Simple Audio Switch - Hum

Discussion in 'The Projects Forum' started by Ix88, Dec 31, 2012.

  1. Ix88

    Thread Starter New Member

    Dec 31, 2012
    6
    0
    Hi, I have a few questions about a stereo audio switch. I've wired a DPDT switch to three 3.5mm audio sockets so that I can select an audio source (from a choice of two) for one output (going to speakers). The grounds from the three sockets are all connected to each other.

    It works as far as selecting an audio source goes, but there's significant hum present. Also one of the audio sources is much much quieter than the other. The hum is not present if I remove the switch and connect one source directly to the speakers. I'm away for a few days so I don't currently have my switch and speakers etc with me to check, but I think that removing the switch also stopped the quiet problem (i.e. I don't have to turn the speaker amp up really high to be able to hear something from either input).

    I was wondering if anyone could point me in the direction of how to fix this?

    Is connecting the grounds of the two inputs and the output the right way of doing this? Or would it be better to use a 3PDT switch so that the two inputs are isolated from each other? There are a few people who have made the same kind of switch that suggest connecting the grounds together (including on this forum I think), but it seems like a 3PDT switch would make more sense?

    My soldering could well be the problem so I will also check that when I can, and possibly just redo it anyway. I'd like to know about the ground question as well though.

    Thanks for any help!
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Are those thwo sources independent, as in different devices? Then you probably have a ground loop, so a 3PDT switch should solve that, you can try it by disconnecting the ground of the jack going to the unused device, and see if it helps.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Connect all the grounds together.

    Did you use shielded audio cable to prevent hum pickup of radiation from mains wiring that is all around you?
    Is the switch and its wires in a metal box connected to the audio grounds?

    EDIT: You also have a problem where the switch makes one audio source level much too low so the switch is connected wrong. Post the schematic of the switch and your wiring.
     
  4. Ix88

    Thread Starter New Member

    Dec 31, 2012
    6
    0
    That's right; they're two different devices. I'll try just disconnecting one ground first like you said, and then use a 3PDT if it makes a difference.


    Aah... I wasn't really thinking, maybe that's why I have this problem and others online don't - I only used unshielded single core wires (the same as others) but I also put it in a plastic box while I think the others used metal. Do you think it would be best to switch to a metal box, or use audio cable (2 core screened cable, right?), or both? Also would connecting the grounds together not create a ground loop like kubeek suggested? The inputs are from different devices.


    Thanks for the replies!
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    I bet there´s one of your problems right there. Without shielded cables and with long runs you will get all sorts of nasty stuff induced into the wires, even without the possible ground loop.
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,362
    Something doesn't jive. You are taking audio output and feeding into speakers.
    Presumably these are 8Ω speakers. Speaker cables do not need to be shielded because the speaker impedance is so low there is very little pickup from external EMI.

    Hence shielded audio cable or a metal box should make no difference.

    Something is getting picked up and being fed back to the low level, high impedance input of your signal source. I would guess there is something wrong with your ground connections.

    Having said that, shielded cable and metal box would reduce the pickup but I would question why is the input source picking up interference. Check your grounds.
     
  7. Ix88

    Thread Starter New Member

    Dec 31, 2012
    6
    0
    I forgot about this bit of your post, sorry. It's just wired as shown in the attached image. Edit: to clarify, the "output" side on the attached image is the amp and then speakers.


    Thanks for the explanation. My audio sources are things like the audio from a PC, and it's connected to speakers via an amp. So is it the amp input impedance that's important in this case, or the speaker impedance? I would guess the amp? And I think it's large so that makes shielding important? I will definitely check the ground connections when I get back in a few days anyway.
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    That is confusing, you said you are swithcing two sources to one amp, but this looks like switching one source to two different amps.Please put some labels in there and show where the speakers are actually connected.
     
  9. Ix88

    Thread Starter New Member

    Dec 31, 2012
    6
    0
    It is switching two sources to one amp. The block labelled output goes to the amp, I probably should have put it on the right side of the image, sorry.
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    So speakers are connected to the amp, so don´t mind their impedance. You will definitely need shielded cables for the connections because the impedances are very high and will catch noise.
    Have you tried disconnecting the ground between input a and input b, and veryfying you don´t have problems with just one source connected?
     
  11. Ix88

    Thread Starter New Member

    Dec 31, 2012
    6
    0
    I think I should show the full system in case something I don't understand has introduced a problem. The switch and connected devices are shown in the attached image. I don't know what the monitor does (if anything) to the audio from the HDMI cable before it outputs it to the headphones. The Xbox audio via the monitor is the louder one. PC is much much quieter. Sorry I didn't include full details to start with, I think this is everything now.

    Thanks, I will try getting rid of the single core stuff. I've tried removing the switch and only having one source connected, no problems like that (with either source). I've never tried disconnecting the grounds inside my switch before and I can't do it now - I'll give it a go when I'm home again in a few days.


    Edit: So far it sounds to me like the best solution is to check the ground connections and replace the wiring with shielded stuff. And maybe go for the 3PDT switch to separate the grounds.
     
  12. MrChips

    Moderator

    Oct 2, 2009
    12,447
    3,362
    Ok, now you say it correctly. You are taking low level signals from the 3.5mm jacks and feeding into the input of an amplifier, not directly to the speakers. (I know it is common layman's practice to call PC amplified speakers just "speakers" hence the confusion). The input impedance of the amplifier is much higher, I would guess in the order of 10kΩ.

    In this case, yes, you must use shielded audio cables and a metal box will also help.
    The accepted practice in a self contained unit, is the shielded audio cables are grounded at one end only.
    However you still need a common ground between the source and the amplifier. In this case you don't have much choice but to connect the shield at both ends of the cable.
     
  13. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The sketch shows no ground connection between thew sources and the switch circuit.
    It also shows no ground connection betwewen tyhe switch circuit and the input of the audio amplifier.

    A mono shielded audio cable has ONE signal wire that is surrounded by a shield of braided wires or metal foil.
    A stereo shielded audio cable might have separate left and right signal wires and each is surrounded by its own shield, or it might have two signal wires surrounded by a single shield.

    Since you did not use shielded audio cable then your ordinary wires are like antennas that pickup all kinds of interference and hum.
     
  14. Ix88

    Thread Starter New Member

    Dec 31, 2012
    6
    0
    Sorry for the confusion. You're right - I was using "speakers" to refer to my amp also (I should have known better, it's a separate amp and speakers, not an all-in-one PC speaker thing).

    I'll replace the single core wire in that case. Should both input shields be connected always to each other and to the output shield, or should I use a 3PDT switch so that only one input shield is connected to the output at once? And would you mind explaining why if possible please? I don't understand what the advantage of all of them connected together would be.


    Ah I didn't know there were different kinds of stereo shielded audio cable - I assumed the single shield case. How is a double shielded stereo cable wired up to the tip, ring and sleeve at the ends? Do the shields just get joined together?

    In my switch, only the left and right audio lines are switched. The sleeves of the jacks of the two sources are always connected to the sleeve of the output. In the sketch the three rectangles (labelled "Input A", "Input B", and "Output") each represent a 3.5mm socket, and I tried to show that the sleeves were all connected by drawing the wires between where it says Gnd on the sockets. Does that make the ground connections correctly?
     
Loading...