Stereo to Dual-Mono headphone wiring.

[Albatross]

I recently made a stereo to mono cable using a few resistors as detailed in another thread on this board and it works a treat. It goes to an ART DTI (that Hugh made me buy) and off to a little amp (in bridged mode) for my mono grot box.

Now.... I would like to take a stereo feed from my laptop, convert to mono and get dual-mono in the stereo headphones.

I've trawled the web and my own head but am confused.

Anyone care to give me a pointer...? Many thanks.

[Sam Spoons]

You need resistors in there to keep L from driving R and vice versa (a simple mono summing lead)



Not sure if those resistor values will work as it doesn't specify it's for headphones......

Then just fit a stereo minijack socket to accept the headphone jack at the other end with the tip and ring wired together so it feeds the mono signal to both L & R

Probably easier to do it in the DAW though.... Reaper has a dedicated mono sum button on the master 'channel' strip.

[Albatross]

of course

Then just fit a stereo minijack socket to accept the headphone jack at the other end with the tip and ring wired together so it feeds the mono signal to both L & R

Easy as that... That was the closed door in my head this morning... thanks, Sam.

Just for info of anyone reading this, I used this config from the Rane site at http://www.rane.com/note109.html for my cable.



But i know your one works too.

Thanks again.

[Sam Spoons]

[Sam Spoons]

I'd still just do it in the DAW though.......

[Albatross]

Probably easier to do it in the DAW though.... Reaper has a dedicated mono sum button on the master 'channel' strip.

It does and i use it all the time. But i often wan't to listen in mono to stuff from say 'media player' and there's no mono there. There are things you can install but, you know.

I wanted this for hardware monitoring where i have two sources going into my laptop L&R line-in. Of course they come out of the headphones L&R too, and its a bit unbalancing for my old brain. I can use software monitoring via Reaper but latency gives me a sort of free slap delay... Its all too confusing... I like cables...

[Hugh Robjohns]

Yep, just wire the tip and ring together at the headphone socket for dual mono.

[James Perrett]

I'd be tempted to use lower resistor values if you are feeding headphones direct unless you have plenty of spare volume or high impedance headphones. Something like 100 ohms would be work.

[Albatross]

Thanks all.

[Albatross]

Just for info;

Headphones are a pretty standard 24 Ohm Sony hifi jobbies and i'm using them with an also pretty standard laptop internal soundcard headphone socket.

Tried the 470R / 20k combination and as suggested the output was pretty low.

So for anyone reading i ended up with 47R and 2K to ground and that gives an almost identical volume level in the headphones.

Thanks again for the advice.... works a treat!

[Hugh Robjohns]

Yes, makes sense. The resistor values in the Rane design above are intended for use in a voltage-transfer line-level interface. However, your headphone-driving application requires a power-transfer interface, which means you'd need much lower mixing resistances, of 100 Ohms or less as James suggested. So scaling everything down by a factor of 10 makes good sense.

H

[James Perrett]

I was going to suggest a lower value but then worried that you might have a fragile headphone output that would struggle with being driven by the other output. 47 ohms sounds reasonable to me.

[Albatross]

I've dug out this old thread because I'm summing two channels (not heaphones this time but a simple diy mic amp and simple diy jfet piezo preamp) to one mono output and am finding that the volume pots for each are effecting each other.

I'm using the 475R / 20k configuration.

Is there a way to to that happening?

I wondered the purpose of the shunt resistor?

Would be grateful for any clues. Many thanks.

[ajay_m]

If you're working in the voltage domain i.e the output will not directly drive headphones, then the summing resistors can obviously be higher. Otherwise looking back into the pot, it can drag down the output from the other side, and in fact for any passive resistive mixer this will always be true to some extent, although obviously if the pots were, say, 1K and the summing resistors were 10K, then the effect would be minimised.

Otherwise you would be looking at an active design, using a low-cost dual opamp, chosen depending on whether you want to directly drive headphones or just another input. There are a ton of devices available, many single supply, a bit of googling will probably come up with a design you can use. Just bang it together on a piece of veroboard.

[James Perrett]

You probably want to use higher value resistors in line with the outputs for this particular purpose. The best value depends on the design of the source and destination circuits but I would guess that you may want to go as high as 5 or 10k to reduce interaction between the volume controls.

