Fred Briggs - Super OCD

briggs · Post by **briggs** » 28 Feb 2012, 16:39

From my blog:

The Fulltone OCD has been an overdrive favourite for many years now. It's got a really nice tone when your amp is set right (I prefer it with Marshalls I must say). Well, I've always thought about modifiying the basic circuit (which itself is based around the old Voodoo Labs Overdrive) to give it a little more variety and use over a wider spectrum of tones.

As you can see the "Super OCD" has a few extra controls:

- Firstly the "Bass" control - it's nicked from Paul C's Timmy pedal and works very well to add in pregain bass for more low end response.

- Secondly the "Saturation" control, this allows you to increase the clipping headroom of the circuit and have an overall "cleaner" tone while dialling in just the required amount of diode clipping.

- Thirdly the "Boost" control, I've configured the second opamp to allow a range of clean boosts to be selected to really allow you to push the front end of your amp into overdrive.

- The final control I've added is a switch to go between 9 volt and 18 volt operation. The 18 volt operation adds extra headroom to the circuit to stop the opamp clipping from fogging up the tone. The 9 volt operation retains the original headroom seen in the OCD.

- Other changes include alterations to the Tone control and a swap of opamp from the TL082 in the original OCD to the Burr Brown OPA2134. These changes all add up to create a really nice overdrive/distortion box. It's got all the original Fulltone OCD tones in there and loads more. If you like the original OCD you'll love the Super OCD

Post by **azrael** » 28 Feb 2012, 16:59

Nice, I recently built an OCD variant with many of the same ideas.
Slightly different tone control though.

beedotman · Post by **beedotman** » 28 Feb 2012, 17:54

Looks really nice!

briggs · Post by **briggs** » 28 Feb 2012, 18:03

azrael wrote:Nice, I recently built an OCD variant with many of the same ideas.
Slightly different tone control though.

The tone control is just one I settled on, there are loads of options really. You could even go for a full TMB type, or a Big Muff type, anything you like at all!

Post by **azrael** » 28 Feb 2012, 19:04

Oh yeah, for sure. The SWTC you used is already a big improvement on the stock one, IMO.

I used a different variation on the SWTC.

crash415 · Post by **crash415** » 28 Feb 2012, 19:25

Fred, I love your pedal mods. The COT50 mod is just wonderful. I have 2 questions on this OCD, basically just to help me understand the circuit better.

The 10k/100microF on the boost stage, doesn't that make a really low corner frequency for a low pass filter?

Trying to understand the purpose of the boost pot. Normally, there is a 150K resistor in that spot on the original, so would this really be doing any boosting?

Thanks

Post by **azrael** » 28 Feb 2012, 21:41

It's 10k/1 microfarad.

The original has like...39k/100n, IIRC. So with a 150K in the loop, it had a gain factor of around 4-ish.

Fred's is variable from unity gain to a gain factor of 10.

briggs · Post by **briggs** » 28 Feb 2012, 22:26

azrael wrote:It's 10k/1 microfarad.

The original has like...39k/100n, IIRC. So with a 150K in the loop, it had a gain factor of around 4-ish.

Fred's is variable from unity gain to a gain factor of 10.

Aye, that's your answer. Here's the formula for voltage gain in a non-inverting opamp - Vout = Vin*((Rf/Ri)+1) So to get opamp gain divide the loop resistor (the setting of the 100k pot + the fixed 10k in this circuit) with the resistor to ground/bias voltage (10k in this example), therefore the min gain is 2 ((10k/10k)+1) and the max gain is 12 ((110k/10k)+1). So with this you can get more or less boost. The 10k 1uF just sets it to flat boost the full range of frequencies, you could probably tailor this to cut out frequencies below 50-60Hz or so if you wished but it doesn't make that much of a difference...

For more info on opamps here's a decent page for reference: http://www.bcae1.com/opamp.htm

azrael wrote:Oh yeah, for sure. The SWTC you used is already a big improvement on the stock one, IMO.

I used a different variation on the SWTC.

Out of interest what variation of the SWTC did you use azrael? The treble boost/cut?

cpm · Post by **cpm** » 28 Feb 2012, 23:48

The OCD tonestack is where it has its characteristic sound. If you change it there is no more OCD, imho, just a more generic clipper (as good as it can be)
Actually, i didnt like at all that OCD tone, way too bassy with my gear

crash415 · Post by **crash415** » 29 Feb 2012, 03:22

I got the gain part, I was looking at the 10k resistor going to the opamp as part of the gain equation.

