A High Gain solution ..... "BLACKFIRE" !

[MartyMart]

Hi
This Blackfire schematic is somewhat different to the earlier one that I have a layout.  Among other things, that one had a low-pass filter (like the Rat).
This circuit is somewhat unlike the usual ones we see. 
From what I can make out, Q1 and Q3 are a neat type of buffer (1M input impedance, negligible output impedance).  The two gain stages, around Q2 and Q4, seem more conventional (common emitters with bypass caps). 

What do the 10k base resistors do, I wonder?  The 0.0022uF cap and 470k resistor pass high frequencies, with fc=150Hz, so deep bass (for a six string) is cut.

From what I can see, the final stage is has a low-pass filter (220k and 0.001uF), but quite a lot of feedback to the base (220k).  Is that right?

Any comments on how/why this works so well?

Joe updated this a couple of years ago, that may explain your "earlier" schem - I dont have that one.
Am I right in thinking Brett, that you were expecting those 10k base r's to be more like 47k - 100k ? so is this
"squashing" down the signal some ?
150Hz cut makes this circuit have zero "mud" at the low end - great !
Not sure how the large feedback is changing the sound ... more "spank" !

MM.

[powerplayj]

Are there any other clips besides the ones which use a cab sim?  I'd like to hear it with an actual amp.  What other effects does it compare too?

[WGTP]

IIRC Joe had trouble with oscillation with just 2 stages when a by-pass switch was used.  By adding a 3rd stage, the output and input were inverted which solved the problem, I think.  The earlier one I built just has the 2 BF stages, but I didn't use a switch.  The BF was a predecessor to the Vulcans and the stages were designed to have some tube like characteristics.  I don't remember/understand how they work, but the 10K resistor had something to do with it.  I think that last stage is similar to his speakers sim.  Note the Vulcans use 3 stages.  Great stuff.  Maybe he will make an appearance.     

[brett]

Hi
I *think* I understand the 10k resistors now.
Without caps and bias resistors to turn on the bases of Q2 and Q4, their base currents flow straight from the emitters of Q1 and Q3.  But the voltages on those emitters too high, and probably too variable, to bias Q2 and Q4 directly.  But a 10k resistor reduces the base current through Q2 and Q4 to workable levels.  Without the 10k resistors, there is only the base-emitter resistance and the 2.2k emitter resistor.  The 10k resistors not only add their own 10k of resistance, they also greatly increase the base-emitter resistance (which is inversely proportional to base-emitter current).

There are concepts in here that we could well use in different places.
cheers

[lovric]

adding to your comment on the last stage of BF. according to JoeD the last stage is a speaker emulator. i understand it is a low pass filter and a positive feedback. it seems to produce a 'hump' or resonance in the low end characteristical of a guitar speaker/cabinet. to avoid the mud in the bass, i guess, one should try to not distort much the low frequencies but still keep their amplitude within the signal.

[Somicide]

question:
if it cuts ~150Hz, how would that affect a guitar tuned to, oh say, C?

[tcobretti]

You'd lose some of the bass frequencies, but not too much, because most guitar rigs aren't set up to generate low frequencies that well.  Also, those frequencies tend to interfere with the bass and bass drum, so it's good to lose them.

[DDD]

Extremely attractive schematic IMO at least for four reasons:
1. No vacuum tubes (absolutely no comment)
2. No OpAmps (no comment)
3. No FETs (comment: predictable operating points)
4. Last but not least: good sound

[WGTP]

In the beginning, there was a link to creating tube sounding audio pre-amp stages that Joe posted.  You might find them by searching.