[james902]

i have too many questions... so here goes...

1. does using a lower volume on the pot increase battery life?

i understand a higher Resistance means a lower current (V=IR), however, i see light bulbs burning off charge via formation of light. does that mean each additional resistance decreases or increases battery usage?

I want to know because i hear of people with pimetas that only lasts for 5 hrs off of 2 9v batteries. While my 1 9v cmoy has been running happily for 2 days straight w/out a hitch.

if someone can explain why pimetas use soo much more charge, i'll be mucho happier!

2. also, what's the average battery life of a basic cmoy with an 9v alkaline battery?

 

[Earwax]

Quote:

Originally Posted by james902 i have too many questions... so here goes... 1. does using a lower volume on the pot increase battery life? i understand a higher Resistance means a lower current (V=IR), however, i see light bulbs burning off charge via formation of light. does that mean each additional resistance decreases or increases battery usage? I want to know because i hear of people with pimetas that only lasts for 5 hrs off of 2 9v batteries. While my 1 9v cmoy has been running happily for 2 days straight w/out a hitch. if someone can explain why pimetas use soo much more charge, i'll be mucho happier!

A pimeta has several extra chips sucking up power. Besides the left & right opamp, there's a ground channel opamp, plus 3 or more buffers. Then the opamps themselves are more likely to be current pigs compared to the ones usually used in a cmoy and biased into class A operation and the buffers may be in wide bandwidth mode.

Quote:

Originally Posted by james902 2. also, what's the average battery life of a basic cmoy with an 9v alkaline battery?

Let's see, a 9V battery is about 600ma, IIRC. A typical cmoy with a 2132 opamp and low power LED might draw about 10ma. So roughly 60 hours if you could actually drain the battery dead. So 48 hours of runtime doesn't surprise me.

 

[Megaptera]

Quote:

Originally Posted by james902 i have too many questions... so here goes... 1. does using a lower volume on the pot increase battery life?

The pot attenuates the audio signal before it gets to the active component of the amp. It's not changing what the active (ie, current-drawing) section is doing to the signal, just what the initial level of the signal would be before it gets there.

Edit: Doh! Forgot about the output voltage. Ignore my misinformation

 

[drewd]

Quote:

Originally Posted by james902 i have too many questions... so here goes... 1. does using a lower volume on the pot increase battery life? i understand a higher Resistance means a lower current (V=IR), however, i see light bulbs burning off charge via formation of light. does that mean each additional resistance decreases or increases battery usage?

Current is everything. You have the answer right there, with Ohm's Law. If the current drops, then the batteries will last longer. So, additional resistance will decrease the battery useage. Light bulbs are just resistors that get hot enough to generate light. They're not really burning off charge by the formation of light, they're doing what all resistors do and disspating power in the form of heat. It just happens that the filament material glows when it gets hot. Measure a 60 watt light bulb with a multimeter and you'll probably come up with something around 230 ohms (if I remember correctly). I vaguely recall somebody (maybe Nelson Pass) designing a power amplifier that used light bulbs as high current resistors to bias the tubes. But I digress (a lot!)

Quote:

Originally Posted by james902 I want to know because i hear of people with pimetas that only lasts for 5 hrs off of 2 9v batteries. While my 1 9v cmoy has been running happily for 2 days straight w/out a hitch. if someone can explain why pimetas use soo much more charge, i'll be mucho happier! 2. also, what's the average battery life of a basic cmoy with an 9v alkaline battery?

Earwax has it right on. Six buffers and two opamps biased to be turned on all the time means that even when there is no signal, the amplifier will be drawing a fair amount of current from those batteries. When you start playing music, the amp becomes pretty hard on a pair of 9V batteries. The case on my Pimeta gets warm, even at low volumes. Heat is power and the power comes from the batteries.

I suppose that you could expect 20 to 30 hours from a Cmoy, depending on the opamp, the headphones and the music.

-Drew

 

[tangent]

Quote:

does using a lower volume on the pot increase battery life?

Yes. That reduces the voltage the amp has to drive into the heapdhones, so it reduces output power, which has to come from the battery.

Quote:

does that mean each additional resistance decreases or increases battery usage?

Thinking about only resistance is insufficient. You need Ohm's law: V=IR.

Let's take an example using the question I answered above. But first, let's rearrange the equation to I=V/R. Let's say V is one volt at full normal listening volume. R is the resistance (impedance) of the headphones, and let's call that 120 ohms. I (current drawn from the battery), then, would be 8.3mA. Let's then reduce V to 0.5V, and we get half the current, 4.2mA.

This is all very simplistic of course: you're not listening to continuous 1V signals, the headphones aren't a constant impedance at all frequencies, and the current that goes to the headphone is just part of the total current drawn from the battery. But it does answer the question, "does current go down if voltage goes down and resistance stays the same?"

Quote:

i hear of people with pimetas that only lasts for 5 hrs off of 2 9v batteries.

That doesn't make sense, and I think it's because you're not giving enough information. Let's assume that you mean 2x9V in series, and that they're alkalines. If that's the case, it would have to be drawing 120mA to hit that battery life!

If instead you mean rechargeables, the mAh rating is 3-4 times lower, so the current would be 3-4 times lower. 30-40mA is a fairly common high-end PIMETA configuration. The low run time in this case would simply be a consequence of running on rechargeables.

You don't have to configure a PIMETA to be a battery-munching maniac. Minimum current draw for a PIMETA with decent op-amps is about 15mA, not much higher than for a CMoy.

 

[13405]

Quote:

does using a lower volume on the pot increase battery life?

Yes, as tangent stated correctly above, unless of course its an amp which is biased into Class A

- I'm unaware of any 'portable' amps which are like this though.

 

[james902]

Quote:

Originally Posted by Megaptera The pot attenuates the audio signal before it gets to the active component of the amp. It's not changing what the active (ie, current-drawing) section is doing to the signal, just what the initial level of the signal would be before it gets there. Edit: Doh! Forgot about the output voltage. Ignore my misinformation

tang: i looked back at the post that said 5-6 hours and they were using two rechargables.

thanks everyone for clearing it up for me. on one last note...

does it make a difference if the pot is before or after the amp-processed signal? I have it after the amp.

 

[Garbz]

Oli the Pimeta can be Class A biased.

So is the Mint and the Meta42

/EDIT: Disregard my rambling!

 

[tangent]

Quote:

i looked back at the post that said 5-6 hours and they were using two rechargables.

I think I know the post you mean, and I think that amp's an abberation. If it's the same one I'm talking about, the guy had the batteries in parallel, which is a bad idea with rechargeables.

Quote:

I have it after the amp.

Bad idea. That makes the output impedance of your amp equal to the pot's setting. Also, it puts the heapdhones in parallel with the pot, which gives nonlinear output impedance. You want the output impedance to be low and as linear as possible.

Quote:

Oli the Pimeta can be Class A biased.

Only the input stage, Garbz, not the output stage (buffers). In a fully class A amp, Oli's right, the current drawn from the battery is constant regardless of volume setting.

 

[james902]

woohoo! more soldering this this weekend!

 

[jboehle]

Are you guys talking about the PIMETA that I have up in the FS forums? It's batteries aren't in parallel, they're wired up in series, just like Tangent's PIMETA assembly guide suggests. I figured the battery life was due to the stacked buffers on the L/R channels, since I don't have it biased into class A.

-Jason

 

[james902]

if i stack two 9v in parallel, will it double the battery life? woah... that would be nice

got my answer solved. thanks to the search button

n/m