Understanding output stages - help?

[dubspecialist]

Hi all,

I'm looking for some advice here - I have successfully modified the final stage in the Arachnid circuit to have a desirable (to me at least) Muff style tone control. What I would like to know is how to implement this successfully in future designs to minimise noise/potential pops. Given that this one was a mod I have simply put the tone control last in the circuit using some perfboard, the LPF/HPF components and a pot. It sounds nice, I'm pleased with it but theoretically would a better practice be to place it before what appears to me to be a decoupling capacitor (C15) and a pulldown resistor (R11)? (see attached image) - Can someone maybe help me demystify this a little, what is the role of the 100ohm resistor (R12) at this stage and would placing the tone control before those last three components (R12, C15, R11) be a better practice?

Any help is appreciated

 

[dubspecialist]

Not sure if anyone is following this but thought I'd update anyway. I tried modding this build in two alternative ways:

1. placing the tone control immediately after the final unity gain stage, ahead of R12. Doing so meant a nice tone when the pot was set but sweeping it resulted in a serious amount of noise as you adjust the pot - interesting! I wonder if this is because it was messing with the feedback path for the opamp stage? Either way it was not useable.

2. just for kicks I placed it after R12 and before C15 - predictably the value of R12 messed with the relationship between the components in my RC circuit (muff style tone control), shifting the cutoff frequencies in an undesirable away.

So, I resolve to leave the tone control at the end of the circuit and am happy with this. I'm curious to know what separates a stage in a circuit like this - perhaps it's a component shunt to ground like R11? Oh well, more testing and research I guess

 

[giovanni]

I do think it’s plausible that putting anything in series before the coupling cap will affect the feedback loop in unexpected ways (some info here maybe).
Not sure why R12 is needed, but it was probably determined empirically to minimize noise or other issues.
My EE knowledge is pretty dusty at the moment but you can probably find some good recommendations for textbooks on the forum to learn about various design considerations.
Good luck!

 

[dubspecialist]

Thanks for the response @giovanni I'll be sure to have a read through that previous thread for reference.

Yes, I should go in search of an appropriate text book. Been enjoying learning by doing so far too though!

 

[BuddytheReow]

Keep in mind that if you're putting in a passive tone filter (Muff in your case) it will suck out a lot of volume. The simple fix is to add a recovery stage after. I would suggest an LPB-1 since it is a textbook common emitter amplifier that won't color the tone too much. I would use this stripboard layout https://tagboardeffects.blogspot.com/2013/08/bmp-tonestack-w-lpb1.html.

Also, C15 and R11 act as a high pass filter. According to this calculator the cutoff frequency is 1.6Hz, which means that it is filtering out low frequency noise. http://www.muzique.com/schem/filter.htm. I would put the tone stack and recovery right before this. The filter may not be 100% necessary, but it would be good practice to keep it in just in case. It is circuit dependent.

Edit: I would also check out the Resources thread in the test kitchen to help you brush up on your EE knowledge. Once you know the basic boring stuff then tinkering gets more fun since you have a better understanding of why you are modding the circuit.

 

[dubspecialist]

Hi Buddy,

Thanks for the response! Yep, I am aware of the "volume suck" that comes from the muff style tone stack. I actually compensate by modding the earlier gain stages to allow for more boost earlier in the circuit.

Yep, I'm aware that the combination of C15 and R11 act as a HPF and the frequency at which the cutoff is set by these values. As mentioned in my earlier posts, I attempted putting the tone control ahead of this stage to undesirable results... which brought me to the question I asked in my second post regarding what behaviour separates parts of a circuit, perhaps a component shunt to ground..?

EDIT: I should be clear that when I say "separates" I am trying to determine what quality in a circuit means that components at one stage do not interfere with components immediately after them. Placing the tone stack in the places mentioned above meant that either it was in series with R12 and as a result the cutoff frequencies of my tone stack were altered undesirably or it appeared to interfere with the opamp feedback path for the final unity gain stage. When I place it after R11 the tone ctrl acts as I would expect. Leading me to idea that a component/resister shunt to ground (R11) prior to the tone control may be the key. Anyway, I'm sure I can resolve this question with some googling...

 

[mdc]

One thing you could try is putting a simple BJT emitter follower in front of the tone section and see if that helps? That's worked for me when muff tone stacks have acted oddly in a circuit.