My New Fuzz - the Starfuzz

[tcobretti]

I was dorking around and came up with this.  It is a very nasty octave up pedal, where the octave itself is somewhat subtle, but the fuzz is out of control.  This was a first draft of the pedal, but it seemed like all my substitutions just made it sound worse. I didn't try it, but I suspect matching the Hfes of the NPN/PNP pair will help bring out the octave.

It reminds me a lot of the Superfuzz.  If you like obnoxious fuzz, you should check this out.  It will take you two minutes to put it on a breadboard.

[John Lyons]

Looks interesting Travis. I'll have to bread board this up and report back.
Maybe a resistance balance to ground would help Q2-3 bring the octave goodies (short of matching HFE)

John

[tcobretti]

That's an interesting idea.  Maybe connect the wiper of a small value trim pot to ground (<1k), and the connect 1 & 3 to the emitters of the transistors.  Then tune for max octave.

Good idea!

[John Lyons]

That's what I was thinking as well. I was thinking of the super fuzz but that one uses the base resistance to a balance pot to ground.
I guess it depends on the mismatch in HFEs. Might be better to match...

By the way, what does the PNP/NPN thing really do compared to  two NPNs in the same configuration?

John

[brett]

Hi
the idea of a PNP-NPN "pair" is an interesting one.  There is some magic in there for sure.
A few years ago, Alfonso Hermida did some SPICE simulations of a NPN-PNP pair in a circuit that was called "the tubey".  I thought he posted the output on the forum, but I couldn't find them with the search function. 

I wonder if anyone has a schematic of the "tubey", or maybe its predecessor, which was an ASCII schematic of just the pair ?
cheers

[Gus]

http://www.geocities.com/tpe123/folkurban/fuzz/snippets.html

pushme pullyou

[brett]

Hi
this is the schematic I was thinking of.
A push-pull pair with AC bypass so that the gain of the NPN device can be raised.


Alf's modelling of the output with rising sine wave input is shown below.  There are a few interesting features:
The "double hump" will produce the octave tones. 
The graph also indicates why it would be a wild fuzz with a large input signal - the harmonic overshoot or "horns" with large input are usually an indication of nasty fuzz.
Like the Bazz Fuss and a few other "synth-like" fuzzes, the positive and negative parts of the signal are not mirrors of each other, especially for the large input signal (the +ve excursions last longer than the -ve excursions)



cheers

[tcobretti]

Gus, I can't believe I never saw that schem.  It's almost the same pedal!  Weird.  I handled the NPN/PNP pair a bit differently, though.  I'm sure his sounds better; Tim is the man.  I would imagine that mine is a bit crazier, though.  I think the lower hfe Q1 will tame the circuit a bit and probly help bring out the octave, as will the smaller caps to let less signal thru.  I tried a gain pot like he has in my design, but didn't like it as much.  However, it sounded very different from what I ended up with so might be fun to try if somebody's breadboarding the circuit.

Brett, that is interesting stuff.  When I first stumbled on the circuit, that was not too far from what I had.  I figured a simple boost stage at the front would make the basic circuit more interesting.  The two must sound very different, because one would have to be on massive amounts of crack to describe the Starfuzz as 'tubey'.

[brett]

RE:gain

I should have  mentioned that the "complete" tubey had an inverting op-amp in front of the schematic above (variable gain from 2 to 22).  That's a heck of a lot less than the gain of 200+ in the front end of your Starfuzz.  Regardless of the gain, you and Tim obviously have a much better approach with a single transistor.

PS Should there be a cap between the collector of Q1 and the pot in your schematic?

[mac]

Hi
the idea of a PNP-NPN "pair" is an interesting one.  There is some magic in there for sure.
A few years ago, Alfonso Hermida did some SPICE simulations of a NPN-PNP pair in a circuit that was called "the tubey".  I thought he posted the output on the forum, but I couldn't find them with the search function. 
I wonder if anyone has a schematic of the "tubey", or maybe its predecessor, which was an ASCII schematic of just the pair ?
cheers

Are you talking about Arsenio Novos's design?


