Technology of the Big Muff - What does what?

[John Lyons]

Ok, seems like a popular pedal and has been for many many years.
Lets figure out what does what. Making a value higher or lower does what?

I'll start the ball rolling.

R1 A Pull-down resistor to drain any DC voltage to ground from C1. Helping reduce "popping"
Any value from 1M to 2.2M is fine, no change in sound.

C1 Sets a high pass filter along with R3. Smaller cap = less bass and a little less signal into the circuit.

R2 Limits the signal in a bit as well as setting the impedance. Bigger lowers the impedance and signal.

R4 Is a feedback resistor which works along with R3, R5 and R6 to bias the transistor. Smaller = less gain
as it lets more feedback flow from collector to base.

C3 is a "coupling cap which forms a low pass filter with R7. Bigger = more bass (and signal) passed.


C2 across the Base and Collector rolls of some high end. Bigger + more roll off.

Next person...

John

[Ben N]

I would add that aside from any tone-shaping function, the purpose of C3 is to keep DC from R6 off the sustain pot R7 and the base of Q2 (which is biased by its own network consisting of R10, R9 and R13). R7 controls the amount of signal passed from Q1, a booster, to Q2, by shunting an adjustable amout of signal to ground.

The components surrounding Q2 are the same as those surrounding Q1, with two important exceptions: the addition of D1, D2 and C6 to the feedback network of the transistor, and the lack of a "sustain" control after it. D1 and D2 are back-to-back diodes that are referred to as "clippers" or "clamps", because they clip the waveform (creating what we hear as distortion) and clamp the dynamics of the signal (creating what we hear as compression or sustain). C6 reduces the amount of bass that goes through the clippers.

[Greg_G]

Thanks for this.. they're always a good read.

I believe C6 is primarily there to block DC through the diodes, which would affect bias.

[station]

R5, R13 and R16 (the emitter resistors to ground) can be lowered to raise the gain, I've heard of people shunting the emitter directly to ground for a crazy - over the top fuzz. Conversely, you can reign in an unruly fuzz by raising these values.

The collector resistors (R6, 11, 17 and 25) were components that saw a lot of different values over the years too. Anywhere from 10k - 22k. Lowering these values also tends to raise the gain.

One last thing, R6 is there to control the lower limit of the sustain pot. without it (like on many triangle versions) you get no sound with the sustain rolled all the way back.

[theehman]

The collector resistors (R6, 11, 17 and 25) were components that saw a lot of different values over the years too. Anywhere from 10k - 22k. Lowering these values also tends to raise the gain.

I've seen them as high as 33K.

One last thing, R6 is there to control the lower limit of the sustain pot. without it (like on many triangle versions) you get no sound with the sustain rolled all the way back.

I don't think I've ever seen a triangle w/o R6.  The lowest value I've seen for it is 820 ohms.

[mattpas]

R5, R13 and R16 (the emitter resistors to ground) can be lowered to raise the gain, I've heard of people shunting the emitter directly to ground for a crazy - over the top fuzz. Conversely, you can reign in an unruly fuzz by raising these values.

I did that on the Creamy Muff on my site. I don't think it gets that crazy but it sounds pretty good.

[Gus]

It not that simple. It has few thing going on a the same time.  Some things to look up

Page 86 TAOE has something to look at.  The added voltage drop across 100 ohm in the emitter leg adds to the Vbe drop

http://www.nd.edu/~hscdlab/pages/courses/microwaves/labs/Agilent1293.pdf    Kind of like figure 6 but with an added emitter resistor.

[ayayay!]

Okay, I've never even looked at a Big Muff before.  (Blasphemy to some, I'm sure.)  But I'd like to know the reasoning, in detail, for the values chosen for the resistors on +9v:  R6, R11, R17, R24, R25. 

Thanks John for starting this! 

[R.G.]

R1 A Pull-down resistor to drain any DC voltage to ground from C1. Helping reduce "popping"
Any value from 1M to 2.2M is fine, no change in sound.

Yep.

C1 Sets a high pass filter along with R3. Smaller cap = less bass and a little less signal into the circuit.

Yep, sets the input high pass along with everything else up to the collector and emitter of Q1, including feedback - see below.

R2 Limits the signal in a bit as well as setting the impedance. Bigger lowers the impedance and signal.
R4 Is a feedback resistor which works along with R3, R5 and R6 to bias the transistor. Smaller = less gain
as it lets more feedback flow from collector to base.

R2, R3 and R4 set up the input and feedback for the transistor. All play multiple roles. R3 and R4 do indeed set biasing. But imagine that you replace Q1 with an inverting opamp stage, the + input being biased else where. That would make the gain of this stage be Rf/Ri = R4/R2, right? That is what is happening here, excepting for the transistor having a lower input impedance and less gain than a purpose-designed opamp. Because of the transistor's "imperfections" the gain is lower than you'd get from an opamp with the same resistors, but the operation is very similar. It's an inverting feedback stage.

