Cheap Mods
Casio DH-100 Resonance

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I can’t seem to keep from modifying this horn.  I don’t know; maybe it’s the fact that Casio created a pretty cool little instrument, then crippled it’s sound to achieve mass market approval.  Except for the CZ line’s Phase Distortion, I’ve never found Casio sounds to be very rich.  It’s taken benders to turn SK’s into phenomenal sounding instruments.  Even the FM Portasounds, as boring as they can be, have a richness that most Casios lack.


The DH has an honest-to-goodness VCF, albeit one with an architecture I haven’t seen elsewhere.  It’s a 4-pole lowpass filter, where the cutoff frequency is adjusted using the variable ON resistance from 4 FET’s.  The breath control envelope drives the gates of the FETs, where stronger breath means higher Vgs means lower Rds(ON) and thus a higher cutoff frequency.  Interesting is that the filter cascade is just 4 simple RC networks in series (the “R” in this case is between the drain and source of a FET), sandwiched by two op amps acting as buffers with a little gain.  There are two feedback paths, one to the input of the output buffer amp and another to the input of the input amp. 


The DH itself modifies the path back in to the output buffer amp for three of the voices (synth-reed, clarinet, and flute), increasing the feedback to give those voices a little resonance boost.  But it’s not much.  If you’re used to programming old analog synths, you know that on most you can push the filter resonance to the point of self-oscillation.  With the stock DH design, that’s not possible.  And the little resonant push that it does do is anemic.


So, this mod adds a resonance control allowing you to increase the VCF resonance so that not only is it musical (i.e., emphasizes the motion of the filtering as it varies with breath pressure; gives some squawk to the sounds, etc) but also will self-oscillate.  Such self-oscillation is a bit unwieldy, but is a lot of fun to play with using breath control.

The Modification

This mod requires pretty much only a potentiometer (though I used an extra resistor just because I didn’t have the right size and value of pot laying around).


How it works:

Resonance is just limited filter oscillation right around the cutoff frequency and it requires a signal to be fed back from the input to the output.


My mod is really really simple:  add a pot between the output of the filter and the input of the filter, effectively bypassing the weak feedback path that’s already there.  If you use a large value pot (>400kOhm), then you can adjust the new feedback path from nearly nothing (very high resistance = basically stock performance) to extreme, piercing squeals (much lower resistance = pure filter oscillation).  My goal was to allow variation between these two extremes, hopefully making the DH more musical in the process.


Parts List

  • 1 small 500kOhm pot (I used a 1MOhm paralleled with a 470k resistor)
  • 2 longish (6-9”) strands of 30Ga Teflon-coated wire-wrap wire with stripped ends

Electrical Installation

  1. Turn the horn off.
  2. First wire the pot.  Tin the stripped ends of the wire with solder and tin two terminals of the pot.  Connect one wire to the center terminal of the pot, then another to an outer terminal (doesn’t matter which).
  3. Remove all the screws and open up the DH.  You only need access to the top PCB.
  4. Find the following thru-holes on the PCB:  R7 and R14.  They’re down near the bell end of the horn, surrounding the 8-pin dual 4558 op amp.
  5. Solder one wire to each thru hole (doesn’t matter which)
  6. Just as a check:
    1. Adjust the pot so the full resistance is between the wired terminals. 
    2. Put the batteries back into the DH, then turn it on.
    3. With the BREATH switch OFF, press and hold down any key on the horn.  You should hear the normal sax sound.
    4. Now rotate the pot slowly.  The sound should get thinner, then…SQUEAL!  Back off the pot.  All is well.
    5. That’s it for the electrical.


Mechanical Installation


I placed the pot on the right side, just above my uppermost slide switch, above the volume knob.  All my controls were already on that side anyway.  Depending upon the size of your pot, you can place it wherever it will fit.  Wherever you decide to place it, make SURE you look out for the support posts that hold the connecting screws.  You don’t want to drill a hole only to find that you can’t use it because the horn can’t be closed with the pot in the way.

I also had to cut the shaft of the pot somewhat to make the knob sit as low profile as possible.





My own comments about this mod:

I have to say that this is the best thing I’ve done to this horn.  It feels a bit like a monophonic analog synth now.  After all, you have a DCO that has a limited-though-variable waveshape (each selectable voice), an envelope generator (your breath + the DH’s follower circuit) that drives the VCA and VCF cutoff, and the switchable vibrato is a very limited LFO for the DCO.  And now you’ve added resonance to the VCF.


Performance:  Setting the resonance knob to a medium-high setting gives some ‘waw’ to the breath effects that you can generate.  If you set the resonance to just under oscillation, you can create some wacky bird calls with mouth shaping and tonguing effects.

What’s interesting is that much of the resonant ‘wowing’ occurs at lower intensity breath settings, where if you blow harder, the main voice dominates the mix.  I can’t say exactly why this occurs; perhaps it's due to the design of the Casio filter?


One more note:  I first adjusted the breath sensitivity to the factory settings via the service manual, but I didn’t like that very much (seemed like I had to blow too hard).  So I messed around with VR1, VR2, and VR3 until I got a setting that I like which is more sensitive and responds to mouthing effects.  It’s here that the resonance can really add to the expressiveness.


Further mods?

I think I’m done.  I did try to make a simple circuit which would take the breath envelope and drive the resistance of the feedback path (voltage controlled resonance, essentially).  I tried a simple FET and put the breath signal to the gate drive (much like Casio used for the “resistors” in the filter path).  I couldn’t get it to work nicely though; the range between ‘no effect’ and ‘full squealing oscillation’ was tiny.  Yes, I could redesign the circuit to be more linear, expand the adjustable range, and be biased appropriately, but that’s a lot of bother.  This simple potentiometer resonance brings a lot of fun to the DH without a ton of work.