From: Arsenio.Novo@mba.org (Arsenio Novo)
Date: 04 Jul 95 23:04:50
Newsgroups: rec.music.makers.guitar
Subject: New Overdrive Circuit
Organization: MtlNet (MBA.org) MBA [514-465-8524] Brossard, QC

Hi,

I've noted that talk on this echo always comes back to the subject of
overdrive distortion.  Whether generated by a vacuum tube amplifier or
a transistor amplifier there seems to be undeniable differences to me
as well.

Lately, I was tinkering with an unusual transistor circuit
configuration I had come upon a few years ago and made a few
modifications to the circuit that turned it into one beautiful
screaming "tube-like" overdrive but without the wall of noise these
things usually make.  When pushed it even makes that distintive
"zoo-zoo" sound...!!!

The original circuit was simply a complementary matched pair of
transistors connected so that all the terminals overlapped.  i.e. both
bases tied together, emitter of the PNP connected to the collector of
the NPN, and the collector of the PNP connected to the emitter of the
NPN.  Thus:
                          ___________o
               2N3906    |      |
                    b  |< e   |/ c
                   o---|------|    2N3904
                       |\ c   |> e
                         |______|____o


This transistor pair is then biased by 2 equal value resistors in each
of the compound legs, one to the positive supply and the other to
ground common.  The signal is coupled to the base pair leg and the
output is picked off either of the other 2 legs.

The result is that the above circuit exhibits the behaviour of a
multiplier over a range of signal values.  It basically performs a
sin function: in other words a frequency doubler.

This doesn't have a very good distortion sound though because it is
rather "burpy and buzzy".  However, lately I was toying with the
circuit when I offhandedly decided to try doing something to it just
to see what would result.

After adding a large cap from the NPN's emitter to ground the thing
went wild on me...  WANGO ZE TANGO!  SUPREMO DISTORTION!  I then
proceeded to refine the circuit a little more and got a better
understanding of what it was doing.

The final schematic follows but first a couple of notes on the
circuit.  The "bias balance" trimmer should be adjusted for a
symetrical clipping threshold of the output signal as viewed on a
scope.  Short of this it can be easily set by "ear" for the most
sensitivity somewhere around mid-turn.

The input should be driven by a lo-z stage if your electric guitar
doesn't have a built-in pre-amp.  You can alter the emitter capacitor
value in a range from 0.1uF to 1uF in order to obtain various basement
characteristics but I found the indicated value is a good compromise.
The input capacitor should not vary much either though because if it
is made too large the circuit goes balistic and cuts out on the tutti.


"Tube-sound" Distortion Overdrive Circuit:

              _____________________________o + 9volt battery
              |                      |
              < bias balance         <
        20K   < <--.  set            < 10K
      trimmer <    |                 <
             _|_   < 220K            |    0.1uF
             ///   <      ___________o----||----o output
                   <     |      |   c
          0.0047uF | b |< e   |/ c 2N3904
input   o----||---o---|------|
                       |\ c   |> e
                         |______|____o-----
                   2N3906            |     |
                                     <    ---  0.22uF
                                 10K <    ---
NB:  2N3906's gain matched           <     |
      with 2N3904's                  |------
                                    _|_
                                    /// gnd common

The operation of the circuit more closely resembles a vacuum tube than
a diode clipper does because of the strong square law characteristic.
This is due to the negative feedback around both base-emitter pairs.
This feedback accentuates the junction non-linear behaviour manyfold.

Thus each transitor drives the other even harder so that the transfer
curve ends up more logarithmic than is typical of a single transistor.
In other words: the clipping is gradual and not abrupt like it is in
the case of a silicon diode.  Typically a lot more 2nd harmonic is
produced as well.  As a bonus the waveform folds over on itself when
the circuit is overdriven!