C3 is a "coupling cap which forms a low pass filter with R7. Bigger = more bass (and signal) passed.

Actually, with R7 and to some extent R8, which appears as a bigger load as the pot rotates to max. At the max setting, it's almost all R8, or rather, C4 and the input impedance of the next feedback stage at Q2.

C2 across the Base and Collector rolls of some high end. Bigger + more roll off.

Correct, in the feedback around the opamp sense. This capacitor appears just like a capacitor to ground but of a value equal to the effective gain with feedback times the real capacitance; this is known as the Miller effect, and this is sometimes called a "Miller capacitor".

I believe C6 is primarily there to block DC through the diodes, which would affect bias.

This is correct; C6 and C9 also have a varying impedance, and serve to lower the gain and hence the amount of clipping at lower frequencies. Making this smaller/bigger, causes less clipping at bass/more bass to be clipped.

R5, R13 and R16 (the emitter resistors to ground) can be lowered to raise the gain, I've heard of people shunting the emitter directly to ground for a crazy - over the top fuzz. Conversely, you can reign in an unruly fuzz by raising these values.

They control the open loop gain, and as such make the approximation to an opamp more/less valid as they get smaller/bigger. With lower open loop gain, you run out of feedback gain faster and have a lower overall gain, which decreases clipping.

The collector resistors (R6, 11, 17 and 25) were components that saw a lot of different values over the years too. Anywhere from 10k - 22k. Lowering these values also tends to raise the gain.

Raising them tends to raise the open loop gain. I'm using gain in the strict sense of voltage out over voltage in, not "gain" the way guitarists say it, meaning more apparent distortion.

One last thing, R6 is there to control the lower limit of the sustain pot. without it (like on many triangle versions) you get no sound with the sustain rolled all the way back.

Yep.

It not that simple. It has few thing going on a the same time.

Absolutely correct. Most things affect other things too.

But I'd like to know the reasoning, in detail, for the values chosen for the resistors on +9v:  R6, R11, R17, R24, R25. 

6, 11, 17 and 25 are chosen for bias point/gain on the transistors in the open-loop sense. R24 is chosen as per the stabilized-bias setup I've typed in here many times to bias q4 properly with R21.

[John Lyons]

Ok, good start folks.
I'll add in the details as they come in.

Keep it simple (as can be) as my main reasoning in doing this
is to break it down for people. So far so good.

john

[R.G.]

What other details are needed?

[frank_p]

What other details are needed?

Well if there was to be a "technology of the BM", the tone control explanations would have not to be forgotten.

[Gus]

To add to R.G.'s post

The first stages gain is a combination of the open loop gain setting of the transistor (has to be lower than 15K /100) and the 470K/39K about X12 inverting feed from a 0 ohm source, a guitar is not a 0 ohm source.
so LESS than X12 inverting because of limited openloop gain and the source Z of the guitar.

If the transistor had a lot of gain like an opamp the gain would be very close to X12 inverting (ignoring the guitar Z).  The input Z would be the 39K and C1 because the node that the base and 470K and and C1 would be at a virtual ground.  The transistor is limited to a max of X150 by the 15k and 100 ohm (it is even less because you need to add re to the 100 ohm and re varies with current re=25/Ic, Ic is in ma).  Also where in the supply range closer to sat or cutoff

The 470K and 100K are a part of the biasing.  The higher the Hfe the less current thur the 470K to the collector node.  The Ic (collector current set by the designer selection of resistor value and collector operation point) "splits' at the collector node some current for the bias resistors and some for the emitter The higher the Hfe the less the loading of the resistor bias string.

There is even more. A lot going on.  IMO it does not lend itself to easy change this for that simple posts

What I suggest you do if you want to understand what does what.  Build the first stage use a 10K as a load after C3 (crude sim of the stages load). Use a regulated supply so the voltage does not change.  Measure the nodes voltages.  Then use a signal gen set to triangle build a sim of the guitar because that adds to the 39K input and set to a pickup you like to the  peak to peak voltage it generates.  Look at the signal with a scope then change out transistors that you measured the Hfe.  Then change the parts one by one maybe remove the 100 ohm emitter resistor.  Take notes maybe even listen.  If you change the bias resistor(s) and want the same gain the ratio needs to stay the same if they are lower value more current is shunted away From the emitter

I will build circuit or fragments to make sure I got the math right and test them with a gen and scope.  

Also If you start to get this you will understand the first stage of the Crybaby like inductor whas.

Here is an idea for the guitar sim think simple, take a pickup and volume control place them in a metal box can etc for shielding feed one end of the pickup from the gen the other end to the "top" of the volume pot, ground to gen and "bottom" of the volume pot and ground of the fragment effect, wiper to input of the effect.

