Yet Another Active Summing Mixer

[synthetic]

Radiance suggested that I start another thread for my project. Here is where I'm at. I took the front half of NYDave's passive summer with the makeup of Forssel's active summer. Questions for the trust:

- How do I calculate the feedback resistor for the makeup stage? I have a multiturn trimmer in there as well for fine tuning.
- Is that OP275 going to be able to handle makeup gain as well as driving the output? Should I add an output gain stage and/or transformer?
- I'd love to swap those OP275s for discrete opamps like a 990 or Forssel 992, but that might need to wait for version 2.0

Let me know what you think, -jl

 

[synthetic]

Hey Radiance,

Synthetic. So much for avoiding confusion.

I'm definitely no expert but I think that it is a better idea to have a 'passive' fader/pot rather than have a trimmer on the opamp feedback resistor. You will have to ask some of the more experienced diy'ers out there.

That is an internal multi-turn trim pot. The idea is to have a fixed feedback resistor (15k?) and have a trim pot in parallel with that for ultra-anal level matching.

- How do I calculate the feedback resistor for the makeup stage? I have a multiturn trimmer in there as well for fine tuning.

The gain of an inverting opamp is -Rf/Ri. So, for example in your summing network, each channel will have -(15//25k trim) / (2k). The overall gain of the summing network is -(all the channel resistors in parallel)/(15//25k trim). If you ask me you'd be better off having a fixed gain of -1 (per channel) for the summing amp or else you run the risk of clipping the amp with a lot of channels. Also you're missing the 2k resistors on the stereo leg of the 4P3T switch.

The 2k resistors are for the mono setting. When the switch is in the center position, the attenuated signal goes to both the left and right channels.

Is that OP275 going to be able to handle makeup gain as well as driving the output? Should I add an output gain stage and/or transformer?

The OP275 is an inverting amp and if you directly connect it to the outputs you're going to have an inverted single ended output. Better add a balanced line driver. If you still choose to do this consider the input impedances of the the next stage. Check out the John Hardy 990 data sheet for application notes.... What is the gain make up stage for?

The gain make up brings the level back up, as you lose ~18dB in the circuit. I was hoping that keeping the circuit fully balanced would avoid the need for a balanced line driver.

 

[macwanj]

Oh shoot! Sorry for all that confusion synthetic...I should have read your post more carefully!

 

[radiance]

No time for a ready made answer .....

http://www.groupdiy.com/index.php?topic=27201.msg326342;topicseen#msg326342

 

[synthetic]

I'm using the front half of NY Dave's summing mixer...

 

[synthetic]

...and the back half of Forssel's active summer.

 

[NewYorkDave]

Every time I see or hear the term "summing mixer", I just want to scream.

The coupling caps on the input and output of the amplifier are shorted out and therefore not doing anything useful.

 

[synthetic]

Scream? For me, I heard a summing mixer shootout on a web board somewhere. They compared 5 summers with an ITB Pro Tools mix. I chose, "anything but #5," and of course that was the ITB mix.

The caps in the schematic are based on the Forssell makeup stage, "use caps if servo amp not used." Skip them? Add them if I have problems? Only important when using Fred's 992?

 

[babyhead]

Synthetic-

You are making a Jackalope.

I think the NYD design allows you to plug in your favorite 30db of gain and call it a mix. I am glad you posted this because it has me thinking...

SummingBus2 is an Active Combining Network. Check out the design discussions on FFs site. There are also threads about this here, a lot lately.

The 992s are badd. If you do the FF design, remove the shorted caps and go with the servo. (The JE16 Mike Preamp is also badd((who is Mike?))

Dave-  but this will sum AND mix as well as buss AND bus.

(I love my mila.)

 

[synthetic]

I already have a mic preamp made with Forssell 992 amps. But I want a dedicated gain stage for this purpose. Perhaps I will use 5532s for the 1980s SSL sound. I'll socket this and listen to 5532, OP275, whatever else I have around. Any other ideas for a makeup stage? My master plan is to build this plus a GSSL mix comp in one 2u box.

I understand that "summing mixer" is redundant but it's also descriptive. If you just say mixer, is it a console? A martini shaker?