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Old 19th September 2009, 11:01 PM   #1
jkeny is offline jkeny  Ireland
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Default How to reduce battery PS Noise

I've tried my 3.3V LiPoFE4 batteries on two different digital units & in both cases noticed a higher noise background than from non-descript regulators. I've put a scope across the battery terminals & see a sine waveform with peaks every 40nS and fluctuating in intensity with the music. What does this tell me - I need a cap across the terminals to smooth out the supply demand?

What's the best way to reduce battery noise & how can I measure it?
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Old 19th September 2009, 11:12 PM   #2
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Originally Posted by jkeny View Post
I've tried my 3.3V LiPoFE4 batteries on two different digital units & in both cases noticed a higher noise background than from non-descript regulators. I've put a scope across the battery terminals & see a sine waveform with peaks every 40nS and fluctuating in intensity with the music. What does this tell me - I need a cap across the terminals to smooth out the supply demand?

What's the best way to reduce battery noise & how can I measure it?
If it is a peak with a steady period - 40nS or otherwise - it is most likely an oscillation. This would also explain higher noise.
It would mean that your amp or whatever oscillates when powered from this battery - the battery may actually have some inductive component. Try putting a cap across the battery but not on the battery side but as close as possible to your circuit-to-be-supplied. Just a few uF electrolytic with some loss, do NOT use a low ESR one, just a regular run-of-the-mill electrolytic.

jd
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Old 20th September 2009, 12:01 AM   #3
jkeny is offline jkeny  Ireland
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Originally Posted by janneman View Post
If it is a peak with a steady period - 40nS or otherwise - it is most likely an oscillation. This would also explain higher noise.
It would mean that your amp or whatever oscillates when powered from this battery - the battery may actually have some inductive component. Try putting a cap across the battery but not on the battery side but as close as possible to your circuit-to-be-supplied. Just a few uF electrolytic with some loss, do NOT use a low ESR one, just a regular run-of-the-mill electrolytic.

jd
Yes it's a sine wave with peaks every 40ns - These batteries are very low internal impedance of about 4mOhm which might have some bearing on this. I powered two different DACs directly from the battery & noticed this noise - in one when the hiss reduced when I turned up the volume to about midway & then started to rise again (without any music playing).

I know some very low esr caps (oscon, SepC, etc) can give rise to oscillation in certain circuits - I'm wondering if this is the phenomena here. Could I test it by putting some small R in series on the + lead from the battery?
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Old 20th September 2009, 01:45 AM   #4
iko is offline iko  Canada
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To eliminate "tooth fairy" effects I'd recommend you hook up a power resistor that draws about as much current as your amp. Then hook up your scope and test again. Tell us what you see.

What amplitude, roughly, is the sine wave you saw when powering the amp? What amp is the load? BTW, when it fluctuates with music, it means the output impedance is such that the music induced current draw results in enough voltage variation that you can see it on the scope. Again, its magnitude would help us get an idea of what's going on.

You know already my alternate response: compare with a good regulator.
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Old 20th September 2009, 02:11 AM   #5
jkeny is offline jkeny  Ireland
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Originally Posted by ikoflexer View Post
To eliminate "tooth fairy" effects I'd recommend you hook up a power resistor that draws about as much current as your amp. Then hook up your scope and test again. Tell us what you see.

What amplitude, roughly, is the sine wave you saw when powering the amp? What amp is the load? BTW, when it fluctuates with music, it means the output impedance is such that the music induced current draw results in enough voltage variation that you can see it on the scope. Again, its magnitude would help us get an idea of what's going on.

You know already my alternate response: compare with a good regulator.
Thanks Iko,
It was a DAC I was powering not a power amp. (I tried 2 different DACs). Can I just put a power R across the leads from the battery - I accidentally touched these wires momentarily with sparks & the thick wires heated up quickly (these batteries can deliver 20A continuously or 60A in 10 sec bursts).
Any suggestions what would be a good ohm value at what wattage - (you can tell I'm a bit clueless)

The fluctuation in time with the music was in the intensity of the waveform on the scope screen, not in any change in the shape of the waveform itself! Does this signify the PS is not capable of supplying sufficient current without voltage drop?

