mono to stereo headphones amp

[snk]

Hello,
I am building an Eurorack synth, and as it is rather portable system, I would like to add a small headphones out module, so I can play it everywhere 

I have found a couple schematics and layout, but prior to building it, I would have two questions :

1- It would be a mono in (synth) > stereo out (phones) module : so what's the best option ?
- making a "Y" wire coming from the synth and feeding both amp inputs (passive, quick & dirty option, but i'm not sure it's good enough?)...
- ... or an active solution to split the signal before going into the amp, like the AMZ Jfet Splitter?

2- I am considering a CMOY amp : it seems a simple and rather popular choice (I want an easy but reliable option, low parts count but good enough sounding module for playing on the go)... But should I need to change anything to have it match the impedance when used in an Eurorack case?


So, for [ mono synth > stereo headphones out ], what would be the best choice? Cmoy alone? Amz Jfet Splitter alone? Amz splitter into the CMoy? Something else?

Thank you in advance 

[merlinb]

From your description it sounds like there is no stereo information, you're just feeding identical signals into each earpiece. So just split the signal and feed into both ears, either with a single driver amp or one for each ear, your choice. So basically a Y splitter, no need to reinvent the wheel.

[snk]

Thank you, Merlin.
Yes, it would be mono in > stereo out (with identical signals into each earpiece).

I don't need a combo mono/headphones, as I already have an output module, but thank you for the suggestion and the link

So basically, I could just go for the AMZ buffer splitter or an adapter version of RoG Splitter blend?

Are there any pros or cons for one or the other (one uses transistors, the other one an opamp)?

I will be making it on veroboard, so may try this layout or that one :
https://www.parasitstudio.se/stripboard-layouts/amz-buffersplitter



Would both be suitable to feed headphones?

[merlinb]

Why do you need any kind of splitter? Surely the synth source already has a low output impedance capable of feeding straight into you headphone driver module? Sounds like all you need is a piece of wire! If you're worried about the module having a low input impedance, just add a single buffer.

The AMZ buffer splitter is a solution in search of a problem. One JFET can easily drive more than one input, the extra JFETs are just unnecessary. I don't really know what the circuit is even for. Similarly, the RoG doesn't actually need two buffers, one opamp can drive many things!

[GibsonGM]

If it's mono going to 2 outs that are the same, it's "Big Mono", not stereo.  Actual stereo is a different thing.
You just want to feed 2 'speakers' in parallel.

If you already have a power amp in your design, then yes, all you need to do is split the signal and send it to each phone.  I would imagine that this could be done as simply as feeding each phone thru a 'fair value' resistor in order to protect the amp's output. 

Can you post any specs for this module, so we can see what resistor would be a good choice?

[snk]

If it's mono going to 2 outs that are the same, it's "Big Mono", not stereo.  Actual stereo is a different thing.

Yes, I was a bit imprecise in my description indeed. Sorry about that.

You just want to feed 2 'speakers' in parallel.

Indeed. I should have said : "I just want to listen to the mono signal on both ears of my headphones" 

Can you post any specs for this module, so we can see what resistor would be a good choice?

Well, this is where it gets a bit tricky
By its modular nature, the final module in the chain can be one thing or another... For instance, it could be the Dreadbox Hades synth (the website doesn't provide impedance specs), a Squarp Rample, an audio mixer module (like this one for instance), etc.
I am planning to build this simple mixer module next week, so if it works as expected, chances are good that I will be using it as the last module in the chain when playing outside with headphones...

[GibsonGM]

I'd just split the out in a "Y" and put a 100 ohm resistor on each side.  The R is just there to make certain there's a load....opamps don't like to be open or TOO low.
 
            -------100R    Left phone 
Module ----|
            -------100R   Right phone

[snk]

I'd just split the out in a "Y" and put a 100 ohm resistor on each side.  The R is just there to make certain there's a load....opamps don't like to be open or TOO low.
 
            -------100R    Left phone 
Module ----|
            -------100R   Right phone

Ok, so just a simple passive circuit, with only the 100R resistor between the module and the headphones? No active splitter, no preamp?
Wow, it sounds so simple 

[snk]

Why do you need any kind of splitter?

