# Help me understand this schematic (Mach 1, Southern Belle)



## Grubb (May 5, 2022)

One of my favourite circuits is the Mach 1/Southern Belle, which I have enjoyed building and messing around with. I'm trying to understand how parts of the circuit work, with an eye to potential mods. Below is the first section common to both circuits, taken from the SB build docs:




I think I mostly understand the negative feedback loop, the purpose of C5, and the clipping diodes. At least in functional terms.

I think R6 and the Drive pot are setting RF for this amplifier circuit.

What I don't yet understand well is what C3, C4, R8 and R9 are doing. I am anticipating that R8 and possibly R9 are involved in setting RIN and that there are some filters at work here, because this is where the Timmy bass control is. I would love it if someone could help me grasp what is going on here.


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## spi (May 5, 2022)

Read section 3.2 of ElectroSmash - ProCo Rat Analysis

Although it's discussing the RAT, I think the same concept applies here.


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## BuddytheReow (May 5, 2022)

Those are high pass filters. The “dumbed down” explanation is that those filters select the frequencies to go through the gain stage


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## Grubb (May 5, 2022)

Thank you. Am I right in reading it this way?

Filter 1: C4 + R8
Filter 2: C3 + (R8+R9 in series)
I'm still not sure how to calculate the input resistance. The way R8 and R9 split into two branches is confusing me. The corresponding resistors are parallel in the Rat schematic so I'm none the wiser. The C3 branch is in series, but I am not sure how to interpret the C4 branch.


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## Grubb (May 6, 2022)

If I draw it like this, it makes me think I can simply add the resistance... 🤔 I couldn't work out how to signal VREF on this phone app so I used GND instead.


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## BuddytheReow (May 6, 2022)

Grubb said:


> If I draw it like this, it makes me think I can simply add the resistance... 🤔 I couldn't work out how to signal VREF on this phone app so I used GND instead.
> View attachment 25979


For the record you can take out C1 or (C2 and R2) and the circuit will still work. It will just sound different.


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## Chuck D. Bones (May 15, 2022)

C4 & R8 cut the bass & determine the mid & treble gain.  C3 & R9 add a little bass back in.  This is a common technique for controlling the mud when the gain is cranked.  Take a look at how the Timmy does it.


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## giovanni (May 15, 2022)

Grubb said:


> Thank you. Am I right in reading it this way?
> 
> Filter 1: C4 + R8
> Filter 2: C3 + (R8+R9 in series)
> I'm still not sure how to calculate the input resistance. The way R8 and R9 split into two branches is confusing me. The corresponding resistors are parallel in the Rat schematic so I'm none the wiser. The C3 branch is in series, but I am not sure how to interpret the C4 branch.


R8 and R9 are not in series. C4 is in parallel with the series of C3 and R9. R8 is in turn in series with that series. That is the total impedance is ((Z3+R9)||Z4)+R8.
One way to look at how it’s filtering: that impedance will allow current at specific frequencies to flow through Vref, affecting the frequency response of the op amp stage.


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## Chuck D. Bones (May 15, 2022)

Where Z3  = 1/sC3 = impedance of C3.  Or you can run a Spice simulation and see what it does.  This is the freq response of the 1st stage with DRIVE set at noon.


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## giovanni (May 15, 2022)

That 350 Hz peak is interesting, way lower than a TS uh?


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## Chuck D. Bones (May 15, 2022)

You mean 3.5KHz, right?

C5 & the DRIVE pot roll-off the top-end.  With DRIVE at noon, it's 3dB down at ~10KHz.  The TS typically has a 51pF feedback cap and a 500K DRIVE pot.

What we're seeing here is the small-signal response.  Once the clipping diodes start conducting, the freq response changes drastically.


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## giovanni (May 15, 2022)

Chuck D. Bones said:


> You mean 3.5KHz, right?
> 
> C5 & the DRIVE pot roll-off the top-end.  With DRIVE at noon, it's 3dB down at ~10KHz.  The TS typically has a 51pF feedback cap and a 500K DRIVE pot.
> 
> What we're seeing here is the small-signal response.  Once the clipping diodes start conducting, the freq response changes drastically.


Oh duh, much higher than a TS or Klon then! I need readers I guess 
How do the non linearities affect the frequency response? I guess it’s just one of those things where you just measure it?


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## Chuck D. Bones (May 15, 2022)

These freq response plots are not all that useful when the circuit is non-linear.  Best way to evaluate the tone in a simulation is to run a time-domain analysis and look at the harmonic content.  Then build it and listen to it.


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## Grubb (May 16, 2022)

Chuck D. Bones said:


> C4 & R8 cut the bass & determine the mid & treble gain.  C3 & R9 add a little bass back in.  This is a common technique for controlling the mud when the gain is cranked.  Take a look at how the Timmy does it.


