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Multi-input amplifier + Wolf's Dragon Foals

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  • Multi-input amplifier + Wolf's Dragon Foals

    Hi all, I'm starting a fresh project and was hoping to get some feedback

    I've had a pair of Wolf's Dragon Foals lying around doing nothing for a few years, that I figure I may as well give to my girlfriend. They are bare MDF currently, so the plan is to finish them off nicely in red tolex, and build a matching amplifier to drive them. Finishing the speakers shouldn't be too hard, so the amp is my main focus.

    You can see a colourful schematic attached below (side note, Lucidchart is pretty awesome!). The general idea is that it's running Sure's popular 2x15W amplifier, and has 3 different inputs to choose from: a physical 3.5mm jack; Bluetooth, or a Google Chromecast Audio. See the links below for info on each component.


    Things I'm wondering about so far:
    1. Will a 60W (12VDC/5A) AC-DC adaptor be sufficient? The amp requires 12VDC/2A, the bluetooth receiver requires 12VDC/1A, Google Chromecast requires 5VDC/1A, and the volume control also requires 12VDC but doesn't specify current (I assumed 1A max). So all that adds up to 53W... should I have more overhead? (Or not, seeing each component's current spec probably has a bit of wiggle room).
    2. I've got all the powered components connected to the same DC jack. It it okay to wire it this way or will I run into any issues?
    3. I think I'm on the right track with using a 4 pole/3 position rotary switch to switch between 3 different audio sources. That said, I'm not familiar with them at all and any advice about them would be good.


    Other stuff:
    • The ground loop isolator may not be needed, I'll have to test the whole setup and see what the noise is like.
    • The volume control is also up in the air at the moment. I'm not sure if I want to include it; might just be easier to only worry about the volume of the source.


    Any input is appreciated, thanks!
    Attached Files
    Last edited by revans; 12-15-2015, 03:56 PM.

  • #2
    Re: Multi-input amplifier + Wolf's Dragon Foals

    1. That supply should be more than sufficient.
    2. You're wiring it correctly. You may run into some noise issues, but that's always a crap shoot - you can never tell without building it first.
    3. You need a 2-pole, 3-position switch as a minimum. The 4-pole switch should be OK. Make sure the switch is of the "break before make" type. Generally speaking, don't run your ground signals through the switch. Connect all your ground wires together at the amplifier input and only run the signal wires through the switch.

    Good luck!
    DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
    The Aria's - RS150-4 (or RS150-8) + XT25SC90
    The Mariposa's - TEBM65C20F-8 + ND16FA
    The Canzonetta's - RS100P-8 and ND16FA
    AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

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    • #3
      Re: Multi-input amplifier + Wolf's Dragon Foals

      I see 1 issue, the volume control should show up in the block diagram before the amplifier. Looks like a slick little system! Are you going to hide the Chromecast in the amp chassis too?
      Electronics engineer, woofer enthusiast, and musician.
      Wogg Music
      Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat, SuperNova Minimus

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      • #4
        Re: Multi-input amplifier + Wolf's Dragon Foals

        Originally posted by tyger23 View Post
        1. That supply should be more than sufficient.
        2. You're wiring it correctly. You may run into some noise issues, but that's always a crap shoot - you can never tell without building it first.
        Yes that’s what I’ve run into on previous projects. I’ll be sure to wire it all up and test it first.

        Originally posted by tyger23 View Post
        3. You need a 2-pole, 3-position switch as a minimum. The 4-pole switch should be OK. Make sure the switch is of the "break before make" type. Generally speaking, don't run your ground signals through the switch. Connect all your ground wires together at the amplifier input and only run the signal wires through the switch.
        That’s a non-shorting switch, right? I didn’t know the ground wires didn’t need to go through the switch, thanks for the heads up. What's the idea behind it, reducing noise issues?

        Originally posted by wogg View Post
        I see 1 issue, the volume control should show up in the block diagram before the amplifier. Looks like a slick little system! Are you going to hide the Chromecast in the amp chassis too?
        Oh, yes it should! Went back to Sure’s documentation & looks like I misread it the first time around. And yes, that's the plan with the Chromecast! So ideally it will end up as a fancy-looking, self-contained, little box that my girlfriend can sling Pandora etc to via her phone.

        I've attached an updated version of the circuit schematic.

        Further question: Ideally (if it actually turns out that I need it), where should the ground loop isolator be placed? After the rotary switch, volume control, or the amplifier?
        Attached Files
        Last edited by revans; 12-15-2015, 03:57 PM.

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        • #5
          Re: Multi-input amplifier + Wolf's Dragon Foals

          Just updating this now that I've got everything hooked up (yeah, it's a bit of a mess of cables...). I designed a test bed with a bunch of modules that all the hardware can bolt onto - so I can move things around and change parts out as needed. It was laser cut at work, and then I did a fair chunk of soldering for all the patch cables.

          Click image for larger version

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          The cheapo rotary switch ...
          Seems to do the job, if with a bit of a pop sometimes. I'm planning on finding a higher quality one for the finished product. I ended up connecting the grounds to the switch too, mainly because I was curious to see how it would pan out.

          Noise/interference...
          Is definitely an issue, whether it's from ground loops or whatever (I don't have a great grip on that stuff).

          Bluetooth:
          Is always silent, regardless of whether it's wired directly to the amp or selected via the rotary switch. It doesn't need the ground noise isolator, makes no difference.

          Chromecast Audio:
          Regardless of whether it's wired directly to the amp or selected via the rotary switch, the chromecast is very noisy. But not an audio hum, more like spikey interference that sounds like when speakers pick up your phone receiving a text. However I've figured it's related to the power supply. When I power the chromecast from a wholly separate microUSB phone charger, then it's silent. Funnily enough, powering the chromecast from this secondary supply induces a whole lot more hum on the other 2 sources. Either way it's irrelevant as I only want to use one power supply. Fortunately, the ground loop isolator essentially removes all the noise That said, I'd rather not have to squeeze in the isolator if possible, so I might investigate using a proper DC-DC isolated power supply instead of my cheapo aliexpress one.

          3.5mm jack:
          There's something kooky going on with this. I thought it was an issue with the jack at first, as whenever I switch to it there is a significant hum. But after some investigation I'm not too sure:
          1. If I disconnect the amplifier from any sources and turn it on, it is silent as expected.
          2. Then, if I plug in an RCA cable to the amplifier's left channel input jack, the left speaker will hum. And vice versa with the right jack/speaker. This is with plain RCA cables with nothing connected to the other ends. Same thing happens with a RCA to 3.5mm cable. Or when I use the terminal blocks.
          It's not a power issue, as if I disconnect all the other powered components, it still produces the hum. What's going on with the amplifier's input here & how can I fix it? Any help would be much appreciated!

          Functionally...
          It seems pretty great! I really like the Chromecast Audio, and being able to sling Google Play Music/Spotify to speakers via a phone. The sound quality is surprisingly good as well! I'm sorta debating what to keep as far as functionality goes though. I definitely want a physical jack and the chromecast. But the bluetooth & volume control are up for debate...
          Last edited by revans; 01-08-2016, 04:38 PM. Reason: Figured some stuff out re. noise

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