Help me understand pan circuit? (API/Yamaha)

[JW]

I've been building some API 536 channel strips into my Yamaha console.

I have a couple questions about the pan circuit.

I've attached a schematic drawing of how Yamaha did it (after their final opamp) It's sort of similar to API, but in an effort to make it even closer to API to see what the difference was, I wired up a 10K pan pot how they (API) did. I attached this schematic as well. It's exactly the same as the API 536 schematic I've come across, but no stereo bus. Instead, it goes into just 8 program buses, with 33K mix resistors.

Anyway, I'm running into a problem of the "pan in" circuit being too low in volume in comparison to "pan off". I see that the 1K (mix level) pot can be used to adjust the level right before the pan pot. I'm assuming this is a trim to match the levels between pan on/pan off. But even with this 1K trimmer zeroed out, I'm about 3dB below the "pan off" circuit.

It seems to be to be an obvious solution to place this 1K trimmer in the "pan off" circuit instead, to trim that down to match "pan on" and then trim the whole thing up a bit with the DOA.

It begs the question, why did API put the trimmer there? In what circumstance would a DOA driving a 10K pot actually be higher in level (and subsequently need to be trimmed down?) in comparison to no pan pot at all? Seems like the trimmer should naturally be on the "pan off" circuit.

In fact, in the Yamaha schematic, I see that 330 ohm series resistor doing just that (I think?)

(Another thing to note, and which I am curious about. Yamaha has the resistors that are going from the pan pot's wipers to ground (1.8K in Yamaha's case) on the 'common' side of the pan switch, which also puts them in the circuit when pan switch is off. API only has them in circuit when pan is switched on. )

 

[JW]

Here's what I built . . . (API looking pan)

 

[JW]

For reference Boji's redraw of 536 scan. The resistors off the pan pot wipers are actually 3.3K, not 33K.

 

[ruffrecords]

I am not quite sure you really have a problem. With the pan centred, the signal is supposed to be 3dB down compared to the level you get hard panned one way or the other or with the channel out connected direct to a bus.

Cheers

Ian

 

[JW]

Hi Ian,

Right, the difference between L/R and center is around 3dB as well (with pan on)

I'm referring to the difference between hard panning the pot (with pan switch 'on' let's say to the left) and pan switch 'off' (but monitoring only the left side.) There is still about a 3dB difference, and these should be able to match with that 1K pot adjustment I would think? That pot is zero'd out though, which makes me think I should try putting it on the (pan off) leg of the switch instead of pan 'on'

With the yamaha pan circuit hanging on the 536 DOA output, pan in/out match. Hmmmmmm.

 

[ruffrecords]

The 1K pot is certainly odd. I cannot see any obvious use for it. What happens if you remove it and connect directly to the pan pot?

Cheers

Ian