distortion box - low end rolloff

[Bob M.]

I've built a distortion unit into an instrument I use all the time that benefits from occasional use of this sound. I started with something loosely based on an MXR Dist + but I've changed quiet a few of the parameters/components over time to adapt it to my use and sound.

Here's my current problem. I play thru a big amp wit a 15" speaker and use alot of low end in my sound which I like alot. When I use this distortion, I sometime get a very low end feedback. I would like to add a filter that cuts out or shunts the very lowest frequencies to eliminate this. I have modded the distortion to let alot more low end pass into the distortion (bigger input cap) but I still have a 'control' problem with the very low end. Most of my mods or variations have come thru copying other distortion circuits. If I get pointed in the right direction, I can 'trial and error' it to fine tune it. Any help would be appreciated.

Thanks,

Bob M.

[Luijo]

Originally Posted by Bob M.

I've built a distortion unit into an instrument I use all the time that benefits from occasional use of this sound. I started with something loosely based on an MXR Dist + but I've changed quiet a few of the parameters/components over time to adapt it to my use and sound.

Here's my current problem. I play thru a big amp wit a 15" speaker and use alot of low end in my sound which I like alot. When I use this distortion, I sometime get a very low end feedback. I would like to add a filter that cuts out or shunts the very lowest frequencies to eliminate this. I have modded the distortion to let alot more low end pass into the distortion (bigger input cap) but I still have a 'control' problem with the very low end. Most of my mods or variations have come thru copying other distortion circuits. If I get pointed in the right direction, I can 'trial and error' it to fine tune it. Any help would be appreciated.

Thanks,

Bob M.

If you have already modded the device for more low end and you are getting feedback in the lower frequencies....the logical step would be to go the other way around.....decrease the low end. The first choice would be the same input cap you've said you' ve increased...now lower it. Do it in small steps so you can control the balance between feedback and low end content.

[Mark Hammer]

The "issue" with the Dist+ is that, as a noninverting op-amp using the ground leg as the variable side for setting gain, increasing the gain necessarily implies raising the low-end rolloff. When people complain about the unit sounding "too thin" the logical step is to change that stock .047uf cap in series with the gain pot for something larger, like .22uf. That solves the thinness problem but clearly in this instance it has begotten other problems at the same time.

I think it is important here to distinguish between fixed and variable low-end rolloffs. You CAN use both at the same time. The increased cap value in the gain-setting portion of the circuit is a kind of insurance against having too little bass at highest gain. The price you pay is that there is too much bass.

Note, though, that there is a cap in series with the output of the chip (just before the clipping diodes). If you reduce the value of that one, you can conceivably arrive at a low-end rolloff point which gets you out of the rumble/feedback problem, yet remains completely independent of the "keep-the-bass" solution related to gain. Whatever objectionable bass has been added in the gain-setting portion will be filtered out by the output cap since it will act as a crude fixed highpass filter.

I don't know which frequencies are causing you grief so I'll just say...experiment.

[David Schwab]

As a bass player I have to comment that all that low end from a guitar through a 15" speaker just makes for muddy sound in a band situation! Unless you are playing in a band with no bass player, you are stepping on other instruments' space.

But do as Mark says... That was going to be my suggestion also.

As a side note, if you have a circuit inside the guitar, with no active circuits before it, you don't need an input cap at all. It's purpose is to block DC voltage. Of course it's also sometimes used to roll off low end, but that changes with the source impedance.

[Bob M.]

Thanks everyone for taking the time to reply. First off, the instrument in question here isn't a guitar. I added some more lows to the circuit because it sounded better and improved to my ears. What I think I'm tryng to say here is that I want to have some extra fullness (than the standard Dist +) but I want to limit the very lowest frequencies, sort of like a microphone with a 60Hz or 120Hz cutoff feature. I might need a sharper slope than 6db per octave, maybe 12 db or possibly even 18 db so I can really notch out my problem frequency range. I'm not sure I can achieve this with just raising or lowering a cap value. I appreciate your comments and help.

Bob M.

[Mark Hammer]

The rolloff does not HAVE to be situated in any one spot. There is an input cap in this circuit to block incoming DC. THAT exerts a 6db/oct rolloff. There is a DC blocking cap on the output and THAT exerts a 6db/oct rolloff. And of course, the cap in the ground leg of the gain circuit also exerts a 6db/oct rolloff. Feel free to tinker with all of them to produce a more finely controlled low end that gives "just enough" bass.

If more precisely-tuned (and *known*) cutoffs are what you want/need, then maybe you need to consider aiming for maximum bandwidth in the distortion circuit, and adding a 2-pole highpass filter with fixed settings after it.

[Bob M.]

Thanks, Mark. That's a great reply with lots of info and food for thought!

Bob M.