The Dannysound EN129 Thru-Zero Oscillator is one of a pair of DIY only oscillator kits from Thonk.  See my post about the Cali Osc, too.

The EN129 Oscillator is based on the through-zero VCO by Bernie Hutchins, published in Electronotes #129.  This is an analog through-zero oscillator with a full set of waveforms, including sine, triangle, sawtooth, pulse and square.  Features also include:

• Coarse and fine tune knobs, driven by internal 9 volt regulators
• Linear through-zero frequency modulation input with attenuator
• Log (i.e. exponential) FM input with attenuator
• Pulse Width Modulation (PWM)
• Symmetrized Ramp Modulation (SRM)
• Panel LEDs monitoring incoming control voltages, pre-attenuation

This VCO does not include an onboard VCA for dynamic depth linear FM, as did the original EN129, but it’s easy enough to use an external VCA.

Construction

The physical design is identical to the Cali Oscillator.  The build guide again gives almost too much detail in the placement of parts, showing a photo for each value of resistor, etc..  Parts are separated into bags, each bag having a label with the part number from the schematic.  It would be pretty hard to get a part in the wrong place.  I didn’t follow the build order exactly, since I like to solder as many parts in place with Kester 331 solder with water washable flux.  As with the Cali Osc, I used heat shrink and thermal compound on the matched PNP transistor pair.  And I encountered the same 1 mm flexing issue with the panel that I described in detail on that other post.  Something I didn’t mention before was the use of light pipes in the panel in front of the LEDs.  I’ve never encountered that approach before.  You have to insure that each LED is aligned behind its light pipe, which is a bit of a pain.  I would solder one leg while using the other as a lever to position it.  You can notice some no-clean flux on the panel board, left from soldering the pots, jacks, switch and LEDs.

A few things to point out on the Main board.  Notice the 1n polystyrene capacitor is a radial lead type.  It’s supposed to be axial, so it can lie down.  The radial leads make it stand up.

I substituted CA3080s from my own stock of Harris Semiconductor parts.  These have worked well in past projects, so I trust them.  (I used a Harris in the Cali Osc too.)  Also I happened to have some LF353 op amps, which is the original part used by Bernie Hutchins for U6 (he used two LF351s), and which is shown as that on the schematic provided by Thonk.  The TL072 provided is an equivalent, but I wanted to use the LF353 anyway.  I’m sure I can’t tell any difference.

Tested Frequency Ranges

This VCO has two initial frequency pots, the large coarse frequency knob and the through zero offset knob.  I measured the following frequency ranges of the coarse knob with the TZ offset knob in each of three positions.

TZ Offset minimum:  Very slow to 2 Hz

TZ Offset center: 76 Hz to 5.5 KHz

TZ Offset maximum:  177 Hz to 13 Khz

Then I added +5 volts to the 1V/octave input.  (And notice that just patching to this input drops the pitch almost four octaves.)

TZ Offset minimum:  Very slow to 6 Hz

TZ Offset center: 240 Hz to 18 KHz

TZ Offset maximum:  567 Hz to 37 KHz

At 37 KHz the sine wave still has a good shape.

Demo Recordings

Waveforms

The first recording lets us hear the different waveforms: sine, triangle, sawtooth, pulse, and square, in that order.  On the saw and pulse I made a manual sweep of the initial PW, SR knob.  The best sawtooth is trimmed to be with the initial SR at zero.  This leaves a little glitch at the other end.  In the center of the SR knob the pitch is doubled.

SRM with an LFO

This next recording demonstrates the Symmetrized Ramp Modulation with an LFO.

Audio rate SRM

Here’s SRM at audio rate, manual changing the depth and varying the frequency of the modulating oscillator.  All modulations in these demos use the sine wave from the Cali Osc, whether normal or LFO.  I did not include a PWM demo, because I did one for the Cali Osc.  Note that the controls for Pulse Width and Symmetrized Ramp are shared.  Both are affected at the same time.

Log vs Linear FM by LFO

Now let’s compare using an LFO to modulate the frequency, first using the Log FM input, and then using the Linear FM input.  You can clearly hear the EN129 reversing through zero.

Through-zero FM

Now the grand finale:  Full through-zero FM at audio rate.  For this demo the TZ offset was set about 10 o’clock.  First we hear the depth manually changed.  Then we hear the effect of modulating dynamically, using an external VCA and envelope generator.  During this I noodled around with the pitches of both oscillators.