Help understanding Thor EQ

[erick4x4]

So I have a Thor that I like, but still feel like I need to tweak the EQ. I have a marshall Plexi, and I am really trying to use the thor as my plexi option when I can't have my amp and rely on random backlines (like when I fly places to play).

So can anyone walk me through what is happening after the mu-amp stage? I am a little confused with the opamp. I see components similar to a tone stack, but would first and foremost love to know what is affecting what just cause I am curious, but most importantly what I could tweak to cut some highs, or even convert to a 2 or 3 knob tonestack.

Thanks guys!

[stm]

The opamp section of Thor is not really a tonestack, but a fixed tone shaping circuit.  Well, in fact it is two circuits in one, which we noticed it was possible to combine the two circuits into a single opamp instead of resorting to two opamps.

The components that go into the (+) input of the opamp form an active 2nd order low-pass Butterworth filter.  This has better high-frequency rejection than the simple two-R and two-C in series used on the previous circuits, when compared for the same -3dB cutoff frequency.  So, if you just want to cut more highs it is a simple matter of increasing the 1nF and 470pF caps to 1.5nF and 680pF, or even 2.2nF and 1nF--the idea is to keep the approximate 2:1 ratio between these caps.

Now let's consider the components around the (-) input of the opamp.  When the BOTTOM switch is CLOSED you get a unity gain buffer from the opamp, which is part of the low-pass filter described above.  When you OPEN the BOTTOM switch, you add a bridged-tee network (BTN) in series with the feedback path.  The BTN has unity gain at very low and very high frequencies, and cuts around a particular frequency, which in this case was chosen around 125 Hz.  As the notch is set around 8 dB down, the opamp has to provide 8 dB of BOOST around 125 Hz in order to still get unity gain at the (-) input.  In summary, the bottom part of the circuit is a fixed bass boost which models a speaker cabinet resonance at this frequency.   If you want more or less boost you can change the 33k to ground perhaps between 10k (more boost) and 100k (less boost) with moderate interaction with the boost frequency.

Hope someone is still awake reading this!

Finally, if you need extra flexibility, you might try adding a Marshall tonestack after the opamp.

[MartyMart]

Continuing on from what Sebastian has said, I did the following to offer a bit of
tone control :
Replace the 33k to ground from the junction of the 22n's with a pot wired as a variable resistor.
I used a 22k pot with a 10k fixed resistor, but perhaps a 50k or 100k may work, IIRC I tried a 100k
and it caused some very "odd" whistling at extreme settings !  beware !
It works well as a treble cut, i wouldn't say that it boosts the top end much, i run it at about 4/5ths
of rotation for a nice warm tone.

MM.

[erick4x4]

The opamp section of Thor is not really a tonestack, but a fixed tone shaping circuit.  Well, in fact it is two circuits in one, which we noticed it was possible to combine the two circuits into a single opamp instead of resorting to two opamps.

The components that go into the (+) input of the opamp form an active 2nd order low-pass Butterworth filter.  This has better high-frequency rejection than the simple two-R and two-C in series used on the previous circuits, when compared for the same -3dB cutoff frequency.  So, if you just want to cut more highs it is a simple matter of increasing the 1nF and 470pF caps to 1.5nF and 680pF, or even 2.2nF and 1nF--the idea is to keep the approximate 2:1 ratio between these caps.

Now let's consider the components around the (-) input of the opamp.  When the BOTTOM switch is CLOSED you get a unity gain buffer from the opamp, which is part of the low-pass filter described above.  When you OPEN the BOTTOM switch, you add a bridged-tee network (BTN) in series with the feedback path.  The BTN has unity gain at very low and very high frequencies, and cuts around a particular frequency, which in this case was chosen around 125 Hz.  As the notch is set around 8 dB down, the opamp has to provide 8 dB of BOOST around 125 Hz in order to still get unity gain at the (-) input.  In summary, the bottom part of the circuit is a fixed bass boost which models a speaker cabinet resonance at this frequency.   If you want more or less boost you can change the 33k to ground perhaps between 10k (more boost) and 100k (less boost) with moderate interaction with the boost frequency.

Hope someone is still awake reading this!

Finally, if you need extra flexibility, you might try adding a Marshall tonestack after the opamp.

Thank you very much. I think the first thing I will try is to use a DPDT switch to give me 2 treble cut options for the 1n/470p caps. I may also tweak the bass boost to be a little more subtle.

And if I still want a traditional tone stack after that would I insert it between the opamp and the 100n capacitor? I know this wouldn't have recovery, but it seems like this circuit has more than enough volume, so I don't think it would matter right?

[stm]

For a more subtle bass boost you may try changing the 100k/33k resistors in the (-) feedback path with 68k/47k, respectively (notice the product of the two resistors is kept constant, i.e. 100*33 ~ 68*47 in order to retain the frequency of the peak).

If the tonestack eats too much signal you may add a second opamp after it with a gain of x2 or x3.  You'd still be using a DIP8 package for the opamp.

[czech-one-2]

The opamp section of Thor is not really a tonestack, but a fixed tone shaping circuit.  Well, in fact it is two circuits in one, which we noticed it was possible to combine the two circuits into a single opamp instead of resorting to two opamps.

The components that go into the (+) input of the opamp form an active 2nd order low-pass Butterworth filter.  This has better high-frequency rejection than the simple two-R and two-C in series used on the previous circuits, when compared for the same -3dB cutoff frequency.  So, if you just want to cut more highs it is a simple matter of increasing the 1nF and 470pF caps to 1.5nF and 680pF, or even 2.2nF and 1nF--the idea is to keep the approximate 2:1 ratio between these caps.

Now let's consider the components around the (-) input of the opamp.  When the BOTTOM switch is CLOSED you get a unity gain buffer from the opamp, which is part of the low-pass filter described above.  When you OPEN the BOTTOM switch, you add a bridged-tee network (BTN) in series with the feedback path.  The BTN has unity gain at very low and very high frequencies, and cuts around a particular frequency, which in this case was chosen around 125 Hz.  As the notch is set around 8 dB down, the opamp has to provide 8 dB of BOOST around 125 Hz in order to still get unity gain at the (-) input.  In summary, the bottom part of the circuit is a fixed bass boost which models a speaker cabinet resonance at this frequency.   If you want more or less boost you can change the 33k to ground perhaps between 10k (more boost) and 100k (less boost) with moderate interaction with the boost frequency.

Hope someone is still awake reading this!

Finally, if you need extra flexibility, you might try adding a Marshall tonestack after the opamp.

Hi, thanks for the info on the 1nf / 470pf swap to cut more highs. I used a 2.2nf/.001uf and it now sounds perfect with my bright roland jc-50.
however, i changed the 33k to ground to a 10k resistor and the thing wistled like a boiling tea pot when the bass boost switch was engaged.
so i just eliminated the switch for now. any idea why my THOR didnt like the 10k?