how to build a feedbacker? (like old df-1 boss pedal)

[mstoppini]

Hi,
many years ago I had a boss distorsion feedbacker pedal.
I remember a weak distorsion sound but the feedback function, when you stomped on the pedal holding it down, were really interesting.
Then I sold it, but I was young a stupid.
I was thinking about recreating the feebacker part of that gizmo, any idea to reproduce it?
I remember someone posted on ampage something about a chip wich could produce a note a number of octave above the note played, a sort of harmonizer, does anybody know what was all about?
thanks,
M.

[Mark Hammer]

The schematic for the DF-2 should be in the schematics archive here.  Yes, the distortion function is nothing spectacular.  

The "feedback" function is essentially a quick and dirty oscillator that locks onto the fundamental frequency of the note being held and provides one and two octaves up (mixable), with a little bit of vibrato for good luck.  

A quick look at the schematic indicates that these "feedback overtones" are always present (i.e., the note tracking circuitry is never "off-duty").  The footswitch system seems to provide a sort of quasi-gating action by feeding a CV envelope (when you hold the pedal down) to the collectors of a pair of NPN transistors (one for each overtone).  

This octave-on-demand arrangement is VERY very similar to what occurs in the MXR Bluebox (now that Francisco Pena's excellent schematic redraw makes it clearer).  In the Bluebox, it is used to reduce sputtering at the end of notes (the envelope has a very fast attack and decay) so that you get as clean an octave *down* as possible.  In the case of the DF-2, the onset of the "feedback" is slowly faded/gated in (and my guess is that C54 partially sets the rate of fade-in) to become audible.  Having one myself, I suspect that holding the footpedal down also "freezes" the sampled pitch so that whatever the pitch detector is now sensing is shut out.

Part of the logic of this system is so that:
a) the "feedback" swells, providing a more effective emulation of the real thing,
b) people will use the fuzz for chords and restrict use of the feedback function for single notes only (the fundamental detector can only handle one fundamental)

On the other hand......

Suppose you wanted to use the effect as a dedicated mono octave-up box.  If the gate was always "on" and the "pitch freeze" disabled (in addition to the built in frequency modulation), wouldn't it provide a tracking 1 & 2 octave-up unit?

I had never really considered this possibility before, but looking at the circuit now, and the E&MM Harmony Generator the other night (hammer.ampage.org), it strikes me that maybe there are some interesting mod possibilities here.  Lift the clipping diodes from their ground connection, keep the pitch extraction/synthesis going full-time, and in theory, anyways, you have a fuzzless octave-up box.


Hmmmmmm......... :twisted:

[Ansil]

i had on eon my site but it is a true feedbacker.   as in it keeps feeding back the signal back on top of its self

[jimbob]

Any other effects out there that can built or purchaced that will feedback a note a few octaves? GnR has that on many songs where the a note is doing this feedbacking and it sounds great to me.

[ExpAnonColin]

I can't find it in the schematics section.  Someone link me

Very interesting, Mark, I had always thought it was some sort of a feedback loop.

-Colin

[idsnowdog]

I wouldn't want any kind of octave thing.  I would think it would be possible to have a regenerative loop type device.  The pedal would split the signal amplify it and add it back to the circuit creating a feedback loop.  The controls could be very simple such as wet/dry, threshold and possibly a presence control to tune the feedback note.  Just enough gain to nudge the amp to feedback naturaly rather than creating a whole new tone.

Snowdog

[Ansil]

I wouldn't want any kind of octave thing.  I would think it would be possible to have a regenerative loop type device.  The pedal would split the signal amplify it and add it back to the circuit creating a feedback loop.  The controls could be very simple such as wet/dry, threshold and possibly a presence control to tune the feedback note.  Just enough gain to nudge the amp to feedback naturaly rather than creating a whole new tone.

Snowdog

again  thats what my above post does. if someone wants to host if drop me an email.

[ExpAnonColin]

Ansil- colin@experimentalistsanonymous.com

Can anyone link me to the DF-2 schem?

