Signal Pass-Through Switch

Discussion in 'The Projects Forum' started by sohcahtoa, Jan 21, 2008.

  1. sohcahtoa

    Thread Starter Member

    Nov 7, 2006
    15
    0
    Greetings fellow enthusiasts. I was wondering if someone could help me with a building block of a larger project. Essentially, I am trying to create a sort of pass-through switch for an audio signal (a gate?). I'm basically trying to use an astable multivibrator to open and close a gate for an amplified audio signal to pass through to a buffer amplifier stage. I tried an experiment just to get going where I had the multivibrator connected to the V+ terminal of my buffer amplifier, essentially turning it on and off. I had a high-pass filter to filter out the clicks this created, but that also has the effect of filtering my audio signal. I know this isn't a good way to do it, but it was just part of the experimentation process to get the project rolling.

    Can a transistor be used to do this? I know you can use a transistor as a DC switch, but I'm not familiar with any configuration for passing AC signals.

    Google has not helped thus far.

    Any help is greatly appreciated!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    There are always more than one way to do that function. You could use a relay to switch the signal. If you want to be more electronic, consider an analog switch that is controlled by a logic level control signal. There is the old CA3140 op amp that has a strobe input that will disable the output. And you can use a transistor configured as an emitter follower. Controlling the voltage on the emitter will cause it to go out of conduction and stop the audio signal.
     
  3. sohcahtoa

    Thread Starter Member

    Nov 7, 2006
    15
    0
    Thank you for the great suggestions. I hadn't thought of an emitter follower configuration (not sure why). I'll give the emitter follower a try, and if that doesn't work out, I'll be placing an order for the CA3140.

    Thanks again for your help.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    That should be easy - just raise the emitter voltage so the base stops conducting, or open it up altogether. 3140's are pretty ancient - Jameco might be the last source.
     
  5. sohcahtoa

    Thread Starter Member

    Nov 7, 2006
    15
    0
    You're right about the CA3140. Only Jameco seemed to have any in stock.

    I just happened upon some discussions of the use of JFETS for this purpose. I found the following link:

    http://www.pic101.com/audiosw/index.html

    It gives some circuit configurations using JFETS to get around the "DC Thump" problem I was experiencing. Just wanted to throw that out there in case anyone's interested.

    Someone also suggested that the emitter follower may end up creating a fair amount of noise in this configuration. I'm not sure how much that might be in practice.

    sohcahtoa
     
  6. Gadget

    Distinguished Member

    Jan 10, 2006
    613
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    You might also want to try a CMOS quad bilateral switch array (such as the 4016 or 4066).
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Gadget beat me to it. I suggest the 4066, as it has lower impedance when conducting.
     
  8. sohcahtoa

    Thread Starter Member

    Nov 7, 2006
    15
    0
    You guys are making this too easy. ;)

    I haven't ordered parts yet, but after reading the datasheet of the 4066, I'm going to grab a handful of them for this project and for future use.

    I'm going to give the JFET analog switch a try as well, just for kicks (and learning).

    Thanks again.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The CD4051 has one signal switched to one of 8 outputs or 8 inputs switched to one output.

    The CD4052 has two switches. They switch a signal to one of 4 outputs or 4 inputs to one output.

    The CD4053 has 3 switches. They switch a signal to one of 2 outputs or 2 inputs to one output.

    Maxim has improved ones.

    They need their inputs and outputs biased at 0V for a dual supply system or biased at half the supply voltage for a single supply system.
     
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