This is on the breadboard right now. I don't usually release a schematic before I've soldered up a working pedal, but I know Jimi Photon was looking for a vactrol implementation of the Harmonic Tremolo.
The Cardinal Tremolo is a vactrol implementation of the Harmonic Tremolo from Fender's 1960-1963 tremolo-equipped amps (the Super, Pro, Twin, Showman, and Concert models). A huge amount of credit for the basic design of this effect for stompboxes goes to RG Keen, who created a FET-based implementation. This vactrol-based implementation has some nice benefits, not least of which is that parts matching is not required, and it uses a far simpler chip-based LFO (or the TAPFLO chip). Additionally, the topology allows a simple switching mechanism to go between the harmonic tremolo and a "normal" tremolo.
How It WorksInput stageYour input signal is amplified by a simple FET gain stage (drain follower). It's got enough gain for a fair-sized boost. This is to make sure there's enough output in cases where you might have a really hard wave form (for example, a distorted triangle wave in the TAPFLO chip), without the necessity of a fourth gain stage. The gain stage before, rather than after, just worked better, epecially with the tremolo mode used for Q2. J201 was used because it had more output, but other FETs are perfectly fine, perhaps with some necessary rebiasing, for anyone who doesn't need or want as much output volume. In general, I used FETs because BJTs were too bright (and had an unpleasant harshness), and because they require additional biasing resistors. If FETs become prohibitively expensive, I might decide to redesign it, but the J201 is still plentiful and cheap.
Q2 tremolo sectionQ2 serves two functions. When the switch is in harmonic tremolo mode, C8 is connected to ground, and together with R4 creates a low-pass filter with a cutoff of 723Hz (which sounded best to me after a LOOOOOT of fiddling). When the switch is in "normal" tremolo mode, C8 is no longer connected to ground and for all intents and purposes Q2 becomes an all-pass stage. The 10K at the input cuts a very small amount of highs, but that's cured by the 1uF bypass cap on the drain. The gain on Q2 will pretty much always be very close to that of Q3. I tried a large number of J201s and they all biased just fine into this rage. J201 was chosen, again, because it had enough gain to ensure that the effect is always at or above unity volume.
The actual tremolo effect is created by putting the LDR in series between two 1uF output caps. When the LED is on, the resistance between the two caps drops and the signal passes. When the LED is dark, the resistance is high enough to block all signal. This is the same as the tremolo method used in my Blue Warbler, and it's also similar to the method used in the Tremulus Lune. I used this method on the all-pass stage to ensure that it would work properly with LFOs that expect the series LDR. (Again, my intention is to build this with the TAPFLO chip.)
Although my testing showed it was not necessary for a working and good sounding effect, you can adjust R7 and/or R5 to make sure the level of Q2 does not overwhelm Q3. I tried all the J201s I had and none of them varied enough in their gain to not produce a phasing sound.
Q3 tremolo sectionQ3 is set up essentially as a treble booster. Very, very small input and output capacitors pick up, for the most part, where the low-pass filter on Q2 leaves off. There is a small amount of overlap, and the output capacitor is also slightly interactive with the output of Q3, so the 1nF is a compromise (2.2nF would have been ideal for the harmonic mode, and 470pF would have been ideal for the "normal" mode).
The tremolo in this section is accomplished by placing the vactrol between gate and ground. This means it works backwards from Q2. Here, more "on" time shunts more signal to ground. The result is that this section is at full volume while Q2's is at minimal volume, perfectly out of phase with each other.
The mode switch grounds the gate of Q3 in "Trem" mode, shutting it off. It was done this way because it made the mode switch a very small and common SPDT switch.
LFOThe LFO can be any capable of driving multiple LEDs. I picked the LFO from CultureJam's Shoot the Moon (a simplified Tremulus Lune) for the example schematic because the LED will spend more time "ON" at lower depth settings.
The TAPLFO chip from Electric Druid will function the same way.
There are probably other LFO implementations that will work. For example, you could use the simple phase shift oscillator from the Magnavibe (also used in my Blue Warbler) by hard wiring the depth to max, and using a blend circuit to bypass the whole effect, but this won't really save many parts (if any) because you need the extra gain stage for the mixer in addition to the LFO parts. You can't just wire it up as normal, though, because the depth pot will not work right.
VERY IMPORTANT NOTE: To function, this effect REQUIRES the VTL5C1 or another vactrol (or LDR) with dark resistance >30M. Smallbear sells a Macron clone of the 5C1 (
http://www.smallbearelec.com/servlet/Detail?no=1296), and the CdS photocell 9203 (
http://www.smallbearelec.com/servlet/Detail?no=711) MAY work.
The mode switchAs noted, the mode switch goes from the harmonic tremolo sound and a normal full-range tremolo by removing the treble cut from Q2 and grounding out Q3.
If you use a center "off" switch, you'll also get a third "Vibe" (or maybe "bright" would be a better description) mode, which removes the treble cut from Q2 but leaves Q3 on. This means that during most of the cycle, the high frequencies will pass unhindered, while the lower frequencies throb. Although it doesn't really sound like any particular vibe unit, it does add some variety, and I couldn't think of what else to call it. It's also a neat effect for playing notes over open strings, so the low notes will have a noticeable tremolo effect applied while the high notes will cut through better.
SNEAK PEAK VIDEO!
Enjoy!
I'm actually going to be building myself a version using the TAPFLO chip, which I've already confirmed works for this design. Unfortunately, I fried my chip doing something stupid and had to order another from Smallbear, so I won't be able to build it for a bit. I'll try to work up some layouts soon, one for the TL072 LFO and one for the TAPLFO. In the meantime, I need to build the other stuff I put on hold to get this designed.
EDIT EDIT: Vibrato mode is a no-go. Doesn't work without a lot of contortions and severe watering-down of the circuit as a whole.
Vibe Mode addition mod
It's possible to add a true pitch vibrato mode, by turning the full-range stage into a Magnavibe stage. It requires a few changes and a DPDT SPST switch: the LDR needs to be connected to the source instead of C5; and then a 10nF cap needs to be connected to C6. Like this:
1 4
2 5
3 6
EDIT: Simplified
1
2
3
1: Source Drain of Q2
2: C5
3: No connection
4: 10nF (other side is connected to the drain)
5: C6
Change C5 to 10nF. With this implementation, the tremolo on Q2 is created simply by shunting the connection of C5 to C6, meaning that the tremolo function is a source follower instead of a drain follower. Once you've made the drain and source resistors match and bumped them up to 10K or 22K, it's largely irrevevant which pin you pull the tremolo from because you won't get much of a boost either way. 22K is a value I know works great with a 2n5457, but I'm not certain about with a J201.
However, you would also have to omit C4, and change R5 and R7 to a matching value (not sure about the exact value -- 10K or 22K would probably both work -- but they have to be identical). The gain on Q3 would need to be changed to match, perhaps by putting a 1K (just a guess on the value) on the source instead of grounding it.
Optional added craziness for use with center off toggle: 56K resistor from lug 3 to lug 1. You'll get a sort of chorus-y, half vibe effect. I suspect that, combined with the harmonic section turned on that this can create some really, really unusual sounds.