Now in the interest of the common good I donate this design to the
public domain for personal use but retain copyright and reserve all
rights for any commercial purpose.  In other words build one for you
and your friend but if you have it massed produced for profit I only
ask a fair share.

Please, do try the circuit and leave any comments in private at my
e-mail address:  arsenio.novo@mba.org

Arsenio
arsenio.novo@mba.org


mac

[John Lyons]

Travis
Here's what I get with a strat and a twin reverb.
Starfuzz Clip

Needless to say, hard gating!

What Are your voltages?
I had to sub in a 2k resistor (or a 160K trimmer from base to ground) to get 4.5v on Q1. I used a 2N5089 as that was handy.
With 10K as per the schematic I got 1.2 volts
I have 1.3v on the NPN/PNP pair collectors with 10K.

John

[Dragonfly]

That's an interesting idea.  Maybe connect the wiper of a small value trim pot to ground (<1k), and the connect 1 & 3 to the emitters of the transistors.  Then tune for max octave.

Good idea!

I did something like this to an octafuzz i worked up a while back with martymart...i'll see if i can find the schematic..it used to be in my gallery, but isnt now....

[joegagan]

woah, that sounds really cool john. nice one,  ,mr. brown!  low parts count . i love it

[ambulancevoice]

wowow, that sounds real real horrorshow i must say!
i gotta build this, after my mondo fuzz

[tcobretti]

Brett - You are right about the cap, I'll fix that on the schem.  It should be another 4.7u.  Thanks!

Mac - that is really cool.  Has anybody breadboarded that to see how it sounds?

John - the voltages I measured were  1.6 - 0.6 - 0 on Q1 and 1.3 - 0.6 - 0 on Q2/3.  If you audio probe he first stage it is pretty dirty, almost an OD.  The misbiased first stage pushes a loud but dirty signal to the weird phase-splitting second stage.  I'm pretty sure the gating comes from the second stage.  It does gate, but it gates in a manageable way IMO.  You get a three or four seconds or sustain on single notes before it gates; power chords ring forever.  I stuck in a 5089 I had and it sounded much the same to me as the MPSA18.  If memory serves, they have similar characteristics. 

Joe - Thanks!

AV - Thanks, also!  Please let me know how you like the Mondo.

[mountainking]

You can probably get rid of or at least reduce the gating by making the second 10k resistor bigger. Maybe sub it for a 100k trimmer and then adjust.

[tcobretti]

I started with a 100k trimpot on Q2/3s collector and it made very little difference.  I don't think those guys are even acting like transistors anymore; I suspect much of what's happening has to do with the diodes in the transistors.  So to speak.  I could certainly be wrong.

I need to try no resistor on Q2/3s collector!

[Barcode80]

not to hijack, but per the post above with the Arsenio Novo circuit, here is a layout if anyone is interested. unverified.

[John Lyons]

I started with a 100k trimpot on Q2/3s collector and it made very little difference.  I don't think those guys are even acting like transistors anymore; I suspect much of what's happening has to do with the diodes in the transistors.  So to speak.  I could certainly be wrong.

I need to try no resistor on Q2/3s collector!

Yeah, something is odd here. I actually smoked a 50K pot trying to bias the Q2/3 stage. The voltage does not really change until you get very little resistance on that collector resistor, then poof and a flash of smelly burning pot (entiometer ...ha ha).

I'll tweak some more on this. Adding a mid cut a la Super Fuzz is in the cards as well for sure.

John

[tcobretti]

You smoked a pot?  (insert marijuana joke here)  I had a 100k that didn't melt, so I am surprised by that. I guess the current moving thru those two transistors must get pretty high!

I was thinking that a warp pot might be interesting, and possibly the two resistor, two cap tone stack on the end of many ROG emulators.  Although the second may make it less fuzzy, which is bad.