# Tremolo Circuit

## Recommended Posts

So, I'm in an electronics class at my school, and we're building our own projects for second semester. My project idea was to use a 555 chip, a transistor, and an op-amp to make a tremolo effect.

I'll try to explain it as simple as possible for those who arent as in depth into electronics as me or some of the gurus around here.

So first we have a 555 chip, which oscillates (varies the voltage at a steady period) at any given frequency, depending on capacitor/resistor sizes. In this circuit we're working between say 2 and 10 Hz (1 Hz= 1 cycle per second).

Since the volume of a guitars signal is determined by amplitude, and not just the level of voltage, applying the output of the 555 chip would not be sufficient for raising and lowering the volume. So in order to understand how I am going to use it, you must understand how to wire an inverting amplifier using an op-amp.

Thats the schematic for an inverting op-amp. The inverting input (the one with the negative sign on it) is used as an input, through a resistor. R1 in this case would be reffered to as the input resistor. The non-inverting input (with the positive sign) is connected to ground as a reference. The output is connected to the inverting input through a resistor, which would be called the feedback resistor. The beta (gain, or amount of amplification) is determined by the sizes of those resistors. The formula is Ri/Rf=B. In this case we dont actually want to amplify the sound since we are working with line levels, so we would use 2 resistors of the same size (I'm using 1ks, however the size doesn't matter).

So in order to change the amplitude, we have to change the Beta, and in order to do that, we must change the size of one of the resistors. But how do we periodically change a resistors size? We can't, however the voltage applied to an NPN transistor's base will change the bias, or internal resistance between the emiter/collector junction. So by hooking the inverting input to the emitter and the output to the collector, and the 555 output to the base, we are effectively steadily moving the resistance between the inverting input and output up and down.

That is our schematic so far. As to the size of R3, we use atleast 1K, to limit the current flowing through the transistor.

I have spent a ton of time trouble shooting and improving this circuit and its still in the works, I still need to test it using an actual guitar. So far all I've used is a function generator (a peice of equitment that puts out a small AC signal) and an oscilliscope (a peice of testing equitment that allows you to view the actual wave that you are hooked up to) and it seems to be working fine. The wave that the function generator puts out is basically a perfect sine wave, and the output of the op-amp is equally as smooth. Right now I'm doing all this testing on a breadboard, and eventually will transfer it onto a PCB.

I will gradually expand on it and let it grow more and more intricate. I believe the first expansion I will do is sending the output signal through a second inverting amplifier because the output of an inverting amplifier is of course... inverted. So sending it through two would give you an in-phase output. Most of the time in a guitar, it doesnt really matter if the signal is inverted or not, considering you're not mixing the signal with anything, but i want to leave myself open if i ever decide to try some parralel effect tricks or something.

Something I might also do is have a switch to switch between the two outputs of the 555, one of them is a square wave and the other is more triangular, so i can send it through either more sudden volume changes or more subtle ones.

I will also use a potentiometer to determine the frequency of the 555 so i can go slow or fast. And of course, I will add a pot to change the depth also.

Any questions/comments would be appreciated, I will probably post the final schematic, including the 555 pinouts and any switches/potentiometers I use, proper voltages and resistor sizes, etc.

##### Share on other sites

hi, it sounds really interesting, simple and cheap to do, great diy. one idea i see is something i dunno if might be of any practical use or has already been done, but could be interesting.

once you have made up all the things to do with the operational and the signal, you could try use something more complicated than just the two outputs from the 555 to controll the level. instead of doing a regular sine, square or triangle wave you could do with something more ellaborated using two 555 or other circuits. i did once a question about this and was introduced into function generation with logical gates. sounded interesting and could be for you, iÂ´ll try to find the link, just in case you wanna have a look. i was trying to do exactly the same as you.

wasn't hard to find, i haven't done that many posts ... hope it is of any use. bye

Edited by cr_XD
##### Share on other sites

My "advice" would be to try it with a guitar before you go any further. A guitar chord is way more complex than a simple sine wave. My concern would be how well it would track the guitar. I haven't seen anyone use a 555 for a tremolo, but it sounds interesting. I hope it works for you!

##### Share on other sites

yeah you could do some intricate stuff with it if you got into binary counters and logic gates, i technically could get into that, because ive built a circuit for timers, and counters, that incorporated the counters and gates, but i have to get this project done by friday, working at most about an hour and 15 minutes a day on it. That means debug it all, expand it to my satisfaction, and transfer it to PCB in about 5 hours total. By then I should be able to incorporate the switch to toggle between square/triangle waves, and get the signal back in phase, and incorporate the potentiometers to adjust it all, but I don't think I will be able to add a whole new chip, let alone two or three.

