Help with audio filter please

[Serratus]

Hi, I have an unusual question.

I am building an electric guitar which has alot of lights in it. One of the things I want to use is EL string, which runs from an invertor which supplies 4KHz AC to the string. Unfortunately, you can hear the 4KHz tone through the electric guitars pickups so I need to filter out 4KHz from the guitars output. I think I need a notch filter (or band-stop filter) to cut out a narrow bandwidth (Q) around 4KHz. But I don't know how to do this! I think that I can maybe use a MF10CCWM capacitor filter, but I don't know how to set it up to do what I want.

Any help gratefully received!
Rich

 

[audioguru]

Hi Rich,
The EL's inverter has a square-wave output not a sine wave. If you notch-out the 4kHz (which might change its frequency away from the notch) then you will be left hearing 8kHz and 12kHz. You could use a sharp lowpass filter and endup with an electric guitar sounding like it is playing over an AM radio or a telephone.

Can't you position the inverter away from your guitar? Then use shielded cable to conect it to your EL?

Can't you use shielded wires to your pickup?

I used an LMF10 as a notch filter in my distortion analyser, built using descriptions of the circuit in its datasheet. It works very well. :lol:

 

[Serratus]

Hi,
Thanks for the info. So what you're saying is that I could use an MF10 as a notch filter to filter out the 4KHz, but that there's probably no point as I will still get the 8kHz and 12kHz tone coming through? (you have to talk in plain english with electronics and me I'm afraid lol).

Regarding you're other suggestions, I have tried the invertor away from the guitar completely, but the pickups still pickup the tone from the AC in the string itself, which obviously I can't shield as then you wouldn't be able to see it! And I think it's the guitar pickups themselves that are picking up the tone, so shielding the wires to the pickups wouldn't make any difference.

Unfortunately it's starting to sound as if I can't use the EL string.

Thanks anyway, if you have any other thoughts then let me know.

regards,
Rich

 

[audioguru]

Make the inverter operate at 25kHz so only dogs would be bothered. EL is brighter at higher frequencies but doesn't last as long. :lol:

 

[Serratus]

Make the inverter operate at 25kHz so only dogs would be bothered. EL is brighter at higher frequencies but doesn't last as long. :lol:

Yeah, I figured that if I can run it at >20kHz then it might not last long lol - might still have an acceptable life though, as it's not going to be on for very long at a time. So do you think I'd be able to mod the invertor I have to make it run higher?

 

[Serratus]

Okay, so it seems I can run the wire at 20kHz which would then not be audible.
BUT, I can't find anywhere that sells a suitable inverter. I've opened up the inverter I have (which is 4kHz) and the board looks pretty simple so I figure I could probably make one. So has anyone got a circuit design for a simple inverter that could power EL string at 20kHz and run from batteries (either 1.5V or preferably 9V)?
Thanks in advance.

 

[audioguru]

Instead of replacing the entire inverter, why not just reduce the value of the oscillator's frequency-determining capacitor on the inverter that operates at 4kHz? Then make sure its output transistors don't get too hot.

 

[Serratus]

Instead of replacing the entire inverter, why not just reduce the value of the oscillator's frequency-determining capacitor on the inverter that operates at 4kHz? Then make sure its output transistors don't get too hot.

Woooooooooooooooooooooosh! That was the sound of what you just said going right over the top of my head lol! Okay, so I'm looking for a cap on the board which is what determines the frequency? Then how do I determine what the cap needs to be replaced with to change it from 4k to 20k?
Thanks again

 

[audioguru]

Okay, so I'm looking for a cap on the board which is what determines the frequency? Then how do I determine what the cap needs to be replaced with to change it from 4k to 20k?

I thought you said the board looks simple. Then if you sketch what it has and how it is wired together, we can help you change its frequency.