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Capcitors for Class D Amp Power Supply Recapping???
Capcitors for Class D Amp Power Supply Recapping???
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Old 16th January 2020, 02:34 AM   #11
MorbidFractal is offline MorbidFractal
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Join Date: Nov 2017
In terms of ripple current rating higher is better at any frequency. Klipsch claim 200W @ 35V which sounds iffy. The absolute voltage of your supply will set the maximum power output into any particular load.

The supply is subject to a ripple voltage. Your 60Hz sine mains gets rectified into a 120Hz...

Picture: V(SINE) is secondary of your transformer. V(RECT) is after the rectifier.

Your capacitors only get charged up when V(RECT) exceeds the voltage stored on them. This is one source of ripple current. That process also means that your DC bus to the amplifier has a ripple voltage on it.

Picture: I(DRECT) is the rectifier current V(RECT) is as before. V(BUS) is the DC supply to your amplifier. You can see that the capacitors are only charged up when the voltage from V(RECT) exceeds the V(BUS) voltage and discharges when it is below.

The peak I(DRECT) current is high, don't place too much value on the value it's a model, but the RMS is lower. Of more immediate concern is the ripple voltage, how much it sags in between charging.

If the bulk capacitance, total uF, is too low then the ripple becomes high and your amplifier will be driven into clip earlier. That affects the claimed rated power. As I suggest I don't think Klipsch are being honest or they are quoting funny numbers.

I(DRECT) is not the whole story as far as ripple current in the capacitors is concerned. On the other side of things the amplifier draws current to produce music.

Picture: I(IAMP) is what the amplifier is drawing from BUS. This is a 1KHz sine wave at the limit of clipping. If you look closely you can see the ripple voltage imposed on the peaks. When V(RECT) is above V(BUS) most of the amplifier demand will be from V(RECT) otherwise it is from the Capacitors.

As a result the current in the capacitors is a mix of two sources. One low frequency, Mains, and one high frequency, Audio.

Picture: This is the current in one, of the four, capacitors. It comprises the mains charging current and the amplifier music current, discharge so it is negative. There is also higher frequency current. That's the ripple current in the amplifier output filter inductor.

If you care then you have to account for the fact that these currents are at different frequencies and the ESR of the capacitor is different at those frequencies resulting in different losses. However...

In all cases everything gets complicated by the fact that in terms of RMS power music, by and large, can be treated as being 1/8th the power of its peak.

200 RMS Watts of Music is only 25 Watts RMS Average of Power. It is this sort of behaviour that allows you to derate certain specifications for components.

As a result Klipsh will likely have designed to a Ripple Voltage specification to achieve a claimed power without clipping and sort of said the ripple current looks OK. Sell It.

End result is that for you your chosen Capacitors are going to do the job and do it better than the originals.

Apologies for the added added confusion.
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Old 16th January 2020, 02:37 AM   #12
MorbidFractal is offline MorbidFractal
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4x4700uF Job done.
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Old 16th January 2020, 08:00 PM   #13
MonoJon is offline MonoJon  United States
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Quote:
Originally Posted by MorbidFractal View Post
In terms of ripple current rating higher is better at any frequency. Klipsch claim 200W @ 35V which sounds iffy. The absolute voltage of your supply will set the maximum power output into any particular load.

Wow, MorbidFractal, thank you so much for all that detailed explanation!



Yes, Klipsch simply marketed this thing as a "200w system", but the subwoofer is rated at 100w and the satellites are rated at 50w each. The amp probably doesn't deliver that.


As for the rest of what you wrote, I think I followed about 50% of it. Is it possible that I am conflating ripple current ratings with ripple voltage, and it is this ripple voltage (not the ripple current rating of a capacitor) that has the potential to create "noise" in the audio path. The capacitor must simply have a ripple current rating high enough able to handle the ripple being delivered to it? Am I to further understand that that it is really impedance/ESR that influence the potential for ripple voltage noise, and not ripple current rating? Is it also true that lower impedance/ESR would naturally lead to higher ripple current ratings as impedance is inversely proportional to current?


Is it also true then, that though the FX series capacitors may work fine and noise-free in my application, that choosing the PW series capacitors may have some benefit in increased operating efficiency and lower operating temperature?


~ Jon
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Old 16th January 2020, 11:31 PM   #14
phase is offline phase  United States
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If it were mine, I would use the largest Panasonic FM, or FC that I could find that will fit on the board in that voltage, and not worry too much about the capacitance rating.

Nichicon PS is a similar part, if you are stuck on that brand, and there are probably many more that will have similar specs. These specs can be found in the data sheet for most models, esr, temp ratings, physical size...

Class D amplifiers arenít the typical audio application, by the nature of their high speed action.
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Old 16th January 2020, 11:40 PM   #15
MonoJon is offline MonoJon  United States
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Quote:
Originally Posted by phase View Post
If it were mine, I would use the largest Panasonic FM, or FC that I could find that will fit on the board in that voltage, and not worry too much about the capacitance rating.

Nichicon PS is a similar part, if you are stuck on that brand, and there are probably many more that will have similar specs. These specs can be found in the data sheet for most models, esr, temp ratings, physical size...

Class D amplifiers aren’t the typical audio application, by the nature of their high speed action.

Thanks. Panasonic FC would fit at 3900uF, but I already ordered Nichicon PW (which exceed all the specs of the original capacitors, so I think I am good).




BTW, that should have been Nichicon FW, not FX in my post above this - I can't seem to edit it, but I want to make the correction for the record.

Last edited by MonoJon; 16th January 2020 at 11:43 PM. Reason: Correction
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Old 17th January 2020, 12:34 AM   #16
phase is offline phase  United States
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Good choice, should work very well.
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Old 17th January 2020, 12:52 AM   #17
phase is offline phase  United States
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Good choice, should work very well.
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