Zobel cap voltage rating

[Khron]

Greetings...

I'm in the process of building a pair of bi-amped bookshelves with Seas L15RLY/P and 27TDC. I'm going to use a pair of bridged Symasym's for each woofer, and a single Symasym for each tweeter.

Now, they'll be actively crossed over at 2KHz, but i also intend on using a Zobel network on the woofer. The part i'm concerned about is the voltage rating for the cap in the RC network. The amps will have +-35 to +-40V rails, and because they'll be bridged, if i understood correctly, the voltage swing doubles, so that gives me a max of 160V peak-to-peak on the woofer.

Currently, i have a small stock of 63V ERO (Vishay-Roederstein) film caps from which to put together the 15uF i need. Given the situation, would it be too risky to use these 63V caps, or would 160V (or more) caps be necesary?


Many thanks in advance

Chris

[AndrewT]

Hi,
the normal maximum voltage across the speaker terminals is |Vrail|
For a bridged design this becomes |2*Vrail|
Your bridged amp running on +-40Vdc could send 80Vpk to the speaker.
But, there is a complication.
If the amplifier output suddenly switches off, the back emf from the speaker can exceed the driving voltage.
It's all about inductors trying to oppose the change.

I would use 80V times a suitable factor.
100V (1.25*) seems a little too close, 160V (2*) would be a lot better. 250V (3*) better still.
15uF is just in reach of motor run/start capacitors. Using a PP version of one of those, although unusual, would be a safe start (they're not too expensive).

[richie00boy]

He's right with 160V peak-to-peak though Andrew, as a worst case.

But your point about inductive spike is also a relevant one.

[Khron]

Well that's no good news, unfortunately.. I only currently have small value caps at such voltage ratings.

The chance of blowing 63V caps in the situations you described isn't negligible, i fear...

[Juergen Knoop]

at 80Vpk your speakers would explode as well
regards

[Nordic]

The very nature of this heavy duty cap, predisposes it more to being certain types... which are also good for mains filtering... paper and film etc, as oposed to the normal selfhealing stuff... whish I could remember the publication now... One of the reasons are that other types can fail as closed circuit...

[AndrewT]

He's right with 160V peak-to-peak though Andrew, as a worst case.

but the speakers are never driven to see the Vpp signal.
The Vpk is the worst case.
Take one amp with output terminal at -40V and the other amp sending +40V to the other output terminal. This is what bridging allows the terminals to do.

Now, the speaker sees +40pk - (-40Vpk) =80Vpk across it's input terminals. The only time the speaker can ever see more than the Vrail to rail voltage is that sudden cut off inductive spike.

[richie00boy]

They will see full pk-pk swing in the case of simply playing at maximum level with for example a full cycle of a sine wave I'm sure music will have the odd full cycle swings here and there.

[AndrewT]

Hi Richie,
each amp has supply rails of +-40Vdc.
each amp can send a maximum voltage to the output terminal of either +40Vpk or -40Vpk.

wire them in bridge mode and the maximum across BOTH terminals becomes +40Vpk to -40Vpk or -40Vpk to +40Vpk.
In both cases the maximum voltage at the speaker terminals is twice what a single amplifier can deliver. i.e. 40Vpk in non bridged mode and 80Vpk in bridged mode.

[megajocke]

You don't have to worry about the inductive spike unless the amp is misdesigned and does not have output clamping diodes. If it doesn't have output clamping diodes and also has VI-limiting it will blow up itself when the protection circuit activates, which is bad design. 80V is the maximum voltage the cap can see. A 100V cap would be perfect.