2 channels of audio crossing over, not sure why

Discussion in 'The Projects Forum' started by BackyardBrains, Apr 10, 2014.

  1. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
    Brief description of project
    I am working on a device that has two audio signals. We have set up the board so that there are two audio out jacks. On the right jack the tip is sig#1 and the ring is #2. It is switched on the other, the left jack the tip is sig#2 and the ring is #1. We did this because we often use Radio Shack's external speaker which takes mono input from the tip. This way we could plug in two speakers and hear the one signal on one speaker and one on the other. Both audio out jacks have the other signal on the ring so that we can send both signals to a mic-in port on a computer and see both signals.

    Also, there are two switches / thumbwheel potentiometers. Both switches power all of the board's ICs. The right pot controls the gain of the right signal (#1) and vis versa for the left.

    I have realized a problem I could use some help solving:
    1. When both switches are on, I am observing the audio crossing-over (hearing both signals on the external mono-input speaker) in one instance but not another. It is working well when I have the right audio jack connected, there is no combining of the signals on the external speaker. When I tried this with the left audio jack connected, I heard with both signals. These two tests were done with both switches on, pots turned up all the way.

    Update: I found that when I connect an audio cable to the right jack (with other end is connected to nothing), the crossover issue of hearing sig#1 and sig#2 (heard on the external speaker with the left jack providing the signal) goes away. The right jack has a tip switch, so that the onboard 8ohm speaker outputs the signal#1 when the the right audio jack is not populated and when this jack is populated, signal to the lm386 (8ohm speaker amp) is disconnected.

    I have attached a picture of the schematic and the board layout.
    Last edited: Apr 10, 2014
  2. Veracohr

    Well-Known Member

    Jan 3, 2011
    Have you checked your jack wiring/physical connections? Might have something backward.
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    I see four audio inputs, not two. What is the input signal, and what is the function of this circuit? Is the IA supposed to be a difference amp?

    Your 2nd stage gain is 220, not 80.

    There is no decoupling on any of the opamps. For a dual opamp running running 47 dB of gain on each side, with power coming from long traces rather than planes, the power needs to be extra clean. Plus, the LM386 is notorious for needing good decoupling.

    Your lowpass corner freq is 1292 Hz, not 3000 Hz.

    Your highpass corner is 339 Hz, not 700 Hz.

    It looks like the gain-bandwidth corner freq is around 7 KHz. This is the point where you have no negative feedback and the amp is running wide open. Are you counting on this for your lowpass filter characteristic?

    BackyardBrains likes this.
  4. Experimentonomen


    Feb 16, 2011
    And the layout is garbage, a typical example of someone using the autorouter function.
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    Ex - Not so sure about the autorouter. Maybe parts of it, but htere are a lot of off-angle runs that had to be hand routed.

    Backyard - Went to your site and saw what this is. More questions:

    When you say you hear both signals, are they equal or is the correct signal louder (much louder?) than the leaking one?

    Why is the 2nd stage gain so high while the much better IA is strapped for a gain of 4?