Vampire Slayer DIY PCB?

[vigilante398]

`I just checked the PCB to the schematic, component ref's do not match so follow value on PCB's I would say.
27k resistors should be 2 off (R1,R2 on PCB)
47k resistor should be 2 off (R5, R6 on PCB)
1M resistor should be 1 off (R3 on PCB)
All else seems fine to me.
Cheers
Mick

That's fair. In all of my projects in the case of discrepancy between the build doc and the PCB, always follow the PCB, the PCB is always correct.

Designators are a crapshoot because while they are often correct, due to my rampant laziness sometimes things are copied between documents (most notably the power supply schematics) without regard to the actual designators used on the PCB.

 

[matmonster58]

Would it be noisy to have the input resistor on a switch?

I've read some people prefer 5.6 meg and some people prefer 10 meg for piezos. I'd like to try both out

 

[vigilante398]

Would it be noisy to have the input resistor on a switch?

I've read some people prefer 5.6 meg and some people prefer 10 meg for piezos. I'd like to try both out

My recommendation would probably be to have the 10MΩ as a constant connection then use a switch to add another 10MΩ in parallel so you can get 10MΩ or 5MΩ.

Back when I did these for customers I had a few requests for 10MΩ input impedance, but I generally hear from piezo players that 5.6MΩ is still high enough to work well.

 

[matmonster58]

Just finished up. I added an 11meg/5.5meg input impedance switch. The 11meg is too bright on upright bass, I prefer the 5.5

 

[Paul.Ruby]

A question for folks who've built this. I was sent one to try out already built and wanted to test it prior to plugging it in. I was surprised to see that the oscillator runs at about 4khz (in the audio range) and then even more surprised to see the power supply noise is on the output signal. Yellow is input. Blue is output. When I saw the blue signal had noise, I probed the 250V supply and see that's where that noise is ending up on the output signal. I have not plugged it into my guitar signal chain yet... But this does not seem expected.

 

[Smrtokvitek]

I think the SMPS circuit is tuned to a frequency of around 40kHz so it looks like you have some bad component value in a timing circuit maybe?
It is a simple way how to generate a 250V supply. You will always have some noise unless you use a more elaborate circuit and maybe a bigger enclosure to have some distance from signal wires. There is no free lunch.

 

[Tassieviking]

Make sure that the 4kHz is not coming from your 9V power supply. ( or 12V Supply )

 

[vigilante398]

A question for folks who've built this. I was sent one to try out already built and wanted to test it prior to plugging it in. I was surprised to see that the oscillator runs at about 4khz (in the audio range) and then even more surprised to see the power supply noise is on the output signal. Yellow is input. Blue is output. When I saw the blue signal had noise, I probed the 250V supply and see that's where that noise is ending up on the output signal. I have not plugged it into my guitar signal chain yet... But this does not seem expected.
View attachment 91159

What power supply are you using with the pedal?

 

[Paul.Ruby]

I think the SMPS circuit is tuned to a frequency of around 40kHz so it looks like you have some bad component value in a timing circuit maybe?
It is a simple way how to generate a 250V supply. You will always have some noise unless you use a more elaborate circuit and maybe a bigger enclosure to have some distance from signal wires. There is no free lunch.

This is what I wanted to know (40khz expected). I'll check the components. For the others, I'm using a bench supply for 12V, which has no noise. The tube heaters are run directly from that supply, so 9V seems low since the heaters want 12.6V.

 

[Paul.Ruby]

Just to follow up... The 555 is running at 53khz. There is considerable noise on the 250V supply and the output, but it is all at very high frequency. Still trying to figure out if the lower frequency noise of the previous pic is real. It sure looks real on scope. I don't hear any whine with the pedal in my signal chain unless I have very high gain after after the vampire slayer, so my ears tell me it's not real. In this picture below, blue is the power supply and red is the output signal (switched from the previous picture). The lower frequency noise seems to be aliasing, but that really surprises me that the scope could consecutively sample enough high or low points to look like low frequency noise.