[OT] 3.5mm stereo to mono adapter with switch?

[John #9]

Tsk! Should've read the OP first. soz!

[tabbypurr]

Signal outputs: no problem. Speaker/headphone outputs: don't do it.

NT

[Dave Plowman (News)]

Stereo/mono switching is normally done at the amp inputs. Not at headphone or speaker output.

Very little is pure left/right anyway. If it is, more likely twin track than stereo.

Really, to do all you want in a decent fashion would need a stereo headphone amplifier box with switching on the input as required. Battery ones can have quite a good battery life.

[The Natural Philosopher]

Yep. Protection will cut in and you will get distortion. However some headphone outputs have series resistors in them

Speakers not.

[Brian Gaff]

So surely a mono 3.5 is two connector whereas the stereo has three. This would mean that you would short one channel out in one case if you plugged in a mono phones system. Maybe this is why they do not make one.

Brian

[Brian Gaff]

Well assuming this, then you would need two adaptors I'm afraid for the reasons in my last message. If you can be sure shorting out a channel does not matter there could be a way, but its very dodgy. You can buy plug in stereo to mono so mono earpieces hear both channels as mono, you can buy a device that has a mono plug and routes the output to both channels of a stereo set of phones. Brian

[Terry Casey]

A simpler - and better way - is to buy a twin stereo adaptor (single male in, two paralleled females out) and a stereo male.

Short left to right inside the male, making sure it is marked clearly to avoid confusion, and simply insert it into the second female to switch stereo to mono.

Others have issued warnings about equipment damage so I won't repeat them here.

[pamela]

That is exactly what I sometimes do but it's a bit of a fragile set up have 3 plugs (including the headphone plug) sticking out of a tiny dictation machine.

[pamela]

I have been trying that (using a spare 3.5mm mono to stereo adapter for the "switching plug" because its build to short out the two signal paths).

However with the particular plugs and sockets I have tried the insertion and extraction is very far from smooth. It's a sort of biggish tug or a good old shove.

One reason I wanted a switch was to make it easy to A/B compare the mono and stereo playbacks.

[Roger Hayter]

No permanent harm maybe, but if and when the two output signals are significantly different you will get distortion, as each output is seeing a very low impedance at the difference signal frequency.

[lew]

I think that "

" would do the trick for getting "stereo" output from the mono dictation machine. A feedback says that it worked with a mono out to a stereo headphone to hear with both ears.

The pic of your adaptor don't make sense to me if the problem was being able to listen with only 1 side of a stereo earphone/headphone. The mono plug to stereo jack do work. Radio Shack had carried all kinds of adaptors & a big shame that they no longer have storefronts.

[Johnny B Good]

Ok, no problem with that requirement. :-)

In which case, you might be better off using a simple mono to stereo adapter plug to solve that particular problem.

That's the problematic situation. You can't just simply bridge the left and right signal lines together without the real risk of creating a lot of distortion. The risk of distortion (and even damage to the amps) depends on what class of source you're plugging this contraption into.

If you're using the "Headphone Socket" on home HiFi kit, this might simply be an attenuated output from the speaker amps where a simple resistor network is used to allow you to keep the same volume setting without burning out a pair of 8 ohm headphones and deafening yourself into the bargain.

The simplest attenuator is just a couple of resistors (a hundred ohms or so each) in series with the headphone socket's left and right contacts. In this case, you can cheerfully bridge the headphone socket contacts with no harm to the amplifier. A more sophisticated version of this is the use of an audio voltage stepdown transformer instead of the simple resistor attenuator.

Although a stereo pair of such transformers will reflect such L to R shorts back to the amplifier output terminals if they were perfect lossless transformers, the usual imperfections (leakage inductance and winding resistance) would typically be more than enough to prevent overloading and distortion.

However, this isn't always the case and a seperate, dedicated headphone driver amp may be providing the headphone socket feed. Whilst such an amp may be limited to no more than 500mW, possibly as feeble as 50mW, output, they will, nevertheless have a low output impedance possibly less than 1 ohm so can suffer similar distortion when their outputs are bridged together (although there's less chance of permanent damage from overloading).

If you can tolerate an additional 10dB or so loss, you can simply connect each headphone channel via a 10 to 36 ohm resistor to the stereo input contacts and wire a switch across the headphone socket contacts to bridge them together. The resistor values are chosen to suit the headphone's impedance and the minimum load impedance specified for the amplifier.

Anything more sophisticated than that will typically involve a seperate headphone driver amp fed from a simple resistive summing network that will allow the headphone amplifier's inputs to be bridged with no untoward effects on the stereo source this adapter is plugged into. If it needs to be portable, you'll have the addition of a battery to power the amp.

If you want to avoid battery powered adapters and you don't mind a little extra bulk, there is a way to avoid such an active solution and its battery power requirement. This consists of a pair of small audio transformers and some cunning switchery in a suitable box, along with the required input and output sockets.

