Basic Mixer - 1646 output stage - seeking critique

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JR.
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Re: Basic Mixer - 1646 output stage - seeking critique

Post by JR. » Sat Nov 07, 2015 6:33 pm

mediatechnology wrote:
You show all the 10k input mixing resistors grounded when that input is deselected. This means the mixing stage IC1a is operating at maximum noise gain all the time. If they are left hanging when the input is deselected, there's less noise if only a few inputs are mixed.

ricardo is right about that. I saw it originally but was focused on the 1646.


What you might consider is putting the 10K on the left side of the switch with the switch "pointing" to the right.
Have the commutator on the left, connected to the 10K Rmix, and the switch stators on the right-hand side.
One stator is connected to the virtual ground mix bus for On; the other grounded for Off.

When de-selected from the bus, the right-hand side of the 10K is grounded.
That configuration lowers crosstalk from open switch capacitance and not increase noise gain unnecessarily when a large number of inputs are de-selected.

You might eliminate one half the OPA2134 and use it on the other channel. As pointed out you can re-define the 1646 outputs to obtain polarity inversion.

JR and ricardo are the console designers so ultimately I defer to them.

emrr is correct about keeping a low impedance drive to the THAT1646.

Fortunately the PGA2320 is buffered and the output stages appear to have quite a bit of drive current capability with a 75 mA Isc.
I try to avoid console design questions since I no longer do it...

yes back-grounding de-selected inputs keeps a constant noise gain but may be preferable for keeping the differential math in balance (to cancel ground noise errors). The operative question is 9X the ein of modern summing op amp worse than one mic preamp at nominal gain (rhetorical no)?

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mediatechnology
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Re: Basic Mixer - 1646 output stage - seeking critique

Post by mediatechnology » Sat Nov 07, 2015 6:38 pm

SSL used quasi-balanced mix buses in the 4 and 6K that required back-grounding to keep it in balance.

I wasn't sure this one was limited to 10 inputs or we just stopped drawing at 10...
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AdAstra
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Re: Basic Mixer - 1646 output stage - seeking critique

Post by AdAstra » Sun Nov 08, 2015 5:47 pm

Extremely helpful, I appreciate all replies. I had missed the last few in a flurry of activity over the weekend. But as you can see in updated schematic, it should be quite simple to add grounds on pin 2 of the relays. The current schematic has the PGA sitting between the to 2134 stages, I figured the safest things was to buffer on both sides (please remember I'm an absolute beginner here:))

Now it sounds like I could eliminate the second 2134 stage and drive the PGA straight from the 2134 output and into the 1646. It would save me a resistor and a cap and shorten the signal path so it sounds like that is the route I should go.

I will post my progress up to reading this anyway, for what its worth. I've got a digital ground plane on the right side of the board under the GPIO expander and darlington arrays. Also a very small signal ground plane under the input connector area (to be fed from another PCB in same chassis). Signal grounds all return to that point and there is a pad to return that to chassis ground. "Dirty" grounds all return to PSU 0v. Digital ground isn't tied to any other ground, as the idea was to return all digital grounds back to the MCU board, and from there to chassis ground. Pin 1s are linked directly to chassis.

I'll get back to work integrating these comments, but if anything is otherwise fishy with the rest of my work I'd love to hear about it.
Thanks to all.
AdAstra.
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Last edited by AdAstra on Sun Nov 08, 2015 6:32 pm, edited 1 time in total.

AdAstra
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Re: Basic Mixer - 1646 output stage - seeking critique

Post by AdAstra » Sun Nov 08, 2015 6:02 pm

Ok, I think I updated the switching as described, here's another drawing:

Also, each bus gets 8 relay-switched inputs, and a couple hardwired inputs, so 10 inputs per bus maximum.

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AdAstra
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Re: Basic Mixer - 1646 output stage - seeking critique

Post by AdAstra » Mon Nov 09, 2015 12:31 pm

Ok, one more update. This is a bit cleaned up and now has a single 2134 stage feeding PGA, then PGA driving 1646. Did a bit of re-routing and cleanup, and made the 15v+- rails bigger.

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