Marc Girard

DIY Big Knob! (Pictures Included!) New
« on: July 06, 2006, 03:49:50 PM »
Hullo all,

Here's my take on the DIY Big Knob!   It's really simple as I don't need much connectivity.  I used an ALPHA 4 decker pots, the very same ones they use on the Presonus Central Station.  That's how I got a hold of the part: I ordered the pot from Presonus' service center... 5$ CDN.  A joke.

So, here's my 25$ CDN Passive volume controler knob with independent high-quality mute switches and fully balanced signal path!  (Wow, I would be good working for P.R. compagnies wouldn't I?)




Flames and comments are welcome!

Coming up soon: GSSL, the revenge!  (my 2nd GSSL)
« Last Edit: January 23, 2014, 05:55:07 PM by Marc Girard »


rodabod

DIY Big Knob! (Pictures Included!)
« Reply #1 on: July 06, 2006, 04:05:41 PM »
What do you use it for?

Roddy
Quote from: tv
punchy fat bastard chip

Assistant

DIY Big Knob! (Pictures Included!)
« Reply #2 on: July 06, 2006, 06:26:14 PM »
simple but nice!

why there are two mute switches?

rove

DIY Big Knob! (Pictures Included!)
« Reply #3 on: July 06, 2006, 08:41:42 PM »
I just built a similar box (not quite as neat!) with two vintage 600/600 attenuators I have.  A dual mono pvc.  I was going to use it to attenuate the outputs of some of my pres that have no output control.  Nice enclosure, I used a rat shack box for mine, where did that box come from?

Paul

Marc Girard

DIY Big Knob! (Pictures Included!)
« Reply #4 on: July 07, 2006, 12:56:03 AM »
rodabod: I use it to attenuate my DAW's D/A outputs.  Right now I have a little Mackie board (1402VLZ-PRO) that does the job but I want to see if it's gonna be more transparent.

Assistant: Why 2 switches?   Because not one! :)  No jokes, I wanted a simple and cheap solution for my knob, that's all parts I found at a local surplus store.   A 4 pole DTDP switch is not easy to find in those places so I opted for a simple solution...   Thanks for the nice words!

rove: I bought that box at the local surplus store.  It's an Hammond 1456 Series.  You can find them here: http://www.hammondmfg.com/dwg16.htm    I wanted the one with wooden sides but I'll wait, they didn't have it in stock, maybe the next one!

Marc Girard

DIY Big Knob! (Pictures Included!)
« Reply #5 on: July 12, 2006, 10:32:41 AM »
News update:  The thing works but not as good as I thought.   The attenuation is not enough for +4dBu operation.  

Is there a way to make the pot attenuate more?  Thanks

manarm

DIY Big Knob! (Pictures Included!)
« Reply #6 on: July 12, 2006, 11:19:02 AM »
Hi

You didn't connect the the Jacks grounds to the pot. IMO, the pot just acts as a "resistive load" regarding to the output of the device you connect to.
If you link the grounds to the free leads of the pot, you'll get an attenuator.
Am I wrong?

manarm

DIY Big Knob! (Pictures Included!)
« Reply #7 on: July 12, 2006, 11:32:23 AM »
Quote
the pot just acts as a "resistive load" regarding to the output of the device you connect to.


:oops: Thinking before writing ! :oops:

The pot acts as a serial resistor between the two devices you connect to your box.
I agree with the rest of what I said  :green:

chrissugar

DIY Big Knob! (Pictures Included!)
« Reply #8 on: July 12, 2006, 11:40:07 AM »
It should work. Probably you wired something wrong. Draw a schem of what you did and we will try to help.

chrissugar
Christian Mike Sugar
        CMS-LAB

Rochey

DIY Big Knob! (Pictures Included!)
« Reply #9 on: July 12, 2006, 12:24:52 PM »
Gents,

Is it safe to assume that the inputs to the speaker amplifer (be it within the active speaker or a separate box) would have near-infinite impedance?

In which case you wouldn't need to buffer the output to make sure you have a low output impedance.

In which case, Manarm is right, if you have the pot set up as a variable resistor, then you're only going to create a potential/voltage divider between the pot (maybe 10K ohm max) and your amplifier (1MOhm input impedance).

In other words, even at max resistance, you'd still drop 1M/1.01M accross the amplifier. 99% of the signal still going to the amp.

What you're trying to do with the pot is create a potential/voltage divider within the Little Knob, which will then be sent out to the amplifier. The amplifier should look like an open circuit to the divider, and therefor not change any of the settings.

Take a look at:
http://sound.westhost.com/pots.htm
and look at the "putting your pot to use" section. There you'll see the difference between "Volume Control" and Variable resistor.

If you've already investigated this - please ignore my post.

cheers

R
Expat Audio Home: http://www.expataudio.com


chrissugar

DIY Big Knob! (Pictures Included!)
« Reply #10 on: July 12, 2006, 01:49:46 PM »
Mark, you say you want balanced, so your have to wire two pots for each channel like in the "Dual-GangVolume Control" situation.

