# Help me understand the ODR Bass Cut Mod



## spi (Aug 2, 2022)

There's a filter in the feedback loop of one of the ODR-1 gain stages that sets the frequency content.  It looks like this.





One common complaint of the ODR-1 is that it is too boomy in the bass, and one mod is to increase the 1K5 resistor value, as is done in Aion's version:





I think this is supposed to be a high-pass filter.   If I'm correct, the formula for the corner frequency is 1/2*pi*R*C.  For the stock circuit, the 820R+82n corner is 2368Hz, and the 1.5K+2u2 corner is 48Hz.

Here's where I get lost:   increasing R should decrease the corner frequency, in this case fully dialing in the 50K pot results in the cutoff of the second corner dropping from 48Hz to 1.4Hz.  *Shouldn't this allow more bass, not less?*

What am I getting wrong here?


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## HamishR (Aug 3, 2022)

Thinking about it. No good will come of it. Believe me I've tried.


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## music6000 (Aug 3, 2022)

spi said:


> There's a filter in the feedback loop of one of the ODR-1 gain stages that sets the frequency content.  It looks like this.
> 
> View attachment 30209
> 
> ...


Are you about to build this or want to mod the PedalPCB PCB?


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## phi1 (Aug 3, 2022)

You’re right that the R in this position determine the corner frequency. But the R also controls the gain. In this position, with higher R reducing the gain. 

Changing the corner frequency from 48hz to lower won’t be audible on guitar anyway, so only the gain reduction will be audible. The 820R+82n path won’t be affected, so turning the bass knob essentially just attenuated anything below 2368Hz. 

This is how the Timmy bass knob works too. 

The electrosmash articles are pretty helpful. I’ve read through parts of the tube screamer one several times. They can be a bit hard to follow at first, but they’re broken up nicely into sections. 

For the aion nobels, 2368hz seems really high to me for the unaffected path, like turning the bass down will cut mids too.  That’s just theoretical, I haven’t tried the circuit.


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## Big Monk (Aug 3, 2022)

phi1 said:


> But the R also controls the gain. In this position, with higher R reducing the gain.



This. Gain reduction = bass reduction in many cases.

With no other mods, gain reduction works to reduce the flubbiness in a stock Fuzz Face circuit as well.


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## phi1 (Aug 3, 2022)

In this case I’m not saying general gain reduction sounds like bass reduction (flubbiness reduction), although that can be an effect. 

In the aion nobels mod, the gain reduction is targeted on the frequencies lower than 2368Hz. (It will have a small effect on the higher frequencies too, since the 1k5 and pot are in parallel with the 820R, but less drastic gain reduction than the effect on the lower frequencies).


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## spi (Aug 3, 2022)

music6000 said:


> Are you about to build this or want to mod the PedalPCB PCB?


I'm about to build the Pro-10.  My question was motivated by the tweak done to their version (which was to add 3rd filter, with a cut-off between the other two).

I am probably not going to mod it, but I was thinking of socketing the resistor in case I ever decide to fine tune it.


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## phi1 (Aug 3, 2022)

Socketing the caps is an idea too, to change the freq of each path without affecting gain


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## Alan W (Aug 3, 2022)

And there's also the opinion that the Nobels is NOT bass heavy—it's relatively flat, compared to many of the popular pedals that have either large mid boosts or trim the bass to keep it from getting mushy. So, people used to those pedals try the Nobels and find it too bassy.


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## music6000 (Aug 3, 2022)

spi said:


> I'm about to build the Pro-10.  My question was motivated by the tweak done to their version (which was to add 3rd filter, with a cut-off between the other two).
> 
> I am probably not going to mod it, but I was thinking of socketing the resistor in case I ever decide to fine tune it.


Socket the resistor would be my suggestion, I have built 2 of the Aion & you can get the same response as a Nobels ODR-1 with the Bass knob at full clockwise position.
I run mine at 2.00 to 3.00 O'clock  meaning 5.00 O'clock is full clockwise.


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## Chuck D. Bones (Aug 25, 2022)

You have to look at a slightly bigger picture.  I started to type out an explanation, but it got way too wordy.  This pic tells the story much more clearly.  The green trace is the gain at IC1-1 with the stock values shown in the schematic below.  The blue trace is the gain when R7 has been increased by 50K.  If we put a C50K pot in series with R7, we can sweep from the blue trace to the green trace.  That's what the Aion mod does.  GAIN is dimed in this simulation.

