Master mixer balanced out using THAT1646?

[RRR]

Hi,

I'm putting a mixer together comprised of 24 preamps feeding into 4 summing mixers (attached) and I want to feed the output of the 4 summing mixers (L1 - 4 Out/ R1 - 4 Out) into a master mixer with L/R gain control and a balanced outputs, and was thinking of using the THAT1646 for the master mixer/ balanced output section.

I was wondering what people thought of doing this, to essentially feed the THAT1646 with the 4 outputs of the 4 summing mixers and include a master gain control, or, do you think it would be better to sum the 4 outputs of the summing mixer into another NE5532 with a gain control, and use the output of the NE5532 to feed into the THAT1646?

Many thanks!

 

[JohnRoberts]

Get your signals squared away and volume controlled before sending to the 1646... I think they want to be driven by simple low impedance sources.

I didn't review your entire schematic, but in passing it is not generally a good idea to put pots in negative feedback loops. For one reason wiper bounce can momentarily look like infinite gain.....

JR

 

[RRR]

Get your signals squared away and volume controlled before sending to the 1646... I think they want to be driven by simple low impedance sources.

I didn't review your entire schematic, but in passing it is not generally a good idea to put pots in negative feedback loops. For one reason wiper bounce can momentarily look like infinite gain.....

JR

Thanks! So shall I get rid of RV9 and RV10, and instead have gain control pots coming off pin 1 and 7 of U2A/B going to ground, with the signal coming out of pin 2 of the pots?

Also, in that case do you think it would be better to sum the 4 summing mixer outputs into another NE5532 with master gain control, and take that output into the THAT1646?

 

[RRR]

I've redone the summing mixer and put together the master mixer part and plan on running L1 - 4 out/ R1 - 4 out of the summing mixers into C13 and C14 of the master mixer - can I run ML1 Out/ MR1 Out directly into a THAT1646? What do people think of this design so far?

Thanks!

 

[k brown]

You're getting some of the basics quite wrong.

As a start, I'd study the basic building blocks of these projects, and adapt as needed (substitute/add the THAT part for the outputs:

https://sound-au.com/project94.htm
https://sound-au.com/project94a.htm

 

[RRR]

You're getting some of the basics quite wrong.

As a start, I'd study the basic building blocks of these projects, and adapt as needed (substitute/add the THAT part for the outputs:

https://sound-au.com/project94.htm
https://sound-au.com/project94a.htm

Thanks - can you please elaborate as to what's wrong?

My mixers resemble Fig. 2 (attached) of https://sound-au.com/project30b.htm but with added panning with RV1 - 6

I don't want volume pots on the inputs as I want to sum multiple inputs and have a volume control that controls all 6 inputs, which is why I've used the scheme above.

 

[Matt Syson]

Having a fixed resistor as the negative feedback and then have your pot in parallel with it (possibly with a capacitor in series to reduce the DC offset that would cause scratchiness COULD be a solution.
The pot should be either a linear or neg Log to get reasonable level control with rotation position. The THAT chips like to 'see' as close to zero input SOURCE resistance so feeding from a 10K pot would probably work but less than ideal.

 

[Newmarket]

As has been discussed above, and has been a recurring subject on this forum previously, the THAT1646 needs to be fed from a low impedance source - the lower the better- to get the best out of it. eg directly from an opamp. I think this is clearly advised on the datasheet.

 

[RRR]

Having a fixed resistor as the negative feedback and then have your pot in parallel with it (possibly with a capacitor in series to reduce the DC offset that would cause scratchiness COULD be a solution.
The pot should be either a linear or neg Log to get reasonable level control with rotation position. The THAT chips like to 'see' as close to zero input SOURCE resistance so feeding from a 10K pot would probably work but less than ideal.

Thanks Matt - very helpful! I'll draw this revision up. I was also thinking, if I lowered the 10K pot in my drawing to a 5K, or even a 1K, feeding into the THAT1646 which has a 5K impedance input, do you think this would still be less than ideal, and preferable to drive directly from the opamp, or could this be acceptable?

 

[capacitorless]

Add an opamp stage (one with very low output impedance) before the THAT1646, and put the 10k pot before that?

 

[JohnRoberts]

Thanks Matt - very helpful! I'll draw this revision up. I was also thinking, if I lowered the 10K pot in my drawing to a 5K, or even a 1K, feeding into the THAT1646 which has a 5K impedance input, do you think this would still be less than ideal, and preferable to drive directly from the opamp, or could this be acceptable?

you can keep asking but the advice remains the same... drive the 1646 from a low impedance (op amp) and don't put pots in NF loops.

JR

 

[capacitorless]

Some of us, definitely myself included, need examples to work from, at least early on. So here's one (search string: that1464 example) that shows a circuit that might get you close:

https://electronics.stackexchange.com/questions/598877/that1646-distortion-on-output-signal

There's a problem with the circuit, that likely has to do with not drawing down bias coming from the preceding OPA134, which (in error) gradually charges up the coupling caps between the OPA and the 1646. Edit: a couple of potential fixes are described near the bottom.