Low pass filter on bipolar power supply

[Yazoo]

A while ago I built an amplifier for a keyboard using a chip requiring a bipolar power supply. I came back to it recently and realised it suffered from 100 Hz hum so I want to add low pass filters to get rid of this. Can I just add an RC filter to both the positive and negative lines with the capacitors connecting to the shared ground line or would this cause problems?

[antonis]

Can I just add an RC filter to both the positive and negative lines with the capacitors connecting to the shared ground line or would this cause problems?

No problem as long as negative supply filter cap positive leg faces to ground..
(and reverse R-C arrangement, of course..)

edit: Could you plz post a schematic of power supply..??

[Yazoo]

This is the schematic. I was hoping to insert RC low pass filters at the V+ and V- points. It's a simple half-wave circuit and I didn't use voltage regulators because it pulls quite a lot of power.
It does work but there is that constant hum I mentioned.

[antonis]

What are your AC and DC voltages..??

P.S. I'd use full-wave rectification..

[Dormammu]

This is the schematic. I was hoping to insert RC low pass filters at the V+ and V- points. It's a simple half-wave circuit and I didn't use voltage regulators because it pulls quite a lot of power.
It does work but there is that constant hum I mentioned.

Yeah man, it's no good.
Use a full-wave rectifier.

[Yazoo]

I used half-wave because the transformer has no centre tap. I could create an artificial centre tap but this would involve re-doing a chunk of the circuit. If I can get away with it, I would prefer to just insert a separate circuit board to do the filtering. I saw an LC filter board for bipolar power supplies on eBay which looks like it would do the job for around £10. What do you think?

[antonis]

Make your own by using resistors instead of inductors..

[ElectricDruid]

This is the schematic. I was hoping to insert RC low pass filters at the V+ and V- points. It's a simple half-wave circuit and I didn't use voltage regulators because it pulls quite a lot of power.

Exactly how much power are we talking about? It's going to make a big difference to the resistors, since the more current, the more voltage they drop (and the beefier the resistors have to be, too).

[antonis]

And the more worse the voltage ripple..

[Rob Strand]

built an amplifier for a keyboard using a chip requiring a bipolar power supply.

I'm assuming you mean a power amplifier?

In this case RC filters are not suitable.
Toroid inductors are just about useless for hum.
Larger caps may help (but not much in the idle state).

I have a feeling you problem is related to the ground tracks.  You haven't separated the noisy grounds and quiet grounds.  You might even have multiple connections to the chassis for the input and output jacks.

The AC input should connect directly to the where the 470uF caps all join.  A single wire from that cluster taps off the caps not along the wire from the input.



While the half-wave doubler connection isn't ideal it can work for low powers and it is unlikely to be causing the hum.

Do you have any preamps in there as well?

You might have a ground loop.  Is the hum present with no keyboard connected?



(Whatever the amplifier) It's important that the inverting ground point, the 22uF cap in this pic, does not connect to/along noisy ground points, and does not connect along high current grounds between the power supply and the speaker.  The 10 ohm in this circuit tries to force the issue but if the circuit is wired with tracks that follow this pattern and the 10 ohm was link it is usually OK too - the 10 ohm can be better though.

[PRR]

I'm assuming you mean a power amplifier?

If so, and a loudspeaker power amp: no reasonable number of 470uFd caps makes a good solution.

You want >1,000uFd for 8r load. Bipolar, two 2,000uFd. Half-wave doubles or triples that: two 5,000+uFd. (Yes, one or two 10-packs of 470uFd would do.)

The plan shown with a speaker power amp has more ripple than DC.

In some long run, a more suitable transformer may be wanted.

[Yazoo]

OK, I've rethought this. I've got a Chipamp I built a while ago and I'm going to use that instead. I had a look tonight and the capacitors are huge.

Thanks for all the advice. As Benny Hill said "learning all the time".