Noob Trying to Build Mono/Stereo Unbalanced 1/8" Switch

[doulos30]

the fast track pro has a mono button on front is it broken?

you just need 2 jacks and a spst switch Ignore the jacks and pretend that green is gnd black is left and red is right



I don't think this will damage headphones the correct way is like this though with a dpdt switch

[Lotus 7]

It's generally very poor engineering practice to ever connect two different outputs (your left and right phone channels) directly together. Doing so effectively drives one output with the signal from the other output unless the signals are exactly the same (perfect gain-matched mono totally in-phase). Additionally, simply shorting the two channels together does not give perfectly summed mono.

Consider the situation when one channel has no output at all so is driving it's channel at a "0 voltage" level and the other channel has a strong signal. The second channel is effectively trying to drive current into a short (0 voltage).

Many, but NOT ALL, headphone output circuits are designed with series current limiting resistors to prevent damage if the two channels are shorted together or to ground. Unless you can confirm that your phone output has the limiting resistors, you should not short the two channels together.

The best solution is to use an op-amp summer that totally isolates the two channels and correctly sums them so a centered, in-phase mono signal is 3 dB louder than it is on either L/R channel.

Otherwise, if you don't care about mono summing in a mathematically "correct way" and just want a "psudo-mono" signal, you can add series resistors to each channel before shorting them together (use values that are approximately the same as your headphone impedance and no less than 32 ohms - 1/2 watt for most gear) and you won't risk damaging the output circuits (usually op-amps). The series resistors will reduce the phone level a few dB and will slightly alter the frequency response of the phones as well as changing the bass damping at the bass resonance frequency, but not enough to bother most listeners.

You really should do the stereo to mono summing in the interface signal before it reaches the headphone output if at all possible.

[Lotus 7]

Re the schematics being posted by doulos30

You keep changing the circuit! This is #3. Please add a different post if you want to do that. It's impossible to address a constantly changing design.

Using 2K resistors as in your "current" version will result in almost no output from any typical low-Z (30 to 60 ohm) headphone. With 50 ohm headphone drivers, a 2K resistor will attenuate the signal 32.04 dB. Not good. No more than 50 ohms is needed.

[Richard Crowley]

That wiring illustration is one of the most confusing things I have seen online in months. The colors of the terminals are completely ambiguous vs. the actual tip, ring, sleeve connections.

Furthermore, even a liberal interpretation of the circuit only sets both sides of the headphones to one output channel, leaving the other floating/unused/unheard. As I understand the original specifications, this does NOT achieve simply combining Left and Right for summed monaural.

Inserting 2K of series resistance will SEVERELY reduce the sound level in the headphones (which have an impedance of 64 ohms.

If you wanted to be safe, you could use 100 ohm resistors there, although, to be perfectly candid, simply shorting Left and Right (Tip and Ring) will not cause any permanent damage or performance penalty.

[Lotus 7]

Originally Posted by gerudobombshell

...What type of issues will I be incurring with this simplistic device? Are there impedance issues? Amperage issues? ...

Yes and Yes.
Repeat: "It's generally very poor engineering practice to ever connect two different outputs (your left and right phone channels) directly together."

Sometimes the "simplistic" approach is elegant, but sometimes it's simply wrong. this is one of those times.

[Matt Syson]

Hi
The sketch saying 'stereo to mono toggle switch' is fine and dandy except that the 2K resistors need to be much lower value as commented. Something like 50 Ohms would be fine and not introduce serious level loss. This assumes that the headphone output is rated to handle headphones down to 32 Ohms or less.
You should NEVER short two outputs of anthing together partly because it could damage the output circuit (although generally unlikely) but it will introduce extra distortion as there is a possibility of one signal forcing the (usually present) current limiting of the other.
As a scheme it is 'mildly flawed' but probably the most practical in this scenario.
Matt S

[doulos30]

i was just using the switch diagram as an example I didn't calculate the right resistance values cause I figured that might be somewhat dependent upon the headphones output impedance

your right the top drawing is in error I was thinking of input instead of output like a radio shack mono adapter

[gerudobombshell]

Hi, thanks for all the information! I've been reading up on the subject. I am trying very hard to wrap my head around it, and I feel like I'm ever so slowly starting to get there!

So, the Fast Track Pro's headphone output's working range is 32 - 600 Ohms. From what I understand, if you short the two live wires Black and Red, that effectively halves the impedance of the headphones...? Or is there a formula I'm not familiar with that I need to apply?...

Thank you again!


PS: The resistors in the diagram... what is their purpose? Are they attenuating the stereo signal so that the mono-summed signal is equal in level? What other attributes are the resistors on the stereo path inducing? Thank you!

[Richard Crowley]

The resistors protect the output circuits of the left and right channels from fighting with each other. Another way of looking at it is that they form a simple, passive "mixer", combining the left and right channel signals.

Most (all?) modern equipment includes these output series resistors inside before the signals get to the output connector(s). That is why it isn't so terribly disastrous as some people here fear to simply short the Left and Right signals together.

As you are reading this sentence, hundreds of people are combining Left + Right with nothing more than a simple, hard-wired Y-cable.

[enginefire]

One thing the diagram got 100% right is using two female jacks, one for input one for output. connect everything up with wires, your device will last a lot longer and you minimize your risk of breaking you headphone out Jack on your interface.

sent from my m-3500 using the gearslutz app

[Matt Syson]

Hi
If people are using simple 'Y' cables to 'mix' left and right, they may not actually damage the gear but it will not be of the best possible quality as for example the 30-100 Ohms on the putputs of an op amp stage serves it's function at maintaining stability and a modicome of 'protection' the op amp will go into current limit at some point.
For this to be OK you need the resistors to be around 1K or thereabouts (line outs).
Dedicated headphone stages that are intended to handle 32 Ohm phones only need to have resistors of say 50 Ohms to protect from instances when left is significantly 'positive' and right is significantly 'negative'. The resultant voltage to the headphones will be zero (equal voltages) but there will be significant current in the resistors..
Matt S