DIY Stereo Tremolo Design

[Phoenixfire]

Hey!
So after falling in love with the sound of stereo tremolo, particularly with variable phases between 'left' and 'right' (like the flux effects liquid tremolo or the 'rt. phase' control on the EHX super pulsar), I've got the idea stuck in my head to design one myself, to my own requirements. Only problem is, I actually don't know how the basic tremolo circuit operates!

If you could share some of your valuable knowledge here, I'd very much appreciate it! I'm keen to learn about LFO design and operation, how to use it to create the tremolo effect, how to add tap capabilities, how to trem two signals with out of phase LFOs while maintaining control over the phase shift, and anthing else you may think is important!

to give some context, the feature set I'd ideally put in is:
*Stereo or mono operation
*Stereo phase control
*tap tempo
*At least square and triangle waveforms
*All the usual suspects (rate, depth, volume, shape skewing)

I'm currently studying electrical engineering (fourth year), so i understand most things about resistors, capacitors, inductors, basic op amp circuits, etc.
I'm happy to spend time reading articles or explanations if there are some particularly useful ones. I'm planning on lots of research before even starting schematic design!

I'm keen to learn, so please be keen to share!

Stuart

[raulduke]

That is one complex tremolo build to start out with!

I'd start with looking at this Tap-Tempo Tremolo circuit:

http://www.musicpcb.com/pcbs/tap-tempo-tremolo

It uses a Microcontroller for the LFO, which allows for tap-tempo, waveshape, phase control etc.

Google will throw up loads of build threads, including the original design thread on DIY stompboxes.

I'd start with building (or even better bread-boarding) something like that (or one of Madbean's tremolo projects) and then work your way up to the all singing all dancing stereo version you have specified.

The audio part of the tap tempo circuit is actually quite simple (look at the two opamps where tremolo in and tremolo out are marked).

An optocoupler (driven by the LFO) is used to vary a resistance in the audio path, thus varying volume.

The first opamp is an inverting buffer, the last opamp is an inverting amplifier (to boost the output if needs be).

In terms of LFO's, I would think a microcontroller based solution is going to be the best bet to get what you are asking for.

[micromegas]

OK. I prefere Steve's explanation to mine


Welcome to the forum Stuart!

I've studied EE myself and once you dip your feet in this hobby you never get out!!

You have a really deep explanation of how a tremolo works in chapter 6 of this book:
http://www.guitarscience.net/papers/guibook.pdf
That one is, however, based in discrete oscillators.

There are a lot of LFO designs out there (synth guys have been use those since the beginning of times) best thing to do is to try some. I recommend you to start with vactrol based tremolos. Search some culturejam's designs and you can quickly find a really good designed LFO you can try.

In a basic vactrol based tremolo, you have an LFO that outputs a sinusoidal/triangular/etc voltage used to modulate a ldr/led combo that somehow controls the gain making variations in volume.

If you want to get deeper into oscillators this is the book: "Practical Oscillator Handbook" by Irving M. Gottlieb, although not every design has a practical application for tremolo.
You can find a lot of useful information about oscilators in "The Art of Electronics"  (Chapter 5 on 2nd Edition).

Doing a quick google search I've also found this:
http://casperelectronics.com/finished-pieces/nova-drone/theory/#sec2


However, if you want tap tempo, you'll probably have to choose the digital way. Electicdruid has a great design that we all have used/heard off for a digital LFO with tap tempo (with code and everything!!). You can find it here:
http://www.electricdruid.net/index.php?page=projects.taplfo

[midwayfair]

This is, well ... a very large topic.

First things first: Tremolo is the rhythmic modulation of amplitude.

You can accomplish this multiple ways. The two used in almost every case are to modulate either the gain of an amplifying device in the circuit or the volume of the signal level before, after, or in between the amplifying devices. This involves the manipulation via a low frequency oscillator (LFO) of a variable resistance element, which can be a photocell (light dependent resistor, or LDR) or a transistor (usually a FET). Amps don't need the variable resistor for some methods of tremolo, but blackface amps use a photocell. (This leads to a lot of confusion about the qualities of an amp's tremolo, which is too much to go into here.)

A third way to alter the volume of a signal is used much more rarely in solid state tremolo pedals (in fact I can't think of a specific example): you can vary the bias of an OTA device like the CA3080 or LM13700. This is how the gain is altered in lots of compressors (e.g. Dynacomp) and OTA phasers use it. This still involves a variable resistor, but it's just hooked up in a particular way.

