Cardinal Tremolo (Vactrol-base Harmonic Tremolo) - schematic & discussion

[midwayfair]

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 Works
Input stage
Your 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 section
Q2 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 section
Q3 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.

LFO
The 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 switch

As 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.

[pinkjimiphoton]

holy sheet, jon, this looks fantastic!!!

thanks man!! sounds like EXACTLY what the doctor ordered!

i will start laying it out tomorrow i hope.

so no home-rolled vactrols? i guess i can buy a couple from steve.

thanks man... this looks pretty good and accessible!

we'll see how vero-friendly it is.

[samhay]

Nice job Jon. I like the TAPFLO implementation, even if (or perhaps because) it is a bit of an anachronism.
You might get away with using 'typical' LDRs if you drop R5 a fair bit. You will lose a little bit of boost and it will not go as deep, but it might work well enough all the same.

[midwayfair]

You might get away with using 'typical' LDRs if you drop R5 a fair bit. You will lose a little bit of boost and it will not go as deep, but it might work well enough all the same.

I'll try that, but off the top of my head I'm not sure it'll help. The equivalent resistor is 2.2K in the Blue Warbler and won't trem with anything else* there either ... not even a little bit. Common sense tells me: "Well, 10M or 1M should be high enough to block most signal just as well as 50M," but that doesn't appear to be the case. Maybe it has something to do with the vactrols' light memory. Part of the problem is I don't really understand exactly why it works in that orientation in the first place. Originally I did it in the Blue Warbler because of the series LDR arrangement in the Tremulus Lune, and only later discovered that no other vactrols work. Most of the time I arrive at circuits by trying literally everything I can think of on the breadboard, so it's hard for me to know how to change some things once I know they work.

Considering that the VTL5C1 also has the really low light resistance necessary for tremolo on Q3 (actually, the low on resistance is almost as rare in roll-your-owns as the high dark resistance), I'm not too torn up. It's a pain to have to order a specific part, I know, but

*Incidentally, H11F1 and the like will function, but often they cause distortion. It's better to use the center pin on the diode side if you try to use these chips. The datasheet makes it look like that pin's not connected to anything, but in practice it works. It would be awesome if someone actually manufactured a real honest-to-goodness chip-sized vactrol at a reasonable price instead of all the photodiode/phototransistor nonsense.

[samhay]

Yeah, after posting I looked up the VTL-5C1 and it has a much lower light resitance that I thought it would - very cool.
Upon futher reflection, it would appear that LDR1B is just one half of a voltage divider with the volume pot and C6 is mostly redundant. If you replace the volume pot with e.g. a 10k, you should get away with using an LDR with 10x less dark resistance as long as its light resitance is quite low.

Even if it works badly with a fet optocoupler that's pretty interesting. They can be made to work as voltage-controlled resistors (e.g. http://www.edn.com/design/analog/4368893/Use-a-photoelectric-FET-optocoupler-as-a-linear-voltage-controlled-potentiometer), but they don't get much love in the stompbox world. I thought pin 3 was NC, so using pin 2 is how fet optos work?

[midwayfair]

I thought pin 3 was NC, so using pin 2 is how fet optos work?

Pin 5: http://www.datasheetcatalog.org/datasheet/fairchild/H11F1.pdf
The "FET" side actually appears to be more like a diode than a FET. I was unable to make it work as an amplifier, for instance, which was a real disappointment because that would have been freaking awesome for so many things ... sorry, I realize that saying "diode side" was confusing since I didn't mean the LED side.

I can try reworking the output sections. I also noticed this morning that C6 was redundant in a trem-only design , as I had been keeping the topology from a prior schematic that just had a vibe/trem mode and then changed some values and added Q3.

I'd have to 10x the output caps if I use a smaller volume pot and ditch C8, and it means no hope of a vibe mode, but if it means a wider variety of optocouplers/LDRs, then I'm certainly willing to give it a shot!

[pinkjimiphoton]

the vibe mode may be the coolest part. tremolo kinda vibes sound completely different from chorus kinda vibes.
jon, any chance of a vid or audio snip?
would love to hear this thing before i commit to ordering some vactrols.

[midwayfair]

the vibe mode may be the coolest part. tremolo kinda vibes sound completely different from chorus kinda vibes.
jon, any chance of a vid or audio snip?
would love to hear this thing before i commit to ordering some vactrols.