Thanks for this Chuck. I can see in the Tommy schematic that one of the resistors is now a pot and no longer a fixed value. I understand this allows you to manually shift the frequency of the filter.

But it raises questions for me, so please forgive my lack of understanding. I think that when the Gain Range switch on the Timmy is engaged, the lower value resistor R12 becomes parallel with R3 and this increases the gain of the amplifier stage. I'm guessing that R12 and C9 also form a third filter. With R12 and C9 in the circuit, is C3 still forming a filter with R3 as it would be with the switch disengaged?


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## HamishR (May 16, 2022)

Chuck D. Bones said:


> These freq response plots are not all that useful when the circuit is non-linear.  Best way to evaluate the tone in a simulation is to run a time-domain analysis and look at the harmonic content.  Then build it and listen to it.


This is exactly what I do. Except for the bit before build it and listen to it.


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## Chuck D. Bones (May 16, 2022)

Grubb said:


> But it raises questions for me, so please forgive my lack of understanding. I think that when the Gain Range switch on the Timmy is engaged, the lower value resistor R12 becomes parallel with R3 and this increases the gain of the amplifier stage. I'm guessing that R12 and C9 also form a third filter. With R12 and C9 in the circuit, is C3 still forming a filter with R3 as it would be with the switch disengaged?



We don't get another filter, the Gain Range sw retunes the filter we already have.  R12 || R3 gives us 325Ω.  C9 || C3 gives us 259nF.  The Gain range goes up by ~10x (20dB) and the C3-R3 high-pass freq goes up by 1.5x.


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## Grubb (May 17, 2022)

Chuck D. Bones said:


> We don't get another filter, the Gain Range sw retunes the filter we already have.  R12 || R3 gives us 325Ω.  C9 || C3 gives us 259nF.  The Gain range goes up by ~10x (20dB) and the C3-R3 high-pass freq goes up by 1.5x.


Amazing, thanks Chuck. That makes a lot of sense. The resistor pairs and capacitor pairs are each parallel pairs, and that affects the overall resistance/capacitance of that part of the circuit, forming a new combined filter.


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## HamishR (May 18, 2022)

Honestly we are so lucky to have folks like Chuck here. I do try to understand the theory and have read a fair bit, but I guess my mind is just not wired the right way. So I'm only partly joking when I say I just build it and listen to it. I do a lot of that. But with Chuck's guidance, dropping breadcrumbs of clues and knowledge, I have ended up with some fantastic results. I'm happy to build stuff to see if it works, especially if I get the results I do with Chuck's help.

I have met many people with superior knowledge to mine. But very few who can share it in such a positive, helpful way.


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## Grubb (May 18, 2022)

HamishR said:


> Honestly we are so lucky to have folks like Chuck here. I do try to understand the theory and have read a fair bit, but I guess my mind is just not wired the right way. So I'm only partly joking when I say I just build it and listen to it. I do a lot of that. But with Chuck's guidance, dropping breadcrumbs of clues and knowledge, I have ended up with some fantastic results. I'm happy to build stuff to see if it works, especially if I get the results I do with Chuck's help.
> 
> I have met many people with superior knowledge to mine. But very few who can share it in such a positive, helpful way.


Agreed, something I love about this place is that it's mostly orientated towards helping folks learn and improve. Doesn't matter if they're 14 or 74. Chuck and Robert are really key in setting the culture here IMO. As a professional educator in my day job, I really appreciate this place for its educational ethos.


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## Chuck D. Bones (May 18, 2022)

You're too kind.

There are tons of folks who make this place what it is...

Too many to list, but you know who they are.


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## Grubb (Tuesday at 7:42 PM)

Sorry to resurrect the thread, but I thought this might be the best place to ask another question about the Timmy bass control. If I reverse the wires to the lugs, will this operate more like a conventional control (CCW = cut, CW = add)? Do I also need to change the taper of the pot? 

The reason for asking is that I've modded a Mach 1 circuit into a pretty good sounding distortion but increasing gain = increasing Bass, and I want to be able to offset this to some degree by rolling some Bass off. I figured a Timmy control would be appropriate given the origin of the Lightspeed/Mach 1.


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## Feral Feline (Wednesday at 11:58 AM)

According to the PDF for the MXR version of the Timmy  the BASS control is already cut CCW and bass-boost CW, but I don't know what the schematic is like for this.

The original Timmy lug order (on a schematic L>R) is 1 2 3, the PedalPCB Tommy is 3 2 1.

As for taper, the Tommy is linear taper so I wouldn't bother changing that. 


Best bet is to breadboard it, and try each taper yourselff and see which you like.



In other words, I don't know.


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