 

Forget about Left/Right. Substitute L with the + of the balanced signal (pin2 on XLR) and R with the - (pin3 on XLR)
Hope it helps.

chrissugar
Christian Mike Sugar
        CMS-LAB

chrissugar

DIY Big Knob! (Pictures Included!)
« Reply #11 on: July 12, 2006, 01:51:52 PM »
Later I discovered a small error in the notations on the schem. Left in should corespond to Left out. The same thing for Right in and out.

chrissugar
Christian Mike Sugar
        CMS-LAB

Marc Girard

DIY Big Knob! (Pictures Included!) New
« Reply #12 on: July 13, 2006, 12:32:38 AM »
Thanks for all the inputs guys, you folks are the best!  After reading chrissugar's schema, I verified my wiring, I think I'm OK.  The only fancy thing I added is DTDP switches for mutes on each side, nothing really extraordinary.

Here's a new picture of my "fixed" knob...  Just added a ground from the 4 decks (PIN #3) of the pot to the ground already in place for the TRS connectors.



Let's hope it does it.  I've got a feeling that the pot does not offer enough resistance when lowered completely...   Even though, it's an ALPHA 10k ohm...  Then again, you guess is as good as mine, I'm totaly green! :)
« Last Edit: January 23, 2014, 05:55:26 PM by Marc Girard »

Rochey

DIY Big Knob! (Pictures Included!)
« Reply #13 on: July 13, 2006, 01:19:53 AM »
when turned to 0, then basic electronics
r2/(r1+r2) Vin = vout

should make sure that your output is 0.

good luck

R
Expat Audio Home: http://www.expataudio.com

chrissugar

DIY Big Knob! (Pictures Included!)
« Reply #14 on: July 13, 2006, 03:57:55 AM »
Quote from: "Marc Girard"
I've got a feeling that the pot does not offer enough resistance when lowered completely...  


Are you sure it is a log pot? I ask just because what you describe is very similar to the situation when you use a linear pot. A log pot should produce 20dB attenuation at mid position.

chrissugar
Christian Mike Sugar
        CMS-LAB

Marc Girard

DIY Big Knob! (Pictures Included!)
« Reply #15 on: July 13, 2006, 08:12:24 AM »
Frankly, I'm not 100% sure it's a log pot...   I couldn't find a pot with 4 decks easily so I ordered the pot from the local Presonus service center.  The Central Station is a passive unit so I figured out that I would be all right using their pot, since there's not a hundred ways to do this...  I might be completely wrong!    The markings on the pot says: (A) C10Kx4  (A) for the Taiwan Alpha Logo.

Tried so research on Taiwan Alpha's web site, no luck really.

chrissugar

DIY Big Knob! (Pictures Included!)
« Reply #16 on: July 13, 2006, 09:44:55 AM »
The easyest way to find out if you have a linear or log pot, is to put the cursor exactly in the mid position and measure the resistance between the wiper and the two ends of the pot. For a 10K pot, if linear you should measure aprox 5k and 5k, if log pot you should have aprox 1k and 9k.
Go measure it and come back with the results. Run! :grin:

chrissugar
Christian Mike Sugar
        CMS-LAB

Marc Girard

DIY Big Knob! (Pictures Included!)
« Reply #17 on: July 13, 2006, 11:29:56 AM »
OK, I mesured the pot with my lame Mastercraft test meter...  Might be a little off, but I tried to adjust it the best I could, THE RESULTS ARE IN!

At minimum position, the pot mesures ~8k ohm (might be a little off)
At maximum position, the pot mesures 0 ohm
At the middle position, the pot mesures 2.5k ohm

So, I guess it's a log pot?   I'm starting to wonder if Presonus didn't add Jedi tricks to their circuitry for their Central Station.

chrissugar

DIY Big Knob! (Pictures Included!)
« Reply #18 on: July 13, 2006, 12:27:27 PM »
It looks very strange, they are not linears but they are either not log pots really. I just measured some minutes ago some  Bourns, Vishay and Omeg 10k log pots.
Bourns total=10.2k mid=1.09
Vishay total=9.95k mid=980ohm
Omeg  total=10.3k mid=950ohm
It looks like log pots from different manufacturer have aprox similar values, so it is a bit odd that 2.5k at mid position for a total 8K pot. It should be at mid position aprox 10% of total value.
Maybe you should use a better quality pot. Or stepped attenuator. I know it is expensive but it tracks perfectly.

chrissugar
Christian Mike Sugar
        CMS-LAB

Marc Girard

DIY Big Knob! (Pictures Included!)
« Reply #19 on: July 13, 2006, 04:43:41 PM »
OK, that's gonna be it, I'm gonna use another pot...  Do you have any for sale?! :)   If not, where's a good source for a 4 decker pot?  Thanks a lot for  your patience and support, it's really appreciated!


 

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