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Author Topic: Converting pots to stepped attenuators  (Read 1182 times)

rodeo

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Converting pots to stepped attenuators
« on: June 23, 2004, 09:22:39 PM »

Hi,

I want to convert some gain pots to series attenuators to be able to match gains. I've found some equations/calculators but they all require the max desired attenuation in db in addition to total resistance and # of steps. I know I can measure the amplifier gain, but I'm confused how this relates to the original pot as its only a log or rev log taper with a specific total resistance  Confused

Is there a way to calculate even db steps without having to measure the resistance of the original pot at every db of gain?

Hope this makes sense Razz  I'd really appreciate any input.

Mark L
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Scott Helmke (Scodiddly)

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Re: Converting pots to stepped attenuators
« Reply #1 on: June 24, 2004, 09:09:21 AM »

Depending on how perfect you want to be...  You could just buy new pots with detents, and maybe add a trimmer to match them up better.
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John Klett

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Re: Converting pots to stepped attenuators
« Reply #2 on: June 24, 2004, 11:11:39 AM »

stepped pots are not too complicated - as volume controls they are used as continuous attenuators so you are just defining specific steps.

if the regular pot is 10K then the total resistance of the resistors in the attenuator is 10K.

I make a map

so in one case I may want a range from a normal "unity" setting and maybe I want a total of +10dB "gain" to -24dB "loss".  Say I have a 24 step switch.  The map might be...

Then I calculate voltage dividers using the total R of the ladder and the amount of attenuation I want.  Then I do some addition and subtraction to get the indivual step R's...  sowhat do I need for -2 atten...  -4dB etc ...  step step step.

feed to attenuator

switch gain         actual
step    position   attenuation

top      +10          0dB attenuation (followed by 10dB gain)
02       +08          -02
03       +06          -04
04       +05          -05
05       +04          -06
06       +03          -07
07       +02          -08
08       +01          -09
09       "unity"      -10
10       -01           -11
11       -02           -12
12       -03           -13
13       -04           -14
14       -05           -15
15       -06           -16
16       -08           -18
17       -10           -20
18       -12           -22
19       -14           -24
20       -16           -26
21       -18           -28
22       -20           -30
23       -22           -32
24       -24           -34
R that stops attenuation at - 24
bottom 0 volts ground - 0

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