Fader Impedance--Calulating for Maximum Throw
Jun 14, 2009 at 3:43 PM Thread Starter Post #1 of 9

Zaubertuba

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Not happy with how my preliminary Szekeres builds are turning out, I'm turning to the Apheared 47 as my next option for a balanced build.

As the whole point of this project is to see how much you can do on a budget, my cheapskate solution for a balanced volume control involves using parallel stereo alps faders (which you can get for as low as $7.0 ea.). Much easier to manipulate than two rotary knobs, and way cheaper than a quad-pot.

Apheared's design calls for a 10k pot, but the output on this circuit is apparently extremely loud, and for maximum effectiveness I need to get the most throw I can out of the faders. I'm thinking 20k pots might accomplish this, but I don't want to go off the deep end and not get enough volume out of the circuit, either.

How do I go about calculating output given different input pot impedances?

EDIT-Sorry about my spelling on my thread title. Yes that should be "calculating". Not enough coffee this morning. :p
 
Jun 14, 2009 at 11:21 PM Post #3 of 9

Zaubertuba

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O.K. - Sorry for such a silly question, then. Guess I'm more of a newbie than I realized.
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...so, achieving maximum desired volume at a given fader setting is more a factor of the gain characteristics of the amp circuit itself?

Your patience as I get "up to speed" is sincerely appreciated.
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Jun 14, 2009 at 11:33 PM Post #5 of 9

Pars

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Quote:

Originally Posted by Zaubertuba /img/forum/go_quote.gif
O.K. - Sorry for such a silly question, then. Guess I'm more of a newbie than I realized.
rolleyes.gif


...so, achieving maximum desired volume at a given fader setting is more a factor of the gain characteristics of the amp circuit itself?

Your patience as I get "up to speed" is sincerely appreciated.
wink.gif



A couple of issues here. First, a pot or fader acts as a voltage divider. The total resistance available is what is specified as the pot value (i.e., 10K, 100K, etc.) Because it is operating as a voltage divider, it is the ratio of the input resistance to the resistance to ground that dictates how much of the input voltage is presented to the wiper or output of the pot. THis is why changing the pot value does nothing in terms of how loud the amp is at given points (for the most part). Larger values of pots are normally used in things like tube circuits, which require higher impedences for matching the particular circuit characteristics.

The question you ask regarding max. desired volume at a given setting relates to what is know as the "curve" of the pot. Audio pots are normally logarithmic (vs. linear). Several log curves may be available. But yes, as you noted, the gain of the amp itself has by far the largest effect on where in the pot rotation (or fader setting) you will find the best listening volumes. Changing the gain of the amp is the best way to control this (if possible). On the A47, the feedback loop controls gain, so decreasing this from the stock gain of 11 (IIRC) to 4-5 or even lower is pretty easy to do.
 
Jun 15, 2009 at 12:16 AM Post #6 of 9

Zaubertuba

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Extremely helpful info., guys. Thank you!
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Quote:

Originally Posted by Pars /img/forum/go_quote.gif
..... But yes, as you noted, the gain of the amp itself has by far the largest effect on where in the pot rotation (or fader setting) you will find the best listening volumes. Changing the gain of the amp is the best way to control this (if possible). On the A47, the feedback loop controls gain, so decreasing this from the stock gain of 11 (IIRC) to 4-5 or even lower is pretty easy to do.


According to Apeheared here, the gain is actually only 3. Still, it doesn't sound too difficult to play with. Is the resistor to play with what one would designate as R4 on the classic Cmoy (i.e. the 10K Resistor between the output and pin 2 of the opamp in Apeheared's schematic)?
 
Jun 15, 2009 at 5:33 AM Post #7 of 9

amb

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Gain = (R4 / R3) + 1
You can vary either resistor to change the gain. Just be aware that some opamps must operate at certain gain or higher to remain stable.
 
Jun 15, 2009 at 2:58 PM Post #8 of 9

deltaydeltax

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Quote:

Originally Posted by amb /img/forum/go_quote.gif
Gain = (R4 / R3) + 1
You can vary either resistor to change the gain. Just be aware that some opamps must operate at certain gain or higher to remain stable.



Where are you seeing R4 and R3? I don't see a schematic anywhere on here. Is there a common page elsewhere you two are referencing?
 

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