The ANDROVOX (Modified Dead Robot Circuit)

[Cybercow]

I’ve been futzing (obsessing) with the Dead Robot circuit on and off for a few months now and I believe I have arrived at very decent and more fully featured use of the HT8950 voice changer chip. Of all the schematics I’ve seen for this project, only two have incorporated a proper power supply so it will play well with other pedals with regard to DC power supply bricks. All of the prior Dead Robot circuits leave out many of the available features the HT8950 offers. None of the schematics for the Dead Robot (that I’ve seen) exploit the built-in LED driver that will light an LED in proportion to the signal strength of the plectrum of the guitar string attack. It turns out that the LED can drive an LDR that is strapped across the “Warp” (“Control” in some circuits, “Frequency” in others) control. This little feature adds a new dimension to the features provided by the HT8950 chip. Also, the “Warp” control can be surmounted by an expression jack for use with a foot pedal.

The HT8950 is an 18-pin DIP voice changer chip that offers a diode-matrix accessible 8-rotary switch to access the different voice settings. The HT8950A is the smaller (16-pin) version of the HT8950 and offers all the same features – except the option to employ an 8-way rotary switch to directly select one of the 8 settings capable in the HT8950/HT8950A chip. In the HT8950A, the 8 different settings are accessible only by using a momentary switch to cycle ‘UP’ or ‘DN’ (down) thru the selections. The HT8950 is getting harder to source, but its little brother, the HT8950A, is still readily available. I will eventually draw up a set of schematics for the HT8950A with the same features as the AndroVox – but without the 8-way rotary switch. Instead, it will have a couple of additional momentary stomp switches to access the ‘UP’, ‘DN’ cycling thru the 8 different options.

But it’s not over yet . . . . . .  there is also a built-in ‘VIB’ option and I’m still exploring how to manipulate that vibrato and I think there is a way to manipulate the LFO frequency. I just haven’t gotten there yet. To see what I mean, check out the datasheet for HT8950/HT8950A and read through it carefully. I think you’ll get the idea.

So with no further ado, here is, what I believe to be, the finalized schematic of the AndroVox . . .

And here’s the a block diagram of the HT8950 chip including a legend of the ‘UP’ / ‘DN’ cycling selection order . . .  .

I had a few PCBs made up after verifying on breadboard, completely mangling MadBean’s very last “ManDroid” PCBs, and trial & erroring many approaches. Even the schematic above is a corrected version of the schematic used in the creation of the Gerber files for getting the PCBs manufactured. I had three small kludges to deal with, but the PCBs I have will still do all the tricks shown in the schematic and description above. I just had to lift a resistor leg, add a wire, insert a cap between lug #2 of the ‘Dry/Wet_A’ pot and R7, and dangle the clipping diodes to add a toggle switch to take them out of circuit easily.

And here’s kludged up schematic for the 16-pin HT8950A voice changer chip. (I will eventually get a proper schematic drawn up.)

[Anonymous]

This is so cool! I played a Death by Audio Robot and wired up a PedalPCB Roboto and love the sound. The addition of a wet/dry mix control and the ability to add an expression jack has me interested in hearing the expansion of sonic capabilities.

I am very interested in seeing what comes of your experimentation with replacing the rotary control as well! Please keep us updated!

[Anonymous]

I would love to hear an audio demo from you!

[Cybercow]

And for when an HT8950 (18-pin voice changer chip) cannot be obtained, the HT8950A (the more readily available 16-pin chip) will provide the same functions, but with momentary actuators to scroll ‘UP’ (TGU) and ‘DN’ (TGD) to access all 8 of the voice changer chip’s selections. The ‘ROB’ button can be pressed to directly go right back to “Robot” mode. I hope to get an audio demo up in the next week.

And personally, I think the ‘VIB’ button is a bit weak, lame & fixed at 8Hz; but the builder can decide whether or not to use it. I still have yet to explore if that 8Hz vibrato frequency can be altered by changing the value of the resistor at the “FVIB” pin (#12 on the HT8950 and #15 of the HT8950A – 100K) on either chip. I suspect trying a trimpot there would only alter the ‘depth’ of the vibrato. But we’ll see.

[Cybercow]

ANDROVOX Part II . . . . . .

This highly modified Dead Robot circuit just continues to amaze me. The more I play with it, the more cool features I discover.

Typically, a ‘Dead Robot’ circuit “sounds” like a highly variable ring-modulator when NOT in the ‘Robot’ mode – with the ‘Blend’ control set at half-way. (The specified “MN10K” dual gang ‘balance’ pot’ has a center-detent.) In ‘Robot’ mode, it sounds precisely like many of the other ‘Dead Robot’ circuit demos you can hear on YouTube. Even the original DBA Robot.

With the ‘Volume’ set to about 50%, ‘Blend’ set fully to dry, the ‘Drive at about 10%, and the clipping diodes turned off, it performs like an adjustable dirty boost. Turning up the ‘Drive’ control then adds a good amount of dirty boost. Turning up the volume adds even more loudness.

Then, turning the clipping diodes on, (with all the other settings the same as above), it “sounds” more like a great, adjustable, distortion pedal.

Sweeping the ‘Blend’ control back towards center, you start to hear the various ‘modes’ that the HT8950/HT8950A chip will generate – with ‘some’ of the dry guitar signal. In all modes but ‘Robot’, this circuit sounds more like a tunable (with the ‘Control’ and ‘Fine’ pots) ring-modulator.

Each of the ‘Mode’ settings (again – other than the ‘Robot’ setting) is a specific harmonic division of the signal input. Depending on the specific non-‘Robot’ mode selected and the setting of the “Control’ and ‘Fine’ pots, there are certain harmonics that will resonate beautifully on specific notes. Those notes are actually tunable according to the ‘Control’ and ‘Fine’ settings – for each of the ‘Mode’ divisions. And while in these various divisions, the clipping diodes and ‘Drive’ control determines just how nasty the output will sound.

Then, with the ‘Control’ and ‘Fine’ controls turned all the way up, the ‘Splode” momentary switch will actuate the optocoupler which, in turn, manipulates the ‘Control’ value withe the nature of the guitar plectrum’s attack & decay dynamics.

And while I haven’t yet added the expression jack and hooked up an expression control pedal, the theory and behavior thus far suggests that an expression controller will allow the user to sweep thru the ‘Control’ value. Inserting an expression pedal will temporarily replace the ‘Control’ pot, rendering control of the ‘Control’ pot to the expression pedal sweep.

As long as the HT8950 voice changer chip has been in the pedal building realm, I am truly surprised no one has come up with something as robust as this specific build of that chip.

And while the HT8950A does NOT have the means for a direct ‘Mode’ selection option by way of an 8-way rotary switch, the ‘UP’ & ‘DN’ momentary switches can cycle thru to each of the division settings – as well as the ‘Robot’ setting. The ‘ROB’ button pads can be used to add a momentary on with for dirt access to the ‘Robot’ setting.

Getting a good demo of this circuit will have to wait till I finish the first build because it is so full featured, to do a demo with no visual reference to the settings is very time-consuming. It will be easier to just show that front of the finished pedal build with the audio in the background so folks can see what does what.

[Anonymous]

We are patiently waiting….  Your descriptions are so valuable  I’m planning to build one of these soon.

[Cybercow]

And now, the long delayed video demo of the AndroVox project build . . . . .

AndroVox Video Demo  (embedded)

[Anonymous]

That’s a wild ride! Fantastic project, documentation, and demo. Finding ways to use more widely available  chips is exactly what the doctor ordered.

Thanks!

[Author]

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