That's what the dry path is doing. The LFO is another consideration. An LFO is created by using the noise naturally present in a device to create a positive feedback loop at a very, very low frequency (below human hearing); the feedback loop will continue as long as gain is 1x or slightly higher. The frequency of the oscillator is manipulated by a rate control. The depth control must be done separately after the LFO; usually it's simply a voltage divider that alters the output voltage of the LFO.

It is most common in modulation pedals to use an op amp. They're easy to fire up, and have a huge rate range before the LFO craps out. However, they have a drawback in that they aren't typically capable of producing anything except square (pulse) or triangle wave forms. You can soften the waveform a bit to make it "sine-ish" but it will never be a true sine wave. To get a sine wave, you have to use a (more problematic)

This is for a mono tremolo pedal. To do stereo, you need to create LFO signals that are out of phase, so that you can modulate two parallel dry paths out-of-phase with each other. The dry paths have separate outputs that go to two different outputs. The LFO signal has to come to rest at lower depth settings with both sides being either at full volume or half volume (meaning it gets louder and quieter).

Having worked on this "a bit" I can tell you that it's not an easy task. However, he's an LFO schematic (which I will be using in a future project) that will give you out-of phase LEDs (for use with photocells) and an adjustable waveform. Vr is adjustable via trimpot to balance the LEDs "on" state:



To use this, you would need to design a dry path with matching/adjustable output or gain levels for the left and right bands that can use a photocell.

Here are a list of useful tremolo circuits for you to look at:

• The Tremulus Lune (from which the above oscillator is directly derived -- just to show you what you can do with that oscillator; the dry path is also useful)
• The Phase 45 (this is where the Lune's LFO came from; also shows a method of modulating FETs)
• Boss Tr-2 (another way of modulating FETs, similar LFO)
• EA Tremolo (for a "bias modulated" tremolo, and a transistor-based LFO)
• Univibe (a "better" version of the LFO found in the EA Tremolo; also shows how to drive a lamp instead of an LED -- not terribly useful for a stereo tremolo)
• RG Keen's Harmonic Tremolo (uses the Univibe LFO and shows a method for phase splitting that LFO to modulate two audio paths out of phase, with the modulation accomplished like the EA Tremolo)
• Craig Anderton's Stereo Tremolo (the LFO can be problematic and the photocell is obsolete, but it's a good starting point)
• The datasheet for the TAPLFO (a tap tempo controller that produces a PWM signal, which is then used to modulate an LED or heavily filtered to a control voltage)

I also have a project nearly ready that uses the TAPLFO and produces inverted LEDs. It's not a stereo tremolo, but it works great and could be adapted for the audio path to a stereo trem with tap tempo.

Start simple. Make a mono tremolo you like the sound of. Make a stereo split you like the sound of. Then worry about mixing them together.

[Phoenixfire]

This is fantastic guys, thanks! I'm now significantly more knowledgeable, which is always great!

It looks like my best starting point is a basic tremolo, probably one based on the TAPLFO ic.
Also it seems like my highest hurdle is also the one I really wanted, a stereo phase control.

Using a pair of TAPLFO ics, I could potentially run one as the 'master', and feed the 'clock out' signal into the tap cv input on the 'slave'. But then is there some way I could delay the 'clock out' pulses by a variable amount, in order to have control over the relative phase?

The other option is probably easier and slightly less 'hacked together', which is to code my own micro-controller with the ability to adjust output phases. its just more time consuming!

my options are limited as it seems there's no way to do tap control without a micro-controller. Has anyone ever designed an op amp oscillator with tap control?

As far as the dry signal path is concerned, my current understanding is that at least two op-amps are needed for a basic mono tremolo. One to buffer the input (probably unity gain), provide tone shaping if desired, going into an ldr/led/vactrol section, and a second to buffer the output, provide a level control, and again adjust tone if desired.
Does that seem like an accurate assessment?

[raulduke]

Jon (Midwayfair) gave some good advice on different tremolo topologies. Vactrol based (like the Tap Tempo Tremolo) is not the only way to get a tremolo effect.

Someone might have designed a tap-tempo analogue LFO, but I don't see what the benefits would be TBH. I'm not aware of one. The circuitry involved would be very complex. You'll have to do your own digging around and see.

As we said, start small and work your way up from there. If you only need 180 degrees phase shift between the L+R channels, then inverting the LFO's signal could maybe work. If you need variable phase shift this becomes more complex.

Electric Druid, the guy who designed and sells the TapTempo LFO chip, might be able to give a few pointers. He might even be able to code you a custom LFO chip if you are prepared to cough up the £ 

I'd say build a few different standard tremolo circuits and go from there...

As with all circuit design, the original idea is the easy bit. Implementing it is the major PITA!