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Building an adapter - dual quarter-inch and 3.5 mm, to 3.5 mm

Discussion in 'General Electronics Discussion' started by Subtox, Mar 7, 2017.

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  1. Subtox

    Subtox

    10
    2
    Mar 8, 2016
    I'm a novice so please excuse my possibly bad phrasing and electronics ignorance...

    I have a Focusrite Scarlett 6i6 audio recording interface (shown here), which has two 1/4" output jacks for studio monitors. I don't have monitors yet but I have a nice set of PC speakers (Bose Companion 3) that I'd like to use. Due to software limitations I haven't been able to feed the audio from the audio interface to the PC speakers through the operating system (still waiting on their tech support for a reply on this).

    So if that ultimately falls through, I'd like to build an adapter to share the speakers between my PC and the audio interface. It would look something like this:

    [​IMG]

    I have found plenty of dual 1/4" to 3.5 mm adapters (like this), but none that also have another 3.5 mm plug coming in. Is this possible by simply splicing together the wires, or does some special circuitry need to happen at the junction point?
     
  2. ramussons

    ramussons

    367
    71
    Jun 10, 2014
    How about combining with this...?

    3.5mm Mic Stereo Audio Y Splitter 1 Female to 2 Male Adapter Cable

    [​IMG]

    saw it on eBay
     
  3. Subtox

    Subtox

    10
    2
    Mar 8, 2016
    Thanks for the reply! It seems like that should work but after more reading I'm led to understand that I should be adding resistors on the tip and ring connections to prevent damage (to the devices and/or speakers? I'm not sure which).

    I found plans for a simple Altoids tin mixer here that I think I'm going to try. This plan is for only 3.5mm jacks but I can easily swap one of them for two 1/4" jacks. I might also add volume knobs.
     
  4. Audioguru

    Audioguru

    3,002
    673
    Sep 24, 2016
    Your wiring might blow up the amplifiers if the pc and the audio interface are both connected together. Added resistors will ruin the sound. Volume controls and a mixer are never connect to the output of an amplifier, they are always at the input.
     
  5. ramussons

    ramussons

    367
    71
    Jun 10, 2014
    Your Audio recording Interface has earphone outputs; they will not drive a speaker; unless the speakers have an amplifier associated with them. If there is an amplifier, it will not load your audio recorder.

    If you have a desktop PC, then it will have a Line Input / Line output. There will no issues using the PC as a source or as a recorder.

    BUT if the PC is a Laptop to be used for recording, it is not likely to have a Line input, only a Mono Mic input with a DC feeding the external electrec mic. In this case, you will need to combine the L and R channels AND reduce the level (attenuate) using external resistors and a capacitor.

    If the Laptop is a source, you'll be using the Line out and there should be no problems.
     
  6. Subtox

    Subtox

    10
    2
    Mar 8, 2016
    The Scarlett is for all intents and purposes a sound card, just external, so my understanding is that both the PC and Scarlett are outputting a similar, non-amplified signal. The speakers themselves are amplified and they have a subwoofer and their own volume control.

    The PC is a desktop and it has both line and mic inputs. I just don't use those jacks for recording. That is the purpose of the Scarlett -- it has lower latency, more inputs, lower noise, etc. (It connects to the PC via USB.)

    So if neither the PC or Scarlett are indeed amplified, I thought I would be able to safely merge their signals into a single set of speakers with a simple mixer. Any schematics I've found have said to use resistors to prevent damage or the signals from interfering with each other.

    I mentioned adding volume knobs to the mixer because the PC doesn't have its own (just software controls), so I was thinking it would be nice to have a tactile knob for the PC. The Scarlett has its own volume knob but I figured I should add a volume knob to both mixer inputs for consistency. Is that too many volume knobs? (Volume knobs everywhere, wooo!)

    Audioguru, you mentioned the resistors will ruin the sound. I have read that using too much resistance will ruin the sound, but I thought there was a happy medium somewhere. Given that my speakers are amplified and the signal sources are presumably not, does that change your assessment? I wouldn't be going from an amplified source's output into the mixer in that case.

    Again, this all assumes the sources are not amplified. I'm not exactly sure of the signal strength of the PC or Scarlett, or how much that matters to the needed ohmage of the resistors.
     
  7. Audioguru

    Audioguru

    3,002
    673
    Sep 24, 2016
    Sorry, you did not say the speakers are amplified and I did not look them up. I thought the audio interface and pc have speaker amplifier outputs to feed the studio monitor speakers you mentioned that must never be shorted together and an 8 ohm speaker fed through a resistor sounds like a bongo drum.

    Try 10k resistors as a simple mixer or use 1k resistors if the 10k resistors reduce the levels too much.
     
  8. Subtox

    Subtox

    10
    2
    Mar 8, 2016
    Thanks for the reply and sorry for the confusion. I've been learning a lot about this in the past few days (and still have a ways to go) and may have left out or assumed certain important bits of info.

    Any monitors meant for this audio interface would be amplified -- something like these, for example -- and I know the PC needs external amplification for anything but headphones or the smallest speakers, so I'm assuming neither device is amplified per se.

    The mixer plan I'm using calls for 1k resistors so maybe I'll start with that and see how it goes.

    If all else fails, as a last resort I'll throw some "cheap" (at least $300) studio monitors on my credit card. But it would be great to make this work with just a few dollars in parts, since I already spent a lot on my current speakers a few years ago.
     
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