Question about Tonestack in Duncan TS Calculator

[pappasmurfsharem]

Is R5(517k) in the scheme below a critical part of the tonestack? I assume it is because changing it's value to near zero basically kills all the curves.

What is the science behind what this part is doing?
Would there be any issue with just throwing a A100k pot for in the out portion, assuming there is enough gain that a recovery stage isn't necessary?


Background: I'm wanting to try various gain stages before this tone stack to see how it might be useful in future builds.

[psychedelicfish]

I'd assume its just for simulating the impedance of the next stage. Near zero value should kill all the curves, because you're shorting the output to ground.

[pappasmurfsharem]

I'd assume its just for simulating the impedance of the next stage. Near zero value should kill all the curves, because you're shorting the output to ground.

I was thinking that as well, I tried changing the 517k to 100K to simulate a 100K pot, but it basically destroys the curves.

[psychedelicfish]

Tonestacks are fussy little buggers, maybe the schematic with the values shown won't drive any load lower than 571k. Perhaps you could try with lower value pots and see if you can go any lower with R5?

[pappasmurfsharem]

Tonestacks are fussy little buggers, maybe the schematic with the values shown won't drive any load lower than 571k. Perhaps you could try with lower value pots and see if you can go any lower with R5?

I found basically the exact info I needed here I think.
http://www.diystompboxes.com/smfforum/index.php?topic=74755.0

I said to scale all the resistors 100K/517K  and the caps by 517K/100K which is roughly .193x and 5.17x respectively.

After doing that and rounding to the nearest values this is what I come up with.

The top being the scaled values, and the bottom being the original.



This seems logical enough am I missing anything?

[gritz]

Hi, remember that some of these tonestacks were taken from valve amps so the resistance values are perhaps higher than we might use in a pedal - that 571k resistance represents the input impedance of the next stage. Often it's appropriate to scale the values by dividing the resistances by e.g. five or 10 and multiplying the capacitances by the same amount.

Remember that if you use a potentiometer as the output then the impedance of whatever the pot feeds will appear in parallel with that pot and may affect the eq by loading it. The output impedance of the assembly might not be fantastically low either, so a long cable run from the output might lose treble, so be prepared to need an output buffer.

Edit: heh, you beat me to it!

[pappasmurfsharem]

Hi, remember that some of these tonestacks were taken from valve amps so the resistance values are perhaps higher than we might use in a pedal - that 571k resistance represents the input impedance of the next stage. Often it's appropriate to scale the values by dividing the resistances by e.g. five or 10 and multiplying the capacitances by the same amount.

Remember that if you use a potentiometer as the output then the impedance of whatever the pot feeds will appear in parallel with that pot and may affect the eq by loading it. The output impedance of the assembly might not be fantastically low either, so a long cable run from the output might lose treble, so be prepared to need an output buffer.

Edit: heh, you beat me to it!

With the current scaled values, do you think a buffer is still necessary? or within reason would this act like a typical pedal setup?

I'm going to be trying a BSIABII gain stage before this.

[gritz]

Hi, remember that some of these tonestacks were taken from valve amps so the resistance values are perhaps higher than we might use in a pedal - that 571k resistance represents the input impedance of the next stage. Often it's appropriate to scale the values by dividing the resistances by e.g. five or 10 and multiplying the capacitances by the same amount.

Remember that if you use a potentiometer as the output then the impedance of whatever the pot feeds will appear in parallel with that pot and may affect the eq by loading it. The output impedance of the assembly might not be fantastically low either, so a long cable run from the output might lose treble, so be prepared to need an output buffer.

Edit: heh, you beat me to it!

With the current scaled values, do you think a buffer is still necessary? or within reason would this act like a typical pedal setup?

I'm going to be trying a BSIABII gain stage before this.

This one?



(my google-fu is a little weak this morning - lack of coffee!)

Looking at the schemo I'd be wary of the high output impedance of the gain stage - that fet's drain (and the unknown final value of the trimmer) will have to drive the modded tone satck and the input of the next pedal. Mebbe a fet source follower buffer between the gain stage and the tone stack as per the cathode follower in the real amp, like in Robert's Spitfire pedal (Q5)

[pappasmurfsharem]

Basically yes.

Except I was under the impression that you needed the 1uF still as a bootstrap cap as seen in this scheme.



But the scheme you posted is the general idea except with the scaled values to use a 100k volume pot which should present a lower impedance to the following stage correct?

Wouldn't using a bootstrap help lower the output impedance of the gain stage?

[gritz]

Basically yes.

Except I was under the impression that you needed the 1uF still as a bootstrap cap as seen in this scheme.

