Basic Mixer - 1646 output stage - seeking critique

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AdAstra
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Basic Mixer - 1646 output stage - seeking critique

Post by AdAstra » Mon Nov 02, 2015 12:08 pm

Hello,
I'm trying to design a simple mixer for utility purposes in my live mixing work. I've pieced together this bus assignment and output stage based on THAT datasheets and so on, but I'm sure its got some problems. Any guidance would be much appreciated as I'm a hack/hobbyist, not a real engineer.
Thanks in advance.
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mediatechnology
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Re: Basic Mixer - 1646 output stage - seeking critique

Post by mediatechnology » Mon Nov 02, 2015 1:35 pm

Thanks for joining us and for the post!

The only thing I see on the schematic that you might want to change is the direction/polarity of the 10 uF OutSmarts caps.
That's due to the phantom protection.

I've started using non-polars there though the terminal voltage under most conditions is low enough that it doesn't matter.
The worst case is an unbalanced output/grounded leg at <<50 Hz.
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AdAstra
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Re: Basic Mixer - 1646 output stage - seeking critique

Post by AdAstra » Mon Nov 02, 2015 1:41 pm

My bad, I meant to make a note about those caps. I intended to use BPs, I just couldn't find an appropriate symbol for the Elna I wanted to use.

Thanks for the feedback. I'm a little shocked, I was expecting someone to say the circuit was a mess.

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Re: Basic Mixer - 1646 output stage - seeking critique

Post by JR. » Mon Nov 02, 2015 3:11 pm

I'll say that if you want,, :lol: but I have already posted comments to this elsewhere.

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

Post by AdAstra » Mon Nov 02, 2015 5:32 pm

Ha, thanks John.

So, forgetting my "simple" adjective for a moment, if I wanted to implement a PGA volume control in this circuit, could I simply insert a PGA2320 between the second 134 stage and the balancing stage?

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Re: Basic Mixer - 1646 output stage - seeking critique

Post by JR. » Mon Nov 02, 2015 5:58 pm

I've never used the PGA part so don't have any experience with that.

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

Post by ricardo » Mon Nov 02, 2015 8:24 pm

Any other reason for IC1b apart from restoring polarity? If not, you can omit it and just change polarity at the output of the 1646
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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.
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Whether the PGA can feed the 1646 directly depends on whether it provides Low Impedance drive. 1646 needs to be driven by a Lo Z .. usually directly from the output of an op amp.

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Re: Basic Mixer - 1646 output stage - seeking critique

Post by Speedskater » Tue Nov 03, 2015 7:52 am

As always, I'm quick to note that:
In an XLR balanced analog interconnect system, the Pin #1 does not connect to the circuit common (aka ground). Pin #1 is a shield and connects to the chassis at the connector.
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Re: Basic Mixer - 1646 output stage - seeking critique

Post by emrr » Tue Nov 03, 2015 3:07 pm

I don't know what a PGAxxxx is, but if it's output Z varies, or is high, I think you'd do better to insert it between IC1A and IC1B rather than before the 1646.
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mediatechnology
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Re: Basic Mixer - 1646 output stage - seeking critique

Post by mediatechnology » Sat Nov 07, 2015 6:10 pm

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.

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PGA2320 Block Diagram. Image courtesy of Texas Instruments.

PGA2302 Datasheet: http://www.ti.com/lit/gpn/pga2320
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