24-pos ELMA switch 10k. Convert to 600R use.

[Studio Mollan]

Hi there,
Iv'e just dropped in a 10k 24pos switch to my desks monitor level controll.
I've been in contact with Jeff att classic API about this already, he sold me the switch wich feels awesome! Jeff is very helpful and i'm sure would give me the values but I rather share it here as theres probably more like me who would like to know this.

It's 10k and the original pot/switch is intended to be 600ohm. It works as it is now but the top 5 steps are extremely loud. Jeff suggested an L-pad in front of the switch. It's useable for sure, a lot better than with the old pot, but i'd like to give the L-pad a go.

So, what resistors should i use? 10k plus 638ohms in parallell makes 600ohms. Is that the way to go? and what would the series resistor be?

Thanks!
/
Emil

 

[ruffrecords]

If the original pot was 600 ohms and the source that fed it had an output resistance of 600 ohms (that's a couple of big ifs) then the combination of the two would have dropped the level by 6dB. That would explain why it got louder when you fitted a 10K pot.

I don't see a need to add an L pad since you are probably feeding it to a monitor amp with a 10K input impedance. SO I would suggest simply connecting the 638 ohms across the 10K pot and nothing else.

Cheers

Ian

 

[Studio Mollan]

Hi and thanks Ian!
I will try this as soon as i got a spare hour in the studio. I'm not sure about the 10k input impedance though. Its a 70:s API desk.

 

[ruffrecords]

Hi and thanks Ian!
I will try this as soon as i got a spare hour in the studio. I'm not sure about the 10k input impedance though. Its a 70:s API desk.

The only input impedance that matters is that of your monitor amp. What is it??

Cheers

Ian

 

[jsteiger]

If Emil's desk is like mine, there is a 2520 in front of and after the control room level switch. The monitor amp won't matter.

 

[Studio Mollan]

thanks for your input guys!
I tried putting a 604R resistor over the 10k switch. The level increase was STUPID! i heard distortion when listening to music and the level was at least 10dB on the first steps of the switch. I also tried a 1k resistor with little difference. And adding a series resistor, 470R, with the parallell 604R gave little difference as well.  I noteced that the level increase almost stopped from the middle and up  to the last step.
As a said to Jeff I'm pretty happy with it as it is. Would be nice to have it as close to API spec as possible though. If not ill just stop the last 2-3 steps and use it as is.
I must say that the sound quality is improved A LOT as compared to using the old potentiometer. Much more than a thought.
/
Emil

 

[ruffrecords]

It is really weird that you get a level increase by putting a resistor across the pot. It sort of implies the pot is in a feedback circuit. Any chance you can post a schematic?

Cheers

Ian

 

[Studio Mollan]

It is really weird that you get a level increase by putting a resistor across the pot. It sort of implies the pot is in a feedback circuit. Any chance you can post a schematic?

Cheers

Ian

I sure can. Will need to find it and take a photo. Probably by tomorrow. Thanks Ian!

 

[Studio Mollan]

It is really weird that you get a level increase by putting a resistor across the pot. It sort of implies the pot is in a feedback circuit. Any chance you can post a schematic?

Cheers

Ian

Ok here's the pictures. The schematics are HUGE. 1m x 80cm ca so pretty hard to make copies. The schematics are also split up in two, one each for the two boards in the CR module.
I'm an amateur here and happy to be corrected. From what i see the four cr buses passes through one 25290 each and the to the 600r pot and then out to the CR ACA via two 5k trimmers. On of them is the dim wish is relay controlled.

Hope you can make something out of this. 
The pot is stated as 600R and followed by individual 5k trimmers and also a relay controlled 5k mutligang pot for dim.

 

[Studio Mollan]

fist phpto of board 1.

 

[Studio Mollan]

second photo. Board 2

 

[Studio Mollan]

zoom

 

[ruffrecords]

Thanks for posting the schematics. It all looks perfectly conventional. Op amp feeds 600 ohm master pot which feeds dim pots which feed trim pots. I can see no reason why slugging the 10K pot with 600 should make the level increase - unless perhaps you wired the 10K pot wrong?

Cheers

Ian

 

[Studio Mollan]

Thanks for posting the schematics. It all looks perfectly conventional. Op amp feeds 600 ohm master pot which feeds dim pots which feed trim pots. I can see no reason why slugging the 10K pot with 600 should make the level increase - unless perhaps you wired the 10K pot wrong?

Cheers

Ian

Hi,
Well i had it in the wrong way at first but them it behaved really strange. I will check the connections again though and try the 600r parallel resistor again. Can you see a electrical reason for NOT using the switch as it is?
/
Emil

 

[Studio Mollan]

Thanks for posting the schematics. It all looks perfectly conventional. Op amp feeds 600 ohm master pot which feeds dim pots which feed trim pots. I can see no reason why slugging the 10K pot with 600 should make the level increase - unless perhaps you wired the 10K pot wrong?

Cheers

Ian

Come to think of it i might have soldered the 600r resistor wrong. The terminals on the 24-pos switch are in, out, ground. I soldered the resistor between in and out. Should it be between in and ground?

 

[ruffrecords]

That explains it!!!! You should wire it between in and ground. No wonder the level went up.

Cheers

Ian

 

[Studio Mollan]

That explains it!!!! You should wire it between in and ground. No wonder the level went up.

Cheers

Ian

That explains it then. Thanks again Ian! I learnt a lot here.
I googled it and found a piece where it says to solder it to out-ground. Will it make a difference?

 

[jsteiger]

Emil

Can you take a level measurement before adding and then after adding the additional R? I would expect it to not make much of a difference considering the buffered configuration.

 

[Studio Mollan]

Emil

Can you take a level measurement before adding and then after adding the additional R? I would expect it to not make much of a difference considering the buffered configuration.

Jeff
I believe you are right. I don't hear a difference in sound and the last step is still a LOT louder as i describes to you. I Will try to do a measurement tomorrow. I'm leaning towards using it as is with a stop at pos 23.

 

[ruffrecords]

That explains it!!!! You should wire it between in and ground. No wonder the level went up.

Cheers

Ian

That explains it then. Thanks again Ian! I learnt a lot here.
I googled it and found a piece where it says to solder it to out-ground. Will it make a difference?

Yes, it will make a lot of difference. It will most likely make the top steps much coarser and the lower ones finer.

I think part of the problem you hae is that the dim pot loads the switch pot anyway, The way it was originally you had a 600 ohm pot loaded by a 5K one - in other words a light load. No you have a 10K pot loaded by a 5K pot - in other words a heavy load. That is what is making the top steps so coarse.

Cheers

Ian

Cheers

Ian