Switch between stereo channel selector and audio amplifier

Discussion in 'The Projects Forum' started by Yarrow, Mar 10, 2009.

  1. Yarrow

    Thread Starter Member

    Sep 18, 2006
    23
    0
    Hi, I have just completed designing a stereo channel selector with the purpose of driving a headphone output. The circuit is able to drive speakers as well, but the sound quality is not much to brag about. Therefore I would like to add a switch to the channel selector that enables switching between an audio amplifier (for use with speakers (let’s call it*speaker mode*)) and the already implemented output (headphones (let’s call it *normal mode*)).

    Link to the stereo channel selector: http://www.electronicsforu.com/efylinux/circuit/feb2003/Add on stereo channel selector.pdf

    There have been done some minor changes in the linked design to fit a 5V supply. The stereo output is driven by a TDA7050 (headphone amplifier).

    I have not done the audio amplifier design yet, that remains for another day. I would just like to implement the switching capability on the channel selector and add the audio amplifier at a later point.

    From what I understand it is not possible to use a normal analog switch to switch between *normal mode* and *speaker mode*. There will be a big bang sound if it is done in that fashion (?). If I am not mistaken the following should be done:

    1) The signal should be attenuated (or muted) as much as possible before it is fed into the audio amplifier?

    2) There should be implemented a delay from when the switch between *normal mode* and *speaker mode* is initiated, to when the signal from the input is fed to the audio amplifier.

    My question is if anyone here has any suggestions on how I could implement such a switching mechanism with delay and attenuation?

    Tnx in advance!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You can get sneaky by doing the switching between amps. I have a power amp for the speakers, and an old Southwest technical class A amp for my headphones. Just turn the selector and turn on the proper amp.

    I would suggest you try a headphone amp, by the way. The sound quality is vastly superior to padding down the output of a power amp.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I don't think any circuit on the website from India works. The circuits are very old and many parts are obsolete.
     
  4. Yarrow

    Thread Starter Member

    Sep 18, 2006
    23
    0
    @beenthere: There is a headphone amplifier on my design. Thats the one I tested the speakes with and got somewhat bad sound quality. What I am after is the swithcing between the headphone amplifier and a actual audio amplifier for speakers while the selector is on and the audio signal is alive.

    @Audioguru: The channel selector has been somewhat tested, and I think the circuit will work, just need the switching mechanism between the headphone amplifier and audio amplifier for the speakers before I can put it down on a pcb and test the entire crircuit.

    I will post my entire schematic tomorrow. (Dont have it on the current computer)
     
  5. Yarrow

    Thread Starter Member

    Sep 18, 2006
    23
    0
    Ok, here is my schematics of the stereo channel selector. All comments/suggestions on the circuit are much appriciated aswell.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    1. I've done the same thing with wafer switches and with relays.

    2. It would be nice to have a narrative from you to walk us through the schematic. I am sure we can figure it out, but an accompanying theory of operation is very nice to have. What we see and what you intended may be different. An explicit walk-through is a big help.
     
  7. Yarrow

    Thread Starter Member

    Sep 18, 2006
    23
    0
    Ok, I will try my best.

    The most of how the circuit works is described in the link I gave in my first post. Basicly the circuit searches (via the NE555D timer that clocks the decade counter HC4017) the inputs and detects(via the dot/bar meter LM3914) an audio signal if nothing is spesified by the user. The user can keep pressing the switch to select channel manually. Based on the output of the HC4017 (the channel selected), the output of the HC4017 is decoden into binary code (via the HC32) and fed into the 7segmet display, displaying the current channel number. The output of the HC4017 also control the 4066 swiches for both channels (stereo). The audio signals are then fed into the TDA7050 headphone amplifier which drive the output. The signal from the TDA7050 also drive the LM3914 dot/bar meter that display the audio levels using LED diodes.

    The switch between the already exsisting headphone amplifier TDA7050 and the audio amplifier i intend to add on later, is the part I am missing.
    I would like to avoid wafer switches and relays to keep a compact solution. Anyone has any ideas?
     
  8. audiomodder

    New Member

    Jan 3, 2010
    1
    0
    Hi, I would really like to build this circuit with the LM3914 you used as opposed to the KA2281, but is there a way to do this with a simpler setup? I noticed your schem has a few extra components, including LED segment displays. Could I just substitute the LM3914 in the first schem for the KA2281? Or is there a way to build it without the dot bar meter but still providing the forward bias needed to T1?
     
    Last edited: Jan 3, 2010
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The 74HC4066 (and the original CD4066) switches are supposed to be biased at half the supply voltage to pass audio safely and without distortion.
    You seem to have them floating or grounded by the input sources.

    You should use a Cmos 555 (LMC555, TLC555 or ICM7555) since the ordinary 555 causes huge 400mA supply current spikes that upsets logic ICs.
     
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