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, especially 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, C7 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, C7 is no longer connected to ground and for all intents and purposes Q2 becomes an all-pass stage. The 10K at the input (R5) cuts a small amount of highs even when C7 is shunted, 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 with the 1uF output cap. 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 original intention was 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 R6 and/or R9 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've used an LFO adapted from CultureJam's Shoot the Moon (a simplified Tremulus Lune) because the LED will spend more time "ON" at lower depth settings. I got lucky in that the bypass resistors on the LEDs as shown produce a much better sine wave sound than the Tremulus or StM are capable of.
This oscillator also works well, and I've built a version using it:
The TAPLFO chip from Electric Druid also works with some adjustment.
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 Bright ("Vibe") 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. 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.
Here's what it sounds like:
Enjoy!
I'm also going to be building myself a version using the TAPFLO chip, which I've already confirmed works for this design.
I use 5457s in most circuits, as I generally prefer the sound.
Although the pinout is correct, the output will be too low for Q2 and Q3 even with rebiasing.* 2N5458 is slightly better, but still not a J201 in terms of gain. It should be fine in Q1; I don't even think you'd need to rebias.
J201s are cheaper, too.
*Note that Q3 is basically already cranked. If you lower its gain, you have to lower the gain of Q2 as well or they will be out of balance.
flipping sweet. who do I have to throw my money at to get boards of this circuit? just point 'em out!
paaaatience. I'm guessing the PCB is about a month and a half away barring any bugs. It's about 3 weeks to get prototype boards, figure a week to build and thoroughly test, 10 days to manufacture, and then a week or so for shipping from China.
Thanks Jon! I've just been waiting and searching and researching different harmonic trems so to have yours come along now is really exciting. It's the main reason a bandmate of mine wants a Strymon Flint and the concept just really does it for me so I'm super excited
Thanks Jon! I've just been waiting and searching and researching different harmonic trems so to have yours come along now is really exciting. It's the main reason a bandmate of mine wants a Strymon Flint and the concept just really does it for me so I'm super excited
You could do Grindy's Tenebrion in the same enclosure and make him a Flint!
Are the bypass resistors for voltage stabilization?
Basically, they siphon off some current.
culturejam wrote:
MADBEAN v.2 wrote:
I'm a little stumped on what R6 is doing....is it a voltage divider with R7 to knock down the brightness of the LED? Imma scratching mah heads.
I didn't get it either, but earthtones had this to say about it after running the circuit in software simulation:
"I was curious about the resistors paralleled with the LEDs, but it appears they allow the LEDs to go more fully dark on the low part of the sweep. That's a pretty neat trick."
Aside from the added range, there's an additional benefit in that the waveform becomes smoother when the bypass resistor is in place. The Shoot the Moon's sine wave setting was better than the Tremulus Lune's simply because of the addition of a single resistor. The effect seems to get better the more similar stages you put in. So the sine wave with three LEDs (two of which are superbright inside the vactrols) and the cascaded bypass resistors double in value each step up make for an even softer waveform than the stock StM had.
After watching your video a second time I have some thoughts/questions...
1. Your build does not have a volume knob, but the schem shows this, why's that? I would just set the output for unity gain. 2. The part numbers your referring to in your write up don't match the schematic. I know what you mean but it's a little confusing. 3. Why tie Q3's source to ground? Odd bias setup for this stage to me. 4. Why fixed resistors on the drains? Did you use a trim pot first then measure the resistance? There is a lot of variance in FETs as I'm sure you know. 5. Why a source bypass cap on Q2? Is the extra gain really need in the harmonic trem mode and not the other trem mode? Q1 would seem like the logical place to bring up the gain.
Sounds great! How finicky is it about optos, would VTL5C9 work?
C9 looks like it might work. It's not as common, and I've never owned one.
ChrisM wrote:
After watching your video a second time I have some thoughts/questions...
1. Your build does not have a volume knob, but the schem shows this, why's that? I would just set the output for unity gain. 2. The part numbers your referring to in your write up don't match the schematic. I know what you mean but it's a little confusing. 3. Why tie Q3's source to ground? Odd bias setup for this stage to me. 4. Why fixed resistors on the drains? Did you use a trim pot first then measure the resistance? There is a lot of variance in FETs as I'm sure you know. 5. Why a source bypass cap on Q2? Is the extra gain really need in the harmonic trem mode and not the other trem mode? Q1 would seem like the logical place to bring up the gain.
Just thinking!
Good questions!
1. The volume control is a trimpot. People can wire it to be external. Transistors aren't consistent; it's just easier to set a trimpot than fiddle with transistor gains. 2. Sorry! I've fixed that. I had made a couple changes to the numbering in my last revision of the schematic, which was after I did the write-up. 3. Lots of gain for the bare minimum of parts. It works every time for me, so there was no need for something more complicated. The Vactrol provides the gate bias -- that might make it look more normal. Normally my go-to source values are more like 560R or 470R. I think the complete lack of bass frequencies in that stage help keep it from distorting, so tying it to ground (and saving one part) was acceptable. 4. Trimpots just don't have good noise performance, so if the bias isn't absolutely critical I'd prefer to leave them off (there are other designs, like the Hamlet, where this is not the case). I used a pot on the breadboard and just took a vaule in the middle of the range that worked for a large number of J201s (which was anything between 2.2K and 10K). Exact bias isn't critical in this design -- you're not overdriving FET stages, which would require a particular gain structure, and Q1 always provides enough output to always get above unity. The only reason to play with biases really is that if your FETs are wildly out of line or if you really want to match the gains of Q2 and Q3. That "could" result in a mild improvement in performance but not a big enough improvement that I think trimpots are worth it. There's something more complicated going on with Q2 and Q3 being interactive, too, but this answer's already turning into a novel. 5. The gain provided by the bypass cap on Q2 is frequency-specific. The way the Q2 tremolo stage and the volume trimmer are wired up, there is always some resistance in series with the output, which cuts some of the highs (also, R5 does this as well, as noted above). Without C4, it sounded a little dull. I just tried a few different things until the frequency response sounded more natural. Boosting the gain on Q1 runs the risk of overdriving the following stages, but iirc in testing it also made Q3 sound a little overwhelming, because now you're boosting the upper mids going into both stages (and I just didn't feel like reworking Q3 at that point). C7 cuts a lot of the same frequencies boosted by C4 at the input of Q2 in harmonic mode, but those same frequencies are boosted by Q3. As it stands, C4 only does what it needs to when it needs to.