[Albatross]

Thanks ajay_m, I'm going to stay passive I think and keep it simple, realise its a compromise but its not that critical.

Thanks again James. I'll experiment with some larger resistors and try and find a good compromise between loss of gain and function.

Thanks very much again.

[Lostgallifreyan]

You can minimise the problem with a passive adder but that means some truly large resistances. This adds noise (is proportional to that resistance), and you'd still need an op-amp to follow, even if just a unity gain follower, just to get a low resistance output that can drive something else without adding complications like HF or LF rolloff because high resistances invite relatively long time constants when interacting with other components in input circuits.

If you are compelled to use an op-amp, you might as well use a standard inverting config to take advantage of the lower input resistances and lower noise.

[Albatross]

Thank you. Plenty to think about.

[Wonks]

If both devices are giving out similar line-level outputs, then a standard passive mixer resistor solution should work, as long as you then feed into a headphone amp driver circuit.

Why aren’t you trying to drive headphones from one? That would be the logical arrangement.

[Albatross]

Thanks Wonks. I have the headphone issue sorted. This is the same amplifier box but this time a mono feed into my little PT2399 delay board.

[Wonks]

I think you’ve rather confused people by not saying that to start with.

So you’re just driving a signal into an effects unit. Just voltage, not power, required.

[Wonks]

Have you got a circuit diagram for the reverb unit?

There seem to be several on line, some quite awful, but the better ones (which may be for similar, but not identical, boards, seem to have 1k or 10k input impedance resistors fitted, so a lot lower than a standard guitar pedal. 10k is fine for a line input but 1k is on the low side.

What you actually have may determine what you need to feed into it.

[Hugh Robjohns]

I've dug out this old thread because I'm summing two channels (not heaphones this time but a simple diy mic amp and simple diy jfet piezo preamp) to one mono output and am finding that the volume pots for each are effecting each other.

Yes, they would.

The headphone stereo-mono adapter is designed for that specific application of dual amplifiers expecting to drive a low impedance, high current load, and the actual load being high impedance and voltage driven. The resistor values are specific to that task.

You have nominally low impedance outputs feeding into a high impedance input, and all voltage-driven. That requires a different approach. Ideally, you need to know the output impedances of both sources, and the input impedance of the destination to calculate the optimum values.

And by the sound of it, the sources output through pots, which means their output impedances are variable from lowish at full volume, to potentially quite high at low volume. That complicates matters....

However, I would suggest starting with series mix resistors of at least 1k ohms, and possibly going as high as 10k ohms. The higher the value the less volume pot interaction... but the greater loss of overall level.

If this is a permanent installation there's no need for the shunt resistor — that was only there in the headphone adapter to maintain a load if it was unplugged from the destination (and to minimise DC popping).

Be aware that passive mixing like this always results in a loss of level, related to the ratio of series mix resistor and destination input impedance, as well as the number of mix inputs. In this case I'd expect around -12dB would be typical....

Hopefully the amp has enough gain to compensate, but if you want to avoid that level loss you'll need to go active with a virtual earth mixer circuit built around an op-amp or two.

[Albatross]

Thanks Hugh, Wonks.

I'm not sure what the impedence of the reverb board is because I can't read the SMD component codes, but its described as 'line level'.

I've got 50k pots on both the piezo and mic amps. They aren't both wide open, probably about 75% guitar and 50% mic - the guitar pre is 6.8k the mic pre 10k but I haven't measured the pot values when they are reasonably balanced to my taste.

I don't know much do I?

I think I'm going to put trimpots on each side, think I've got some 10k ones and try and find the sweet spot and then put the resistors in permanently.

Sorry if I'm rambling, been to the dentst with my wife and its a bit fraught here.

Thanks again chaps, I'm always grateful.

[Albatross]

I think you’ve rather confused people by not saying that to start with.

So you’re just driving a signal into an effects unit. Just voltage, not power, required.

Sorry about that, but I did say it at the top of the last page I think.

But no, we got the headphones sorted ages ago - we got the power sorted on another thread - I shoud have probably started another thread for this issue.

This is the last piece of the puzzle for my busking amp which is pretty much a leather seated box piano stool on wheels. Ha! I sit down and people laugh at me... bit like here really.