Still have a question on the filter in the boost stage. the 10k/1micro has a corner frequency of 16 Hz. You are saying this is a high pass filter? I always thought that was a low pass filter with a cap to ground, or is it different because the cap is going to Vr?

briggs · Post by **briggs** » 29 Feb 2012, 09:29

cpm wrote:The OCD tonestack is where it has its characteristic sound. If you change it there is no more OCD, imho, just a more generic clipper (as good as it can be)
Actually, i didnt like at all that OCD tone, way too bassy with my gear

The "Tonestack" it just a lowpass filter, there's nothing special about it, it's corner frequency is adjusted via the switch and the amount of roll off is set by the pot. The SWTC does exactly the same thing but with more grace... And it's too bassy with your gear - That why I added the Bass control, specially for you

crash415 wrote:I got the gain part, I was looking at the 10k resistor going to the opamp as part of the gain equation.

Still have a question on the filter in the boost stage. the 10k/1micro has a corner frequency of 16 Hz. You are saying this is a high pass filter? I always thought that was a low pass filter with a cap to ground, or is it different because the cap is going to Vr?

It's not working in the typical manner that a low pass would. Basically in the non-inverting setup any signal that the opamp sees at it's inverting input that is also present at it's output gets cancelled back to unity gain. Now, the low pass is in the feedback loop of the opamp, so it's sending those frequencies (anything higher than 16Hz) to ground/Vr (same thing in this config) therefore the opamp doesn't see them at it's inverting input and amplifies all those frequencies above 16Hz. So all those frequencies just stay at unity gain, that is exactly how the 100pF cap does it's job too, it passes back high frequency content (corner freq determined via the setting of the gain/boost pots) to the inverting input and cancels it's gain back to unity. This is how tubescreamers work, they just use diodes with a clipping threshold of 0.7 volts to do the cancelling, incidentally it's also why it's called soft clipping because the signal isn't cut off at 0V but is just reduced in gain back to where it started at unity... I hope that helps, though some of the explanation is a little muddled

TimothyT · Post by **TimothyT** » 29 Feb 2012, 10:20

hey, since it's on a topic on OCD and i've been wanting to mod my OCD, hope this is the right thread to post, if not, sorry

I've realised the bass is really heavy for the v4, is there anyway i can mod it in a way to become less bassy?

briggs · Post by **briggs** » 29 Feb 2012, 10:30

Well, I believe the V4 has a 68nF cap in the loop to Vr on the first op amp, if you want less bass reduce the value of that cap.

TimothyT · Post by **TimothyT** » 29 Feb 2012, 10:34

alright! Will try it soon, thanks for the info!

crash415 · Post by **crash415** » 29 Feb 2012, 16:51

Thanks Fred for the explaination. It makes sense after reading that link about opamps.

Keep up the great work!

Post by **azrael** » 01 Mar 2012, 02:35

briggs wrote:Out of interest what variation of the SWTC did you use azrael? The treble boost/cut?

Yep, that's the one.
The OCD has plenty of bass to spare when its operating a bit like a high pass filter, and honestly needs a lot more treble.
Stock, the OCD has always had the tone control on full blast for me. SO dark.

Maybe you can answer this for me briggs, I've never really understood it.

So with a 1M pot, and a 2.2k resistor, the theoretical gain is like 450. But it obviously isn't that high. I believe this is due to some sort of limiting due to the treble roll off cap? Is there an equation for this?

Thoughts on rail to rail opamps, in pedals like this, also?

briggs · Post by **briggs** » 01 Mar 2012, 09:55

azrael wrote:
briggs wrote:Out of interest what variation of the SWTC did you use azrael? The treble boost/cut?
Yep, that's the one.
The OCD has plenty of bass to spare when its operating a bit like a high pass filter, and honestly needs a lot more treble.
Stock, the OCD has always had the tone control on full blast for me. SO dark.

Maybe you can answer this for me briggs, I've never really understood it.
So with a 1M pot, and a 2.2k resistor, the theoretical gain is like 450. But it obviously isn't that high. I believe this is due to some sort of limiting due to the treble roll off cap? Is there an equation for this?

Thoughts on rail to rail opamps, in pedals like this, also?

The 220pF cap just sets the corner frequency for the treble feedback (and therefore gain reduction) in tandem with the 1M pot (It's value is the variable which means as the gain increases the treble roll off is greater). The formula is the same as all frequency roll of calcs. This chart may come in use too - it shows you how AC signals of different frequencies "see" capacitors (As resistors!): http://analogguru.an.ohost.de/techstuff/Reactance.htm

Seeing as though at max gain the corner freqs of the 2k2/68n network and the 1M/220pF network overlap you can see how the gain is reduced from the theoretical gain of 450 as no AC frequencies will see this.

beedotman · Post by **beedotman** » 03 Mar 2012, 19:56

Here's layout that I've drawn today. Version without charge pump.
Not verified yet... Remarks, suggestions, corrections are welcome!
Pls remember to add protection diode (I've forgot about it during drawing).

sinner · Post by **sinner** » 03 Mar 2012, 20:11

That's really cool layout!

roseblood11 · Post by **roseblood11** » 03 Mar 2012, 22:41

but why do you bend one leg of the saturation trimpot? solder it and add a trace cut right to it - improved stability...