Just saw this. Holy f'ing sh*t! Nice job Jon! Been hankering for one of these for a while and was trying to get around to making one of the rough sketches out there work.
I see the low-pass filter set up by R5 and C7, the high-pass filter set up by C3 and R7, and the series capacitance between C2 and C6 isn't changing frequency response from either cap. So C2 looks like a normal coupling cap to me. Am I missing something? Could one sub that for a non-polarized cap in a smaller value?
I see the low-pass filter set up by R5 and C7, the high-pass filter set up by C3 and R7, and the series capacitance between C2 and C6 isn't changing frequency response from either cap. So C2 looks like a normal coupling cap to me. Am I missing something? Could one sub that for a non-polarized cap in a smaller value?
Yeah, I realize it's a bit weird. There were a few things going on: First, I didn't want to have to use electrolytics for both C2 and C3. Also, it was easier to change just C2 when I was working on the breadboard than to change C2, C3, AND C7 (C2 is big enough to do pretty much nothing to C7) to get the best full-range sound.
As far as whether it needs to be that large -- 1uF had a noticeable loss of bass. 2.2uF wasn't really that distinguishable from 4.7uF, but I took a gamble that people have more 4.7uF caps in their parts drawers. Sometimes things are chosen to make it an easier project for everyone to build.
---
We're really close to having the PCB run ready to go. A prototype board from OSH park has been built and confirmed, but we're working out some issues with the FETs ... apparently Tayda's J201s are lower gain than Fairchild ones, so we'll probably end up suggesting different drain resistor values in Q1 (at least) to deal with that bit of annoyance.
Joined: Sun Nov 18, 2012 5:13 pm Posts: 4780 Location: Soddy Daisy,TN
midwayfair wrote:
Morgan wrote:
Question. Does C2 have to be so large?
I see the low-pass filter set up by R5 and C7, the high-pass filter set up by C3 and R7, and the series capacitance between C2 and C6 isn't changing frequency response from either cap. So C2 looks like a normal coupling cap to me. Am I missing something? Could one sub that for a non-polarized cap in a smaller value?
Yeah, I realize it's a bit weird. There were a few things going on: First, I didn't want to have to use electrolytics for both C2 and C3. Also, it was easier to change just C2 when I was working on the breadboard than to change C2, C3, AND C7 (C2 is big enough to do pretty much nothing to C7) to get the best full-range sound.
As far as whether it needs to be that large -- 1uF had a noticeable loss of bass. 2.2uF wasn't really that distinguishable from 4.7uF, but I took a gamble that people have more 4.7uF caps in their parts drawers. Sometimes things are chosen to make it an easier project for everyone to build.
---
We're really close to having the PCB run ready to go. A prototype board from OSH park has been built and confirmed, but we're working out some issues with the FETs ... apparently Tayda's J201s are lower gain than Fairchild ones, so we'll probably end up suggesting different drain resistor values in Q1 (at least) to deal with that bit of annoyance.
Totally stoked about this project and I don't know why, as a bassist I do have a need yet, but I'm going to make one.
_________________ Pedal building is like the opposite of sex. All the fun stuff happens before you get in the box.
I don't think Josh made an announcement about his recent PCB releases over here at BYOC, but: Cardinal PCBs are available from 1776 Effects now. It's red and has a birdie on it, which I love, and Josh did a great job on the layout.
Woohoo! Ordered! Thanks for letting us know Jon, and also thanks for making this available.
By the way - my next amp build I'm planning is going to be a deluxe reverb hybrid with brownface era harmonic trem. Using your leg work, I'm going to see if I can make the trem on that amp switchable between harmonic trem and bias-vary trem. Should be fun! Although if it works, I guess I'll be selling my cardinal trem.
Woohoo! Ordered! Thanks for letting us know Jon, and also thanks for making this available.
By the way - my next amp build I'm planning is going to be a deluxe reverb hybrid with brownface era harmonic trem. Using your leg work, I'm going to see if I can make the trem on that amp switchable between harmonic trem and bias-vary trem. Should be fun! Although if it works, I guess I'll be selling my cardinal trem.
The trouble is that if you use the standard brownface trem in the amp, it's modulating the bias of the power tubes. You can't simply switch to a different form of tremolo in that case, and if you used Fender's harmonic tremolo scheme, you wouldn't really have a good easy way to switch to brownface. You might, however, be able to conjure something using a single preamp tube in place of the FETs in Q2 and Q3, and then creating an appropriate (perhaps lamp driven?) LFO. Then you can do the same switching scheme. In fact, if you come up with an amp-appropriate single-preamp tube version of this circuit, I think there's a lot of people who would love you longtime.
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I'm guessing the PCB is about a month and a half away barring any bugs. It's about 3 weeks to get prototype boards, figure a week to build and thoroughly test, 10 days to manufacture, and then a week or so for shipping from China.
Nice job Jon! Been hankering for one of these for a while and was trying to get around to making one of the rough sketches out there work.