Sorry, I think I wasn't clear enough : the synth output is mono, and the headphones are stereo.
- If I plug the headphones straight into the synth output (which is designed to go to a mixer channel, for instance), I hear the signal only on one channel. So, this is my primary goal : splitting a mono output so I can hear the sound on two channels.
- Also, the synth output is supposed to go to a mixer or an amp, not to a passive device like headphones. So my second goal is to gain-stage / level-match the synth output volume with what the headphones would expect(if that makes sense).

[ElectricDruid]

I think there's a definite role here for an "output module". Such things already exist of course, but in this case, the requirement is plenty simple enough to build one.

Eurorack modules have certain assumptions. They often feature +/-5V signal levels (10Vpp) and they often have 1K resistors in series with the output for protection. After all, we're talking about a system where you can plug anything into anything. There's no guarantee people won't connect outputs togther, or connect several inputs to an output or whatever else. They also generally expect 100K input impedance.

If it were me, I'd look for a simple headphone amp schematic, and I'd tweak it to meet typical Eurorack specs  - 100K input impedance and at least 10Vpp levels. There's *more* headroom than that on the +/-12V eurorack supply, so that's not hard to do. Then whatever you want to output can get plugged into the "Headphone output" module and you're good. The modules get to see something that they're expecting, and you get a reliable headphone output, since it's the same one you always use.

There's a ton of stuff out there that does this that you can borrow design ideas from. Pick'n'mix! -

https://www.sweetwater.com/store/detail/Behr305EQ--behringer-305-eq-mixer-output-analog-eq-mixer-aneurorack-module
https://www.sweetwater.com/store/detail/Outs--intellijel-outs-eurorack-stereo-line-and-headphone-out-module
https://www.sweetwater.com/store/detail/Phones1U--intellijel-phones1u-headphone-and-stereo-output-1u-eurorack-module

HTH

[snk]

Thank you, Electric Druid!

I think there's a definite role here for an "output module". Such things already exist of course, but in this case, the requirement is plenty simple enough to build one.

Yes, that's it 
I know the Erica synth EDU out, and a couple others, but for instance the Intellijel GetOut or Befaco Out costs 140€... and I think I can build a simple headphones out module quite easily

I'd just split the out in a "Y" and put a 100 ohm resistor on each side.  The R is just there to make certain there's a load....opamps don't like to be open or TOO low.

I tried your suggestion, and it works quite well. Thank you : it's much more comfortable to listen to my synth experiments with both ears 

Eurorack modules have certain assumptions. They often feature +/-5V signal levels (10Vpp) and they often have 1K resistors in series with the output for protection. After all, we're talking about a system where you can plug anything into anything. There's no guarantee people won't connect outputs togther, or connect several inputs to an output or whatever else.

This is why I was looking for an active circuit : to (hopefully) keep things in check a bit better... I'd rather fry a TL072 or a 5532 than a pair of headphones 
I also want to be assured that the output level will be sufficient in any situation.

If it were me, I'd look for a simple headphone amp schematic, and I'd tweak it to meet typical Eurorack specs

Yes, this is why I posted the CMOY circuit above, thinking it would be both simple to build and to understand (and therefore to tweak). I don't want anything fancy or high end, just a safe solution to get the synth sound on my both ears 

[antonis]

Sorry, I think I wasn't clear enough : the synth output is mono, and the headphones are stereo.

There is no such "stereo" thing in headphones..

There are 3 wires (Left-Right-Ground) coming out of headphones plug..
Just split your signal to Tip & Ring of headphones plug (with 100R series resistors, as well said by Sir Mike) and Sleeve to GND..

[

[PRR]

Wow, it sounds so simple

It IS just that simple. I have just "Y"ed sixteen outputs from one signal. No sweat with ordinary chips.

Yes, synths "usually" observe simple rules to allow mass connections without any thought. 'My' ARP had busses and I could feed 20, maybe 50, inputs from one output. 1K outputs, 100K inputs. Each added input causes 1% loss of signal and you can hardly hear 10% drop, can always pot-up a 30% drop.

The Cmoy is NOT a good choice here because it can't share a power supply with any normal circuit; it went all-out on a fully floating battery concept. Also it was aimed at 300r phones which are rather unusual today. A NJR LM386 will take common single-rail power and put more than enough power in modern headphones (unless you sit with the brass section). You don't even need two chips: the one '386 can easily drive two ears.

[snk]

Thank you, PRR, Antonis, Electric Druid, GibsonGM, MerlinB and others!
So it will be a simple passive Y with 100R resistors  !