Originally Posted by Victor Drath
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.
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.
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