PSU capacitor selection

[phreeky82]

This is regarding the capacitor size to use, not the type etc

I have a kit amp (well two, dual mono) I've used for a long time, it puts out around 70wrms @ 4ohm according to specs and seems about right I suppose. The power supplies I also got the "matching" kit as recommended by the designer, which is just a 4700uF 50v cap on each rail (+/-40v or so from the PSU) per PSU.

I've been playing around making another PSU for a small project and calculating cap size to use and found how it can be calculated based on the desired max ripple voltage.

I would seem (to me) the caps I'm using for my PSU are relatively small for the amp power, and I have noticed that, just for fun, when I tried using one channel for a sub that it didn't perform so well, and thought this may be down to either the caps or transformer not having enough "juice"......???

Each module is running from a separate 135VA transformer btw (sounds silly but i bought a single amp module to start with, was just starting out).

Should I up the cap sizes? I've got spare matching caps sitting here, so I could probably rig them up in parallel to double the capacity, fair idea?

[lndm]

Doubling sounds reasonable. Also, paralleling like values would halve the esl/esr.

[phreeky82]

cheers mate, gave me something to read up about regarding capacitor selection etc (i.e. undesirable properties of a capacitor)

[AndrewT]

Hi,
I recommend 2mF to 3mF per amp peak of output current.

For 70W into 4ohms this equates to +-12mF to +-18mF per channel.

Your 4m7F could be tripled or even quadrupled for fully extended bass performance.

You may find that mid or treble performance will drop off with these recommended cap values.

There is another criterion for PSU cap selection. The RC time constant of the PSU (cap times speaker impedance) should be at least one octave below the input high pass filter on the amp.
i.e. 4m7F & 4ohms gives RC=19mS. This limits the high pass bass filter to higher than 9.5mS. If this has been scaled correctly your bass response will be -3db @ 17Hz and -1db @ 34Hz or higher.
This cut in bass response will be very noticeable with wideband speakers. I can easily hear a change in input RC from 35mS to 100mS with 5inch bass/mid driver. This is theoretically way below the low frequency hearing limitations of our ears.
On this basis I have been recommending an input filter of 80 to 100mS and this requires the PSU to be greater than 160mS to 200mS.

Lots of room for experimentation.