I had already started to assemble the parts for a 3.3V Salas regulator - the version 1 to compare to these batteries but I wanted to iron out any kinks in the battery use first
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Old 20th September 2009, 02:22 AM   #6
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Quote:
Originally Posted by jkeny View Post
I've tried my 3.3V LiPoFE4 batteries on two different digital units & in both cases noticed a higher noise background than from non-descript regulators. I've put a scope across the battery terminals & see a sine waveform with peaks every 40nS and fluctuating in intensity with the music. What does this tell me - I need a cap across the terminals to smooth out the supply demand?

What's the best way to reduce battery noise & how can I measure it?
Consider that LiPOFE4 batterieshave a pcb attached to help with charging and discharging. Your PS is not 'just a battery'.

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Old 20th September 2009, 02:24 AM   #7
jkeny is offline jkeny  Ireland
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Originally Posted by pheonix358 View Post
Consider that LiPOFE4 batterieshave a pcb attached to help with charging and discharging. Your PS is not 'just a battery'.

Terry
No the batteries don't have a pcb attached - they are naked, so to speak
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Old 20th September 2009, 02:52 AM   #8
iko is offline iko  Canada
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Originally Posted by jkeny View Post
Thanks Iko,
It was a DAC I was powering not a power amp. (I tried 2 different DACs). Can I just put a power R across the leads from the battery - I accidentally touched these wires momentarily with sparks & the thick wires heated up quickly (these batteries can deliver 20A continuously or 60A in 10 sec bursts).
Any suggestions what would be a good ohm value at what wattage - (you can tell I'm a bit clueless)
Yes you can put a power R. What you need to know is the approximate current the DAC draws, so that you recreate the conditions of the initial test somewhat. Let's assume the DAC draws about 100mA at idle (probably not, but so you can see the calculations). Assuming your batteries provide 5V (you better measure the voltage), then the value of a resistor that draws 100mA with 5V across it would be
R = 5/0.1 = 50 ohm
rated for at least 5 * 0.1 = 1/2 W

You should see no sparks.

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The fluctuation in time with the music was in the intensity of the waveform on the scope screen, not in any change in the shape of the waveform itself! Does this signify the PS is not capable of supplying sufficient current without voltage drop?
I'm not sure I understand what you mean by "in time with the music." With a preamp or power amp, if the voltage in fluctuates in the rhythm of the music, it's definitely a sign of a "soft" power supply, pointing to not having a stellar output impedance. I don't know why a DAC would behave that way, I don't have any experience with them.

Quote:
I had already started to assemble the parts for a 3.3V Salas regulator - the version 1 to compare to these batteries but I wanted to iron out any kinks in the battery use first
Hey, it's simple enough to do a point to point and convince yourself once and for all. You probably invested some in the batteries, so it might be hard to let go. But there'll be always that question sign... what if?

P.S. And I would also do what Jan suggested earlier.

Last edited by iko; 20th September 2009 at 02:56 AM. Reason: Add ps
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Old 20th September 2009, 02:56 AM   #9
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Old 20th September 2009, 03:22 AM   #10
jkeny is offline jkeny  Ireland
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Originally Posted by ikoflexer View Post
Yes you can put a power R. What you need to know is the approximate current the DAC draws, so that you recreate the conditions of the initial test somewhat. Let's assume the DAC draws about 100mA at idle (probably not, but so you can see the calculations). Assuming your batteries provide 5V (you better measure the voltage), then the value of a resistor that draws 100mA with 5V across it would be
R = 5/0.1 = 50 ohm
rated for at least 5 * 0.1 = 1/2 W

You should see no sparks.
thanks again - I'm not sure what current the DAC draws - it's run off the USB power so I'll go for 50mA - the battery is 3.3V so about 66ohm for R should do it, I think.

Quote:
I'm not sure I understand what you mean by "in time with the music." With a preamp or power amp, if the voltage in fluctuates in the rhythm of the music, it's definitely a sign of a "soft" power supply, pointing to not having a stellar output impedance. I don't know why a DAC would behave that way, I don't have any experience with them.
What I mean is the DAC is playing through an amp and when a bass drum sounds I see the brightness of the trace on the scope get brighter (I'm not sure about this scope & I'm new to using scopes so I don't know if this is a faulty scope or PS fluctuation?)


Quote:
Hey, it's simple enough to do a point to point and convince yourself once and for all. You probably invested some in the batteries, so it might be hard to let go. But there'll be always that question sign... what if?

P.S. And I would also do what Jan suggested earlier.
This is just my first time to use the batteries so even though I want to build a Salas reg, I wanted to give them a fair hearing. They're $10 each so not bad really (about the cost of parts for the reg, I think - of course a battery charger is needed)
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