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Power Supply filter capacitors
Power Supply filter capacitors
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Old 6th December 2017, 04:32 PM   #41
Mark Johnson is offline Mark Johnson  United States
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Power Supply filter capacitors
The Akitika kit is a stereo power amplifier using one LM3886 per channel. Unusually, it includes a (discrete component) voltage regulator on its own PCB with its own heatsink. This provides unusually stable supply voltage with unusually low output impedance (compared to unregulated supplies typically used with chipamps).

The construction manual, including schematics and theory-of-operation discussion, is online and freely downloadable. Might be worth 3 minutes to peruse it and appropriate the ideas you deem good.
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Old 6th December 2017, 05:01 PM   #42
Fast Eddie D is offline Fast Eddie D  United States
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Originally Posted by DF96 View Post
Physics tell us that bigger caps (within reason) give less ripple; less ripple is a good thing for sound reproduction.
That is true, but you're making the assumption that the caps have a reasonable impedance curve when you make this statement. In reality, caps vary a lot in this regard and in general larger caps have higher impedance at higher frequencies.

Power supply caps almost always have good characteristics at ripple frequency (120 Hz) but not always at higher frequencies. One nice thing in this regard is that with the ubiquity of switching power supplies, many improved caps are now available that do have lower inductance and esr. I've used them in power supplies for line level circuitry and they work perfectly. I don't see why bigger ones couldn't be paralleled for a power amplifier supply. I will investigate this as it relates to 1875 and 3886 chips once I complete about 20 other projects . In fact I did just this to a piece of vintage (90s) equipment (see above) and the results were gratifying to say the least.

So with awareness of this situation, it is possible to design around this drawback. It is imperative to at the very least place smallish value electrolytic caps (say 220-470 uF) as close as possible to the chip's supply pins. It even points this out in the datasheets.
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Old 6th December 2017, 07:22 PM   #43
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Fast Eddie D View Post
....................
So with awareness of this situation, it is possible to design around this drawback. It is imperative to at the very least place smallish value electrolytic caps (say 220-470 uF) as close as possible to the chip's supply pins. It even points this out in the datasheets.
It's more important that the HF decoupling is located as close as possible to the chip's supply pins and that the current route around the loops is kept very short and very compact.
The MF decoupling you describe, then fits around the HF decoupling and necessarily must have longer, higher impedance connections.
you seem to have misread the datasheet/s
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Old 7th December 2017, 05:05 AM   #44
Fast Eddie D is offline Fast Eddie D  United States
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It's more important that the HF decoupling is located as close as possible to the chip's supply pins and that the current route around the loops is kept very short and very compact.
I know that. I solder them directly to the chip leads on the underside of the board. Your chip is sure to be unstable without this. I have some similar to this ECW-F2224JAQ Panasonic Electronic Components | Capacitors | DigiKey that I use.

Quote:
The MF decoupling you describe, then fits around the HF decoupling and necessarily must have longer, higher impedance connections.
you seem to have misread the datasheet/s
Of course. I wasn't clear, but the whole discussion was about what size of electrolytic caps to use.

Do you think the inductance is necessary, or just a consequence of the necessary layout? I've only built a few prototypes and always put the MF decoupling as close as possible to the chips, less than an inch. Always worked with no funny stuff.
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Old 7th December 2017, 07:58 AM   #45
AndrewT is offline AndrewT  Scotland
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The inductance, leading to increased impedance, is a consequence of the size of the components. One can make the routes short, but ultimately the component size prevents making zero loop area.

I have even fitted the smd HF decoupling on the top side direct to the pins of a chipamp. Can't get shorter/smaller loop areas than that.
Then fitted the electro cans as close as possible. probably 10mm away leaving >20mm loop length.
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Old 7th December 2017, 08:35 AM   #46
OnAudio is offline OnAudio
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The decoupling cap also doubles as an RF filter, it compensates for resistance and inductance between the chip and the electrolytic s improving high frequency transients. Its decoupling is what improves channel separation
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Old 7th December 2017, 08:43 AM   #47
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Checkout the layout here P1 preamplifier and the review here P1 preamplifier
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Old 7th December 2017, 10:01 AM   #48
00940 is offline 00940  Belgium
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@ ggidzinski : the lm3886 has nothing that makes it "allergic" to a high capacitance supply and only some kind of anthropomorphic thinking can lead to conclude that higher capacitance lead to a slower sound (whatever that means). Trust your "old school" intuition on this.

Btw, if we take layout examples, this is pretty much ideal and to the point.

Decoupling caps are c14-15, c3-13, c4-12. It all goes to a groundplane under the board for low impedance.

See LM3886 Done Right. I'm not pushing Tom's products (I think these particular boards are a bit overpriced to be true) but the engineering is solid.
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Old 7th December 2017, 10:31 AM   #49
DF96 is offline DF96  England
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Quote:
Originally Posted by Fast Eddie D
That is true, but you're making the assumption that the caps have a reasonable impedance curve when you make this statement. In reality, caps vary a lot in this regard and in general larger caps have higher impedance at higher frequencies.
By "ripple" I meant mainly the low frequency stuff, up to a few kHz. That is what a reservoir cap is meant to deal with. Higher frequency is dealt with by smaller filter or decoupling caps.
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Old 7th December 2017, 01:18 PM   #50
Mark Johnson is offline Mark Johnson  United States
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Power Supply filter capacitors
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Originally Posted by OnAudio View Post
Checkout the layout here P1 preamplifier and the review here P1 preamplifier
The ground plane layout could be improved considerably by (a) refusing to route signals on the "green" layer {e.g. both ends of R28} ; (b) judicious use of zero ohm resistors {e.g. C13 to C19}
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