But nevertheless I would be interested in reading up on it. Shoot that link my way.

Edit:

Sorry Paul, I was in the middle of my post when u posted. Yeah, I plan on bringing my guitar into school on wednsday to test it, but it should work the same. I don't really know much about a guitars signal, but as long as its an AC signal, at line level, centered around ground, then shouldnt the concept carry on and have the same effect?

The thing I'm worried about is my amp doesnt have an effect loop, and I don't really want to risk messing up my amp by modding it to put one in. So when im using it with my amps distortion, you won't be able to hear the tremolos effect so much as a change in the distortion level, unless you make the depth huge. Does anyone have a solution for this?

The reason I used a 555 was at first I was going to do an extremely simple and sketchy version of a tremolo effect using a relay switch, and running the signals in paralel to affect depth, it was horrible, and with some thought I came up with the idea of changing the feedback resistors value using a transistor. That allows me to get much more diversity out of it, and its solid state rather than mechanical. I also saw a tremolo schematic on another website (i think it was small bear electronics) that used a 555, but I didn't just use that schematic, because I'd rather get some experience coming up with the ideas on my own, rather than just copying someone elses ideas.

Once I test it using a guitar I will let you guys know how it turns out. Thanks for the comments.

Edited by silvertonessuckbutigotone
##### Share on other sites

uh you know i did this with a 555, and a led/ldr and made a nice vibrato. best of luck.

##### Share on other sites

Just a suggestion - it might be easier to simply vary the input amplitude with your variable resistor (in other words, use the transistor as the grounded leg of a voltage divider on the opamp's input). Probably wouldn't hurt to buffer the input as well, but it's certainly not necessary.

Also, FYI, a JFET is somewhat easier to implement as a voltage controlled resistor than a bipolar transistor, IMO - this page may help (it's a little math-heavy, but not too complex). Most classic pedals use this shunt to ground method, some without any active gain makeup whatsoever. As always, YMMV.

##### Share on other sites

uh you know i did this with a 555, and a led/ldr and made a nice vibrato. best of luck.

Now that sounds like it could work with an LDR/LED combo. I'm not sure about routing complex guitar signals straight thru a 555, though.

##### Share on other sites

Paul, I think he's simply using the 555 as a LFO to control the virtual resistance of a bipolar in an opamp's feedback loop (basically a simple VCA), and not applying the guitar signal directly to the timer, a la John Hollis' Crash Sync.

Silvertone, keep in mind that the key to classic tremolo is that it's a periodic attenuator, that is, at maximum depth, it should lower the signal level significantly (at least 20dB, IMO) at the bottom of its sweep, and never boost much above unity at the top - that should eliminate some of the problems you've had using it in front of a distorted amp. BTW, this can be done a lot more simply with discrete components (see the modded EA tremolo at Runoffgroove.com, or transmogrifox's rEAgeneratedTremolo for more ideas), but use whatever you have on hand. Obviously, this is just my opinion, based on what little I know about classic tremolo circuits, so take all this with a block of salt - as always, YMMV.

##### Share on other sites

Paul, I think he's simply using the 555 as a LFO to control the virtual resistance of a bipolar in an opamp's feedback loop (basically a simple VCA), and not applying the guitar signal directly to the timer, a la John Hollis' Crash Sync.

Ah, I see.

keep in mind that the key to classic tremolo is that it's a periodic attenuator, that is, at maximum depth, it should lower the signal level significantly (at least 20dB, IMO) at the bottom of its sweep, and never boost much above unity at the top

That's part of what is confusing me. A 555 is a hard turn-off/turn-on device, isn't it? If so, wouldn't that make for a very rough, choppy tremolo effect? I know that would appeal to some who are looking for that type of sound, but I'm not into it much.

this can be done a lot more simply with discrete components (see the modded EA tremolo at Runoffgroove.com, or transmogrifox's rEAgeneratedTremolo for more ideas

Yeah, the EA Tremolo is a good one. Simple and it sounds very much like the classic Fender tremolo.

##### Share on other sites

A 555 is a hard turn-off/turn-on device, isn't it? If so, wouldn't that make for a very rough, choppy tremolo effect? I know that would appeal to some who are looking for that type of sound, but I'm not into it much.

