Scaling tone stacks and the dual rectifier

[hans h]

Hi community,

I've been pedal-obsessing about the Mesa dual rectifier for a while now. I've successfully made a flexible preamp based on the dual recs frequency filtering cutoffs and asymmetric clipping. This preamp does the input stage and clipping parts of the amp. Now I'm looking into a fun low wattage '' power amp'' to go with the preamp.

Idea for the power Amp is mosfet gain stage for some sweetness but no obvious clipping, followed by a mosfet buffer, then a scaled version of the dual rect orange channel tone stack into a lm386-n4 chip set up for wide frequency amplification (ruby for bass mods by Prr on this forum).

I ran into the limitations of my current knowledge regarding scaling of tone stacks. I would think reducing all resistors by a factor x and increasing the caps by a factor x would do the trick nicely, given the formula for the - 3db point of a filter. In my schemo I used x=10 (see first attachment below). This should work because the Amp has Zout of approx 1kohm and the mosfet buffer approx 25ohm.

Then I ran into the Dr boogie schematic where the pots are scaled down, but the caps and sloping resistor are not scaled?! Attachment 2 shows the recto orange channel on top and the Dr boogie on the bottom.

Is the Dr boogie '' wrong'' or am I misunderstanding the scaling?

[anotherjim]

I would agree with you that the Dr Boogie's values are wrong.

[hans h]

Thanks Jim!

I'll go ahead and implement it as per my schemo. One additional question: I could also eliminate the mosfet buffer stage: mosfet gain stage - > tone stack - > presence - > volume pot (just like the actual Amp). In this case Zout would be 1k2, similar to the amp's ~1k. I'd have to use high valued resistors though (like the Amp). What do you reckon to be noisier: the additional mosfet buffer in my current design or the high valued resistors in option b?

And a third question: is the lowly 50k input impedance of the lm386 a problem with my current setup or with the 'option b' setup? You need input impedance at least ~10ntimes greater than the R of the filters right? Is the mids pot with its 2k5 a problem with the subsequent lm386?

Or is this well-visible in an ltspice Sim? It's easy to do a 50k Zin opamp after this tonestack in ltspice.

Thanks in advance, hans

[Vivek]

Loading of a tone stack due to too low an impedance of next stage can be easily seen in LTSPICE.

It might also be visible in TSC IN THE WEB

[Steben]

I agree with Vivek: use the online TSC.

off topic I wonder why you would not use only a buffer into a single LM386 stage? If power amps clips, main distortion will come from the LM chip anyway and the other distortons are already in the preamp?

[anotherjim]

Indeed, I did not notice the 386 stage - which is 50k? Really needs a buffer but at a push, change that series resistor up to 47k  so the stack now sees 100k and rescale that extra filter accordingly.
Is "P" meant to be Presence? If so it isn't really doing that anyway unless it's countering a matching presence boost earlier in the circuit.

[hans h]

Welcome to the thread Steben and Vivek!

The lm386 is set up for low gain (pins 1-8 not connected) so can handle quite some input voltage before it clips. I have a volume pot and tone stack loss before the lm386 so there's no problem there. Already tried the mosfet boost stage straight into a ruby modified for bass guitar which I have lying around. There was no clipping of the lm386, at least at lowish volume settings. As to why I use the mosfet: the ruby for bass is a bit to clean ('boring') to me. It needs some sparkle from not overly apparent overdrive in the mosfet. I set up the gain stage such that my guitar at full volume yields only a hint of overdrive, barely audible but enough to make it shimmer.

P is indeed presence, but I think I'm leaving that out since I already have this presence control in the preamp pedal. BTW I know it is not a classic presence control, but it is the way 'presence' is used in the dual rect.

Thanks for the tsc suggestion. Ihave the offline version on my pc. Are there additional features in the web version? I'll play around to see the response compared to the actual Amp. If it doesn't work out I may just go mosfet boost - > 10kA vol - > Ne5532 side a set up for inverting buffer - - > tone stack scaled divided by 10 - >  ne5532 side b as noninverting buffer (Zin max 360 k) - > lm386.

[Vivek]

https://www.guitarscience.net/tsc/info.htm


Jatalahd was kind to add few Baxandall and the Blackstar into the web version, at my request about 2 years ago. Thank you once again Jatalahd !

[hans h]

Ah wow had seen the baxandall in tsc but not the blackstar (ISF if I recall correctly?). I'll definitely check the online version as well then.

[hans h]

The online calculator version is neat! Thanks Jatalahd, ~arph and TheseGoToEleven!