The transformer can provide galvanic isolation between the headphones and the driving amp's terminals. The virtue of this feature is that the individual transformer secondary windings feeding each headphone earpiece can be rearranged by clever switching so as to act in series to feed the bridged headphone socket contacts with a mono signal. This removes the 'shorting effect' on the driving amp but it does raise the volume by 6dB unless you also include an additional half voltage tapping point on each secondary to compensate using additional switching circuitry.

Despite the extra bulk compared to a cheap AAA cell powered amplifier, the transformer version does have the virtue of being a low loss and completely passive (no batteries required) solution. Furthermore, the primary windings can also employ switched taps to widen the scope on the range of sources it can be plugged into (for example 0, 10 and 20 dB attenuation settings would likely suffice for pretty well most situations).

[The Natural Philosopher]

Total bollocks.

The transformer will not have nearly enough impedance to prevent effective 'shorting'

[pamela]

Radio Spares don't give an option of 3.5mm mono on their adapter selection tool. Stereo mad, I tell ya!

A decent box would do all I want and do it well but I would never use it because it's too bulky.

[pamela]

Just reflecting......... once upon a time we didn't assume all headphones were low impedance as we do nowadays. The first few headphones I ever bought were high impedance. As high as 2,000 ohms in the case of my Sennheiser HD424 although I think my AKGs were 600 ohms. I don't think many headphones these days have those sort of impedances.

Using attentuators or other solutions you mention may have their advantages but a mono to stereo (or stereo to mono) 3.5mm jack adapter doesn't contains resistors. Instead those adapters short out the two stereo signal paths and while there are many tecnical objections to this, they work well enough in practice.

I would be interested to know if perhaps there is significant distortion present that I am not noticing. To check this I would need to make a mono recoding (probably one that's difficult to hear well) and then play it as dual mono through two stereo channels while shorting and not-shorting the two signal leads. Perhaps I will get around to this although I'm not expecting it to show much difference. Who knows.

The audio source I'm listening to is speech from a dictation machine used as a meeting recorder (sometimes while walking about) and the speech I hear doesn't seem adversely affected by any additional distortion.

I record in stereo but will playback in mono if there is rustling noise from handling the recorder which often gives a swooshing sound from one channel to another when one part of the machine gets rubbed and then another part. (If you see what I mean). Using a lavalier mic is too impractical and anyway may not prevent rustling noise.

If the machine is hand held and we are moving around then the shifting stereo sound stage on playback can be a distraction to hearing what is being said.

So if overall audibility is poor then I may need to switch from mono to stereo for a few seconds to hear which version is clearest which is why I want a quick to use switch. Swapping mono-stereo adapters in and out whenever I have moments of unintelligble speech can slow down the whole process of hearing a recording.

At other times when there is no movement or rustling then stereo playback is best.

Dave Plowman suggested using a box and it would work well if I am always sitting when I play the audio. However I don't always sit down to listen because some of the meetings can run to hours and I will do other activities while I hear the recording on a portable player.

So I started off by thinking that if my portable player doesn't have a mono-stereo switch then maybe I could find a headphone adapter plug with one built in. However it's getting a bit more involved than that!

[Dave Plowman (News)]

Do you own the machine this is to be used with? If so, it would likely be easier to add a channel selector switch to give Ch1 to both, Ch2 to both, mono and stereo.

[Roger Hayter]

The above scheme would be *more* satisfactory with high impedance headphones. Where it would fall down would be with, say, 4 or 8 ohm headphones. But I don't think such things are common, nor are the amplifiers in modern miniature electronic devices designed to drive them.

If the signal on the two channels was identical then shorting the two channels would not cause distortion. It is only to the extent they are different that distortion occurs.

In practice for speech everyone just shorts the two channels together and it doesn't, as you say, cause much of a problem. Perhaps the simplest thing would be to get a short male to female 3.5mm extention lead, and run it through an inline switch such as for a table lamp wired to use one pole only to join the channels. In mechanical terms it might be necessary to pinch two loops a couple of inches apart on the extension lead, catch each loop in the cable clamp each side of the switch without breaking continuity of the lead running round the switch. Then on each side of the switch on the internal side of the clamp remove the outer sheath and tease out one of the channel wires (left one side, right on the other) and cut and insert both (stripped) ends twisted together in one terminal of the switch, using corresponding terminals on each side. The continuity of the two wires would not be broken, but one pole of ths switch could be used to join left and right.

"Figure of eight" type stereo lead might be easier to do this with.

[Dave Plowman (News)]

It would depend on the design of the power amp driving the headphones. A decent power amp will have a very low output impedamce. Which is why you can get into problems paralleling them together. Each amp then provides a load lower on the other than it is designed to drive.

Of course if absolute quality doesn't matter, the headphone amp may already have a series resistor etc in the output to protect it from abuse. In which case you might be able to parallel them OK.

[Carlos E.R.]

And distortion. Proper summing of both channels can not be ensured to happen.

[Carlos E.R.]

But the amplifier is not designed for that different output impedance, so it is, at least, working out of specs. Probably distorting as well. Maybe not. It has to be calculated or tested for each case.