Loud humming 50 Hz noise, what's wrong with my Ambika? [SOLVED]

Hi guys.
So I’ve finished my second Ambika build, but this time something went wrong. Each time a hold a note very loud noise appears, which completely ruins the sound the more notes are played at the moment. in the mp3 file attached You can hear it very clearly, especially in the end, where I’ve turned off the oscillators. It sounds like 50 Hz with some harmonics above.


I’ve got 9VAC 50 Hz 1A, which I guess is enough even for Ambika with VFD. Regulators are recommended: 7908ACT, LM2940T, 7805ACT.
What else must be checked? With this noise the synth is almost unusable :frowning:


EDIT: Got 9 VAC 1.9 A PSU, the noise is gone!

Use a scope to check that the +8V, -8V and +5V rails are flat.

Is that the first unit you build with a VFD? I designed the power supply for a LCD which draws about 100mA, so please check the current requirements of your VFD module and make the appropriate changes to Ambika power supply circuitry. The schematics indicate how much current can be drawn from each rail. Drawing more will cause motorboating.

Could the VFD be making that noise? Which filters did you use?

thanks for participation, guys!

I’m close to buying an oscilloscope, but can’t find budget one. It may show ripples in my case, right?
Current consumption is 300mA (390 max). Too much?

@mmarsh, I dunno. 6 tubeohm’s boards installed, but they were working flawlessly on a previous motherboard iirc.

Forgot to mention it’s looks like the noise can be filtered, so that at cutoff=0 the noise is dull, and with the filter fully opened upper harmonics are audible.

For a new budget scope, google Rigol 1052E or 1102E and read reviews. Both can be had new for under $400 USD (1052E may be flash-able to 1102E spec.s).

I have an 1102E and am pretty happy with it. It’s not perfect, but it’s hard to fault it at this price. They seem very popular with hobbyists (like myself).

The Tubeohm is known to have some noise with OLED screens, as I recall. Did the previous MoBo have aVFD?

mmarsh: Ambika and Shruthi are very different beasts, power-supply wise, the problem is not due to the Tubeohm filter board but to the VFD sucking too much current.

This is not a noise problem, but a motorboating problem.

What kind of VFD? I am using a Noritake and have not noticed anything.

@pichenettes - gotcha.

thanks again everyone!

codymac, thanks for great suggestion, I may get this one in one-two months!mmarsh, nope. It was built from kit, this time I’m sourcing myself
sammy123, noritakepichenettes, unfortunately I won’t be able to try another PSU in two weeks, but when I inserted only one voice card, looks like the noise was absent, so I guess the solution lies there


> but when I inserted only one voice card, looks like the noise was absent, so I guess the solution lies there

@endorfinity but that could just be because with only one voice card there is less power consumption, thus no motorboating. just a thought.