Fix treble control to be centered on 1.6 kHz.

[Aelfad]

Hey, Kaz,

To sum up: are these your final changes to the MP-1 tone stack?

- Presence: low pass @ 6 kHz
- Treble: a) -12 dB @ 1.85 kHz, b) -12 dB @ 5.35 kHz.
- Mid control: a) -3 dB half-power (?) @ 320 Hz b) -3 @ 1.6 kHz.
- Low: -3 dB high pass @ 195 kHz

I surely don't have the best grasp on electronics but how did you manage to change two frequencies for treble and two for mids?

Also, could you share you initial 31-band EQ curve that you wanted to be mimiced in MP-1?

[Kazinator]

Hey, Kaz,

To sum up: are these your final changes to the MP-1 tone stack?

- Presence: low pass @ 6 kHz
- Treble: a) -12 dB @ 1.85 kHz, b) -12 dB @ 5.35 kHz.
- Mid control: a) -3 dB half-power (?) @ 320 Hz b) -3 @ 1.6 kHz.
- Low: -3 dB high pass @ 195 kHz

I surely don't have the best grasp on electronics but how did you manage to change two frequencies for treble and two for mids?

Also, could you share you initial 31-band EQ curve that you wanted to be mimiced in MP-1?

The Mid and Low look right.

Treble is centered on 3.1 kHz, with a bandwidth around from 2.5 kHz to 4.0 kHz, those being the half-power points.I carefully tweaked this not to have a wide volume range so the steps are small.

I swapped an R-C pair in the Presence so that it is a two-pole high pass, rather than bandpass. -3dB is at 5.0 kHz. I tend to have it at -8.

Here are LTSpice screenshots of all the simulation work for the Mid and Presence. In both images, the red bode plot corresponds to the IMPLEMENTED circuit.  I think in the Presence, the green one is stock. I experimented with a higher and lower cutoff (around 6 kHz snd 4.5 kHz) before settling on 5.

You can see the component values, but the part numbers don't correspond go the MP-1 schematic, unfortunately. After swapping them so many times, I basically had the layout memorized. :)

If you look at the Treble, you can see that there is a "runner up" circuit ("out4") plotted in aqua blue. Compared to the red plot, the only thing is that the peak goes higher. The center freq is exactly the same. I built that, like the blue one, but the steps were still too large.  I wanted an in-between boost between 0 and +2 for a less pronounced touch of bite.  If you compare the schematics, the only thing different is one of the feedback resistors: 8.2k vs 5.1k. In that Sallen-Key-with-feedback architecture, the feedback controls the "Q" of the filter without changing the frequency center.  (None of the configurations shown here corresponds to stock; sorry.)

[Kazinator]

Here are the LTSpice screenshots of circuits and frequency plots for the work with the Mid control.

The one I went with is not red; it's the aqua-colored highest curve. This filter kernel actually goes to +3dB. This allows it to have quite a range for an aggressive cut. I do use it at -12!

Again, none of the curves shows the stock configuration here. All mods that I tried.

[Kazinator]

In post #14 in this topic, I wrote about having Marshall-conditioned ears. I looked into that a little bit again. Basically, I had settled on the Mid circuit values by ear: simulating and building it a number of times, adjusting by ear. I hit upon that 320 Hz to 1.6 kHz dip with a 715 Hz center.

Just today, I banged up a simulation of a JCM 900 tone stack, with the pots set at 50%. If we look at the points in the dip that are about -3dB down fom the highest treble level on the right, they are almost bang on the same: around 315 Hz to 1.6 kHz, center at 700 Hz.

The Marshall tone stack will move the dip a bit to the right when we crank the Mid down, and it takes away overall treble also. We don't get any interactivity of that sort on the MP-1 EQ, whether stock or redesigned.  The Mid stays centered where it is, and the Bass and Treble are too far away to have much of an influence.