basic audio mixing circuit help

Discussion in 'The Projects Forum' started by af8567, Jan 8, 2012.

  1. af8567

    Thread Starter New Member

    Aug 8, 2010
    3
    0
    Good day,
    I need some basic help on an audio mixing circuit please.
    Scope- I have 2 audio devices each with stereo headphones output. I want to take each into a project box (mono to an xlr- left/ right pin 2. pin 1/3 shorted to each other for ground). Each needs an independent volume control. The signals post volume pot need to be combined to goto 2 outputs. 1 output is another volume pot to a headphone jack. The other straight to an xlr connection as an output to an external powered speaker i have (no volume control needed).
    I have a 4.7k pot for the first stage volume for each input with a 10k resistor on the output leg. The signals are all soldered together then sent to another 4.7k pot for the headphone control and straight from the headphone pot input to the xlr output for connection to the power speaker (headphone volume should NOT control the volume that goes straight to the xlr output). The circuit works except for when I crank up the headphone volume the 2 inputs are drastically reduced. Also I am losing overall gain in the circuit which I am thinking is because of too high Ω volume pots for the inputs. I know there is a very basic concept here that Im missing. Anyone help? I would love to not have to power this circuit.......Thank you in advance for any help
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,055
    3,245
    It's too difficult to follow what you are doing from your description. Post a schematic.

    Headphones are normally low impedance, such as 32 ohms, so obviously the high circuit impedances you have will not work well with that.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Mixer circuits normally contain amplifiers to make up for losses, and to isolate the signal sources from one another. Without amplifiers, signals will be able to pass between the signal sources - in an extreme case this may do damage. Crosstalk can be reduced by attenuation, but then the output will be low.

    Signal level settings will also interact if the signals are simply mixed using pots, as the impedances will change. Thus, as you have found, turning up the headphones reduces the through level.
     
  4. af8567

    Thread Starter New Member

    Aug 8, 2010
    3
    0
    I'm giving it hell!! I hope this makes sense
     
  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Shorting the left and right channels directly together at each input is unkind to the signal sources. It is likely to cause distortion, and could do damage.

    There should not normally be resistors in the ground leads. (If you take them out, you should add at least a few tens of ohms in series with each input, assuming we start with inputs designed for modern headphones).

    Above all, you need amplification to make this work without everything interacting.
     
  6. af8567

    Thread Starter New Member

    Aug 8, 2010
    3
    0
    I found by adding the resistors changing either input pot did not affect the other volume pot level. Ok so its clear adding a couple pots to this is not gonna work. So my next thought is to look through one of my circuit books and cobble something basic circuits together. Would this be the right track- A summing amp at each input to sum the stereo into mono. Add an amplifier circuit to each of the inputs then off to the volume pot. Next another summing amp to combine inputs 1 and 2. Off to the headphone volume pot then to the 1/4" jack. The external powered speaker would get its signal from the input side of the headphone volume pot. Does this sound like a good start (gonna try to draw a schematic today). Thank you all for your continued help!
     
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