signal tremolo effect (audio)

[transcendent12]

Alright, let it be known that this be my first own initiative electronics project. I'm not well versed in practical circuit design, though I am studying electronic engineering. I'm trying to apply some basic knowledge to make something ...well.. basic.

Idea is to make a guitar tremolo effect pedal, all this does is take the signal from the guitar and then amplitude mix it with sine/triangle/square waves to give not a pitch waver but amplitude waver. I don't have much idea how current effects pedals do this.

Here are my initial thoughts. Basically I'd be grateful to just nail down some basic accepted ideas and begin to make a start.

1) What's the best way to take a signal and modify it's amplitude? Initially I thought I could have an old school IC wave form generator working as a local oscillator then mixing it with the input signal. The frequencies would be pretty low in region of 0.1Hz to like 30Hz is all that is needed for a guitar tremelo. My hope would be that the local oscillator would be centred around zero but with adjustable offset. So the depth of the tremolo could be altered. i.e. so it doesn't necessarily have zero periods when sine wave passes 0V.

2) I could use a microcontroller to implement a tempo counter. I was thinking like an arduino or something else which has a counter and count the physical taps on a button. This would have to somehow set the frequency on the signal generator. Multi tiered switch could be used to multiply the speed by 1x, 2x, 3x etc. But analogue control of speed would be necessary also.

Are these initial ideas feasible? Thank you very much in advance. I hope something can come of this to further my practical knowledge.

 

[crutschow]

For the mixer you could use an analog multiplier. They are easy to use and readily handle the frequencies of interest. You feed the audio into one input and the tremolo signal into the other.

You would need to add a DC bias to the tremolo signal so that you still get an output with zero tremolo. This can be done simply by capacitor coupling and a two resistor divider between the + power supply and ground

A microcontroller should work fine to control everything.

 

[transcendent12]

thank you for the quick reply. I'm glad the ideas made sense at least. I shall take some time to formalise the ideas. And research some figures.

 

[EN0]

I play the tremelo affect on my guitar! One of the pieces I am now playing is Recuerdos de Alhambra, which is probably on Youtube or something. I am interested in how your final project works out!

 

[transcendent12]

This can be done simply by capacitor coupling and a two resistor divider between the + power supply and ground

What is the capacitor for? Is that assuming AC power supply? Wouldn't be needed for DC yes?

And EN0, I shall try my best to not make this just a pipe dream project! And that classical piece is awesome. Actually, I don't really play guitar, it's my brother who plays (more rock). The tremolo pedals you can buy are pretty expensive. like £160+ ($250). I was just thinking to myself it is probably something I might be able to make with my engineering knowledge for about half! Just looked up those analog multipliers suggested earlier, each of those is already like £25. Will definitely have to shop around.

 

[Nigel Goodwin]

The tremolo pedals you can buy are pretty expensive. like £160+ ($250).

I suggest you try looking elsewhere - £19.99 inc. VAT.



You can't make one for anywhere near that price.

 

[canadaelk]

Attached a schematic from the National "Audio Handbook" of 1976. I hope that helps! E

 

[crutschow]

What is the capacitor for? Is that assuming AC power supply? Wouldn't be needed for DC yes?

The capacitor is to AC couple the tremolo signal to the multiplier input while blocking the DC offset created by the resistors.

 

[transcendent12]

Surely if we're talking like 0.1 Hz oscillations, the capacitor would have an impedance which wouldn't let the signal through into the multiplier input? Wouldn't it just appear as a slowly changing DC level?

For example, one of the analog multipliers has a multiplying transfer function of (x1 -x2)(y1 - y2)/10V + Z2 (can be modified from 10V to 3V i think Z2 is some other feedback connection to fiddle gain etc. so can be ignored for now) This is differential so if x1 is the wave generator say 1v p-p sine 0.5V offset, y1 is the guitar signal and finally x2 is 0.5V and y2 is 0.5V. This would give the required effect would it not?

 

[#12]

Please, sir, you are mistaken about the range of tremelo frequencies, or maybe that was a typo. .1 Hz would require 10 seconds for the tremelo to cycle once. Think, Crimson and Clover (Tommy James and the Shondels) at about 3 HZ. I don't think you'd ever need less that 1 Hz and I think I remember the top range on a Fender amplifer as 30 Hz...but it might be lower.

 

[transcendent12]

yeah you're right, 0.1Hz is a bit extreme. That's actually the limit of the signal generator sine waves for normal playing you'd probably use 3-5Hz I guess. But still, at those low frequencies i'm not sure how AC coupling with caps would work. Seems safer to do things differentially if solves the problem.

 

[transcendent12]

Ok here's a specification I've written up. Some pretty interesting problems and stuff have come up.