There is actually an uncommonly used output on pin 6 or 2 (in free running astable mode, 6 and 2 are connected together) that is a triangular wave. Thats why I will be using a switch to control the shape of the wave. I think I will get use out of both shapes equally.

LK, what do you mean use the transistor as the groudned leg of the voltage divider? Could you draw up a schematic so I get what your saying.

##### Share on other sites

...what do you mean use the transistor as the grounded leg of the voltage divider?

Something like this:

Or you could use an opto coupler like Ansil recommended, or an OTA chip like the 13600, or even a dedicated VCA chip - there's always more than one way to skin that proverbial cat. Each solution is going to give you a slightly different sound, so it's all about what you want, what you have, and what you're willing to do to get the sound you want. Personally, I prefer simple and clean, but if you can find something that nails that old funky cheap tube amp bias wiggler sound, I'm gonna need at least two of 'em as soon as you go into production.

Oh, and just for fun, a pair of back-to-back diodes in an opamp feedback loop will logarithmically reshape your triangle wave into a fairly decent imitation of a sine wave. Is it worth the trouble? Try it, if you get a chance, and let us know.

##### Share on other sites

so for some reason the transistor method stopped working, and i couldnt quite get it back to what i wanted, i might have accidentally wired it wrong in the first place and gotten the outcome desired. So, on a whim i decided to try the LED/LDR combo instead and in fact it worked and i was planning on testing my circuit anyway so i had my guitar and amp already there. it works great and i'll probably finish the dessign process tommorrow and then start soldering it up. I'll post some sound clips as soon as i get it together in a box etc. I'll also make a schematic for you guys eventually.

Edited by silvertonessuckbutigotone
##### Share on other sites

ilike the 555 with a led ldr combo. i was able to get it to do some fun things, and i only like a few tremolos anyway, but this was the closest thing i could get to ahhhh shoot what was that really famous trem everyone goes on about. anyway i like it a little better but i like mine for anything i would use trem for. i think the 555 is an interesting chip. lol

but on the same hand i am insane

##### Share on other sites

A schematic of what you did would be interesting to see.

I know very little about using the 555 for circuits like a tremolo. My comments were based more on a "guitar-synth-in-a-box" thing I was designing a few years ago. I had trouble with it tracking the notes equally on all of the strings anywhere on the neck, and I was thinking that the 555 might respond in a similar fashion. This discussion does give me an idea for possibly completing that project, though...

##### Share on other sites

ansil did you find a way to change the depth in the LED LDR circuit? I've got mine to where it basically turns the volume completely off but for some reason i cant figure out how to set it to a specific depth. The way i have it hooked up is in the inverting amp i have the photoresistor in parralel with the feedback resistor.

##### Share on other sites

Simply putting a variable resistance in series with the LED would probably give you a depth control by varying the LED's drive current. If that doesn't work, try using a volume pot setup to drive the LED (pot wired top to 555, wiper to LED, bottom to ground). I still think wiring the photoresistor as a ground shunt in front of a fixed gain stage is going to be quieter and more well-behaved, but that's just me.

##### Share on other sites

actually lk thats what i did, and you are right.

##### Share on other sites

ok so i got everything working and all. however, there is a clicking/popping noise when the 555 oscillates. I know the popping is coming from the 555 circuitry and not the op amp circuitry because when i unplug the LED and just have the 555 simply oscillating into nothing, he noise still is there. electronically the only thing connecting the audio circuitry and the 555 circuitry is the power supply, I have a feeling its noise from the flip flop of the 555 or the transistor turning on and off. I cant figure out how to get rid of it though. This is how I have the 555 set up.

For Ra I'm using a 1K, and for Rb I'm using a 100K Pot. For C I'm using 10uF. I'm using pin 3 for the output. If anyone has ideas, please let me know. Thank you, your help is appreciated.

##### Share on other sites

You're probably getting the pop coupled through the power supply, that is, the current demand from the timer chip is sucking too much out of the opamp power supply and generating a transient - using a 7555 or one of the other CMOS timer chips will help, and so will isolating the power supply lines (so the 555 power leads and the audio circuit power leads meet only where the battery hooks into the board), and bypassing both the battery itself and the opamp power pins with caps is always a good idea. Without seeing the rest of your circuit, it's difficult to be more specific - can you draw up a complete schematic, or at least a schemo of the other half of it?

##### Share on other sites

Hey, just remembered this - Laurier Gendron has a page on tremolo and vibrato that has a 555/LED/LDR tremolo on it - take a look:

It uses the pulse output, and includes a pulse width adjustment option.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
• Home