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Chip Amps Amplifiers based on integrated circuits

Power Supply filter capacitors
Power Supply filter capacitors
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Old 7th December 2017, 08:36 PM   #51
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by DF96 View Post
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.
What's important is that there's electrolytic caps on board with the chip. Many (probably most) consumer grade designs, from cheap "hi-fi" systems to receivers, do not have an intermediate set of decoupling caps. They have one large pair of reservoir caps located close to the output devices, be they integrated or discrete. These caps have to cover a wide frequency range of decoupling because they are usually complemented with only a small value (0.1 to 0.22 uF) capacitor. I know this because I have repaired, hacked, and disassembled for salvage many recycle bin units.

DIY types usually put the power supply circuitry on another board. I know I do. This necessitates the use of smaller (220-470 uF) decoupling caps on board with the chip.

Is there an advantage to the DIY way? Idon't really know. But I for one mix, match, and reuse stuff I build since 90% of them are prototypes anyway, so it's an advantage for me.
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Old 8th December 2017, 08:53 AM   #52
AndrewT is offline AndrewT  Scotland
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The fast current changes are supplied by the on board local supply rail decoupling capacitors.
The very fastest by the HF decoupling and the slower changes by the MF decoupling.

The PSU is separated from the amplifier by long traces/cables. The smoothing capacitance can only meet LF and DC current demand due to the inductive reactance of those longer connections.

You have to interpret the current demands of the amplifier you are building and include HF and MF decoupling that can meet those demands.
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Old 9th December 2017, 07:53 AM   #53
Mark Whitney is online now Mark Whitney  Netherlands
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Quote:
Originally Posted by ggidzinski View Post
Which design do you advocate, the one with the snubberized/35,000uF design or the last one with no filter caps at all and only the bridge output directly to the chip amp pins with the chip amp only decoupling?

George
With the right layout, a 1000uF capacitor will only have about 0.1Ω to 0.2Ω at 1MHz. The bigger problem is the impedance at low frequencies. Look at what happens below 500Hz.

But then again, shouldn't we be asking, at what frequencies do the capacitors work at and why.
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Old 9th December 2017, 09:28 AM   #54
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by ggidzinski View Post
Which design do you advocate, the one with the snubberized/35,000uF design or the last one with no filter caps at all and only the bridge output directly to the chip amp pins with the chip amp only decoupling?

George
The last scheme can be a recipe for disaster. While the filter effect (in terms of ripple voltage) might be sufficient with only close-chip caps, you have all the huge charge current spikes running at your PCB locally. With the slightest error in layout these spike currents will couple directly into your signal path and ruin all the performance. Actually, this a circuit design & layout challenge of the tougher variety. It can be done reaching "OK levels" of error if you are a seasoned expert in the field. Not recommended, that is.

Like other have suggested, locate the main reservoir electrolytics off-board, no bypasses (so you don't ruin their damping ESR). Then place smaller caps (value about 1/10th of the main caps) at the chip. These also don't need snubbers/bypasses when correctly chosen (eg paralleled multiple smaller caps) and using a good layout.

Don't be tempted to use only small film caps of way too low value, because this will end up in an supply impedance (what the chip is actally seeing) like in below screen-shot. It shows the impedance of a 150nF film cap in parallel with a 1000uF electro which is located 30cm away and connected with a set of twisted wires. The inductance from this wiring rings with the 150nF and results in a supply impedance peak of about 6ohms(!) at 1MHz, not a good idea (and with more capacitance it gets even worse as the peak wanders down in frequency)
Note that @1Mhz the impedance is actually lower when the 150nF cap is removed, not what one would expect...
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Last edited by KSTR; 9th December 2017 at 09:34 AM.
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