[teemuk]

The tone control – really simple, just visualise this:

The signal path splits in two parallel RC filters, one is a low-pass filter (R18 & C12) the other one is a high-pass filter (C10 & R20). The potentiometer is connected between the outputs of these two filters and acts as a "fader", panning between the corresponding output of each filter. At one far end of the dial, most of the signal comes through the low-pass filter, at the other far end the signal mainly comes through the high-pass filter. In the mid-position of the dial the output signal is that of the two filter outputs combined. Usually the -3dB points of the filters are a bit different, which results into a mid-range notch.

[MattXIV]

The main question I've always had looking at the Big Muff is why there are clipping diodes in more than one stage?   It seems that anything that gets clipped in the Q2 stage will be clipped even more in the Q3 stage, making the Q2 stage's clipping irrelevant.

I'll add my descriptions for the tone stack and output:

C10 and R20 act as a high pass filter - Larger caps and smaller resistors decrease the relative amount of bass on higher tone knob settings.  Larger caps and larger resistors increase the output level of the filter at a given frequency.

R18 and C12 act as a low pass filter - Smaller caps and larger resistors decrease the relative amount of treble on lower tone knob settings.  Smaller caps and smaller resistors increase the output level of the filter at a given frequency.

The relative fcs of the filters detemine whether the combination of the two results in a mid cut (high pass > low pass, the result for stock components), a mid pass (high pass < low pass), or neutral mids (high pass = low pass).

R19 blends the output of the filters.

C11 is a coupling cap and also acts as high-pass filter with R21.

R21 and R24 work as a voltage divider to set Vb for Q4.

Q4 provides some gain after the tone stack.  Decreasing R22 or increasing R25 increases the gain of this stage.

C13 is another coupling cap and also acts as a high-pass filter with R23.

R23 is a voltage divider used to set the output level.

[Eb7+9]

I posted this a long time ago and Aron used to have it archived somewhere
funny how nobody mentions this key resistor :

R12=8k2

replace by a 10k trimmer, and set to taste - it affects the overall "dirt/sensitivity" response more than anything IMO
I've woken up old BMP's by tweaking this guy alone ...

R12 does two things, mainly it allows more drive into the second clipper stage and also, to a lesser extent, slightly reshapes some of the NFB going on in each stage around it ...

eons ago I wired up a BMP with trimmers replacing every resistor except for a few obvious ones and switches with multiple caps ... as expected most of clipping occurs past a certain gain level and then doesn't really change much - that's because the diodes dominate the clipping picture - even using high gain NPN devices doesn't necessarily give you more girth to the grind ... you can get a bit more edge from the gain stages if you eliminate the emitter feedback in the gain stages (R5, R13, R16) but not that much ... my feeling is that the stock resistor and cap values were well chosen - tweak R12 and that's as good as the circuit gets IMO

in response to the doubling of clipper stages - that raises a good question ... I find the BMP's dirt is not as focused as some similar fuzzes, I think as a result  ... but it is what it is - certainly serves as a design data point ...

[thereverend]

holy crap i'm actually learning something. all these explanations are making me see the schematic completely different than i did before. thanks for starting this!

[teemuk]

As for doubling up the clipping stages... Well, for a given amount of voltage gain (in total) it results into harder clipping than a single clipping stage would provide. Also, the signal to the second clipping stage does pass through the low-pass and high-pass filters involved, which will then alter what gets clipped in the second diode clipping stage and in what proportion. It's a complex process and the outcome depends a lot on shaping of the frequency response ("voicing") and gain / overdriving levels involved.

Think about it. If this kind of "cascading" of overdriven stages was futile all tube amps would practically have just a single gain stage.

[R.G.]

I skipped the tone circuits because Jack's done those to death at his site. It's an overlapping blend between a single-RC high pass and low pass. Twiddling the overlap gets you a whole lot of different over/underlaps.

As to multiple clipping stages, the real story is in how hard each clipping stage is driven over and above the clipping knee. For the most interesting clipping you want the waveform driving the clipper to spend as much time in the bending-over-but-not-flat-topped part of the clipping. This is simpler to do if you have a previous clipper stage to keep things in bounds so you know where the clipping's going to go. And with more than one stage, there is plenty of gain available to get a lot of clipping.

And, as Teemuk says, and I have said for a long time, the outcome depends on the shaping of the frequency response both *before* and *after* clipping.

[John Lyons]

The main question I've always had looking at the Big Muff is why there are clipping diodes in more than one stage?   It seems that anything that gets clipped in the Q2 stage will be clipped even more in the Q3 stage, making the Q2 stage's clipping irrelevant.

The clipping in this arrangment is sometimes called "soft clipping". The diodes are a feedback loop just as in the Tube Screamer.
This is a good bit softer than diodes to ground as in the Rat, Distortion +, etc etc.
With only the first clipping stage in the BM the fuzz is fairly weak. Add in in the second clipping stage and you get the
classic BM "violin like Sustain" If you have an audio probe you can listen to the collector of the first clipping stage to hear
what one clipper stage sounds like.