After finding a couple of triodes lying around I decided to build a valve preamp. I've built a simple boost converter based on a 555 timer to generate anode voltage. schematic pcb

The circuit is powered from a 9V mains adapter. VR1 is adjusted so that I get approximately 200V output.

'HV' output goes directly to the anode of the valve through an RC-filter (R=33 Ohm and C=220uF).

When I probe the output of the amplifier I get a lot of noise from the boost converter (here is example of the noise superimposed on a 4.4kHz sine wave):


When I disconnect the power from the boost converter, the noise disappears and the signal is perfectly clean (while there is some charge left in the output capacitor).

How do I filter out this noise? I thought that the RC-filter would be sufficient, but apparently it's not.

edit: I removed a 33 Ohm resistor from the RC-filter and added an LC-filter. Here is the schematic of the output:

enter image description here

The amplitude of the spikes now decreased, but the noise is still present. noise2

  • \$\begingroup\$ C2 is bulk; where is bypass? \$\endgroup\$
    – Jon
    Feb 22, 2015 at 1:16
  • \$\begingroup\$ I probably missed it, thanks. I bodged in a 100nF capacitor, although it didn't help much \$\endgroup\$
    – Ashton H.
    Feb 22, 2015 at 1:25
  • 1
    \$\begingroup\$ I see at least a 3rd order passive LC filter network in your future, and I wouldn't be surprised if it's 5th order. \$\endgroup\$
    – markt
    Feb 22, 2015 at 1:39
  • \$\begingroup\$ One thing to note is that if the ground cable of your scope is longer than ~3 inches, a heck of a lot of that noise is not real. Try putting a short wire between the nearest ground and your scope, and see if that changes the signal. \$\endgroup\$
    – 0xDBFB7
    Dec 15, 2016 at 23:14

1 Answer 1


My first attempts at fixing this would be:

1) add 100n ceramic bypass cap across C3

2) use another 220uH inductor and 220uF / 100n capacitor as a LC filter network on the HV out.

  • 1
    \$\begingroup\$ Adding an LC-filter helped, although some noise is still present. I edited the question. \$\endgroup\$
    – Ashton H.
    Feb 22, 2015 at 2:18
  • 1
    \$\begingroup\$ Add a 220uF cap across C7. \$\endgroup\$ Feb 22, 2015 at 2:25
  • 1
    \$\begingroup\$ I've added a 470uF cap and the signal is almost perfect now, thank you. \$\endgroup\$
    – Ashton H.
    Feb 22, 2015 at 2:43

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