In the manuals for Monotrons it has a warning under the headphone jack entry: Do not connect a cable with a monaural plug to the headphone jack
Why? And is it why my Monotrons’ volumes have dropped so low
I have other questions (like what if I just stick an adapter on a mono cable to give it a TRS plug…) but feel like a should get a grasp on the ‘why’ first.
Just bought a replacement Delay. Sure, they’re cheap but I still don’t want to trash another. Bad karma etc
With a mono output you could not plug headphones in, the audio would be on one side only, so they have to use a dual mono output.
I have a trs to ts cable that I use. Not ideal afaik (cables shouldn’t sum signals together?), but never had problems.
Yep that’s why.
Hosa makes a 1/8" TRS to 1/4" TS cable that solves the problem.
Thanks both!
Why does the damage occur? And what’s connected to what within a TRS to TS and how does that stop the damage?
I have some cables that are like almost that already - except it’s TRS to a single RCA - I can stick a phono>TS adapter on those but is that good enough?
I think the answer to why the damage occurs is phase cancellation but happy to be taught otherwise
I don’t think phase cancellation has anything to do with it. It’s a dual mono signal (=2x the same signal), both sides are perfectly in phase with each other, because it’s the same source, just duplicated. Duplicate a track in your DAW or on your OT - what will happen is the signal will become roughly 3 db louder.
Regarding possible damage: You can split audio with a cable, but you can’t sum it. You’ll need a least resistors. A simple cable can’t properly sum and it can damage some circuits. I’m not sure if in this case this would really be a problem (we’re not summing two different signals, we’re basically summing the same signal) and many people use a trs to ts with their Volcas and Monotrons.
There was an article that explained it pretty well. If I only could find it…
Yeah basically you are sending the ring signal to ground, which is why the signal is lower. Same with Volcas as @Schnork pointed out.
What do you think about the risk of damage to the output circuit on Volcas/Monotrons when using a trs to ts cable?
When summing two channels that do different things - maybe one channel is silent at one point while the other plays full volume - potentially there could be pretty big voltage differences and afaik that definitely can damage an output circuit.
But in this case, it’s the same signal, but some say even tiny voltage fluctuations could damage a circuit…
I think it is unlikely but still possible, especially if some other wrong connection was made. It also can cause problems with sync on the Volcas if the audio cable is TS-TS.
Yeah. I thought so, too. That’s why I didn’t even bother with soldering a proper summing cable.
Ideally you’d be summing with a mixer (maybe get a small passive mixer to do the job), or hey, just learnt you can actually buy summing cables for a decent price nowadays. 
Just search for “stereo to mono summing cable 1/8’ male TRS to 1/4’ male TS”.
If you don’t want to risk any damage, do it the proper way.
Sounding good
Regarding potential damage. One thing that springs to mind is that the headphone jack will be a TRS socket with three terminals. When you insert a TS jack fully into the socket the sleeve of the jack will connect to the ring terminal of the socket, shorting it to ground. So the channel that feeds the ring (either L or R) will be grounded which could, depending on the Monotron’s output stage, cause damage. Most modern gear has protection against shorting an output to ground but it’s still worth considering.
Yes! Not adding the protection would make sense with the Monotrons’ low price vs other gear. Also I’ve never seen the warning in other gear’s manuals - plus I do mix and match cables and adaptors all the time on things with no bad results until now
That could very well be the reason for the warning.
The schematics are available from Korg, if anyone wants to have a look…
I‘m not really familiar with the different methods that potentially can be used.
Full schematic from Korg:
Relevant section:
The TPA6111A2 datasheet lists “thermal and short-circuit protection” among its features. My guess is that using a TS cable would just silence or seriously attenuate the Monotron and not damage it.
@Clancy - did using a TS cable actually damage your Monotron, or does it work properly when you switch back to TRS?
That’s what I’m asking really. Two of my Monotrons have much lower volume than they once did. That’s permanent & whether using speaker or TRS (which is what I use - but wondering if I sloppily used TR at some point in the past). New batteries don’t fix it.
The other symptom on one of them is that the volume dial does almost nothing - can’t turn down to silence can’t turn it up. I mean it turns but you just get a sort of mild fluctuation
The controls do what they’re supposed to so it’s not like the filter has got stuck near closed
Maybe they’re just old (10 years).
Now that I’ve replaced the Delay with a new one I just wanted to get a bit more clued in because I generally like to ‘misuse’ audio things. But I also don’t like throwing away dead electronics lol
That sounds more like a bad volume pot to me.
I have the OG Monotron (non-delay) from the month they first came out (2010?) and when I turned it on last year it was as loud as I remember. I can’t recall whether I’ve plugged a TS cable in, but it has seen light use.
I’ve only run my Monotron delays through TRS cables - they are fun in sequence.
FWIW, I’d rank myself as extremely junior when it comes to circuits and DIY: I’m very competent at soldering and following instructions, but I’m usually too impatient to do the math or setup pspice and have little practical experience with specific chips. Hopefully someone else who has more practical or theoretical knowledge can chime in on whether shorting the output of a TPA6111A2 to ground could be harmful. I’m doubtful, but I don’t want to ruin anyone’s gear with my amateur guesswork.
Just dug out my old Monotron Delay to see if I can integrate it prior to a reverb and I’m having pretty much the same symptoms as you. Volume is low and the wheel doesn’t change it much, and most disappointingly the feedback control is pretty ineffective, I only get maybe 6 audible repeats with it at max. That’s me just testing with my phone plugged in and then onto the interface, both with TRS 3.5mm cables.
I suppose it’s pretty much had it… 
Might as well crack it open and see if there’s an obvious problem with the pots coming loose or something. Voiding the warranty on a ten year old piece of £30 gear isn’t an issue!
I did open it up last night but there was nothing obvious to me that looked like a problem. It seems like there’s enough similar reports around the net about these symptoms in particular that it’s probably just a common issue with a chip or capacitor I guess? The main common factor seems to be trying to use it again after a long period of storage.
I could try probing at it with a multimeter but I’m not sure if it’s worth my time. I have a standard old Monotron that has been stored in the same place and that one seems to work fine, it’s much louder at least.
