OD-1 with opamp buffers

[micmac]

Hi all,

Fan of the SD-1 here. I built the PedalPCB Oddity and am actually liking it a lot. Sure, no tone pot, but the way it's set is fine. It's tight (Tuco from Breaking Bad comes to mind). And one less knob to worry about.

After I built the Oddity I put the circuit on a bread board. I replaced the first BJT buffer with a J201, also increased the biasing resistor to 1M for a bit more input impedance. Sounded the same, a little less background noise. Then I just changed both transistors out for a dual FET-input OPAMP, because why not.

I'm aware that at first there was an OD-1 with a quad opamp version and that people are raving about it. Also found a writeup by analogman where he said "I found the main difference was capacitors, several smaller value capacitors were used, sucking away low end and making a thin tone."

But when I go around the schematics of two OD-1 versions and do the math on the high and low pass filters, although the caps differ in value, the results are actually exactly the same, except for the high pass at the output, which doesn't matter.

He adds that the quad opamp doesn't matter a whole lot ("it sounded a little better but almost the same aswith just the capacitor mods").

Well, I certainly drew my own conclusions from all of this The quad opamp version had a different opamp and about half the input impedance. Other than that it's the same.

I also tried a mod from Mark Hammer at diystompboxes, where he replaced the 720Hz filter with one along the lines of the ODR-1. But that made the OD-1 go from tight to flubby, so I reverted it again.

I ended up with a bunch of small utility mods, nothing that changes the sound of the OD-1. Except, I added a socket for a small pF cap into the feedback loop of the drive stage, in case I want to tame the high end a bit in the future.

Be careful, the picture with the cuts is already mirrored!

 

[micmac]

Actually, let me add one more thing. I was also playing with status LED switching on the breadboard, because I had seen people add a cap sort of into the LED resistor, like here for instance. So I tried it out, but I could still hear the switch action.

Then I had this idea to do the switching not directly on the ground connection, but instead between the LED and the LED resistor. And that actually worked quite nicely, most often I couldn't hear the LED switching at all, only sometimes a little noise came through.

I'm sure everybody has seen a 3PDT wired for true bypass. So just check the switch in the picture for the difference. Left column is input, right column is for LED switching. Normally we have a jumper between the lower left to the lower right and ground connected to the middle lug of the LED column. But below I just connected ground to the lower left plus the LED resistor, which then goes to the right and connects to both the bottom and the middle lug. So now the LED switching happens behind the LED resistor, causing less noise.

Try it, you might like it!

 

[jubal81]

Putting the 100pf capacitor in parallel with the drive knob makes the low pass filter drop as you raise the gain. At max gain, the cutoff frequency is about 1.5Khz. At minimum gain it's like 48khz.

It's going to get a lot darker as you turn up the gain.

 

[micmac]

I know

 

[micmac]

Actually, 100p is barely noticeable, even cranked. I sort of liked 330p. But I play Volume high and Drive at 9 o'clock, so that's why. But I mean I just put a socket, it's not baked it. Can be changed to taste.