socket's tip spring - right now (while still normaled) those two contacts are shorted anyway.

• you can wire a TRS plug as a fully insertable listening plug

(shorting T and R in the plug)