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Old 12th January 2013, 02:58 PM   #1
CraigP is offline CraigP  United States
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Default output capacitor 'size'

Hi all,

I'm working on learning more about electronics and amps. I've built a simple LM386 based amp. I'm using the example from the datasheet. The circuit is built and works pretty well! I have a few questions I was hoping someone could help me with.

I've tried 3 different output capacitor "sizes": 330uf 470uf and 10,000uf. they all work. How would I determine which I would *want* to use? For some reason I was thinking that the larger cap would cut off higher frequencies? Anyway, any thoughts or comments would be welcome.

A quick follow-up question. I first ran this circuit off my benchtop homemade power supply. It is a old laptop power supply that goes into a LM317 based circuit that allows me to "dial in" my voltage. When I use this to power my amp it "thumps" and sounds like crap... When I use a 9v batt, things are perfect. Thoughts on that one?

I included a photo of the circuit and the 3 capacitors I tried.

Thanks for any help folks!


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Old 12th January 2013, 03:04 PM   #2
djoffe is offline djoffe  United States
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Bigger output cap allows more low frequencies to speakers.
Half power frequency is

f=1/(2*pi*C*Rspkr)

As to the thump on the LM317...perhaps the supply voltage is collapsing on signal/current peaks. Too many variables to say for sure from far away.

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Old 12th January 2013, 03:30 PM   #3
DF96 is offline DF96  England
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With such a small speaker not mounted on a baffle you are unlikely to get much bass below about 200Hz or so anyway. You would need to use an output cap below about 100uF before you began to notice anything.
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Old 12th January 2013, 03:33 PM   #4
Bigun is online now Bigun  Canada
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Good for you in doing some experiments to learn !!!!!

In this application, a bigger capacitor should be better because not only does it allow lower frequencies to pass through with less impedance, it will also have less distortion. But you are right that some big capacitors can affect high frequencies due to increased inductance - I don't think that will be an issue here. The real thing is, can you hear a difference ? With the small speaker you show in the photo I doubt that any real differences will be obvious and you can use any of those capacitors.

Not sure about the thump. Many amplifiers produce a small thump when they are first tuned on, especially those with large output capacitors. It depends on the details of your power supply. Not sure why it sounds like crap, but as djoffe said, a stiffly regulated supply might not be providing enough current on the peaks.
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Old 12th January 2013, 03:59 PM   #5
CraigP is offline CraigP  United States
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Thanks for the ideas guys. I made my power supply some time ago when I was just learning... wait... I can't use that as a time reference as I'm still "just learning"... Anyway. I noticed I don't have any output capacitors on the LM317 power supply. I'm guessing this thumping then is as you suggest the supply collapsing. I'll tried to add a capacitor on the supply just for kicks. I also have a Radio Shack shelf speaker called Minimus 77 from WAY back in the day. I'll hook that up to the amp and try all three caps again. Maybe I'll hear the bass difference. I'm pretty sure that are 8Ohms also so I should be good.

Thanks again guys! I'll let you know what I find out.
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Old 12th January 2013, 04:14 PM   #6
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If you are going to leave the speaker unhoused, it won't be able to reproduce bass of any consequencel. Your 470uF cap will thus be plenty. If you cut a hole in small box of a size equal to just the speaker cone and attach the speaker to the inside, as in a typical PC speaker or radio, and then seal the box, you might hear things with a bit more depth.

I can't understand why the power supply isn't better than the battery (other than noise) since the LM317 should be capable of almost an amp current with say, a raw 16V DC source. Laptop supplies are quite substantial so I think you need to double check the regulator circuit and test what current it can supply into some heavy resistor without burning anything up.

I assume the regulator is on a fair sized heatsink as without this, it will shut down irregularly and that will make odd sounds too!.
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Old 12th January 2013, 06:59 PM   #7
jaycee is offline jaycee  United Kingdom
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220uF output cap is plenty for these amps, but it is good that you try different ones to see the effect

A battery typically supplies a nice smooth voltage, whereas any kind of transformer does not. In addition an SMPS supply produces plenty of noise which will probably upset the chip. To help with this, put a 220-470uF capacitor on pins 6 and 4 of the chip, as close to the chip as possible. Alongside that, use a 100nF capacitor - a ceramic will do. This will "decouple" the chip's power supply and should keep it stable.
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