-Colin

[Ansil]

VERSION TWO YOU GOT MAIL COLLIN.........    THANKS AGAGIN

[ExpAnonColin]

Courtesy Ansil Gregory

Ewww... a relay

-Colin

[Ansil]

if you dont' put the an external foot switch but rather use a switch on the box itself then you dont' need one.  then all you need is a momentray switch.   or you could use a electronic switch if you need an offboard pedal..  the whole idea when i first started trying to make this was to use a roland sustain type switch cause i had one available to me.  using the relay rated at twice my supply voltage, caused it to take a moment for the relay to kick in and a moment to kick out when i stepped off of it.   i wanted this cause i didnt' want to have a instant squealy i wanted it to work for it.   i will redraw a non relay version for you. and send it out.

[ExpAnonColin]

Courtesy Ansil Gregory

Here's "version 2", sans relay.

-Colin

[Ansil]

thanks again collin let me know what you think if you decide to build it.

[ExpAnonColin]

The LDRs are effected by envelope LEDs, no?

-Colin

[R.G.]

To get a usable feedback pedal, you have to include the vibrating string in the overall feedback loop so that the string controls the frequency of the note you hear.

If you make just an electronic positive feedback loop not including the guitar string, then you will hear whatever frequency the electronics wants to run at, not the note being played.

The Boss pedal used a phase locked loop to track the input signal from the string, then as Mark noted held the frequency.  It's really a frequency sample and hold, not a feedback pedal.

[ExpAnonColin]

To get a usable feedback pedal, you have to include the vibrating string in the overall feedback loop so that the string controls the frequency of the note you hear.

If you make just an electronic positive feedback loop not including the guitar string, then you will hear whatever frequency the electronics wants to run at, not the note being played.

The Boss pedal used a phase locked loop to track the input signal from the string, then as Mark noted held the frequency.  It's really a frequency sample and hold, not a feedback pedal.

Right, when I owned mine I always considered it more of a synthesizer with pitch controlled by the input rather than a feedbacker.  I wasn't sure if Ansil's would work as such.

-Colin

[Mark Hammer]

Yup, let's be perfectly clear: the DF-2 is a feedback emulator.  The way the synthesized overtones swell (and I suppose the actual timbre of the synthesized overtones) sounds sort of like feedback.  The nice thing is that it does not require any particular volume level, either at input or from the speakers, to do this.

To add to RG's point, once it tracks the note and holds it, you can actually play over the droning note, which is a nifty feature.  In that sense, it is very much NOT a feedback-generating device.

I have gotten curious, though, about the prospects of making it the "up" equivalent of a 1/2- octave-down box.

[smoguzbenjamin]

It sounds like fun though

[Doug H]

I remember when we first saw this schem and went through and analyzed it in one of Aron's previous forums. Interesting idea, although I think synthesizing a pseudo-feedback tone for a guitar seems kind of cheesy in some ways. But if, as Mark is thinking, the idea could be exploited to do other things, that is pretty interesting...

Ansil's idea will probably produce a lot of squealing, but not the kind of "guitar feedback" it seems like you want. As R.G. mentioned, you have to get the mechanical string vibration in the loop too. In one of Aron's previous forums, someone was working on a device that used a small speaker that connected to the guitar headstock and induced mechanical feedback and sustain at low volume levels. From what I remember the soundclips were pretty impressive. That ends up being more of a wiring and "human factors" kind of challenge than an electronic one, but it seems like it is certainly doable.

Doug

[R.G.]

I have gotten curious, though, about the prospects of making it the "up" equivalent of a 1/2- octave-down box.

Leave it to you Mark to home in on the really useful feature.

That's the concept that makes the E&MM Harmony Generator useful - selective application!

Consider this: Build a Harmony Generator, with the available +/- octaves, plus major/minor thirds, fourths, and fifths, and all. Mess with the switching to make it hold a unison/octave/third/etc. when the button is pressed, let it go and go back to tracking when you let the button go.

Dang, Mark. How do you do that?