Large cap ripple current question (rebuilding an amp) - Page 1
Forum section: electronics / projects

author: mkiijam
author_rank: Regular Contributor
timestamp: September 04, 2025, 12:54:36 am
content: Hi Guys,I'm doing a full recap to shotgun a bit of audible 120Hz buzz (very low level) in a 7 channel power amp. It's using the Nippon U36D589 33,000uF 80V 51mm X 121 screw terminal. All of the mechanical specs are pretty important as a PCB mounts to them. I could 3D print spacers etc, so not the end of the world I f I can't find a size that is exact.My issue is: The Nippon was rated at over 14A in ripple current and the closest thing I could find so far is a LNR1K333MSEB from Nichicon, but it is only rated at 9.2A. In my mind this won't work or may heat up etc... It brought up to my brain maybe there is a way to know what this means and calculate if the 14.2A was way over kill to begin with?

author: jwet
author_rank: Frequent Contributor
timestamp: September 04, 2025, 01:56:53 am
content: That is an enormous ripple current spec. The ripple current will settle down to be less than or equal to the steady state output current basically. The idea is you have this pulsating AC source (full wave rectified Tx output), you are are pushing that into the cap each cycle. With no load, it will be a peak detector and have no ripple, as load is increased, the cap will be providing current between full wave peaks. I=C dv/dt and dt is 8.3 mS. at 33000 uF, the cap will maintain a voltage with ripple dominated by the output current. An important spec is usually ESR of the cap, I don't know what you're trying to achieve. The other unknown is what your amplifier's power supply rejection ratio is, if it has feedback, it should have good LF PSRR. You could feed it with a "perfect" lab supply and then slowly introduce 120 Hz ripple to see where its becomes audible.If you want to understand the interaction, I would run a simulation with realistic values for transformer output resistance, diode drop and peak currents. In audio circuits, reservoir caps have to take not just RMS currents but peak currents to maintain the main high rail for the power amp. You don't specify the magnitude of the hum but this would let you understand the requirement. If the original cap is good, the fix might require a more complex filter than the just a huge cap working against the impedance in the circuit. You might consider a choke.Well designed amplifiers should have a PSRR of 60 db or more at 120 Hz, this doesn't take much gain bandwidth.You could also measure the current going into the existing cap. An AC current oscilloscope probe would be the best way to accomplish this- they're a bit pricey but a great to way to see problems like this.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 04, 2025, 03:18:21 am
content: Quote from: mkiijam on September 04, 2025, 12:54:36 amIs this a new/recent problem and have you confirmed the capacitor has degraded ?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2025, 08:08:25 am
content: The ripple current can be well larger than the DC current. With a simple recfier and filter capacitor one normally assumes a power factor of about 1.7. So an AC current for the transformer of 1.7 times the DC current to the output. With current in relatively short pulses much of the current will go to the capacitor and than back out slow. So one would get the AC current in and the DC current out. So expect up to 2.7 times (likely more like 2.5 times) the DC current for the capacitor RMS current.The 120 Hz component of the capacitor current is limited to 1/2 the DC current, but the RMS current can be higher.Residual 120 Hz voltage at the filter capacitor is only one way to get hum. Another way is from the large current peaks and than either inductive coupling or a shared ground impedance (ground loop).

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2025, 10:18:41 am
content: Input capacitors are selected based on the required capacitance to hold up the voltage between AC cycles, which almost always makes them have a much higher ripple current rating than necessary. The exceptions are small input capacitors where huge amounts of AC ripple can be tolerated, like with off-line battery chargers.This is also why the much smaller output capacitors wear out much earlier than the input capacitors, which are huge in comparison.