Anharmonic Tremolo - yes another harmonic tremolo, but with op-amps

[merlinb]

I think there may be some drawing errors in your schematic, as there are some bizarro connections around U1B and U2A. Note the LDR isn't blending between anything- it's just in series with the output of U2A.

[samhay]

I think there may be some drawing errors in your schematic, as there are some bizarro connections around U1B and U2A. Note the LDR isn't blending between anything- it's just in series with the output of U2A.

That will teach me for trying to use Eagle properly - thanks Merlin, good spot. I have updated the schem and replaced the .sch linked in an earlier post.

[Hemmel]

That will teach me for trying to use Eagle properly - thanks Merlin, good spot. I have updated the schem and replaced the .sch linked in an earlier post.

For those of us who don't have Eagle, could you repost the schematic in an image please ?

Thanks !

[samhay]

Sorry Hemmel - I updated the schematic in the first post at the same time. It should be correct (or at least more correct) now.

[pinkjimiphoton]

It seems that, largely thanks to Jimi Photon, we now have a rash of new Harmonic Tremolo designs (well two). Jon (midwayfair) beat me to it, but here is another approach to getting something that sounds (something)like the old Fender harmonic tremolo. As I am late to the table, I bring an added bonus - you can vary the crossover frequency of this design. It is still on the breadboard - and has been for some time now - and I need a push to get it off the bread board and boxed up. It should fit in a 1590B, but it will be fairly snug. At the moment, it has four pots (rate, depth, crossover frequency and level) plus two switches (treble/harmonic tremolo and squarewave/sinewave LFO). These are not cast in stone.

How it works: U1b and U2a form a fairly typical crossover circuit that splits our signal into bass and treble components. We then pan between them using a quick-and-dirty LDR-based voltage divider, which is modulated using an LFO-driven LED (or a vactrol).

Notes:
- U1b is an almost-Butterworth 2-pole Sallen-Key LPF and the 'frequency' dual-gang pot can vary the frequency of this filter from ~100Hz to ~10kHz. Whether this range is entirely useful is debatable, but I like it. Note that because I can't find a C100k dual-gang pot, this works backwards - clockwise = lower frequency.
- U2a is a differential amplifier that creates our HP (treble) signal by subtracting the outuput of U1b (i.e. the bass) from out clean amplified signal from U1a. This means we can easily control the crossover frequency by only having to wory about varying the LPF corner frequency. This is a compromise as the HPF is not as steep as the LPF, but it keeps us out of voltage-controlled OTA territory.
- The 'treble/harmonic' switch essentially determines the gain of the HPF, which in turn determines whether we have tremolo'ing of both the bass and treble (harmonic) or only the trebble. I think Jon's design does something similar and it is a nice way to add a little shimmer.
- We pan between the LP and HP signal using a voltage divider. One half of the voltage divider is a LDR, which is modulated with an LED being driven by an LFO. This method is discussed a little bit in another thread (http://www.diystompboxes.com/smfforum/index.php?topic=102414.0) and was chosen over a more complicated OTA-based solution. Basically, it is a low parts-count way of doing the job. It isn't perfect, and you do not get a particulalry clean crossover frequency, but it works pretty well in this application. The trimmer is used to set the resistance of the other half of the voltage divider to be similar to that of the LDR when the LFO is a min 'depth'. Note that when you mix the LF and HP signals equally, you get an almost-flat response.
- the LFO (U3a) is the standard Phase 45 LFO design, which is used almost everywhere you want to wobble something. I have used slightly different component values here because I have a lot of 100k resistors and because I'm contrary. If you want more control of the LFO, the 'Tremulus Lune' LFO (among others) is a drop-in replacement.
- U3b buffers the LFO and acts as our depth control - it has gain from 0 to 4.5. I used this approach as, for the LDR to work as a voltage divider, we want it to have an intermediate resitance at minimum depth. To achieve this, we want, at minimum depth, the LED pointed at the LDR to be as bright as it is in the middle of the LFO swing at max depth. I gave U3b gain so that at high gain the LFO is clipped by the op-amp (U3b) rails - the LFO gets progressively more square as you turn up the 'depth'. I like this, but have also added the option of a switch to use a 'proper' square wave LFO, which sounds pretty cool too.
- As seems to be my way, the 4 audio op-amps are all DC-coupled. I have tried uncoupling them and can not hear any good reason to do so.
- I have tried using both a home-rolled LED/LDR combo and a Silonex NSL 32SR3 'vactrol'. Both work equally well and you can get a pretty deep tremolo out of this design.
- The power stage is a little more elaborate than usual as I really don't like LFO noise. The LFO and audio sections are isolated from each other and the LFO ground should be kept away from the audio ground as much as possible too. On the breadboard, I can still hear a little LFO noise, but I think this is a product of the spaghetti of jumpers involved. I guess we won't know until it is boxed up, whether this approach is worth while.

I will post a soundclip this evening and any suggestions are most welcome.

Cheers,
Sam

holy cow, sam, this is awesome dude!!! well done!!!

[samhay]

Jimi - thanks for the motivation. I have to say that I am really enjoying the harmonic element of the tremolo - makes it an entirely different beast.

[Hemmel]

Sorry Hemmel - I updated the schematic in the first post at the same time. It should be correct (or at least more correct) now.

Wow I'm still kind of lost ...
It thought I'd see a connection between the top part (with the depth and rate) connected to the input/output circuit...

Does the top part only have an impact on the LED that affects the LDR ?
I really need to try this out ...

[midwayfair]

Does the top part only have an impact on the LED that affects the LDR ?
I really need to try this out ...

