trim plus multiple outputs

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eric appla

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I have a task to trim balanced line level +4dBu  +-6dB and split it into 3 separate outputs.

My current design is using THAT1246 balanced line receiver (-6dB) followed by passive switched attenuator (-6dB at zero setting), OA with +6dB gain and 3 separate THAT1646 to drive the outputs.

First question is, Is there any benefits in adding the switched attenuator into NFB OA gain setting network versus having the gain fixed and switched attenuator at the output?

Above does the job, BUT I had another idea to keep things balanced all the way and just use dual opamp and have common variable resistor to ground for both the positive and negative parts of the balanced line. This would also give me +-6dB trim and very few parts involved, if I use for example OPA1642 powered from +-18V I will theoretically get even better distortion performance and I dont see nay issues with headroom either, the question is how to do the split to 3 outputs but again just a resitor network and output buffer using similar OA like OPA1642 will do the job and its lot less parts.

Both solutions work and have low enough distortion but which one is better? What do you see as pros and cons of each solution?



 

abbey road d enfer

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eric appla said:
I have a task to trim balanced line level +4dBu  +-6dB and split it into 3 separate outputs.

My current design is using THAT1246 balanced line receiver (-6dB) followed by passive switched attenuator (-6dB at zero setting), OA with +6dB gain and 3 separate THAT1646 to drive the outputs.

First question is, Is there any benefits in adding the switched attenuator into NFB OA gain setting network versus having the gain fixed and switched attenuator at the output?
  There could be a very minor improvement in terms of noise. This would probably imply an inverting stage, since the gain of a non-inverting stage cannot be less than unity. You would need to reverse polarity somewhere else.

  Above does the job, BUT I had another idea to keep things balanced all the way and just use dual opamp and have common variable resistor to ground for both the positive and negative parts of the balanced line. This would also give me +-6dB trim and very few parts involved, if I use for example OPA1642 powered from +-18V I will theoretically get even better distortion performance and I dont see nay issues with headroom either, the question is how to do the split to 3 outputs but again just a resitor network and output buffer using similar OA like OPA1642 will do the job and its lot less parts.

Both solutions work and have low enough distortion but which one is better? What do you see as pros and cons of each solution?
What do you think will happen if you connect one of the outputs to an unbalanced line?
 

eric appla

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Oct 9, 2017
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abbey road d enfer said:
What do you think will happen if you connect one of the outputs to an unbalanced line?

very good point, I'm scratching the idea, it is the most obvious things one don't see when looking too much into detail!
 

dogears

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You could use a quadrifilar transformer for distribution if you have enough umph to drive it.
 

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