Mod to Fix the High Freq Roll-off of the MF-101 LPF

[varice]

It was brought to my attention that the MF-101 Low Pass Filter has a roll-off of high frequencies, even with the filter opened up all the way or when in bypass mode (note: the MF-101 does not have true bypass, the Drive circuit still affects the signal in bypass mode). I tested my MF-101 and confirmed that there is a slight but audible cut of high frequencies. I had used a workaround for a while (a parametric EQ to boost the input signal highs to compensate), but I decided to find and fix the problem in the MF-101. I found that there is a low pass filter built into the audio signal output buffer. Removing C21, located near U1 in the upper left corner of the PCB, restored the high frequency response.

Obviously, this low pass filter was added for a reason. My guess is that it was to help tame the high frequencies that the MF-101 can produce when the resonance is turned up until MF-101 oscillates. If you decide to make this modification, be extra careful and heed these warnings from the manual:

@Moog

A WARNING!
If you turn the RESONANCE knob past “8”, the filter in your MF-101 will oscillate. That is, it will produce a tone of its own. This is normal for a lowpass filter of this sort. In fact, synthesists have prized this effect for the past thirty years!
However, you should be aware that it is possible to overheat your speakers if you set your MF-101 to oscillate and then leave it on a single frequency at high volume for an extended time. With no pedal connected to the CUTOFF jack, the oscillation frequency will be between 15 Hz and 12 kHz, depending on the setting of the CUTOFF knob. With a pedal or external control voltage plugged in to the CUTOFF jack, it is possible to drive the oscillating frequency up as high as 40 kHz! Some speaker systems, especially precision studio monitors or audiophile speakers, may be vulnerable to high-volume steady tones at some frequencies within the MF-101’s range,- even frequencies that are too high or low to be audible!
You can protect your speakers by being aware of your MF-101’s output when the RESONANCE knob is at “8” or above. Switch your MF-101 to BYPASS when you are not actually playing it.

ANOTHER WARNING!
You’ll find that your MF-101 sounds really good. You may be tempted to crank up the volume so you can really hear it. That’s OK, as long as you keep a lid on those high frequencies (2 kHz and above) that will stress your ears if they’re amplified too much. Like any high quality electronic musical instrument, your MF-101 is capable of producing a wide range of frequencies. It is up to you to use that frequency range in a careful and professional manner.

[seank]

I wasn’t aware that the 101 had this problem. A few other dudes, myself included, have this problem with the 107. Care to take a stab at that?

[varice]

@seank

I wasn’t aware that the 101 had this problem. A few other dudes, myself included, have this problem with the 107. Care to take a stab at that?

Sorry, I don’t have one (yet!), so I can’t help with that. I read the topic about that last weekend and that inspired me to finally find what was causing the problem with my MF-101.

The MF-101 roll-off is very slight. Since this box does not have true bypass, you wont notice the roll-off by just switching between bypass and Mix knob at zero. The easiest way to hear the roll-off effect is to compare the original audio signal with the box output in bypass mode.

[ColorForm2113]

: edit: sorry I have to stop reading posts as I’m going to bed. Cuz I don’t read the correctly or all the way through