Capacitors

To calculate the capacitor voltage rating for crossovers, follow these steps:

1. Determine the maximum voltage that the crossover will encounter during operation.
2. Consider the peak voltage of the audio signal, which is typically higher than the RMS voltage.
3. Apply a safety margin, usually 1.5 to 2 times the maximum voltage, to ensure reliability.
4. Select capacitors rated for at least the calculated voltage to prevent breakdown.
5. Check the capacitor's temperature rating, as higher temperatures can reduce voltage tolerance.
6. Ensure the capacitor type (e.g., film, electrolytic) is suitable for audio applications for optimal performance.

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So, if an amp is rated at 1,000 watts into 8 ohms.

1,000 watts / 8 ohms = Voltage squared

=89.4 volts RMS

89.4 volts x sq rt 2 = 126 volts peak

126 volts peak x 2 (margin of error to be conservative) = 252 volt rating for the caps.

If you substitute 400 volt for 800 volt caps, they are still both way over-kill from most anything you will ever encounter.

Thank you everyone for your information and input. This all goes with my thinking but I wanted to check with others! Love this forum!

I assume we are discussing a vacuum tube circuit since these voltages are not normally present in solid state amps with the exception of some class D switch mode Power supplies which can generate 400v DC from rectified mains so caps maybe rated at 400v in the primary side or PFC stages.

I guess some high-power class A/B amps can generate voltage swings that can get near 300v so filter caps maybe rated at 250v or often higher.

So, if my assumptions are correct, the answer is DO NOT DO IT

For coupling and decoupling Caps NEVER downgrade voltage rating in fact its better to over spec min 20-30% I went for 50% over rated

Electrolytic Caps are more commonly found in the decoupling path and these are more prone to catastrophic failure i.e. explosion.

Film Caps are not commonly known for explosion but Dielectric failure and short circuit could damage components around them and as they vent their contents and gas.

Typically, a 400v electrolytic cap in a 450v circuit will likely work for a while but will be under stress its whole working life and will degrade and fail prematurely.

I replaced my coupling caps as part of a total rebuild of my wonderful Grant Lumley ST70 vacuum tube stereo power amp, the originals were rated at 1000vdc an already good decision by the designer since the amp sees voltages in excess of 600v. I went for the highest voltage film caps I could get using the same uF. They happened to be the superb Mundorf Silver Gold Oil Film Caps rated for 1200vdc.

If they are filter caps then they must be rated well above the peak voltage they will experience. Within My Lumley ST70 I replaced the twin large in series 450v Filter Caps which had served the Iron well but had shrunk and were now not anywhere near their original spec but were another good choice in its design with new twin Nichicon 500v in series effectively an even bigger improvement in head room.

All other non-signal caps i.e. cathode bypass/bias supply etc.. were replaced with equal or larger voltage but will still be an improvement since they are Audio note Kaisei and Nichicon FG electrolytic capacitors.

There is one rule of thumb in tube electronics over rating voltage in capacitors is GOOD under rating is BAD

toddalin

"If you substitute 400 volt for 800 volt caps, they are still both way over-kill from most anything you will ever encounter".

That advice is valid for speaker crossovers or low-voltage solid-state circuits.

I think the question maybe related to Vacuum tube Electronics

For tube amplifiers.Good practice rule — never downgrade voltage rating, overspec by 20–50% — is the correct approach.

It depends in speakers or in electronics , if in electronics 

The u.f has to be the same and hopefully better distortion ratings 

The volt at minimum the same size in there  a bit bigger voltage 

Even better if you have the space ,most companies design for average quality

Not modding upgrades, I have 25+ years in  upgrades