Audio - mono to dual mono

[whiterabbit]

Hey all!

I have a phantom-powered condenser mic that I'd like to make an adapter for to convert the mono signal into dual mono. What is the best way to build this?

 

[KrisBlueNZ]

Hi Erik. We meet again

You have a single phantom-powered condenser mic with a 3-pin Cannon XLR connector? And you want to adapt it so it can plug into two inputs on a mixing desk or similar, and produce the same signal into each input?

I haven't tried this, so YMMV ("your mileage may vary"), and I'm not an expert on this subject.

Start here: https://en.wikipedia.org/wiki/XLR_connector#Three_pin_-_audio

I think you can connect all three wires from the mic to corresponding pins on both connectors.

This will cause a small earth loop, so you might want to insert a 10 ohm resistor in series with the wires to pin 1 on one of the plugs that plugs into the mixer to avoid that.

Both mixer inputs will provide phantom power to the mic, but this shouldn't be an issue, since they both get their supply from the same voltage. The resistance between each signal conductor and the phantom power supply will be halved, because there will be two resistors in parallel. These resistors are typically 6.8 kilohms, so the parallel connection will reduce them to 3.4k. This may slightly increase the phantom power voltage at the mic, but it won't damage it because it's limited to 48V by the power supply itself. It just means that less voltage will be dropped across those resistors, leaving more of the 48V available for the mic.

The only possible problem is the input impedance. A balanced mic input has a (differential) input resistance around 2~3 kilohms. Connecting two inputs in parallel halves this resistance, down to 1~1.5 kilohms. The microphone needs to have an output impedance that's significantly lower than that. Do you know the output impedance of the microphone? They're usually around 150 ohms or less.

The microphone's output impedance and the input impedance that it's driving effectively form a voltage divider. If the output impedance is much lower than the input impedance, little signal is lost across the output impedance, and most of it is available at the mixer's input. This is how it's supposed to be. As the output impedance increases towards the input impedance, the amount of signal lost increases towards 6 dB. So you may have some loss of signal level. It's possible that the frequency response and/or distortion of the microphone could be affected too.

 

[whiterabbit]

Hi Erik. We meet again

Good morning! Always a pleasant meeting, haha.

You have a single phantom-powered condenser mic with a 3-pin Cannon XLR connector?

Yep!

And you want to adapt it so it can plug into two inputs on a mixing desk or similar, and produce the same signal into each input?

Exactly. I can digitally spread the 1-channel recording to two channels, it's just an extra step that I've long wondered how to avoid.

I haven't tried this, so YMMV ("your mileage may vary"), and I'm not an expert on this subject.

Heh, you haven't led me astray yet so I'll take your word with a grain of salt, but I'll still take it.

Start here: https://en.wikipedia.org/wiki/XLR_co...ee_pin_-_audio

Reading now....

Done.

I think you can connect all three wires from the mic to corresponding pins on both connectors.

This will cause a small earth loop, so you might want to insert a 10 ohm resistor in series with the wires to pin 1 on one of the plugs that plugs into the mixer to avoid that.

Are you thinking specifically of this configuration? [attached]

What I'm planning on doing [read: hoping to do] is having the XLR female connect to a female 1/8" stereo audio jack so that way I can have the option of then connecting the mic to my audio interface as RCA inputs, or directly to the built-in 1/8" audio jack on a computer. Do you think that would still be acceptable? My hope is to build this in a way that gives me more than one option, in case I at some point need to rearrange how my interface is connected (which is rare, but does happen occasionally). And yes, a more capable interface is on my wish list for the future

Both mixer inputs will provide phantom power to the mic, but this shouldn't be an issue, since they both get their supply from the same voltage. The resistance between each signal conductor and the phantom power supply will be halved, because there will be two resistors in parallel. These resistors are typically 6.8 kilohms, so the parallel connection will reduce them to 3.4k. This may slightly increase the phantom power voltage at the mic, but it won't damage it because it's limited to 48V by the power supply itself. It just means that less voltage will be dropped across those resistors, leaving more of the 48V available for the mic.

The only possible problem is the input impedance. A balanced mic input has a (differential) input resistance around 2~3 kilohms. Connecting two inputs in parallel halves this resistance, down to 1~1.5 kilohms. The microphone needs to have an output impedance that's significantly lower than that. Do you know the output impedance of the microphone? They're usually around 150 ohms or less.

The specs indicate 100ohms.

This is the mic I have: http://www.amazon.com/gp/product/B0006H92QK/ref=wms_ohs_product?ie=UTF8&psc=1

Also, because my interface is extremely basic I have an external phantom power supply:
http://www.amazon.com/gp/product/B000XUUXB8/ref=wms_ohs_product?ie=UTF8&psc=1

The microphone's output impedance and the input impedance that it's driving effectively form a voltage divider. If the output impedance is much lower than the input impedance, little signal is lost across the output impedance, and most of it is available at the mixer's input. This is how it's supposed to be. As the output impedance increases towards the input impedance, the amount of signal lost increases towards 6 dB. So you may have some loss of signal level. It's possible that the frequency response and/or distortion of the microphone could be affected too.

I think even a loss of 6dB would be alright with me. Since anything I'm recording I run through Ableton, I generally end up doing a fair bit of digital distortion which should easily mask a small amount of noise/distortion. And if it doesn't work out then I can just record in single mono as I have been, so no worries!

 

[KrisBlueNZ]

Are you thinking specifically of this configuration? [attached]

Yes, exactly.

What I'm planning on doing [read: hoping to do] is having the XLR female connect to a female 1/8" stereo audio jack so that way I can have the option of then connecting the mic to my audio interface as RCA inputs, or directly to the built-in 1/8" audio jack on a computer. Do you think that would still be acceptable?

