Hello Hordijk System

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Re: Hello Hordijk System

Post by Adze » Mon Mar 30, 2020 2:43 pm

Maybe some of you don’t know this (and I find it very useful) you can calibrate Twin Peak filter so it can self-oscilate. I found about this when I went to The Hague to pick up my first system - Rob has presented me the system and told me that he still has to calibrate the filter. I liked the sound as it is and I’ve asked him to leave it as it is. This is really interesting feature as you have two cutoffs so you have two different pitched sounds.
Here are the instructions (written by Rob) if you want to try it:

The Twinpeak has three trimmers, each filterhalf uses one of these trimmers to set the resonance and the third trimmer is to set the balance (silence point when equally tuned) between the filters when resonance is at its lowest. Of you unscrew the panel and pull the top of the panel forward you can adjust the trimmers from above with a fine-blade screwdriver.
The middle trimmer (as seen from the top) is the balance trimmer and the left and right side trimmers are for adjusting the resonance. After adjusting the resonance trimmers and then the balance trimmer you have to check the resoannce trimmers again and maybe adjust them a bit more, this as the balance trimmer does slightly influence the resonance trimmers.
What you do is that you first set the cutoff frontpanel knobs of both filters to a 11:00 to 12:00 o’clock position. (The knob-scale divisions are like the hours on a clock). Then fully open the resonance knob. Then you turn the left trimmer until one of the filters starts to oscillate (I think in a counter-clock direction). Then turn it back until it just not oscillates anymore. Do so for the right trimmer as well. You will notice that the most sensitive point for oscillation is in this 11:00 to12:00 position of the frontpanel knob. I have noticed myself that the oscillation point is slightly temperature dependent, so turn the trimmers back about half a turn extra to not be surprised on a hot day. If you have trimmed these two trimmers you can fully close the resonance knob, apply a signal with rich harmonic content, e.g. a sawtooth or pulse at e.g. a low C-key pitch, turn the cutoff knobs fully to the right, and trim the middle trimmer for the lowest amount of audio leaking through.
The filter should exhibit relatively long ping times, longer as the 24dB, but never get into oscillation if you slowly open a cutoff knob. That is the best setting. First oscillations appear around 400Hz, so that is why the 11:00 to 12:00 o’ clock position is used when trimming.

This is a little piece I made with Twin Peak as an only sound source (and some noise in the background):

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Re: Hello Hordijk System

Post by ear ear » Mon Mar 30, 2020 4:03 pm

He throws the ink first and works out what it is later. "I've got to turn that into parrots." - Ralph Steadman

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