line mixer ch.

[syn]

Hi

   i'm thinking of ~10ch of this. I have most of the parts already.
Do I need U2 buffer at all?
I'll be going into fet DOA for summing.
Any advice most welcome.
Deliberately skipped mute , PFL and snd/rtn circuits, for the sake of clarity.
Assume that unconnected opamp pins have needed surround circuits.
Thanks to Radiance, Rod Elliott and Fred Forssell for the "inspiration".
Thank you

EDIT: typo

 

[sahib]

Currently U4 is driving the panpot network consisting of R30, R35 and R2 . Beyond that is running into  high impedance (fet) so this will dominate as the load.

When the wiper of R2 is in the middle U4 sees an impedance of 4.15K.

When the wiper is in one extereme it sees an impedance of  2.64K.

If the U2 is removed then the panpot network impedance becomes parallel with total aux impedance which is 2.5K.

This time when the wiper of R2 is in the middle U4 will see 1.56K

When the wiper is in one extereme it will see 1.27K

5534 is designed to drive 600 ohms and the lowest impedance is over twice that, so it should not be a problem. But it will be even a cheaper design as the panpot arrangement is already the cheapest you can do.

 

[syn]

sahib
          thnx so much. i thought i could do without the buffer (u2). is that pan pot topology considerd to be "bad"? any better topologies to look for?
is this OK design for a good line input ch.? how about that 1k for the ground buss?

 

[sahib]

Hi Syn,

You are welcome. The panpot arrangement is not particularly considered too bad but it can be done better. However, if you are bothered with the cost of U2 then it won't be of interest to you because it will require a dual pot. The next step up will require a dual opamp.

I can't really say good or bad as it is not the complete plan. I would think it would be more interesting if you included the rest. I have only noticed the 1k on bus ground now. I don't get it. You are the designer. Is it for a psuedo balanced output?

 

[syn]

Hi Sahib,

              i'm not about the cost on this one, i just want to get the  best parts count/performance ratio. I'll explain what is on my mind:
I have 28 ch. of DA+some synths that i want to mix OTB. I need line level input into fader into pan pot (6aux- 2 pairs switchable- i won't need
more than 4 at the same time on a given ch.) than to Summing Buss. After the master fader i'll buffer and  send the signal to monitor through Igor's monitor controler and to AD input for recording masters, using two THAT 1646, one for each task.
I need to nail the pan pots as I only today started to learn about them. I belive Forssell's Summing buss is O.K. Know little about mixer construction.
Thank you very much.

 

[abbey road d enfer]

As it is, the pan-pot has only 1.5dB attenuation in the center position, which is incorrect by any standards.
Don't know why it is so, maybe to try fixing the inherent problem of this pan-pot arrangement, which is its high attenuation and consequent poor S/N performance of the overall system.

 

[sahib]

Well, this is the best part count/performance ratio and it is not really good as Abbey also pointed out. Keep U2 as it does not cost much but better performance. Replace the panpot network with a dual 10K linear pot between the signal and ground. To bend the law of the pots wire 3.9K  between the wiper and high side of each pots. Also 10K series resistor on the wiper output (from D. Self) . It is a better arrangement than what you have and only costs a fraction more.

Edited.

 

[tmuikku]

... To bend the law of the pots wire 3.9K (from D. Self) between the wiper and high side of each pots...

Hi syn and others!

NewYorkDave panpot schematics shows some maths too.
http://electronicdave.myhosting.net/miscimages/panpot.gif

I'm going to build small line mixer too and have been lately brainfarting on simple designs, as you may see from the question below:
Since you've got only post fader auxes, could it be possible to ditch both U2 and U4 if fader is replaced with stereo fader and moved before line receiver chip? At least SSM2141 line receiver can drive 600ohm load. There wouldn't be any gain with the fader though, I'm not sure if there is line receiver chips with gain more than 1.  Input CMRR would decrease since faders aren't perfect, but how much (I don't know how good match stereo faders have vs. cost)? There would propably be some DC draw from the fader, just put coupling caps?

 

[syn]

I'm not sure if there is line receiver chips with gain more than 1.

THAT 1246 can be  configured for +6dB of gain. The rest i don't know.

 

[syn]

Thank you all for your kind replays.I did some research for the last couple of days (Orban, Self) and I'll go with active pan pot arrangement. However, regarding mix bus, 
wouldn't current feedback topology be a good choice for the summing bus amp as it's inverting input has low impedance?