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step attenuator conundrum
step attenuator conundrum
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Old 21st April 2007, 03:06 PM   #1
tl507083 is offline tl507083  Canada
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Location: Halifax
Default step attenuator conundrum

Here's a challenge for you. I have acquired 3 rotary switches, each with 3 wafers and 16 switch positions. I want to build a volume control for my Gainclone. I would like the quality to be as good as possible, ie, signal passing through as few resistors and soldered joints as possible at any one time. I would also like it to have a sensitivity of 1dB over a range of 60dB or so as my amp needs to cope with varying sources. I'm not concerned about how many resistors I will need to purchase or how fiddly it would be to construct.

Does anybody have any idea how I might do this? I was thinking of using one switch for a central volume control and using the other two each as a trim for the L & R channels in a similar way to the kit you can buy from tubecad.com
( see http://www.tubecad.com/Three-Switch_...nuator_PCB.pdf )

Any comments, suggestions or alternative ideas?
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Old 22nd April 2007, 08:49 AM   #2
Mailliw is offline Mailliw  Canada
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Shout out from Halifax!

60 positions is a little excessive isn't it? Seems like a lot of work, why not go with the kit?
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Old 22nd April 2007, 10:11 PM   #3
I_Forgot is offline I_Forgot  United States
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A challenge indeed!

How do you propose to get 60- 1 dB steps from a switch with 18 positions?

If you need such fine control AND some L/R balance capability, use an attenuator chip controlled by a uC. Accurate channel matching, small steps and lots of them, remote control capability... What more could you want?

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Old 23rd April 2007, 10:01 PM   #4
AndrewT is offline AndrewT  Scotland
Join Date: Jul 2004
Location: Scottish Borders
Hi Tl5,
that glassware two stage attenuator looks pretty good in the hand.

Something they forgot to mention.
The second stage attenuator should use an impedance about power amp Zin/10.
The preceeding attenuator stage should be second stage Z/10.
The source should therefore be about Power amp Zin/1000.

The closer you push these matching impedances the more inaccurate the 36 steps becomes in the two stage attenuator.

Even so, it's a clever way to reduce the number of resistors and solder joints on the way thro' and still incorporate a fine balance control
regards Andrew T.
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Old 7th May 2007, 02:47 PM   #5
tl507083 is offline tl507083  Canada
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thanks for your comments everyone. Still haven't decided what I'm going to do but the input was useful.
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Old 7th May 2007, 03:40 PM   #6
FastEddy is offline FastEddy  United States
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The Bottlehead folks make a more modest step attenuator, 7 or 8 position. They offer them as suppliments or replacements for their ForePlay tube pre-amp kit ( http://www.bottlehead.com/et/parts/parts.htm ... 2/3rds down the page ).

" ... For use in active circuits the basic 15K series design is re-implemented as a shunt attenuator, often regarded as the most transparent sounding attenuator configuration, putting only one "padding" resistor in the signal path. This additional padding resistor may be chosen from among four values included with the kit, to give the nominal 19dB gain of the original Foreplay preamp circuit (with Anticipation upgrade) a more flexible range of gain. You can choose from 9 db (close to being an industry standard), 3dB, -3dB or -12dB maximum overall gain with the change of one resistor. ..." ... & low cost @ US$40 (for 2)+ shipping ... and they really are quiet.
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