Can I use a 2k pan pot for API style mixer?

[JW]

Hello,
I have a question or two that I've asked a couple times but still maybe need some help.

At the output of the 2520, in a mixing console of 32 channels, with 33K bus resistors, are we okay with a 2K pan pot? Imagine while looking at the schematic below that everything up to the output of the DOA is API. But then into the Yamaha's 2K pan pot, and the resistors to ground from the pans wiper are actually 270 ohm instead of 1.8K ohm. This gives a -4.25db pull down in the middle of pan. Am I okay here with loading? And can someone check my math in regards to total load on the output of the DOA:

8X33K program bus resistors=4.12K
6Xaux buses (33K+10K vol pot=43K) In parallel, 43K X6 =7.16K
So, 4.12K in parallel with 7.16K=total load of 2.62K

So, a 2K pan pot would be in series with this total load correct? Is this a fine load for the output of a 2520, considering the API ACA summing network would be coming after those 33K summing resistors?

 

[boji]

In some older API desks, A1 is expected to drive a 1k fader and aux. Even if your 10k fader was placed after IC10, FB/Echo pots are 10k, so adding a 2k pot shouldn't be a problem.

 

[JW]

Thanks very much Boji.

Out of curiosity, calculating total resistance for the 536, (8+2 47k resistors) in parallel=7.8K. This in parallel with the 5k pot is 3.05K. I'm guessing about the 1K pot and dual 10K pan pot though if I'm adding them into the equation. That would be series resistance right? But it's either on or off with the pan switch.

If the total load is 3.05K, I'm right around that amount with the Yamaha load as well=cool! But, if I'm considering the API main mix circuit, with pan engaged, would the potential load not be 3.05K +1K+5K(half of pan)=9.05K ? A bit more of a load than Yamaha, but I'm guessing, going between the beefy low impedance output of 2520, and the ACA receiver, either is probably fine? But that's a guess.

And, API or Yamaha, I'm not sure how the pull down resistors on the pan pot factor into the total load. API is 10K pan with 3.3K pull down.

At my most extreme (haven't settled on this value) but with this 2K pan, 270 ohm pull down resistors get me -4.25dB. I'm not sure what concern those 270 ohm resistors might be. It certainly effects gain.

Link to your 536 redraw Console signal flow, input cards, general discussion on API styled DIY mixer

 

[boji]

270 ohm pull down resistors get me -4.25dB.

If the plan is to modify the yammy pan circuit, a standard 10k linear taper pot with 3.3k / 2.2k law resistors is recommended.
Test with both pot/law values and compare the sound?

 

[JW]

Hi Boji,

I did try that pan circuit, as well as how Neve does it with no resistors from wiper to ground. Also the API one, with only resistors to ground. Regardless of slug values, with a 10K pot, I am unable to reconcile my gain with pan (in the center) switched in, and pan switched out. Also, for some reason (i'm guessing loading) the pan is down like 10 dB or something in the middle with the pan circuit from your last post.

I'd like the same gain between pan on/off because I don't have a master bus. Rather just the 8 programs like you see in the schematic. I've chosen 7&8 program buses to install the CAPI ACA main mix bus in. So, you see, it's API up to the 2K pan pot, and then API after the 33K bus resistors, but for whatever reason, 10K pan isn't working for me. 2k is though, so maybe I should just quit complaining.

 

[ruffrecords]

With these circuits you will never get the same gain pan in (centred) as with pan out. You will only ever get pan out gain equal to pan in fully left or fully right. If you want pan (centred) to be the same as pan out then you need to switch in extra bus resistance for pan out to drop the level on the bus by 3dB.

Cheers

Ian

 

[JW]

Thanks Ian

I was coming to that conclusion. To match them, one only needs a 330 ohm series resistor in the Yamaha circuit w/ 2K pan pot and and 1.8K pull down resistors. You can see it right before the pan pot on the schematic.

