Audio buffer and 'fan out' question

Discussion in 'General Electronics Chat' started by Mike33, Apr 3, 2010.

  1. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
    Hey guys,

    Just a quick question regarding audio buffers. Ok, you can connect an opamp as a 'virtual earth' with such a high input impedance that it allows you to work it as a mixer, inputting several signals simultaneously without degradation. Great property of such a high-impedance device.

    What about on the other end? Since the buffer should be able to drive a large load, can you run, say, 2 or 3 different (high input Z) distortion pedals from the output of a single buffer? I see splitter circuits that usually use more than 1 device for their output - I am wondering why - will you get interaction if you try to drive more than 1 device at the output?

    Thinking about parallel signal processing and trying to keep things simple here.
    Thanks for any thoughts!
  2. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    Most opamps have a max output current of 20mA. If you want a peak output voltage of 5V and the supply is 9V then the minimum load resistance is 5V/20mA= 250 ohms.
  3. kkazem

    Active Member

    Jul 23, 2009
    You're correct that using one audio buffer amp as an input to several other devices like distortion pedals may have some interaction, it depends mostly on the devices that you would be plugging the amp into and since op-amps are cheap and come in quad packages for nearly the same price as single op-amps, why not take one output and feed it into several unity-gain buffers so you are guaranteed isolation and it will be free from interaction with the loads. The unity-gain buffer doesn't even need any resistors, but make sure to bypass the power supply pins with a 0.1uF cap as close to the power supply pins as possible.
    God luck.
    Kamran Kazem
  4. Mike33

    Thread Starter AAC Fanatic!

    Feb 4, 2005
    Thanks, guys. I can't think of ANY effect circuit having an input impedance of <250 ohms!! {Fuzz face? lol} Maybe 10k, in a BJT-based pedal. So if this was used to create a parallel effects chain, you'd go lower, maybe 4-5K....

    Parts ARE cheap in this regard, KKazem - I am thinking of space on the PCB more than anything, and 'keeping it simple'. The best way to test this is to build a buffer and feed 2 pedals, then recombine the signal with another opamp mixer (TL072 would do nicely). If one continued to load the buffer, you would expect to reach a point of diminishing returns....
    Mainly I wanted the 'theory' behind why I see several models of "splitter/blender" circuits using multiple buffers at the front end. I guess to offer more current and reassurance that things won't interact! Especially if a player had 5 or 6 parallel....