Do you mean C3? There's no bootstrapping (i.e. unity gain positive feedback) twixt gain stage and eq there. C3 just prevents DC present at Q3's drain from appearing across the tone and volume pots.

Bear in mind I'm not wearing my glasses though and this screen is awfully small...

[pappasmurfsharem]

Do you mean C3? There's no bootstrapping (i.e. unity gain positive feedback) twixt gain stage and eq there. C3 just prevents DC present at Q3's drain from appearing across the tone and volume pots.

Bear in mind I'm not wearing my glasses though and this screen is awfully small...

And this is why you guys' are here. Because I'm a goof and know nothing.

So what is or how would I calculator the impedance at the drain of Q3 so I know what to place in the Zsrc of the calculator to see it's effect on the tone stack, and what effect would this impedance have on objects after the tone stack+volume pot?

[gritz]

So what is or how would I calculator the impedance at the drain of Q3 so I know what to place in the Zsrc of the calculator to see it's effect on the tone stack, and what effect would this impedance have on objects after the tone stack+volume pot?

That's the $64,000 question! The output impedance is effectively the adjusted value of the trimmer hanging off the drain in parallel with the impedance from the drain to ground. The drain impedance will intuitively be very high - imagine pulling the drain voltage upward from it's set point (e.g. 5 volts, or whatever). Any change in current into the drain would be reflected by a change of current through the 5k6 source resistor, which would raise the source voltage relative to the gate (which is tied to ground). However, raising the source voltage relative to the gate would make the fet want to flow less current, so the fet's drain is effectively a constant current sink, with a very high impedance. It's not unreasonable to assume that the output impedance of the entire stage approaches the adjusted value of that trimmer - probably tens of kilo-Ohms. [/non-math explanation]

[pappasmurfsharem]

So what is or how would I calculator the impedance at the drain of Q3 so I know what to place in the Zsrc of the calculator to see it's effect on the tone stack, and what effect would this impedance have on objects after the tone stack+volume pot?

That's the $64,000 question! The output impedance is effectively the adjusted value of the trimmer hanging off the drain in parallel with the impedance from the drain to ground. The drain impedance will intuitively be very high - imagine pulling the drain voltage upward from it's set point (e.g. 5 volts, or whatever). Any change in current into the drain would be reflected by a change of current through the 5k6 source resistor, which would raise the source voltage relative to the gate (which is tied to ground). However, raising the source voltage relative to the gate would make the fet want to flow less current, so the fet's drain is effectively a constant current sink, with a very high impedance. It's not unreasonable to assume that the output impedance of the entire stage approaches the adjusted value of that trimmer - probably tens of kilo-Ohms. [/non-math explanation]

So let's say hypothetically of course.

That I was impatient and already grabbed the components for the stock tone stack above (the non adjusted value one) is there anything I can do? Just place a buffer at the end? Or would it be one before and after the stack?

Are there any reasonable options? This also assumes that I don't have a 500k output for the volume.

Just hypothetical of course, surely I wouldn't have jumped the gun.... Surely

[gritz]

Ah, that is a bit of a challenge. Another trip to the store might be best in the long run.

[pappasmurfsharem]

Ah, that is a bit of a challenge. Another trip to the store might be best in the long run.

You make me sad... So be it!.... Come Patsy...

I hate waiting for Tayda shipments.

[B Tremblay]

I recommend putting a buffer after that tonestack.  Change the output impedance to 1M on TSC and see what happens.

[pappasmurfsharem]

I recommend putting a buffer after that tonestack.  Change the output impedance to 1M on TSC and see what happens.

Would there be an issue with the interaction between the gain stage and the tone stack?

I assume you say change it to 1M to stimulate the input if the buffer?

[B Tremblay]

I recommend putting a buffer after that tonestack.  Change the output impedance to 1M on TSC and see what happens.

Would there be an issue with the interaction between the gain stage and the tone stack?

I assume you say change it to 1M to stimulate the input if the buffer?

The interaction will not be an issue.  Yes, 1M to represent a buffer input impedance.

[amptramp]

I try to design with an input buffer and an output buffer to ensure there is no interaction between the controls and any source or load impedance.  That way, you can put a pedal in a pedalboard with any other pedals and it behaves the same way no matter what you are connected to.  If the controls are between the buffers, there is no interaction with external stuff.

[pappasmurfsharem]

So I tried this out didn't build the buffer yet although I did try putting a boss pedal after the volume pot didn't change the sound much, perhaps I should put it before the volume pot. works pretty well. The BSIAB is definitely too trebly though the controls all work but there is just so much treble content that you have to keep the treble knob fairly low.

And as with the stock BSIAB it just missing that Juicy bass. Any ideas on changing the filtering somewhere else in the circuit to get some more meaty bass before the tonestack?