Yes. The circuitry on IC3 is an LFO (low frequency oscillator), wired so that it oscillates the brightness of an LED. The audio path contains a light-dependent resistor, the resistance of which is varied by the LED. They're still connected by the light from the LED. It's not like photons moving through air are that different from electrons moving through copper. Right?

[Hemmel]

I took electronics for 3 months and that was 13 years ago, when I smoked a lot more green stuff than I do now.
There are some aspects of circuits I understand, others that elude me. I'm having a hard time understanding how just one resistor can tell a circuit how to oscillate through depth, rate, and even if it should do so by sine or sqaure waves...
I hope breadboarding this will help me understand...

I found an old LDR and when I tested it, it showed in almost total darkness 500K, and in direct sunlight, 50 Ohms. Will it do for this circuit ?
I understand I'll need to put the LED/LDR pair in a small box or something, to prevent outside light sources to have an impact.

[artifus]

I found an old LDR and when I tested it, it showed in almost total darkness 500K, and in direct sunlight, 50 Ohms. Will it do for this circuit ?
I understand I'll need to put the LED/LDR pair in a small box or something, to prevent outside light sources to have an impact.

most commonly available ldr's tend to go from a few K to a few meg so that should be fine - if you check the schematic again you'll see that the ldr is paralleled by a 1m resistor...
yes - the more light the less resistance, the darker it is the closer to one or two meg the ldr becomes. hence the need to house the contraption in a light proof box for consistency. beware time - aka - response.

also: http://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdf and http://www.fairchildsemi.com/an/AN/AN-118.pdf

[samhay]

Hemmel - your LDR should work ok. It doesn't need to have a particularly large swing in resistance to give a decent tremolo effect, and for reference, mine are spec'd at 8k-24k at 10 lux & 1M dark resistance. You should be able to set the trimmer to make most LDRs play nice with this circuit, but I guess we may have to wait and see.
To hopefully clarify how it works. If you have two signals connected to pins 1 and 3 of a variable resistor (pot) and you listen to what comes out of the 2nd pin as you twist the knob, you have a basic stereo panner. This is how this circuit works, but instead of a pot, the LDR is one half of a voltage divider and the other half is the trimmer. The LFO, which is the top half of the circuit, makes sure the LED pointed at the LDR changes brightness in a rhythmically interesting way. As the brightness of the LED changes, the LDR's resistance changes and we 'twist' the voltage divider, thus panning between two signals. In this case, our two signals are bass and treble and we get something approximating a harmonic tremolo.
There is some advantage to electrically separating the audio and LFO sections of the circuit, not least that it can help to keep noise from the LFO down.

Jon - I think a physicist would argue there are a few difference between photons and electrons, but in this case they both get the job done.

artifus - nice job noticing the 1M resistor (R12). It is actually not really required in this circuit with the LDRs and vactrol I have tested it with, but I figured I should add it as I have always used something like this to linearise the LDR and to make sure it plays nice. This can always be tweaked if need be.

[midwayfair]

lawdy. Started on the layout and it's going to take me a bit. I used a quad for the audio path but might go back to a pair of duals to save my brain. I honestly think the layout challenges have something to do with my reticence to work with op amps.

[samhay]

lawdy. Started on the layout and it's going to take me a bit. I used a quad for the audio path but might go back to a pair of duals to save my brain. I honestly think the layout challenges have something to do with my reticence to work with op amps.

Yep - op-amps can be a little more awkward to layout than discretes, but it probably comes down to what you know best (which in your case seems to be things that come in a TO-92).

I have most of a vero layout done, which I might try first. Will post once I am fairly certain it will both work and fit in a box.

[pinkjimiphoton]

if you're working up a vero, i'm gonna hang back then... i'm looking at this shaking my head going...man!!!

personally, i hate using vero on anything with more than a chip or two... it gets to be a drag trying to lay out. the clone theory was a nightmare, and i'm STILL trying to get all the noise and crap out of it.

[samhay]

if you're working up a vero, i'm gonna hang back then... i'm looking at this shaking my head going...man!!!

personally, i hate using vero on anything with more than a chip or two... it gets to be a drag trying to lay out. the clone theory was a nightmare, and i'm STILL trying to get all the noise and crap out of it.

This is bigger than I would generally try on vero too, but it is a baby board next to some of digi2t's layouts, so I figured I would give it a shot.

[pinkjimiphoton]

can't wait to see it.  this thing sounds great and i'm a lazzzzzzzzy man sometimes.

[duck_arse]

sam, is the value of C10 1uF?

[samhay]

sam, is the value of C10 1uF?

Yes, but you probably will not hear much difference if you try 100n.

[samhay]

It took a little while, but I have a vero layout that should both work and fit in a 1590B. I have split the LFO and audio sections to seperate boards, which saves quite a lot of space as the LFO board should fit vertically on board-mounted pots. Will check a couple of dimensions and do another trace before I post.

In the meantime, I stumbled upon the Baja Vibrotrem on the other forum. Harald (SabroTone) has done a vero layout too. It looks to be a fairly faithful op-amp implementation of the original Fender harmonic tremolo, but I haven't found any audio clips.  

[nocentelli]

It took a little while, but I have a vero layout that should both work and fit in a 1590B. I have split the LFO and audio sections to seperate boards, which saves quite a lot of space as the LFO board should fit vertically on board-mounted pots. Will check a couple of dimensions and do another trace before I post.  

Excellent. I started a vero layout, but it grew to 21x21 holes very rapidly. Spliting it into two boards seems like a great idea.