If I have a 10K pan pot, then I need a much more dramatic series resistor (can't remember exactly, but something like over 2K from memory) I don't know how much is okay, but let's say for example that with a 10K pan pot and 2.2K pull down resistors, I needed a 3K resistor in series in the 'pan out' circuit to match pan in. This then requires me to boost the DOA via the feedback, and then I'm nearly shorting the feedback resistor, which when fixed in the API circuit is supposed to be 20K.
Also, with 10K pan pot, attempting this, I noticed reduction in level with the addition of each bus added in. Like, another -2dB with each additional bus selected. That can't be right. It just makes me think Yamaha did this balancing act correctly for the final total load they have on the output of the channel's DOA. And they needed a 2K pan pot to do it. They're Alps pots. Long shaft, like 25mm or something, Log A/C pots, not linear. 2K. It's hard to even find 10K with a shaft that long.

 

[boji]

I can attest that with the larger value panpots on 24ch and 47/39k bus resistors, Jeff's virtual ground ACA/Bo delivers a hefty 2 mix output. It also helps that every tx output stage from channel to ACA adds either +3 or +6dB gain.

 

[JW]

Yep, I definitely have the interstage transformer. It just occurred to me another scenario that I hadn't been thinking about that sort of forces me toward using Yamahas pan pots. I only have one pair of Capi ACA buses. So, building api into yamaha is okay, but if I change out pan pots, then the way the input strips react with my stock yamaha buses (auxes etc.) is going to be different. Drrr. I've already had to adjust gain of the ACA input to match my other buses. I'm having to adjust the 20k feedback resistor in the input strip's output DOA down to around 8k to equal my other Yammy channels. I figured up to this point that all this is quite within both the Yamaha and API opamp's capabilities, as the feedback (gain setting) of 2520 can be anywhere from 200 ohm to 20k ohm depending on it's purpose, and even in the Yamaha schematic, it's something like a 2K fixed feedback resistor. I think I sort of have to go with one or the other here. Either cater to Yamaha's existing load (and gain requirements thereby) or go full bore, and actually create a true master bus.

As of right now, I've wired up the exact 536 line circuit you posted Boji, with Yammy's DOA's running +/-24vdc. But I have the capi kits for 2520's and an acopian +/-15vdc power supply. The idea was to do 8 channels and get an appropriate idea of what is the difference between the Yamaha channels and the 536 channels in practice. I'm at 4 right now.

 

[boji]

I've already had to adjust gain of the ACA input to match my other buses.

Perhaps adding some intermediate aca's would allow Yam/Api gain matching (ie distributed summing).

 

[JW]

Thanks Boji. I have been thinking about that last post for awhile now, how to implement, so that when I activate "pan on" the outputs of all 8 busses go to one set of intermediary aca, like those boards you're showing, but with "pan off" then going to another pair, with different gain, but matched in the end for the final master ACA input.

Anyway, what are the schematics (circuit numbers?) I'm looking for that show what API is doing between those 1-8 bus summing resistors (47K) and the input of the stereo summing ACA? I'd also be interested in what their echo out circuits look like.

 

[boji]

Glad to hear you're considering it. Douglas Self's 'Small Signal audio design' has a section on distributed summing which includes a spreadsheet showing improved noise reduction by channel count.
CAPI's block diagram that compliments his ACA pcb's is what I followed. It really is as simple as the diagram shows.

As for echo returns, schematic #'s, and what's going on after the 47k bus resistors, I never got around to making returns...however I imagine they are similar to a channel card minus aux, of course. Echo returns would accept balanced or unbalanced inputs, much like the line-in section going to A1 on the 536.

 

[JW]

I found the 528 schematic in the tech. documents. Different from the 536, in that it has a 1.5K pan pot, if I'm reading this right. Hmmm. These circuits look almost identical. So, I'd imagine 2K is fine, to answer my original question.

Just speculating, but I wonder if the 2488 (528 input modules) had a different enough load per channel (on the ouput DOA) than the 3232 (536) to justify a different pan pot? Perhaps the cumulative loads of more sends and buses or something? I don't know. Haven't studied it too much though.

But if the question is, do I go to extreme modification lengths just to have a 10K pan pot, when API apparently used 1.5K as well, then maybe I'm answering my own questions.

Gotta pick up the Doug Self book. It's probably the most referenced book on this forum.