Information on oscillators (used in some tremolos)

[Tombaxter97]

Hi guys,

Searched about a bit on some information on the oscillator and what actually makes the thing work for a trem like Schaller Hearthrob.

Wondering whether anybody has any links to articles or references to books or anything with the mechanism driving this?

[armdnrdy]

The magical mechanism is called an LFO. (Low frequency oscillator)

Look to chorus, phaser, and flanger circuits.

An LFO generally involves one or more op amps...although an oscillator can be produced with CMOS ICs as well.

[Tombaxter97]

ah cool thanks.

[GibsonGM]

Hi Tom,

I'd do some reading, some looking at schematics...check out questions others have asked....look up "EA Tremolo"....."Easyvibe"....

Then, you're sure to have some really interesting and specific questions!  So come back and ask, and we'll do our best to answer them!

A quick intro:  In a typical use for a LFO, there will be a driven element like a small light bulb or LED...this of course goes bright > dim >bright > dim with the voltage the oscillator outputs.    This is used to control the resistance of an LDR (light dependent resistor) which can be used to vary that gain/volume of a circuit stage (tremolo), or to shift the frequency of various filter stages (phase, flange, univibe etc...).   You are just using the LFO to make a 'remote controlled' resistor, that does some useful work... 

[Scruffie]

IIRC the Heartthrob is a phase shift oscillator.

[armdnrdy]

A quick intro:  In a typical use for a LFO, there will be a driven element like a small light bulb or LED...

Let's not forget an LFO driving a JFET as in the aforementioned EA Tremolo....or multiple JFETs as in most of the popular phaser circuits

It looks like the Heartthrob utilizes a NPN transistor based oscillator.

http://www.diystompboxes.com/smfforum/index.php?topic=71288.0

[PRR]

> oscillator and what actually makes the thing work

Mike, amp, speaker.

Turn-up. Mike signal is amplified out the speaker, gets back to the mike, gets amplified again, and again, and again, until it howls as loud as it can.

The mike/speaker affair is familiar to everybody here, but you can do it with just an amplifier. If you have ever built a HIGH gain pedal, and it won't stop squealing, you've invented an oscillator.

Memorize:

* Amplifiers oscillate
* Oscillators don't

Amplifiers don't always oscillate, as we know. To howl we need gain and we need a phase flip.

OK, howling PA and squealing pedals are not what you want for trem. We "tune" an oscillator by slugging it down with caps, "cutting highs" so it can only howl sub-bass.

As Scruffie says: "phase shift oscillator" is your search-phrase. There is extensive literature on analysis and design. TIP: plagiarize, don't re-invent. Any possible PSO can be stolen from existing plans. If you don't like the frequency, change the caps (not the resistors).

[Tombaxter97]

Great, now I now what to actually narrow my learning onto.

Thanks a lot.

[Tombaxter97]

Great, now I know what to actually narrow my learning onto.

Thanks a lot.

[bool]

Wondering whether anybody has any links to articles or references to books or anything with the mechanism driving this?

It's chicks. A deeply seated urge to impress chicks. To make their chemicals ebb and flow to the rhythm of your LFO.

[poiureza]

Amplifiers don't always oscillate, as we know. To howl we need gain and we need a phase flip.

You sure it's not the opposite ? 
If you have phase flip you actually cancel the whole feedback thing when it gets back at the input after a loop, no ?

[PRR]

> You sure it's not the opposite ? 

OK. Simple 1-stage amplifiers already flip phase, so another phase flip is needed for positive feedback.

You are correct that the general case includes non-inverting amplifiers which do not need another flip.