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Old 15th March 2012, 07:16 PM   #1
fualcr is offline fualcr  Canada
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Wink How to Mult (aka split) a line signal into 8 identical buffered outputs... properly!

Hi Guys, I thought I would ask the collective genius for some pointers on a little problem I would like to solve: Splitting/sending a +4dBu line-level signal 8-ways(!). The only real requirement is superb performance = ULTRA-low distortion and noise

So I drew up a preliminary schematic (below) and I'm would love to hear if anybody could suggest any improvements to the design or at least clarify some points which have been nagging me:
  1. Is there any concern about all 8 output op amps sharing the same bias current path to ground via the single 47kohm (labeled R3)?
  2. Will enabling/disabling an output using the switches (relays actually) result in the injecting of audible pops/clicks into the *other* enabled outputs, given the super-high input impedance of the op amps?
  3. Would there be any benefit (stability/performance-wise?) in mildly "isolating" each of the 8 output buffers with a small 47ohm series resistor on each of the inputs of the op amps at the point labeled Point "A"? Or perhaps one node before point "A" just before the switches?
  4. Have I missed anything? Is this a recipe for disaster?
I have specifically tried to avoid large resistors in the signal path to keep noise to an absolute minimum , and I might also add a HF rejection filter before the single input op amp.

So if anybody has thoughts on this, or better yet, experience, I'd love to hear it.. Many thanks!

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Last edited by fualcr; 15th March 2012 at 07:22 PM.
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Old 15th March 2012, 09:50 PM   #2
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With a DC coupled circuit you just might get clicks from the 8 relays.
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Old 15th March 2012, 09:53 PM   #3
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After the first amplifier feed the other amplifiers from 2.2K resistors. Then have the relay short the input to ground through a capacitor. Also put a high value resistor across the relay. You also can put the relay on the output.
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Old 15th March 2012, 10:44 PM   #4
fualcr is offline fualcr  Canada
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Hi Guys thanks for the input so far. I guess I should've mentioned the previous stage was going to kill all DC (not shown). For the record, it's not the pops/clicks on the output being enabled/disabled that bothers me, it's the pops/clicks on all the other 7 outputs when 1 input is enabled/disabled. For example, I wouldn't want whoever/whatever is receiving Outputs #1 thru 7 to be disturbed with an audio pop/click if Output #8 is being enabled/disabled.

@simon7000 - is your idea of 2.2k resistors is to tackle this issue? The only thing is I'm trying to keep resistors as large as 2.2k out of the signal chain since this is a super-duper low noise system, aiming for better than -130dBu noise ($$$ LME49990 opamps and ground planes!). A 2.2k resistor in the chain would compromise this with its Johnson noise. Is there any reason the isolation resistors need to be even as large as 2.2k (or be there at all)? As for the mute capacitors, I don't believe they are needed if I don't care about pops/clicks in the output channel being switched...

Below is the updated schematic showing where I believe @simon7000 was suggesting the 2.2k resistors. I have labeled them "isolation resistors" for lack of better words... Any additional insight is much appreciated.

Click the image to open in full size.
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