Parallel resistance calculation and changing a fader's value

[jsteiger]

Oh technical slutz,

I’m gonna see if I can ask this clearly. I have been researching building some new cards for my console. In doing so, I’m calculating the loads for many different nodes. This math got me curious about modding my input channels. I am planning on 4 individual pots for the echo sends instead of just 1-5K pot. I charting out the different loads place on the input opamp, it got me thinking about the faders I replaced. Some of you chimed in to help with that. I’m happy to say, I understand a bit of the math now! Remember, the stock 440 faders (in my desk) were 600ohm. I changed them with more obtainable 5K faders. I have noticed how the load the opamp sees, drastically changes when plugging in 5K.

Soooo, a question! Can I add a 681ohm shunt resistor just before the 5K fader, to give the same parallel resistance load as the singal 600ohm fader has?

I have calculated that the lowest possible load the 2520 sees at this stage is 472.13ohms. The highest possible load is 584.07ohms.

The range is 1535.12ohms to 4074.07ohms with the 5K fader in place of the 600ohm fader. It seams like API tried to stay around a 525ohm load in all possible situations at this point in their circuit. It appears that they had their reasons.

I’m trying to stay as true to their old school design as possible. NO pun intended what so ever.

What do you guys think??

Thanks, Jeff

[David Kulka]

Fader math can be a little ambiguous and in the end, I think this is mostly about the tracking. I don't think you'll run into any issues with headroom, distortion, etc. My main goal here would be that the new faders to track the same way as the old ones.

Putting aside the question of impedances and loads, the tapers of your new faders might not match the tapers of the original API's. In fact, they probably don't. So really there are two X factors here.

It would be nice to see a schematic or at least know the load impedance on the wiper of the fader. If it's high (5-10K or more) everything will probably be fine. If it's low (2K or less) the fader may be overly loaded down, with the result that the center of its travel will have too much attenuation. (Do the fader wipers go straight to a patchpoint? I believe that some API desks were built this way.)

In the end, this will probably be a trial and error kind of thing. Sure, you can shunt the fader with a 681 ohm resistor, and the parallel resistance will be closer to 600, but you might not need to, and this probably won't help with tracking anomalies.

A little confusing but to summarize, the input impedance of the fader is *probably* not critical, though it depends on the circuit. The fader output should have the impedance match it's designed for, but it still may not match the silkscreening on your desk or match the old faders.

Penny & Giles once published a paper all about fader impedances, I have it somewhere, maybe I should pdf it and post it.

[JohnRoberts]

+1

I am not familiar with specifics of the API but in general solid state electronics will perform better when driving lighter loads.

I would not expect any degradation from using a modern fader value, and perhaps some improvement.

JR

[nosebleedaudio]

Another advantage of a lower value pot/fader is you can, if needed, add more pots to the same point and not load the op amp, ex; 4- 5K pots will not load the 2520, I would not be concerned about maximum loading on the 2520.
I would also suspect that many designers used mostly 10K and 5K for this reason and they bought pots at a higher quantity, getting a far lower price...

[jsteiger]

Thanks for the posts and input gentlemen. BTW David, great work on my RMX...she is sounding very sweet!

I have already changed the faders from 600ohm to 5K. That was actually done a while ago. A tech/friend helped with a new legend. The desk came with Auditronics facia's so the legend was wrong from the beginning. One day I will get around to all new fader facias.

Anyhow, I guess I'm more looking to make sure I am understaning the math correctly. In trying to learn and understand my input modules, I had noticed that the load on the 2520 appears to always be around 600ohms.

Can someone double check a few of my calculations to see if I am understanding?

For me, I am most unsure about the node when the Echo Pre-Fader is selected.
This is what I came up with. I calculated the Level pot at 100% or 5K.

1 Echo engaged, node=48.7K
2 Echos engaged, node=29.2K
3 Echos engaged, node=22.7K
4 Echos engaged, node=19.5K

I think I understand the rest of it

Thanks again guys, Jeff

Here's a quick and rough schematic of this part of the module.

[jsteiger]

A little "check my math" bumpage.