I'll try to film something tonight. I had basically spent 20+ hours straight on this by the time I finished the schematic last night and it was too late -- and I was too tired -- to film anything.

I'll just be able to do the basic sound at the moment, so no messing with the LFO except speed. I didn't have enough space on my breadboard for the chip-based LFO without removing all the TAPLFO stuff, so I'm just using a phase shift oscillator until my new TAPLFO chip arrives from Smallbear. I already know that LFO works the way I need, though, because I've built multiple pedals that use it. If I have time, I'll see if I can get the vibrato mode switch up and running (and balanced) first, too.

[samhay]

I think (and I may be very wrong) that if you bump C9 up to 10n and drop the volume pot to 10k then you will still get all 3 modes to work reasonably well.

[pinkjimiphoton]

i hate to ask such a dumb newb question,
but on the schem it shows vactrol a and b.. is that the a side with the dot?
or do i need two of them?
waiting to pull the trigger at small bear...
thanks jon!

[midwayfair]

i hate to ask such a dumb newb question,
but on the schem it shows vactrol a and b.. is that the a side with the dot?
or do i need two of them?
waiting to pull the trigger at small bear...
thanks jon!

Sorry! I went through a bunch of iterations of this schem for the LFO and forgot to mark it. those are the LDR sides -- the long legs. They are not polarized, just like a resistor. D2 & D3 are the LED sides (the ones marked with the dot or + sign for the anode).

EDIT: Also, video in 1 hour. Uploading now.

[pinkjimiphoton]

awesomer!

thanks, now i know what i needed i think... so vactrol a and b are the two sides, correct?

i'll order from smallbear.

[midwayfair]

thanks, now i know what i needed i think... so vactrol a and b are the two sides, correct?

Yes, the "a" is the LED side, and the "b" is the LDR side. 1 and 2 designate the two separate vactrols. Is it clearer in the updated schem? It'll be really obvious once I finish a layout ... not going to happen till this weekend, though.

Some updates in the corrected schematic:
1) Increased the output cap on the input stage (C2). Testing with rested ears showed this to pass all frequencies better.
2) Dropped the 1uF cap between the drain of Q2 and the LDR. Not only does it still work, but it sounds much better.
3) Unfortunately, my attempt to get it to work with a 10K volume pot also failed. It drops the output significantly, and the treble side sounds broken ... really, really weak for some reason. Part of it is the interactivity between Q2 and Q3 -- they seem to sort of reinforce each other's gain if that makes any sense.
4) The vibrato mode a la the Magnavibe is a no-go, so I've crossed out that whole mess in my original post. It dilutes the effect, breaks the treble side, doesn't sound particularly good, and frankly I'd much rather have an excellent sounding harmonic tremolo with a really good "normal" tremolo option than a watered-down pitch vibrato + watered-down harmonic tremolo. The good news is that this means that the effect is back to needing just a SPDT for the mode switch (optional center off for a "bright" mode).

Unless some sort of inspiration strikes between now and the time I build a copy of the circuit to confirm, this is the final schematic. I want to stress that until someone else breadboards or builds it that it's unconfirmed.

[midwayfair]

Video added!

Very basic -- I'll do a better demo after I have a working pedal in hand.

[Jdansti]

Just in time for the Conclave! 

[samhay]

Sounds really good.
Nevermind about the absolute requirement for the vactrol - sounds like it is well worth it.

[midwayfair]

Hey, folks, here's an alternate LFO that will do a nice true sine wave:


This'll give it a mode like a brownface and the other mode will have the "right" wave form. It doesn't get quite as deep as a chip-based LFO so it won't have a massive throb, but it's a decent choice for a simpler build with fewer parts.

[pinkjimiphoton]

holy sheet, jon, this is amazing...thank you sir!!!!!! i am gonna try and work up a vero for this thing, and get to building it as soon as i can get a realistic chance.. thanks !!

this thing sounds like the sounds i hear in my head.

[midwayfair]

this thing sounds like the sounds i hear in my head.

Yeah, I was poking around in there the other ....



I .... I mean .....

Oh good!



(I need a tinfoil hat emoticon)

[pinkjimiphoton]

careful, lotta black holes in there....