How do feedbackers work?

[dschwartz]

Hi all.. i´m intrigued with how a feedbacker works, like the line 6 "Dr. distorto"..
Is not a compressor, nor a sustainer, not a physical/magnetic device..the pedal gives feedback (or the illussion of it) and allows to mantain sustained notes forever..

Any ideas, thougts, knowledge about this effect?

[stm]

Hola Daniel!

Check Boss DF-2 (distortion and feedbacker) pedal here: http://www.freeinfosociety.com/electronics/schematics/audio/bossdf2.pdf

Principle of operation is simple:

1) Lock a PLL oscillator to the note you want to extend (during 1 second or so)
2) Freeze the VCO control voltage (now the original note can die away but you already got an oscillator running at its frequency)
3) Blend the locked note gradually into the output
4) For extra bells and whistles you can add some slight amplitude modulation on top of the synthetic note

I played a DF-2 a couple of times around 1987-88 and it sounded OK. Modulation on top of the overtone added more realism to the "feedback simulation".

[JasonG]

Man that is smart.  Could you disable the distortion and just have an " auto tune" oscilator ? be*inger makes a clone.

[dschwartz]

WOW gracias Seba!

What kind of modulation do you mean? i didnt get that part...
now checking out the schem..thanks!!!!!

viva shile mierrrr

[stm]

For our luck the different sections of the DF-2 schem are properly identified, which is of great help to understand what's going on.

I rotated the schem so it is in the "normal" or horizontal page orientation.

On the top-center there is a FUNDAMENTAL EXTRACTOR, which you may already recognize from Boss OC-2 octave pedal.  Its purpose is to obtain a squarewave that will feed the PLL chip (IC7).

Notice just below the Fund. Ext. section you have the ENVELOPE GENERATOR, which just produces a voltage that rises slowly when the frequency is being acquired. This voltage determines the amplitude or volume of the feedback signal.

On the top-right, you have an LFO.  Its output slightly modulates the VCO control voltage by means of R62 (1 Mohm).  This is the modulation I referred to.  It is a slight frequency modulation around the captured frequency.  I made a mistake before when I mentioned it was an amplitude modulation since I didn't study the schem before speaking my mind.

For those who aren't familiar with this pedal, the footswitch operates in a slightly different manner.

Pressing the pedal briefly would change from distortion to bypass and vice-versa (like doing a "single-click" with a mouse).  Pressing and holding the pedal (comparable to a mouse "click-and-drag" operation) initiates the frequency capture sequence and then as the amplitude of the VCO output grows at the collector of Q11, you'll start hearing the simulated feedback (which will be slightly modulated by the LFO).  Finally, releasing the pedal restored normal operation (either distortion or bypass, depending on the current mode previous to pressing the pedal).  The feedback worked even when the pedal was in bypass, IIRC.

Looks like a nice project if it could be simplified.  Imagine a stripped-down version containing only the feedback simulation circuitry.

Cheers.

[JFX09]

Hey, I have this DF-2 pedal, it's actually the only one I've ever owned untill recently 

Let me know if you want a gut shot or any info about parts and such.

jf

[dschwartz]

what happen if you change the note youre playing while feedbacking?

it get confused or something?..

[Mark Hammer]

I own a DF-2 and Dr. Distorto.  The Distorto is digital so I won't bother venturing a guess about its algorithms.  As you might expect, it is more reliable in its tracking than the DF-2, and also more flexible in its controls.

As Sebastian notes, the DF-2 locks onto a note when the foot pedal is held down.  That's why the schematic shows a considerably more complex switching circuit than what you normally see in Boss pedals.  Typically, each momentary press reverse the state of the switch.  Here, a brief tap does something different than holding it.  One of the implications is that if the circuit has to wait to figure out if you just tapped or if you're holding it  down, then it needs to wait in order to start doing the feedbacker function.

In turn, there are two implications of that.  One is that the feedback function sneaks in gradually.  Of course, so does real feedback so that's not a flaw.  The implication is that because it takes a little while to lock onto the note and fade in, it is not going to follow your playing that closely.  Indeed, you can use the feedback tone as drone to play over if you want, which is kind of cool.

All feedbackers are monophonic.  In the case of things like analog octave-down units, being mono-only is a sort of weakness/flaw and lack of capability.  In the case of feedbackers, it's because you normally don't experience polyphonic feedback since feedback is, by definition, highly resonant.  So, whether analog or digital feedback simulation WILL be monophonic.

[JFX09]

Used sparingly and not held for too long it's not bad. Having some mean of sweeping the overtone knob would be great. It's a bit tricky anyway, cause if you miss it can sound kind of daft. But that can be good too I guess 

[dschwartz]

hmm..i saw some fd2 demos and..well, is not what i want..

maybe i´ll try to make that fernandes sustainer project..but i LOVE the sound of my PAF PRO on the neck

[nooneknows]

Hey, I have this DF-2 pedal, it's actually the only one I've ever owned untill recently

I have a DF-2 too! it was the first dist pedal I had back in the '80s, then it was stolen in unlucky day in 1990. Two years ago I bought it again on ebay, when I saw it again I couldn't resist: it doesn't sound that good as I remebered during my school days but, it was the first...

I was thinking about modding it, using the feedbacker part only, a clean unity gain with the oscillator in: has anyone tried this?

[Mark Hammer]

The DF-2 *is* a fairly bland pedal as a distortion.  But there are all sorts of things you can do to sizzle it up without negatively affecting the feedbacker aspect.  The topography of the distortion section is really not all that different from a DS-1, so if you can make a DS-1 sound more to your liking, you can do the same to a DF-2.

[Processaurus]

Does the DF-2, when you hold the switch down and it starts with the fake feedback, does it do a sample and hold type thing where it just plays the note you were playing back when you first pressed the switch down, or does it continue to track your playing?

A simplified version could be doable with true bypass and a second switch to trigger the "feedback".  There are probably some really interesting mods that would be possible in a bigger box with some more room, since it isn't really a feedbacker, but a funky synthesizer.

[Mark Hammer]

Mine is buried under a pile of crap in a garage that is a little too cold to sift through right now, but if memory serves it locks onto a note that you hold for long enough when the treadle is held down.  If I remember correctly, you can actually use it as a drone note to play over.  Keep in mind the pitch tracking circuit has to be "told" that "I want you to listen carefully right now and do what I'm doing".  That "instruction" comes from holding the treadle down.  Unless you lift the treadle and then hold it down again, the feedbacker circuitry will not look for another note to lock onto.  In that respect it is a bit like the Slow Gear and similarly "dumb" devices that are unable to recognize that you are now undertaking a different musical event unless you allow them to reset and make it painfully clear to them.

[dan5150]

I recently picked up a DF-2 on ebay. Mainly becuase I remember playing one in a store way back in the 80's and have always wanted one.
I agree with Mark in that the distortion itself isn't great. (I plan on modding it to sound more like my keeley DS-1, which I really like).

When the feedback function is engaged, and you continue to play over the "dronning" note, the sound quality is bit more fuzzy and "lo-fi". I am currious if it will stil act that way after I mod it for better distortion tone or not.

-Dan-

[km-r]

now i someone could just separate the feedbacker circuit from the dist and switching circuit of the df-2 id build one as a stand-alone feedbacker... 

or someone could lend me their df-2's  kidding!