balanced fade and pan

How would this work, if fed from a low impedance, transformer balanced source?



I still need to work out the make up gain, and I'm hoping that my assumption is correct when I assume that R5-R8 are the summing resistors. The only few references to this orban style pan pot I've seen show it feeding a virtual ground active summing stage... any reason the stage can't be passive summing with makeup gain?

i have no need for bus switching or in place solo... a simple mute and pfl perhaps, but i'm not even thinking about those at the moment. just thinking about summing 24 channel from the tape machine so i can *listen* to it, and pan it ;]

Since i need very few frills, I figured that fully balanced would be perhaps a good idea since I lack the experience to design a really low noise active ground setup... my hopes are to bring each side (L/R) to their respective busses and add shunt R's to bring the impedance down around 150 ohms and feed a mic pre for now.... around 175 ohms or so across the +/- inputs to the mic pre?

this is really similar to NYD's (thanks dave) "balanced mix network" although the fader/attenuator is 10k rather than 5k, and I'm pretty much hoping I'm close on the pan pot. i just figured I'd see what knowledgable people thought about the values, as they were all just chosen to match 10k parts. not so easy to find a good price on 100mm log faders or pc mount dual lins (esp. with center detent) aside from 10k. colin at audio maintenance has these parts at good prices is why I'm hoping to be able to use them

thanks if anyone has any comments

billy

btw, I should clarify 'low source impedance' as 'less than 15 ohms' or so... the circuit feeding this is a modification i'm working on of the api 325 circuit, and also claims +20dBm into 600 ohms, FWIW.

The opamp may become a big fet bloak instead of a 2520 provided it can provide the same or greater power... i'm not positive, because i'm still reading throught the thread, and there many revisions, but i'm pretty sure that it can deliver way more power, so shouldn't be a big problem.

The pan law is dependent upon whether the output resistors are terminated into a virtual earth summer or simply combined. For a known configuration (i.e. 24 channels), you can assume the load for simple summation as being the other 23 channel’s source impedance in parallel (note: this impedance will vary with individual pan settings).

The pan law will also change with input trim/fader setting and fader kill will ultimately depend upon part tolerances, and channel balance, perhaps even the balance of other channels.

Make up gain will be greater for the simple combining case, but noise gain is the same for both approaches (funny how that works). If you feed the simple summation of bus + and - feeds to a balanced preamp input you will need some 24x gain plus makeup gain for pan insertion loss (varies), and perhaps additional loss due to termination impedance if a common mic input (typically 2k or so) is used.

JR

i guess bottom line is that I'd like to use a couple things to my advantage which would compromise other designs... namely no in place solo, no bus switching or multiple bus assignment, and relatively low number of channels, and sources that are already transformer balanced with a low source impedance and plenty of output drive.

would the fader circuit in an app like this be better off as a U pad with 10k series R's and the fader wired shunt as a variable resistor? That would be -10dB at full blow, but perhaps that would be a way to keep the load on more consistent, and in turn the pan law?

i'm still not sure if all four resistors in the that type of pan circuit need to be the same, or if, for example, R5 and R6 could be different or not for the purpose of summing. the circuit, all in all, looks to me like 'almost' Tee attenuators, but not quite.

or perhaps i could shoot this one out: what would be the best way to accomplish balanced virtual earth summing?

i think that may be smarter.... i'd imagine careful grounding is still important however. i remember reading another thread, and someone mentioned something like that, using a mat04 into two discrete op amps. that sounds like the type of route i'd like to take, but i'd love to read an opinion. i would wonder where a differential stage would come into place with summing situation like that however... with one output transformer, it sounds like it would eventually rely on the destination, whether a tape machine input or a power amp line input, to differentiate the signal.

also, would a quad pan pot take me further? a balanced implementation of the dual lin with slugging resistors, or something like daves pan pot. i'm sure there are tracking compromises with that one, but i believe there are consoles out there that use it, at least one that i can think of.

I'm getting dizzy (dizzier?) from the competing design philosophies between your and Gus's combining approaches. Yours appears to be it's OK to be lossy as long as it's balanced (?). While Gus is preoccupied with noise.

This all demonstrates how many ways you can skin that cat, while there may be a little fuzz left on Kitty after all these gyrations.

To answer your specific question. To go to a virtual earth (and balanced) combining topology you just use inverting opamps at both + and - buses then either run those two outputs into a simple differential amp to convert to single ended, or just use the two outputs as the two legs of a balanced output.

Regarding the input feed to the pan network keeping the source impedance low and constant is useful for a stable pan law .

JR