Help with Hum - Hum Help - Aikido

[JesseG]

Not sure if this should be here on in Power Supplies, but here goes...

I am doing my best to fis up a friend's octal Aikido. The power supply needed some tlc, so I built a new one. It runs fine, but with an objectionable amount of 120Hz hum.

Put the scope on the power supply and I see a solid, waveform-perfect 2V sawtooth at the B+ point.

Here's the map - any ideas?

[SY]

What does the sim in PSUDII predict? If it's much less than 2V, I would strongly suspect your grounding. Can you diagram how that was done?

[JesseG]

PSUDII predicts a 2V P-P sawtooth on the C following the choke (fancy that!)

The question is, how do I get rid of it from the Aikido's output?

Signal grounding is star at the input jacks. Power grounding is on the chassis.

[SY]

Well, you could put a regulator after the raw supply (that's what I did for the two Aikidos I built). You can also try nulling the noise- the voltage divider values for the nulling circuit that John suggests aren't exact and the adjustment to get a decent null is pretty sharp. So you can change one of them out for a resistor that's, say, 20% smaller than nominal in series with a trimmer that's, say, 40% of nominal, then have at it.

[fhs]

You clearly need smoothing capacitors on both sides of the choke.

[VictoriaGuy]

Originally Posted by fhs

You clearly need smoothing capacitors on both sides of the choke.

Well, something 'more' , I'd think. One choke and one cap (94uF) isn't much filtering, especially for a preamp. Even old table radios and guitar amps had more filtering than that.

Is there something special about the Aikido that lets it use noisy B+?

[Hosam]

I'm not so professional but I suppose coaxial cable from to the ground could help you to solve the problem if it come from AC noise. anyway you'll not loose anything if you try it even it simple.

[DF96]

Excuse my ignorance, but what is supposed to be the purpose of the diode across the choke? This will bypass the choke on the downward swings of the AC, so destroying the whole point of a 'choke input' supply, by killing the choke current.

[SY]

Originally Posted by VictoriaGuy

Is there something special about the Aikido that lets it use noisy B+?

Yes, it uses a noise-canceling tweak. That's its "claim to fame" and the origin of the "Aikido" moniker.

[DF96]

If you short out the choke you will get a 300V supply with similar ripple. Then RC smoothing can drop and smooth it, but it would have high DC impedance.

If you remove the extra diode you will get a 200V supply with less ripple, and probably a lower output impedance. You could boost the voltage a little by adding a small capacitor before the choke.