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Shunt attenuator in ZSLA-1 (Read 1779 times)
crazy bill the eel killer
Ex Member



Shunt attenuator in ZSLA-1
12/20/05 at 18:02:14
 
I've been looking at Welbournes remote controlled shunt attenuator for volume control and input selection. I know that both input and output impedance varies with attenuator position, but I've also read that it is usually not critical in most situations. I'm thinking of replacing the alps 100k pot in my ZSLA-1 with this unit. The max output impedance of my sources is 600 ohms. Anybody know the math on how the shunt numbers vary with attenuator position, and if this unit would work with the ZSLA-1. I'm thinking it shouldn't be a problem, as cathode followers have very high input impedances, but I'm just learning this stuff and I'm not sure. I'm also concerned about loading down the source. I would like to put a tant or Riken ohm in the series resistor if I can source the proper value.

As always, any help would be appreciated.

Bill                         Cheesy
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Eli Duttman
Ex Member



Re: Shunt attenuator in ZSLA-1
Reply #1 - 12/21/05 at 02:04:58
 
Bill,

The rule of thumb is that the I/P impedance of a driven unit be at least 10X the O/P impedance of the driving unit.  Your 600 Ohm stuff will be OK into 10 KOhms.

Replace the stereo pot. in the ZSLA-1 with 120 KOhm fixed parts.

How are you going to plug the hole removing the pot. leaves?

44 bucks for a regulated (switching) 5 VDC PSU?  Not me.  A 12.6 V. CT filament trafo driving 2X Schottky diodes, filtered by a large 'lytic yields excellent "raw" DC.  A 7805 3 terminal regulator "locks" the voltage.  Follow the regulator IC with a 4.7 muF.  'lytic in close proximity.  A 2nd 4.7 muF. 'lytic inside the controller's housing finishes things off.

All the fancy stuff for remote control does not change the fact that this is a passive control center.  So, SHORT, LOW capacitance cables between the controller and the ZSLA-1 are a MUST.
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crazy bill the eel killer
Ex Member



Re: Shunt attenuator in ZSLA-1
Reply #2 - 12/21/05 at 12:36:59
 
Thanks Eli,

If I just replace the 100k pot with the Welborne unit(which I'm considering because of the simplicity of the installation), I'll build the PSU as you describe. Thanks for the info. As for plugging the hole, I would probably build a new chassis of wood surrounding the existing one, and just add to the length to accomodate the new control. So any wiring would be routed through that hole, which I will probably have to enlarge for a suitable grommet. I'm still considering using a Dact unit, and possibly a TVC. The PEC carbon pots also look very intriguing, and if I could figure out a way to attach a motor I would probably go with it. The TVC is also high on my list, but the cost obviously is prohibitive. The nice part about it though is I could try it passive into the SE84CSEX, and then feed it into the ZSLA-1 first and then compare the two topologies and see which sounds " best ".

Thanks again for your help.                

Bill          :D
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