No, I don't think so. The mic has a balanced, low-impedance output, which should be fed into a balanced input. RCA inputs are unbalanced, and jack inputs on standard sound cards are unbalanced too. You should be using a unit with an XLR connector.

These used to be available as stand-alone mic preamps with unbalanced outputs, but nowadays it's easier and tidier to integrate the analogue-to-digital into the preamp, so it plugs straight into a USB connector. I found two products on Amazon that look good:

http://www.amazon.com/MXL-MICMATEC-Preamp-Condenser-Microphones/dp/B000VZ8WC2/ref=sr_1_2
"MXL MICMATEC": USD 40; mono; XLR (balanced) input; no gain control; provides phantom power.

http://www.amazon.com/Art-ART-USB-Dual-Pre/dp/B002KEAT78/ref=sr_1_12
"ART USB Dual Pre": USD 80; stereo; XLR (balanced) and 1/4" jack (unbalanced) inputs; separate gain controls; provides phantom power.

If I was doing general stereo or mono audio recording, I would go for the latter option. It's really an external stereo sound card (input only) with XLR and jack inputs.

My hope is to build this in a way that gives me more than one option, in case I at some point need to rearrange how my interface is connected (which is rare, but does happen occasionally). And yes, a more capable interface is on my wish list for the future

Given that you shouldn't be feeding a balanced signal into an unbalanced input, I think the future is coming faster than you expected USD 80 seems like a reasonable price for what you get. How does that fit your budget?

I think even a loss of 6dB would be alright with me. Since anything I'm recording I run through Ableton, I generally end up doing a fair bit of digital distortion which should easily mask a small amount of noise/distortion. And if it doesn't work out then I can just record in single mono as I have been, so no worries!

Personally, I would always record a mono signal as a mono track. I'm not sure why you would want it as a stereo track - the system I use (Sonar) uses mono and stereo audio tracks interchangeably; a mono track fed into a stereo input on a virtual effects processor is just automatically sent to both channels. Recording a mono signal in stereo just wastes disk space.

 

[whiterabbit]

Thanks!

Way to shatter my hopes and dreams, Kris!!

Hahaha, but seriously though- thank you for explaining that to me. Even if I can't do exactly what I wanted I'm glad I know why

I really don't do too much recording anyways since most of what I do is with synths and pre-recorded samples. Ableton (as far as I know) doesn't automatically duplicate mono signals to the empty channel; maybe they've done something new with that in Live 9? I haven't played with it much yet. But I'll just do it manually if I have to (or give Sonar a try!). Was mostly just curious about the possibility of doing it analog.

The dual preamp looks pretty handy. I saved it to my Amazon wishlist to bookmark it in case I ever decide to do more recording! If that ever happens hopefully I'll have the budget to also get an audio interface that is anything-but-bottom-shelf, hahaha.

Thank you sir as always for your thorough response/explanation!!

 

[KrisBlueNZ]

Sorry to disappoint :-(

Sonar doesn't so much duplicate a mono signal to an "empty channel"; if you feed a mono signal into a virtual effect, it just feeds the mono signal into both left and right channels. It also has a channel mixing module that you can "connect" between the source signal and the virtual effect, to control how the input source is presented to the left and right inputs of the virtual effect. It gives you the ability to pan, and with a stereo track, you can exchange channels, reduce the stereo image width, etc. I expect Ableton would have something similar.

 

[whiterabbit]

Nah, it's cool- no worries! Sometimes things just don't work the way you expect them to, I guess.

Maybe I used the wrong terminology when I used 'empty channel'; I just meant outputting the mono signal to whichever of the two L/R outputs that was not originally playing the signal.

Ableton DOES have a similar built-in effect called 'Utility' http://www.loopmasters.com/blog/2712_23.jpg

I know, I know... SOOOOOOO hard to use SOOOOOOOO many extra steps.... haha. I definitely sometimes tend to over-complicate things sometimes when I try to simplify! (Like that Transmogrifier I've been talking about- "I need to organize my music library!" 'Ok, why not just do it by hand?' "Because this way will be easier!" *spends three months coding*) Granted, there's nothing like a challenge to provide me with endless entertainment!

 

[KrisBlueNZ]

Maybe I used the wrong terminology when I used 'empty channel'; I just meant outputting the mono signal to whichever of the two L/R outputs that was not originally playing the signal.

Are you recording a mono signal as a stereo track? You should be able to record each track (actually, each clip) as either mono or stereo. If you're recording as mono, you choose whether to use the left or right input channel of a stereo source (e.g. a sound card), and the other channel isn't recorded at all. If you record a mono source as stereo, you get a recording that has silence in one channel, which wastes disk space and causes that problem when you feed it into an effect with a stereo input.

Ableton DOES have a similar built-in effect called 'Utility' http://www.loopmasters.com/blog/2712_23.jpg

Good. If you've already recorded mono sources as stereo in existing projects, you may be able to use that utility to bounce the used channel from your stereo clip to a new mono clip, then you can delete the stereo data to save space. Or Ableton might be able to convert a stereo track to mono. Have a look in the help for info on mono tracks.

 

[whiterabbit]

Are you recording a mono signal as a stereo track? You should be able to record each track (actually, each clip) as either mono or stereo. If you're recording as mono, you choose whether to use the left or right input channel of a stereo source (e.g. a sound card), and the other channel isn't recorded at all. If you record a mono source as stereo, you get a recording that has silence in one channel, which wastes disk space and causes that problem when you feed it into an effect with a stereo input.

I'll check my configuration this weekend. I must be recording in stereo, though. One channel is empty.

It is thoroughly amazing how little I know about this stuff even after a couple years of playing around with it. Knowing how to ask the right questions is definitely key, and I seem to tend to learn the hard way, haha.

Thank you as always, I'll let you know how this goes...