Is this normal? (OCD 9 vs 18 volt headroom question)

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stevejamsecono
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Is this normal? (OCD 9 vs 18 volt headroom question)

Post by stevejamsecono » Wed Apr 04, 2018 10:25 am

So I recently started using my OCD at 18 volts just to try something different, and although it sounds different and cool it seems to have had something of a cascade effect. I have been running one of the older SD Pickup Boosters after my OD pedal as a solo boost, and suddenly said boost is WAY louder than it used to be. I was wondering if this was a typical byproduct of the "increased headroom" of the OCD at 18v, or if I should be concerned. Note that the older SD Boosts have a minimal setting of 6db so I can't really turn it down to compensate for it if this is the case.

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Re: Is this normal? (OCD 9 vs 18 volt headroom question)

Post by my bloody television » Wed Apr 04, 2018 12:24 pm

I just started running my OCD at 18v as well. I'm really liking the added headroom and clarity...I've always run the gain at pretty much zero but thought that it was still a bit too much gain for my liking. At 18v I seem to have hit the sweet spot boost I was looking for. I run a Fulltone '69 right before it and have not noticed an increase in volume when they're both engaged from before. I switch between the two pedals for different tones though, I don't really use one to boost the other for solo sounds except on maybe one song and it's not really a volume boost, more of a gain boost.

Have you tried reversing the order to hear what it sounds like pushing the OCD with the SD?

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Re: Is this normal? (OCD 9 vs 18 volt headroom question)

Post by oid » Wed Apr 04, 2018 12:50 pm

stevejamsecono wrote:
Wed Apr 04, 2018 10:25 am
I was wondering if this was a typical byproduct of the "increased headroom" of the OCD at 18v
In the OCD and other opamp based distortions the supply voltage is a limiting factor in headroom. Most opamps can output fairly close to their supply voltage so if you have an opamp running on 9 volts will get a bit over 8 volts on the output. Say that opamp is wired to have a gain of 20 and you feed it 1 volt in, that is 20 volts, but it can not swing beyond the power supply rails, so we get a heavily distorted 8ish volts on the output. Double the supply and we now have about 17 fairly clean volts on the output.

In the OCD you also have clipping diodes (MOSFETs wired as diodes) on the output to further limit the output so you do not get the full output of the opamp, but the clippers are biased up to raise the clip point and this bias voltage also goes up with an increased supply voltage.

So in this case you do increase the headroom, no need for quotes here. I do not have the schematic handy at the moment so going from memory, I think in this case you do actually double clean headroom by going from 9 to 18 volts.
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