Stepped attenuator

[fdegrove]

Hi,

But it doesn't explain... why?

Dear Mr. Spock has already given the answer(s) to that Q, Sire.

Cheers,

[EC8010]

If you use a lower value pot, it loads the preceding stage more heavily and causes its distortion to rise. But if you use too large a value of pot, its output resistance combined with the input capacitance of the next stage forms a low-pass filter that encroaches upon the audio band and causes treble loss. 100k is a good compromise value in most situations.

[DrDeville]

Ah, your explanations, including Frank's
appeared after my query to Spock

Thanks guys, I'll ponder!

[DrDeville]

Fascinating, Captain!

Okay, let's see if I understand this.

Referring to Crowhurst's Audio Classroom article:

http://www.audioxpress.com/resource/...ss/ga699ac.pdf

(especially figure 1), this is what I've come up with.
Please excuse me for being detailed.

Potentiometers are basically voltage dividers, with
a series element (in the signal path) and a parallel
element (shunting to ground). Thinking of a pot
inserted between Figure 1's two stages, the pot's
series element (Rps) is in series with Cc, and the parallel
element (Rpp) is in parallel with Rg.

Now, the higher the pot's impedence rating,
the higher the resistance of both these elements
for a given attenuation setting.

The pot's series/signal element Rps increases the
first stage's output impedence. So, if the pot is too
large, Rps's interaction with the second stage's
input capacitance (Miller, etc.) causes HF roll off.

The pot's parallel/shunt element Rpp shunts Rg, so if
the pot is too small it effectively reduces Rg,
reducing the first stage's gain and interacting with
the coupling capacitor Cc to cause LF roll off.

Edit: Oh, and one more thing. At high attenuation
settings Rps increases and Rpp decreases,
so both effects are worst at high attenuation
(low volume) settings.

Does that make sense, Captains?

Best,

George "Nameless Ensign On The Landing Party" Ferguson

PS Nice kitty! I gotta get me an avatar.

[dhaen]

Assuming you can drive it, a lower value will also give lower noise.