Let's examine the green trace in a little more detail.  See where the green trace starts to roll-off above 3KHz?  That's the effect of C3.  At lower GAIN settings, the roll-off starts at a higher freq.  The gain drops below below 3KHz because the impedance of C4 increases as freq goes down.  The gain starts to level off around 700Hz as R7 & C5 become dominant.  Below 200Hz, C5's impedance is large enough to come into play and the gain start dropping again.

Make sense?


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## spi (Aug 26, 2022)

Thanks for the writeup and plots.   I'm sure it makes sense but I'm still confused.  From my recollection of electronics class (some 30 years ago), a path through a capacitor to ground allows higher frequencies to have an easier time passing to ground (and hence it acts as a low pass filter).  What confuses me about this arrangement is that it acts more like a high-pass filter because the lows are being taken away.  Like you said, below 200Hz the C5 impedance is high--wouldn't a high impedance mean less signal to ground, so more low frequency present?

I'm curious if you can add the pro-10 mod to your model and plot it along with the green and blue lines? Would be interesting to see what it accomplishes.



y).


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## phi1 (Aug 26, 2022)

You’re right that higher frequencies have an easier time going through caps than lows. But, In that portion of the op amp circuit it, less impedance = more gain.

Attached is a very basic picture of the gain calc. Now if there is a cap in series with R2 (there basically always is in pedals), then we’ll think of it as impedance (Z2). Impedance is basically a way to express resistance that changes depending on frequency.

For low frequencies, Z2 becomes high (because low frequencies struggle to get through the capacitor). If Z2 is high, then gain is less according to the equation.

For high frequencies, Z2 is lower (basically just R2), because the cap doesn’t impede highs. So gain is higher.

The familiar F=1/(2piRC) governs the cutoff frequency of this filter.


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## spi (Aug 26, 2022)

Ok, I think I'm starting to get it:  It sounds like these filters aren't acting like tone knobs, instead they're playing a role in setting op-amp gain, which is set by the impedance in the feedback loop (so varies by frequency).  Is this a fair explanation?


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## Chuck D. Bones (Aug 26, 2022)

Yes.


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## spi (Aug 28, 2022)

I took some time this weekend to learn enough LTSpice to try to simulate this gain block.  Not sure if I succeeded, but I have some plots.   

Here's the stock ODR-1 (gain knob roughly midway):





Here's the Aion mod, with the bass cut fully up:





And here's the Pro-10 mod:






The Pro-10 seems to shift the peak of the gain about 3kz to 2kz.  It still has a pronounced bass:  At 100Hz, they're both around 30db.  But the stock peaks at 38db, while the Pro-10 peaks at 45db, so the bass level relative to the overall frequency range seems to be slightly diminished.


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## Chuck D. Bones (Aug 28, 2022)

Looks good!

For better resolution and smoother curves, make your AC sweep statement look like this:

.ac dec 1000 20Hz 20KHz  

This says to perform a decade (logarithmic) freq sweep.  1000 point per decade, from 20Hz to 20KHz.   SPICE runs very fast when performing an AC sweep because it only has to calculate the DC bias points once.  Calculating 3000 points (3 decades x 100 pts per decade) takes only a few seconds.

You can easily turn off the phase plot; it's usually of no interest and clutters up the display.  To do that, just right-click on the phase scale (right hand side of the plot).  When the dialog box pops up, click the Don't Plot Phase button.  You can turn it back on the same way.  Every time you start LTSPICE, it defaults to displaying phase.

Maybe you already know this:
The GAIN pot is A-taper.  Most audio taper pots are 15% taper, which means that at noon, the resistance ratio is 15%.  At noon, an A100K pot will measure 13K from pin 1 to pin 2 and 87K from pin 2 to pin 3 because 13K / 87K = 15%.  It will never be exactly those values because pots have loose tolerances.


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## spi (Aug 29, 2022)

Thanks, I'll use those sweep settings next time.

Like an idiot I forgot to save the file before I closed it, so don't recall what I actually did.  I think I used the octave sweep.

And I think I just used 100k in where the gain pot is, because ODR-1 uses a A250K and I wanted something in the middle of its range.  But I just checked and see the Pro-10 chose to use A100K there, so the last plot is at the end of its range.

I selected OP07 for the op-amp (doubt that it matters though), and I left the clipping diodes out of my model.


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## Chuck D. Bones (Aug 29, 2022)

OP27 is another good choice.  The diodes don't do anything in the AC analysis, so it's usually fair to leave them out. The only exception is Ge diodes because they are leaky and affect the gain.  Leave them in when you run and AC analysis or the results will be wrong.  In AC analysis, SPICE calculates the bias points, then calculates the transfer functions and outputs the freq response at every point in the circuit.  Unlike the TRAN analysis, the signal level is essentially zero in an AC simulation.


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