My 1980 V5 Electric Mistress is making weird space noises - audio

[Delicieuxz]

Here's an audio clip of what I mean.

https://youtu.be/kDIuS2Ys4Zo

Is this what's called clock-noise, and is it something that can be fixed by replacing some components? If so, I'll probably take it to a tech and not try to troubleshoot it myself. My skills are mostly just soldering together kits that come with instructions.

I also get the regular sound from the EM, but these kind of sounds are present in addition to it.

[DrAlx]

Heterodyne noise.  What else is in your signal chain, and do you get the noise if you only have a simple chain of  guitar, EM, and amp?

[Delicieuxz]

Heterodyne noise.  What else is in your signal chain, and do you get the noise if you only have a simple chain of  guitar, EM, and amp?

Hi. When I read that and after I looked-up what it is, I thought it's an interesting idea but that it's unlikely to be what's happening with my setup - because I've used my EM in a variety of situations and thought I'd even used it on its own, and always remembered some amount of this type of sound.

However, after trying the EM on its own, just the guitar -> EM -> amp, the noise was gone! I then tried it on my current pedalboard layout and didn't hear it, either. But I'm always changing which pedals on my board, so I recreated the layout I had when I made that video clip, and the sound was there again.

I've found that noise is present when my Analogman Bi-Chorus is before in the signal chain (I haven't tested it after), whether or not the Bi-Chorus is engaged. I've also found that noise is present if I have my ADA Flanger engaged ahead of the EM - but if the ADA Flanger is not engaged but still present in the signal chain, that noise isn't present.

I've had the EM for a bit over a year and I thought it needed servicing. It's good to know that it (I presume) doesn't, after all.

Thanks!

[DrAlx]

It is "heterodyne noise". 100%.  I have heard enough of that noise to know what it sounds like.
To be more precise, what's often referred to as "heterodyne noise" in a flanger would be more correctly described as noise due to aliasing.

Heterodyning is what happens when you combine two signals at different frequencies and end up with a "beat" frequency. So for two given high frequency signals you would get a single beat frequency output (e.g. two 100 kHz and 103 kHz signals are both outside the audio band but combine to give a 3kHz beat frequency in the audio band).  The noise you have is not like that.  You will notice that there are multiple points in the sweep that give rise to the chirp sound rather than a single point in the sweep.

Aliasing is when you produce a lower frequency signal (the audible chirp in your case) because the input signal to the BBD contains an unwanted frequency component (typically in the tens to hundreds of kHz) which is then undersampled by the BBD to give an unwanted audible noise at the output of the BBD.
The unwanted frequency component at the input to the BBD (in the tens to hundreds of kHz) is coming from previous stuff in the signal chain.  Usually from a pedal that contains a clock of some sort or a charge pump.

The only way to stop that chirp is to have an anti-aliasing filter before the BBD (i.e a LPF with a cut-off frequency of half the sample rate, or better yet of 1/3 the sample rate).  The EM does not have that sort of filtering before the BBD.  The light filtering is part of its airy sound.  It is also the reason it is more prone to the sort of noise you are getting than a flanger that has more heavy filtering before the BBD such as an Ibanez FL9.

You said the pedal always had a little bit of that chirp noise.  Well the original 18V EM has a LFO/VCO design that is prone to giving those chirping noises even with the pedal on its own and nothing else preceeding it in the signal chain.  At least that is what I found for the V2 EM.  I never tried a V5 so I don't know if that is as bad as the PCB has different layout.

There is one thing you can do to improve the situation and that is to add 820pF in parallel to the 6k8 resistor on the 4558 in following picture
http://www.metzgerralf.de/elekt/stomp/mistress/images/1980-electric-mistress-v5-schematic.gif
That will filter out some of the unwanted RF noise on the input signal to the BBD without massively changing the tone of the pedal.

You can do a simple test.  Find that resistor on the board and just hold a 820pF or 1nF capacitor across it using your fingers and see how much quieter the chirps get.

EDIT: In fact you don't even need to find the resistor.  That resistor goes between pins 1 and  2 of the 4558, so just try holding the capacitor between those 2 pins.


See this page
http://www.metzgerralf.de/elekt/stomp/mistress/electric-mistress-v5.shtml
for pictures of the front and back of the board so you can work out which pins correspond to 1 and 2 of the 4558.