PI-Filter

[wuffwaff]

Hi there,

I´m thinking about modifying the powersupply of my power amp with a PI filter. It now consists of a 450VA 2x30Volt transformer and 2 x 50 x 1000 microF per channel. I would like to change this to 2 x 15 x 1000 microF, 2 x 2mH, 2 x 50 x 1000 microF per channel.

Would this be a usefull size or should the first capacitor be bigger? (The amp is biased at 1 amp but can draw up to 15A eff.)

I´m thinking of winding 4 air chokes of about 70mm diameter,20mm thick.

Is it neccesary to shield these from the rest of the amp?
What material would be the best to do the shielding (if neccesary)?

william

[Nelson Pass]

You only have to watch the ripple current
rating of the primary capacitors - it needs to
be about 2-3 times the DC use you expect.

By the way, I've really come to like the ERSA
inductors available from Zalytron, as they have
low resistance and can take a lot of DC before
they saturate.

I have not had to shield the chokes, but a little
distance from the low level circuitry, say 6 inches
or more, is always a good idea.

[wuffwaff]

Thanks for the quick answer!

Since I build the casing for this amp about 15 years ago (when still at university with all the beautifull milling and drilling maschines) and it has undergone quite a lot of changes inside it will be very difficult to keep the chokes 6" away from the low level circuitry (the casing´s only 440 x 400 x 150 ) More like 3" maximum, so I´m afraid I´ll have to shield them.
The question is with what.

For the capacitors I will use Panasonic FC types wich are rated at 2.2A @100kHz for a 1000 microF/50V capacitor. I´ve tried them in my preamp and cd and they sound really good while being less expensive as an ELNA Cerafine or Silmic.

william

[diyman]

Anybody tried using as chokes the foil air core coils wound by Solo and alpha normally used in speaker crossovers? They come in a variety of values and their flat shape makes mounting easy and is space saving.

ckt

[leroy]

I'm wondering if there's a formula so I can calculate the voltage ripple when using a PI-filter, anybody?

[Elso Kwak]

Hi All,
I use two 10000 µF and two 22000µF caps and two 15 mH chokes in my Ampzilla. The 10000 's are at the rectifier side.
I started with the 2.5 mH coil from the Klipschcrossover but with the 15 mH the sound is better. The inductor I use is from www.intertechnik.de part#1340279 , has a DC resistance of 0.57 Ohm, ferrite core.
The ripple is almost too low to measure on my scope.
Hope this helps.

[P.Lacombe]

William,

An inductor of 2 mH represents an impedance of 1.25 ohms at 100 Hz. This value is too low.

In order to obtain a good efficiency with your filter, you have to use inductors of 10 mH and more. But air inductors of such value are very big and expensive. That is why iron-core inductors are used for this purpose. Iron-core inductors don't requires shielding, and can be diposed anywhere in the amplifier cabinet.

Regards, P.Lacombe.

[Nelson Pass]

Actually 2 mH works fine. The lowest frequency it will encounter
will be 120 Hz, and the impedance there will be about 1.5 ohm.

( Z = 6.3 * L * Freq )

The cap which follows the inductor needs to be low impedance
at this frequency ( Z = 1 / ( 6.3 * C * Freq ). A 10,000 uF cap
will be .13 ohms, so we are already seeing about 20 dB
reduction at 120 Hz.

Keep in mind that much of the noise in the ripple is at even
high frequencies, and they are attenuated even more.

[AudioFreak]

it will see 100Hz if the amp is run from 50Hz AC mains supply like we have over here.

[GRollins]

If you want more inductance, run two (or more) inductors in series. Presto! All the inductance one could ask for.

Grey