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potentiometer resistance and source load ? (cmoy)

post #1 of 12
Thread Starter 

Hi, I'm about to start building a cmoy and I was wondering if someone could explain how potentiometer ratings (10k vs 20k) relate to load on the source?  I'm assuming it's similar to how a 4ohm speaker draws more power from an amp than a 8ohm speaker, correct?  So using a 10k pot in a cmoy could potentially affect a portable music player in what way?  Could it drain the player's battery faster than a 20k pot, put more strain on it's signal path, or maybe cause some distortion in worst case?

 

I started wondering this after noticing how my music player's battery runs down quicker while using it in my car (connected directly to an amp via a cable) than it does while using headphones.  This seemed odd since it dosn't have the strain of physically driving headphone speakers when it's plugged in my car, but I'm guessing it has something to do with resistance, so could the type of potentiometer I use have a similar effect?

post #2 of 12

The battery draining faster does seem strange. I don't know enough about electronics to have an explanation for that.

 

But the pot's ohm value determines how quickly it goes from soft to loud as you turn the volume knob. A higher value pot will increase more gradually. If the value is too low you might find it hard or impossible to get a comfortable volume before it jumps to being too loud. You should balance this with the gain on your amp. Personally I like having as much usable range on my volume knob as possible. So even for a gain of 2, I still like to go with a 50K pot. YMMV.

 

post #3 of 12
post #4 of 12
Thread Starter 

Hmm...  that's something I didn't know.  Not sure if it answers my query though, or maybe I'm just not understanding it.  But thanks for the reply, I was starting to hear crickets.  :p

 

Heh.  ...tangent to the rescue here?  ...anyone?  :)

post #5 of 12
Quote:
Originally Posted by Victor Drath View Post

I was wondering if someone could explain how potentiometer ratings (10k vs 20k) relate to load on the source?  I'm assuming it's similar to how a 4ohm speaker draws more power from an amp than a 8ohm speaker, correct?

 

Actually, it's simpler because you don't have to take sensitivity into account, as you do with speakers. A pot is a pot is a pot, when it comes to questions like this.

 

In circuits like the CMoy, the entire pot resistance is across the output of the source. Ohm's Law tells us that 10K across a 1V signal (typical line level) requires 0.1 mA. A typical portable music player battery might be an amp-hour or so, so if driving the pot were the only demand on the battery, we can expect it to be able to drive that pot for about 10,000 hours, which is more than you should spend at work in a year.

 

So, the answer is no, a downstream amp's input resistance has no substantial effect on PMP battery life.

 

Quote:

So using a 10k pot in a cmoy could potentially affect a portable music player in what way?  Could it drain the player's battery faster than a 20k pot, put more strain on it's signal path, or maybe cause some distortion in worst case?

 

None, no, no and no, in that order.

 

Quote:

I started wondering this after noticing how my music player's battery runs down quicker while using it in my car (connected directly to an amp via a cable) than it does while using headphones.

 

I think you're doing fuzzy science. Some experimental errors I'll bet you haven't taken care of:

 

  • Ensure that the PMP's screen stays on exactly the same amount of time in both cases. The screen represents a load on the battery probably two or three orders of magnitude above that of the downstream pot.
  • Listen to the same tracks in both cases. Variance in drive current due to differences in the audio signal will be scarcely more significant than the load represented by the pot, I suspect, but variances in the bit rate of differing tracks might well be significant, since it affects how often the player has to pull data from storage. If you listen to more music (say, 192 kbps on average) in the car than at home where the balance shifts toward podcasts (say, 32 kbps mono), the higher current demand on the flash RAM while in the car could be enough to explain a difference.
  • Perception: you may not be comparing apples to apples in terms of hours spent in the car vs at home. Driving while listening to music requires more of your attention than listening to the same tracks to relax in a recliner at the end of a long day. Time spent in the car will therefore seem to pass faster, so you might be spending more time in the car running your PMP than you think you are.

 

I could probably think of more, but the main point is, you need to do a proper controlled experiment before you go making decisions based on it.


Edited by tangent - 3/26/11 at 2:50pm
post #6 of 12
Thread Starter 

So there's really no advantage to using anything beyond a 10k pot in a cmoy, right?

 

You're right about the battery life issues, there could be a lot of variables, maybe even my slightly buggy sansa.  I'll try to set up a proper experiment in the near future.

post #7 of 12

Since you use the cmoy with a source that can drive ( poorly ) 32 ohm headphones it's possible to use even lower values for the pot ( like 1k or 4.7k ) , values higher than 10k are useful only if the source requires it . Commercial amps usually have an input impedance of around 50k because it's a "safe" value for most sources .

post #8 of 12
Thread Starter 

Can't really say I came across too many audio taper pots in those values while I was researching.  Besides that, everything I've read says not to go lower than 10k, and many post I've read suggest 50k or 20k would be a better choice, with remarks about reducing load on the source.  I guess my question was what that actually means, and if it matters at all in a cmoy.  From what Tangent seems to say above, the answer would be no.

 

The sources I'd be using are just simple stuff like a sansa/ipod, cd player, dvd player, etc.  I don't have any tube equipment at the moment, but may in the future.

post #9 of 12
Quote:
Originally Posted by Victor Drath View Post

Can't really say I came across too many audio taper pots in those values while I was researching.  Besides that, everything I've read says not to go lower than 10k, and many post I've read suggest 50k or 20k would be a better choice, with remarks about reducing load on the source.  I guess my question was what that actually means, and if it matters at all in a cmoy.  From what Tangent seems to say above, the answer would be no.

 

The sources I'd be using are just simple stuff like a sansa/ipod, cd player, dvd player, etc.  I don't have any tube equipment at the moment, but may in the future.


I think someone is confusing "load" on the source with impedance that may combine with a source's output cap to create a high-pass filter.  This will reduce the amount of bass your CMoy will see from the source.  It may not be an insignificant effect, depending on the size of output cap your source may have.  (That's if it has any - many sources don't.)

 

For instance, let's say you use a BantamDAC with 1uf film output caps (standard for many builds).  Here's a graph that shows exactly what will happen with frequency response using different pot impedances:

 

fc-charts-1.0uf.gif
Chances are, the sources you mention are not going to cause this - since they're usually designed to output to much lower impedance earbuds, etc.  However, as you can see from the graph above, there are occasions where the pot impedance can become important.

post #10 of 12
Thread Starter 

Ah, I see, thanks.  Does this also apply to input caps used in an amp, or only output?  If any of my questions sound silly, I'm new to DIY.  I've got lots of wiring and soldering experience, but this cmoy is going to be the first device I've actually built.

post #11 of 12
Quote:
Originally Posted by Victor Drath View Post

Ah, I see, thanks.  Does this also apply to input caps used in an amp, or only output?  If any of my questions sound silly, I'm new to DIY.  I've got lots of wiring and soldering experience, but this cmoy is going to be the first device I've actually built.


It applies anywhere in the signal path that has a combination of capacitance and resistance in series.
 

 

post #12 of 12
Thread Starter 

Thanks

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