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Casio DH-100
Resonance

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Background:

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

Turn the
horn off.

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).

Remove all
the screws and open up the DH.
You only need access to the top PCB.

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.

Solder one
wire to each thru hole (doesn’t matter which)

Just as a
check:

Adjust the pot so the full resistance
is between the wired terminals.

Put the
batteries back into the DH, then turn it on.

With the
BREATH switch OFF, press and hold down any key on the horn.  You should hear the normal sax
sound.

Now
rotate the pot slowly.
The sound should get thinner, then…SQUEAL!  Back off the pot.  All is well.

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.

Sax_NoRes.mp3

Sax_MediumRes.mp3

Sax_HighRes.mp3

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.

.