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PostPosted: Fri Feb 07, 2014 2:50 pm 
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I've spent the last couple of evenings fooling around with various modifications to my build of the new Parametric Overdrive. My starting point was the mods described by Keith on pages 29 & 30 of the ParaOD instructions, followed by some e-mail dialog with Keith on various ideas for some additional things to try. These included:

  1. Various resistor/capacitor combinations at R22/C11 together with the additional parallel RC pair in the two spaces shown in blue on page 29. This allows you to tailor the frequency response through the gain & clipping stage, as well as changing the maximum gain.
  2. Substituting a 5K trimpot for the R25 resistor to investigate the effect of varying the series resistance on the diode clipping distortion character.
  3. Installing three toggle switch selectable diode combinations for the D2 clipping diode to give symmetrical clipping and two different flavors of asymmetric.
  4. Changing the stock B100K (linear taper) drive pot to an A100K (audio taper) to improve the usefulness of the control at lower gain settings.

I socketed a bunch of the component positions for this experimentation, since it makes trying various caps & resistor values so convenient. Here are my comments for the various mods I tried:

  1. I spent the most time playing around with the RC combos in the op amp feedback loop. If you're familar with the Tube Screamer gain & clipping stage, you'll recall that the component values in the RC pair not only defines the bass rolloff frequency (it's a high pass filter), but the resistor value is directly related to the overall gain of that op amp stage. The ParaOD goes one better by offering you the option of installing a second RC pair in parallel to the first (those blue spaces on pg. 29), very much like the ProCo Rat circuit uses. Therefore, there are two high pass filters present in the op amp feedback loop, and the overall gain depends on the reciprocal of the sum of the reciprocals of the two resistor values. Confusing? Check out this excellent article on how the Rat works if you want to see how the math works: http://www.electrosmash.com/proco-rat

    Keith recommends the use of a 1K resistor and 1uf cap in the second RC position (along with a 22n cap at C1) in order to boost the stock pedal's bass response a bit and give an almost perfectly transparent overdrive. At the same time, the two 1K parallel resistors cut the overall resistance of the two pairs in half, which essentially doubles the maximum gain of the op amp stage. I found that the extra gain made the already sensitive Drive pot even touchier down at low gain settings and more difficult to dial in lighter amounts of overdrive. Consequently, I tried a number of different resistor and cap combinations in the two spots, using higher resistor values to keep the gain down near the stock pedal value, and then adjusting the paired cap value to maintain the same filter frequency rolloff values. Keith and I weren't really sure if these component value changes wouldn't have other unexpected effects on the pedal's tonal response, but I really didn't hear any change in the frequency response and the gain was reduced as desired. But even then, the low end of the Drive pot's sweep was touchier than I like....but more on that later.

  2. Something Keith and I discussed briefly was the possibility of generating asymmetric clipping by changing the value of one of the resistors that are in series with each of the clipping diodes (R24/25). Rather than just changing one to a different value of fixed resistor, I installed a 5 Kohm trimpot in one of the positions (again, doesn't really matter which). This allowed dialing in a higher or lower resistance than the stock 1K value. Surprisingly to me, when I tried out the trimpot set-up, I could hear almost no difference in tone across the whole sweep of the trimpot. So I just dialed the resistance back to 1K and left it at that.

  3. For the switchable clipping diode setup, I removed one of the two clipping diodes from the PCB (doesn't matter which, but be sure to keep your polarities straight!) and hooked up an on-on-on DPDT toggle switch wired to three different diode combos (1x1N914, 2x1N914, 2x1N34A) on a small separate piece of vero board mounted aginst the underside of the top of the enclosure. The switch is installed in the space between the PCB and the cover plate of the enclosure, coming out through the side adjacent to the Drive control. This set-up gives all three of the clipping diode combos mentioned in the pedal instructions, rather than having to limit the pedal to only one.

  4. The last change I made was to address the Drive pot's sensitivity at low gain settings. This was a no-brainer: if the gain comes on too fast with a linear taper pot, try an audio taper. Most similar overdrives use an audio taper pot for gain control anyway. I asked Keith why there was a B100K in there and he said it was just a matter of staying true to the original Pearl pedal configuration. So I changed it to an A100k pot and that did the trick nicely--the gain is now easily controllable at the low end, but comes on strong in the second half of the sweep. This allowed me to go back to Keith's recommend resistor and cap values in the parallel RC pairs and have the higher maximum gain available. Nice!

So at this point, I'm very happy with the performance of the pedal and I don't plan to make any further changes. AAMOF, when I get around to finishing the enclosure, I'll probably remove the sockets on the PCB and solder in the the components I settled on. To briefly summarize where I ended up:

  • Implemented Keith's recommended mods # 1, 2 and 3 on pg. 29 of the ParaOD instructions.
  • Installed a 3-way switchable clipping diode set-up.
  • Changed the Drive control to an A100K pot for better low-end gain control.

Here are a couple of close-up photos I took of the modifications:


Attachments:
ParaOD_mods_3.jpg
ParaOD_mods_3.jpg [ 119.64 KiB | Viewed 3091 times ]
ParaOD_mods_1.jpg
ParaOD_mods_1.jpg [ 124.13 KiB | Viewed 3091 times ]

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PostPosted: Mon Aug 01, 2016 10:35 am 
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Joined: Sun Feb 22, 2009 1:33 pm
Posts: 24
Great info as usual, DVM.
I'm going to incorporate some of your results into my build. I'm going to use different clipping diode options via a GuitarPCB Roto-Tone board/switch to get one additional option - just have to figure what the extra one might be.
Questions I have regarding the BYOC mods 1,2, and 3 you suggested after all of your noodling are why do both BYOC mods 1 (tighter low end) and 3 (more low end) and similarly mods 2 (less gain) & 3 (more gain)? They sound like they would counteract each other. Also, do you think adding a switch for the various C1 input capacitor values would be worthwhile?
Thanks.


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