Harmonic spectrum of Harmonic Percolator

[e.dunajevas]

Hey, there are multiple sources online that mention that Harmonic Percolator circuit suppresses odd-order harmonics. Is that really the case?

I did some LTspice simulations on basic 100Hz sine wave (see circuit diagram and green frequency spectrum chart), and saw that that actually odd harmonics are more prevalent. Am I missing something here? I also experimented with different frequencies and results are similar.

Also for the reference I have measured the same thing in DAW with HP-2 pedal (chart with orange line), and I see the same thing, odd harmonics are higher than the even ones.

[Mark Hammer]

I've made three so far and the distinctive quality is that there is a certain "bloom" to the harmonic content.  In other words, it changes as the note/chord is sustained.  Not sure that an LTspice simulation would capture that.

[bmsiddall]

C1 and R4 values don't line up with the Giblet schematic.

[Vivek]

I've made three so far and the distinctive quality is that there is a certain "bloom" to the harmonic content.  In other words, it changes as the note/chord is sustained.  Not sure that an LTspice simulation would capture that.

It is possible to study time varying things like this, if one takes the trouble to set up properly.

One way could be to feed input of 1000mv, 500mv, 250mv, 125 mv

And do a FFT for each

And superimpose the graphs


Another way would be to use a behavioral voltage as input, set it up to gradually vary from 1000vm to 10mv

And then specify time intervals for which you want the FTT.

Sometimes, LT Spice cannot generate final graph by itself, but can generate data tables, which can be better graphed with XL

I did an analysis of Frequency content of clipped sine wave for different levels of clipping using the third method. It is equivalent to a graph showing how the harmonic content changes as a note dies

[Mark Hammer]

Yes and no.  Remember that what changes over time is not JUST the signal amplitude, but the frequency content as well.  Moreover, the pitch of the note fundamental/s is not stable, since we tend to bend them a little when picking hard.

[Digital Larry]

This is one of the few circuits I'm motivated to build.

If your goal is to suppress odd ordered harmonics, build an active rectifier.  Full wave also known as "octave fuzz".

[e.dunajevas]

Great suggestion Vivek, thanks!

I did a simulation from 10mV to 1V input amplitudes and created this chart.




Probably it explains (in some way) the "bloominess", as note amplitude decays (going from right to left in the chart), even harmonics are actually emphasized. So I guess this could be the explanation I was looking for.

[pinkjimiphoton]

its all about that freekin big cap on the e of the two q's.
that's where the "percolation" happens.

i remember one time i built one of these, and was hot swapping q's... and it would still pass signal til the cap discharged. weird circuit!

[iainpunk]

i strongly believe that the whole ''odd and even harmonics'' is a load of crap.

the wave of a guitars signal, even when strumming a single string, is not symmetrical anyway, so there will be a mix of even and odd harmonics to begin with. even when a magical string that moves in an exact sine wave is picked up by a normal pickup, the output signal will contain harmonics because the pickup measures change in proximity and this proximity is not a linear measurement.

then there's Bias shift through asymmetry of the gain stage caused by the nonlinearity of transistors and the feedback biassing used here, that also comes in to play. this is a time-dependent effect, as you are charging or discharging the input capacitor of the gain stage, this partially accounts for the ''bloom'' of notes as well.

cheers

[Mark Hammer]

Yeah, I don't particularly care which harmonics are produced when.  What I like about the Percolator is that they change.  It's not exactly the same as, say, pulse-width modulation of a synth oscillator, but it yields a similarly pleasing "animation" to the tone when they DO change.

[pinkjimiphoton]

its all mojo.... don't you guys know that? lol

[ElectricDruid]

i strongly believe that the whole ''odd and even harmonics'' is a load of crap.

I don't think it's *entirely* a load of crap, but I certainly agree that it's well over-played. While it *might* be true if guitars were a nice, pure signal source, in reality they're nothing like that, as you pointed out, and consequently, the whole thing about odd and even harmonics in any subsequent distortion stages is somewhat a moot point.

The *theory* is pretty clear though. In synth-world, this is called "waveshaping" and the shape of the function determines the output harmonics. Waveshaping theory also gives us some clues towards waya that changing volume levels can produce different harmonic mixtures.

HTH

[Rob Strand]

Quote from: iainpunk on Today at 11:09:19 AM

    i strongly believe that the whole ''odd and even harmonics'' is a load of crap.


I don't think it's *entirely* a load of crap, but I certainly agree that it's well over-played.

I think there is a large degree of crap to it.

If you set-up this experiment,
           
           /-->   invert -->   distorter -->  invert ---\
Input ---                                                         Mixer  ---> (x 1/2 optional) --> out
           \------------->   distorter  ------------- /

It maintains the general character of the distorter but there are *no* even harmonics.

Where the even harmonicness comes in is the nature of the intermodulation.

[mac]

its all about that freekin big cap on the e of the two q's.
that's where the "percolation" happens.

Jimi, I agree.
Try this. It's a pita to set it right, got 8up with two germs 

Q2e -- 10k pot -- n/c
          | 
          | -- Q1e
          |
         22uf -- GND

There is a schem in my gallery but it's down right now... HTTP ERROR 500

mac