Using DPDT for switching between sound card and AMP - HUM

Discussion in 'General Electronics Chat' started by deepbit, May 19, 2018.

  1. deepbit

    Thread Starter New Member

    May 19, 2018
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    Hi all,
    I am trying to build a box to switch between my sound card (connected to a MAC, MAC is grounded) and my guitar AMP (grounded too)

    I am using a DPDT and I've connected it as the following schema indicates.

    It generates humming, so I am not sure whether I should try to ground it or I've read as well trying to use a 3PDT.

    Any ideas of what solution can get rid of that humming?

    Thanks
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Where is the relay located with respect to its signal source and load?
    How is the relay powered?
    Are you using shielded wires?
     
  3. deepbit

    Thread Starter New Member

    May 19, 2018
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    It is all in a wooden case, no powered at all and the cables are not shielded.

    I only want to switch between to outputs, does it require powering?
     
  4. ElectricSpidey

    Member

    Dec 2, 2017
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    That switch looks like a reversing configuration, where did you get that schematic?

    I’m thinking your schematic should look more like this.

    A_DPDT.jpg .

    Or this...I wasn't sure what you wanted at first.

    A_DPDT_2.jpg
     
    Last edited: May 19, 2018
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  5. deepbit

    Thread Starter New Member

    May 19, 2018
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    I got it from here
     
  6. ElectricSpidey

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    Dec 2, 2017
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    I think the switch in the video has the common connections at the top of the switch, but that is not where convention places them in a schematic.

    From what I can make of it, the bottom schematic I posted is what he is using.
     
  7. ElectricSpidey

    Member

    Dec 2, 2017
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    And if you are trying to combine a "mono" guitar to output from both channels of the sound card, you will need some resistors.
     
  8. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    +1 for the first diagram in this post. Unless I've misunderstood the thread starter's intent, this looks like the solution (assuming the switch has its common connections in the middle, like most I've seen.)
     
  9. Tonyr1084

    Well-Known Member

    Sep 24, 2015
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    Maybe it's me but I'm a little fuzzy on the details. (or lack of)

    I'm assuming you have an amp. Likely a mono amp and you want to play your guitar through it at times and at other times you want to play music from your computer. There's also some assumption that you may want to play both at the same time.

    On the assumption you want to play one OR the other, a DPDT switch should be sufficient. In one configuration the switch applies a stereo signal to a mono amp. And yes, you're going to need a few discrete components (resistors). With the switch in position A (lets call it) you play your guitar. In position B you play music from the computer. Combining the stereo output of the computer into a mono input is pretty easy. You just want to be sure to do it in a way that doesn't permanently hold both left and right channels in a monophonic manor. When you switch to B you don't need to worry because you've only got a single source. But also keep in mind that the output from the computer is a much higher voltage than that from the guitar. Unless you're running the guitar through a pre-amp.

    Now, if you want to play music from the computer and accompany it with the guitar then you'd probably be better off building a mixer. They're not difficult; and will give you an opportunity to learn a little about audio electronics. I can't really shed much light on the subject because I've done so very little with audio. But one thing I DO know is that the hum you're picking up is because you're using unshielded wires and a wooden cabinet. You might be able to build the whole thing and put it in an Altoid's can ( mint can ) and use the metallic property as a shield. But most undoubtedly you're picking up AC hum from lights in the room and anything else that has power. Doesn't even have to be on. AC present in the walls can give you plenty of hum. In fact, I use that very nature to detect some circuits; mostly continuity on my O-Scope. Checking whether a switch is working or not and such. When any electronic component is grounded, unless it has high impedance, it will bring the scope down to a straight line. Good for also testing long lengths of cable. If there's an open, probing one end and grounding the other end, if you have an AC sine wave of sorts then you have an open circuit. If it's not open you will get just a straight line on the scope. But I'm betting you don't have one. That's OK, it's not necessary. All you need is a good soldering iron, a small metal can to house your electronics (for shielding) and a circuit board with the appropriate components. Depending on how complex you want to make it - that's your choice. For now, switching between source A or source B, a DPDT switch will do nicely. Just shield everything and you'll be good to go.
     
  10. Tonyr1084

    Well-Known Member

    Sep 24, 2015
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    Here: If you want to play one or the other, NOT BOTH at the same time then this will work. The blue box is an Altoids (or other) metal can with shielded wires. The shields are soldered to the can itself. You may have to sand the surface where you want to solder to; there may be lacker finish which will prevent you from soldering. The green box represents a DPDT switch (drawn in a more traditional manor). Throwing the switch throws both levers from A to B or back. But you can't play both sources at once. For that I'd recommend building a mixer.

    Guitar.jpg
     
    Last edited: May 20, 2018
  11. MisterBill2

    Well-Known Member

    Jan 23, 2018
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    The shield connections are very important and this posted drawing does not adequately indicate the connected shields. AND, likewise, shielded wire is required if hum is to be avoided.
     
  12. Tonyr1084

    Well-Known Member

    Sep 24, 2015
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    Really? I thought the chassis grounds soldered to the case of the metal can and the comment on sanding the spot where the shield is to be grounded made that sort of evident.

    The whole reason for using shielded cables and a metal can connected to those shields is what will eliminate the hum. Failure to ground will mean failure to defeat the hum. As it is I didn't address the value of the two resistors. In fact, looking back I see that I neglected to even indicate they were resistors. I was focused on the shielding. And yes, shielding is critical.
     
  13. ElectricSpidey

    Member

    Dec 2, 2017
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    Notice in the video the guy is using a plastic case, that is because having 2 inches of unshielded wire is generally not a concern for induced noise…generally…not absolutely, depends on conditions.

    No, I would be more concerned about proper switching on the ground lines, and other various incorrect wiring. (Judging from the supplied schematic)

    Now if the connecting cables are funky…then all bets are off. (also the leads from the connectors to and from the switch need to be short)

    Hum can be caused from a number of different problems, so I wouldn’t jump to any conclusions.
     
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  14. ebeowulf17

    Well-Known Member

    Aug 12, 2014
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    Oops! After rereading the original post and diagram a few more times, I think I misunderstood the original intent. There are two basic possibilities:
    1. Want to play guitar, with choice of sending signal to either computer or amplifier.
    2. Want to hear sound through amplifier, with choice of hearing either guitar or output from computer.
    I originally thought the intent was number 2, but now I'm starting to think it was number 1. Either way, I think ElectricSpidey has laid out the options nicely.

    I also agree that, although shielding is important, I doubt that it's the fundamental issue here. The wiring of the switch in the original post drawing makes no sense whatsoever. If the switch is rewired as drawn in one of ElectricSpidey's drawings (whichever one matches the original intent) I strongly suspect that it will work as planned, and audible signal will dominate. If there's also some hum, proper shielding of the switch enclosure should be addressed, but the switch wiring has to be fixed before nuances of shielding.
     
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