Bosstone magic. Low input impedance - like the fuzzface and fuzzrite.

[brett]

Hi.
After Paul M's recent Bosstone build and report, I've been looking over the circuit, trying to see the source of the magic.  One thing is the low output impedance, which is a feature of several famous pedals, including the Tubescreamer and Rangemester.  That's probably important for some valve amps, but not always.

Interestingly, the Bosstone has two large bias resistors in series (1M and 220k).  What are they doing, I asked myself?  Like the Fuzzface Q1, there is no feedback resistor on the emitter, so it seems that the base current needs to be very low (a few microamps) so the collector current doesn't get too high, send battery, and drive the input impedance too low.  BUT even with more than 1M of resistance, the collector current is still going to be 1mA or more, and the input impedance is maybe 6 kohms.  6 kohms.  That's hardly there.  The Fuzzface is low at 20 kohms, and famous for "loading" or "dragging down" the pickups, giving a "compress then bloom" effect.

I've also realised that the Fuzzrite has no emitter resistor on Q1 and has similar bias resistance to the Bosstone (2 x 470k instead of 1M and 220k).  It has excellent fuzz and compression.

So 3 of the great fuzzes all have no emitter resistor on Q1 and low input impedance.  Magic found?  Partly, I'd say.

Now I need to un-learn that JFETs and op-amps make good input sections because of their high input
impedance.
cheers

[Doug_H]

Now I need to un-learn that JFETs and op-amps make good input sections because of their high input
impedance.
cheers

You can lower Zin of a JFET ckt via it's bias network. With a non-inv opamp ckt you can adjust the value of the resistor that ties "+" to Vr to lower Zin.

[brett]

Hi Doug
Yeah.  It's just that after looking at so many schematics with 1M resitors doing those jobs, it'll feel weird trying a 22k. 

[Paul Marossy]

So 3 of the great fuzzes all have no emitter resistor on Q1 and low input impedance.

Very interesting finding. I wonder if that has anything to do with the Boss Tone being so "touch sensitive"?

[Gus]

I think there is more to it than that.

In the AOE look at the pages with the grounded emitter circuits.  If you put say a triangle wave into one the gain changes a lot with input signal level, more gain close to sat less gain close to cutoff.  So you are generating more distortion  with larger inputs.

The rangemaster is closer to cutoff.  The FF has the 1st C closer to ground.  R.G.'s writeups

So much going on in "simple" circuits

Here is something to try
a opamp
   in to a .22 cap and maybe a 100ohm or so resistor in series to the - input,
a 47K in series with a 200K or so  pot wired as a variable resistor, opamp out to - input as the gain control.
output to a cap to a 100k vol.
+ input 4.7k to 10k to 1/2 power supply voltage

I have not built this an would need to check how stable it might be with the input cap of the cable.

An opamp clipper with the EQ a FF has the ff has an EQ by having the guitar as part of the gain setting parts.  The gain will be the feedback R divided by the mess inside the guitar(resistance and inductance of the pickups etc.) it will only work right connected to a passive guitar or bass.

look up mixers and summing

[Paul Marossy]

I think there is more to it than that.

In the AOE look at the pages with the grounded emitter circuits.  If you put say a triangle wave into one the gain changes a lot with input signal level, more gain close to sat less gain close to cutoff.  So you are generating more distortion  with larger inputs.

Well, that's what I was hinting at - I could be wrong (and probably am), but I was thinking that the absence of the emitter resistor would mean that more current would be able to move thru the transistor, and this disortion would occur easier with a large input, and less would occur with a smaller input. That's what I was thinking could account for the touch sensitivity...

[brett]

Hi Paul, Gus
It's great to have TWO gurus read one of my posts.

I totally agree with the "small voltage to cutoff" comments.  In fact, I've often posted that one advantage of germanium transistors in the Q1 position is the tiny 0.3 V that the base sits above ground, while silicon transistors are at 0.7 V.  Personally, I really like that "going asymetric" sound as volume goes up, and I agree that it's a big part of the sound of these fuzzes. 

But here's the interesting bit:  There's a compromise between asymetric distortion and impedance.  We could choose silicon transistors with twice the hFE of germanium ones, so that the available headroom is the same.  I *think* we don't generally do that because hFE = 70 is where the best impedance is.  Choose hFE = 140 in a silicon fuzzface and you should get a similar amount of asymetric distortion as a Ge one with hFE=70, but you get double the input impedance.  I doubt that the silicon one with higher hFE would have the appeal of the germanium one. 

In fact, the most popular Si fuzzface, the Axis fuzz, is usually built with 2N2369As with hFE near 70.  Undoubtedly this does sacrifice some distortion in favour of something else, which I suspect is low input impedance.

thanks for the comments.  they've certainly kept me thinking.

[R.G.]

Here's another bit for your mulling:

The input impedance is not only low, it's variable, since it depends on the transistors's operating point, which varies with the signal.

[brett]

3 gurus! 
RG, do you mean the variation in hFE with Ic ?
Last night, when looing at hFEs at low Ic, I noticed how some devices are very responsive, and others not.  If I recall, the 2N222A and power devices such as the BD139 are very responsive.  I've always liked the BD139, and Gus proposed using power devices way back when.
cheers

[Paul Marossy]

The input impedance is not only low, it's variable, since it depends on the transistors's operating point, which varies with the signal.

Now that's very interesting! I never really thought about it like that, but that does make sense.

[brett]

Hi
Anybody know how fast hFE changes with Ic ?  Almost instantaneously?  If so, portions of signals further from ground than DC bias lift Ib, Ic, hFE and input impedance, resullting in higher pickup output AND higher gain, while those closer to ground have low Ib, Ic, hFE and input impedance, resulting in lower pickup output AND lower gain.

And the effect should run opposite to the available headroom.  ie the "ground" side of signals will have much greater headroom, but cause reduce pickup output and gain.  I think ? If so, that's a very cool trick of nature.

Datasheets should reveal some devices that will really respond. (ie strongly sloping hFE vs Ic graphs in the 1mA region).
thanks