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[mumbles]

Hi.
I'm trying to figure out the best way for an unbalanced line level signal to go from a 600 ohm passive fader to a 10K pan pot.  Do I need something in between them or not?  Buffer amp? just an amp? nothing?
After the pan pot, it goes to a summing amplifier, if that makes a difference.
Any advise is helpful.

Seamus

[Geoff_T]

Hi

Not enough info...

Your pan pot will need slugging resistors to achieve a law of 3dB down in the middle. The value of the slugging resistors depends on the value of the mix resistors. The value of the mix resistors is pretty open but you need to know it.

I think that your pan pot will load the fader, but the significance depends on answers to above. You can work it out mathematically.

Generally, I'd advise sticking an amp in there (like an emitter follower)... bummer if you want this 100% passive!

[mumbles]

Thanks Geoff.  You are always a great source.  That actually helped us out a lot.  Apparently, what we need are some 1k8 resisters, and we should be ok.

Thanks,
Seamus

[Geoff_T]

mumbles wrote on Tue, 17 May 2005 18:26

Thanks Geoff.  You are always a great source.  That actually helped us out a lot.  Apparently, what we need are some 1k8 resisters, and we should be ok. Thanks, Seamus

Hi Seamus

Imagine the pan pot cranked fully one way.

You have two 10K sections = 5K

You have a 1800 resistor

You have (guessing) a 15K bus resistor

So, 1/((1/5000)+(1/1800)+(1/15000)) = 1216.21 ohms

1216 ohms will put a lot of loading on a 600 ohm fader.

[mumbles]

I see.  Well, we have some pan pots from a Neve that we are using after some Gliss faders from a Quad Eight board.  My tech said that he found that the pan pots have the slug resisters that you mentioned.  As for the mix bus resisters, I'm not sure if he's taking that into account or not, to be honest, and I'm already over my head (short trip)...

Seamus

[Geoff_T]

mumbles wrote on Wed, 18 May 2005 08:09

I see.  Well, we have some pan pots from a Neve that we are using after some Gliss faders from a Quad Eight board.  My tech said that he found that the pan pots have the slug resisters that you mentioned.  As for the mix bus resisters, I'm not sure if he's taking that into account or not, to be honest, and I'm already over my head (short trip)... Seamus

Hi

The 15K bus resistor makes a teeny difference to the total loading but, in reality, 1K8 suits 10K or 7500 bus resistors... you'd use 2200 ohms for 15K.

Neve used 1K stepped potentiometers made up of individual resistors and the bus switches selected dummy loads to present a constant impedance to the stepped switch.

They eventually changed over to putting a B338 between the level pot and the panpot/bus switches.

You'll find that your fader level will drift slightly as you rotate the pan pot. Whether this matters is another matter!

Again, you can work it out similar to I did in the previous post.

[mumbles]

My mistake.  There is no slug resister in the pan pots that we have.  My tech did send me a chart that says if the mix bus resister is 15K, then use a 2k2 / and if the mix bus resister is 10k, then use 1k8.  Given this info, I guess my tech knows that the mix bus resister is 10k...

As far as level "drift," is this what you would need a buffer, emitter follower for?  To keep it constant?  

Seamus

[Geoff_T]

mumbles wrote on Wed, 18 May 2005 10:53

My mistake.  There is no slug resister in the pan pots that we have.  My tech did send me a chart that says if the mix bus resister is 15K, then use a 2k2 / and if the mix bus resister is 10k, then use 1k8.  Given this info, I guess my tech knows that the mix bus resister is 10k... As far as level "drift," is this what you would need a buffer, emitter follower for?  To keep it constant?   Seamus

Hi

I wonder if the chart is one of my tech sheets?

The thing is, you may not notice the drift because of level shifting as you rotate the pan pot.

However, if you want absolute level correctness and have faders with scales, there will be small errors between the fader's dB scale and the actual attenuation, especially when panned hard one way and the fader just below maximum.

[mumbles]

Ok, so I'm pretty sure that the mix resistors are 10k.  We're using Electrodyne acn's and Siemens V275's.  We know that the input impedance of the Siemens are 10k.  I guess what we're going to try is getting the 1k8 resistors and also try putting a emitter follower in there (Purple Audio's RLA2) in a couple different places and see what happens.

I wouldn't be suprised if it's one of your old tech sheets.  I'm not sure where he found it.  The knobs on the pan pots we have are grey and about the diameter of a quarter.  Well, most of them have knobs, anyway.  I'll have to track some more down at some point, I guess.

Seamus