Simple, passive way to remove high frequency interference from low power audio

Question

I have used the innards of a Korg Mini-KAOSS Pad to add some amusing effects to one of my electric guitars, but in order to do this I had to remove the original input connectors and wire my pickups directly to the input circuitry.

The input voltage is fine, however I do now have a problem with a high pitched digital sounding whine when using effects. My assumption is that the input connectors probably had some type of rolloff circuitry (almost certainly passive) and in removing these I am getting interference from the high frequency oscillators in the DSP circuitry.

As part of my remediation I am increasing shielding, but realise I probably need to add a filter to remove higher frequencies.

tl;dr - is there a simple circuit (ie probably just an R/C) that I can pop in to roll-off rapidly above the usual frequencies I would get from my guitar strings?

Answer 1

Can you show us a picture of exactly what you removed? (and maybe the input circuitry) Any filtering will likely take place after the input amplifier, so it's possible something else is happening.

It certainly wouldn't hurt to add a simple RC filter just in case though, then if the problem persists you know it's something else.
If you do this I would go for something like Olin suggests but adjust the values for your guitar pickups (e.g something like 100k for R1) which will probably be higher than 600 \$\Omega\$.

For line level connections, old equipment was often matched 600 \$\Omega\$ in/out. Nowadays however it's usually Lo-Z out to Hi-Z in. The Hi-Z input is often 10 k\$\Omega\$, but can be higher. You could measure the resistance across your Mini KAOSS input (be sure to do it after any DC blocking cap if present) to find out exactly what you are working with. YOu could also measure your pickups DC resistance to get a rough idea there too.

I can only see a line in connection, which will probably be expecting a lower impedance signal than your guitar can provide. Have you tried putting your guitar through a preamplifier/DI box?

There may possibly be a DC blocking input cap you may have removed, which might cause strange things to happen.

Here is a clip of a typical guitar amp input to give you an idea of what it expects to see. Note the 1 M\$\Omega\$ input resistor.
I'm no expert on guitar (passive coil) pickups but I think they are commonly around 5-20k (varies with frequency)
A piezo pickup will be much much higher.

Answer 2

The frequency of the noise will be important and it would be nice if you had an idea of what it was as this will determine the rate at which you will need to roll off.

In any case a simple Butterworth filter might do the trick. You can implement it in a few ways but as far as I have seen the most popular are the Cauer topology and the Sallen–Key topology.

Cauer is passive, implemented only with RC combinations and Sallen-Key is active, requiring an operational amplifier.

Answer 3

Yes, a simple passive low pass filter will likely help. The problem is it will be somewhat dependent on the source impedance. The filter can be made more complicated to deal with this in various ways, but here is a simple design that is likely good enough:

R2 and C1 form a simple low pass filter. The -3dB rolloff frequency of this filter is

f = 1 / 2 * Pi * R * C = 34 kHz

However, that assumes the input voltage has zero impedance. Any impedance of the input source will be effectively added to the R in the forumula above and lower the filter rolloff frequency. If you know the guitar pickup impedance, then you can adjust R2 or C1 accordingly. You want to make the rolloff at least 20 kHz, but still low enough to usefully attenuate the unwanted high frequencies.

Many audio pickups and the like are designed for 600 Ω load. R1 is there to present a reasonable load to the input signal and also to make its impedance somewhat predictable to the R2/C1 filter. You have to check what load your pickups really want. As I said, 600 Ω wouldn't be surprising but it could easily be rather different too. The values in the circuit above are rough examples if indeed the source is around 600 Ω and the input impedance of the amplifier is "high". I'm trying to show you how this filter works so you can adjust it to whatever your conditions really are.

For the circuit above, the signal impedance as seen by the R2/C1 filter can only be from zero to 620 Ω. That gives a -3dB rolloff range of 21-34 kHz, which should be good enough. Note that if the actual source impedance is really much higher than 620 Ω, then R1 will attenuate the signal a lot and you won't get much out. Again, you need to adjust the values to your actual case.