Spec:

• Multiple wave shapes - sine/triangle/sawtooth/square
• tap tempo setting
• speed adjuster (multiplier)
• wave depth adjustment (amplitude of wave generator)
• wave offset adjustment (DC offset)
• Beat accents for different time signatures (micro controlled)
• true bypass
• possibly normalising feedback to account for volume changes
• possibly adjustable tremolo filter to make it work only on higher frequencies
• 18V power supplied - was originally 9 but wave generators need higher

Parts Considerations:

Wave generator - Intersil ICL8038 / XR2206
Minimum voltage for both is 10V, so 9V won't cut it. XR2206 gives better precision, but 8038 seems better documented and it's what i did my initial cut figures. Essentially, in order to get this tap defined tempo. There is basically an RC time constant which determines the frequency of oscillation of the wave generator. Obviously tempo calculation is going to be done by microcontroller. So I'm considering using switched capacitors to simulate variable resistors. Typical atmel AVR on say an arduino can work up to e.g. 14MHz. This gives plenty of flexibility for capacitor / resistor sizes. Resistor size cannot be too low or too high as charge current for the wave form generator is best between 10uA to 1mA (problems at 1u and 5m)



Further updates later I hope it's not against the rules to keep updating ongoing projects like this. Maybe someone will find it interesting.

 

[crutschow]

yeah you're right, 0.1Hz is a bit extreme. That's actually the limit of the signal generator sine waves for normal playing you'd probably use 3-5Hz I guess. But still, at those low frequencies i'm not sure how AC coupling with caps would work. Seems safer to do things differentially if solves the problem.

You can couple low frequencies, you just need a large capacitor to keep the -3dB cutoff well below you lowest frequency of interest. You mentioned a slowing changing DC level, well that's the same as low frequency AC.

As you suggested, with a differential input, you can use one input to apply the offset. That eliminates the need for a cap. But you need the offset voltage on only the tremolo input if you want an output with zero offset (so zero signal with any value of tremolo will still give zero out. You mentioned an offset on both inputs and that will give an output offset of Xoffset times Yoffset.

 

[Brevor]

Tremolo

Years ago I worked for a large musical instrument company. We had tremolo circuits, They all used a photocell excited by a small bulb driven by a low frequency sine wave oscillator. These work very well, You don't need analog multipliers and complex circuitry to do this. Maybe try Googling "tremolo circuit" you would probably find several you could use. You most likley would not be able to tell the difference between driving the tremolo circuit with a sine wave or a triangle wave.

 

[crutschow]

Years ago I worked for a large musical instrument company. We had tremolo circuits, They all used a photocell excited by a small bulb driven by a low frequency sine wave oscillator. These work very well, You don't need analog multipliers and complex circuitry to do this.

It likely used a CdS variable resistance type of photocell. That circuit would be somewhat non-linear, but I suppose it doesn't matter for tremolo effects.

I wouldn't consider using a multiplier as requiring a "complex" circuit since the multiplier is just a single IC. It likely requires no more parts, and perhaps fewer, than the photocell circuit when you add in the driver for the bulb and the amp for the photocell.

 

[transcendent12]

photocell is a nice idea which is used even in high end models. Probably to be closer to old school tremolos as well as providing isolation for the output. However, I don't think it would simplify things that much + I'd lose some flexibility on max freq etc. Differential inputs really can make things quite interesting when using switched capacitors from the microcontroller!

totally feature packed tremolo. I will have to ask my brother what he actually wants though. Don't want to make a pedal he can't use because it's got 15 different buttons/ knobs.

 

[transcendent12]

Power supply question. Ideally 9V input jack is used. Please take a look at this data sheet for a switched capacitor charge pump voltage converter.

http://cds.linear.com/docs/Datasheet/1054lfe.pdf

Pages 9 and 10. You'll find basic voltage doubler and also a bipolar voltage doubler. i.e. 9V input -> 18V output or 9V input -> +-18V output.
18V positive is ok, requiring me to AC couple to low frequencies. But i'd prefer to avoid electrolytics and their uni polar requirements (would be slightly forced the wrong way with output guitar signal)

Also on those data sheet pages, there is just a simple voltage inverter. Am i able to create a +- 9V power supply by say, connectiong V+ to +9V from DC jack, GND to 0V on the 9V jack and -9 to the output of voltage inverter?

I hope this isn't too confusing. Feel free to tell me this is a bad idea/ not needed. Thank you!

 

[audioguru]

Why do you need a plus and minus supply? Any opamp works from a single positive supply if its input is biased at half the supply voltage.
But the minimum voltage for an XR2206 function generator is 10V and a single 9V battery drops to 6V.

 

[transcendent12]

Why do you need a plus and minus supply? Any opamp works from a single positive supply if its input is biased at half the supply voltage.
But the minimum voltage for an XR2206 function generator is 10V and a single 9V battery drops to 6V.

I figured if i only had a positive supply voltage my output would be centred at half of the supply voltage? i.e. at 18V it would centre at 9. And yes 10V needed for function generator is one sided power supply minimum. Also circumvented either by voltage doubling to 18V or bi polarising 9V to +-9V the IC8038 function generator i know off the top of my head centres its output at the middle of the supply.

Let me know if my logic is flawed! If not i'll need to know if voltage inversion is sufficient for bipolarisation as described above...

 

[Mr RB]

Lots of tremelos and ring modulators are made using a resistor and a mosfet, with the fet forming the lower half of the voltage divider. The voltage to the fet gate sets the "volume" coming out of the divider.

It's good for high frequencies (ring modulators) and is fine for tremelos too if your control waveform is smooth enough. Generally getting good linearity is not needed or desired.