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Adding a LED to old EHX Graphic EQ?

Thu Oct 01, 2020 4:28 pm

I've got this great old big box EQ with a mains plug. Only issue is that there's no bypass indicator. I found this site with some details: http://falseelectronics.blogspot.com/2016/11/electro-harmonix-graphic-equalizer.html

Any clue where I could tap in to power a LED? Would change out the switch to make it work unless there's a more convenient way. Also -- on the site linked above, his unit came with a non-stock master volume knob. That would actually be a pretty handy add. Any clue where to stick a pot (and what might be a good value) in the output section to make that work? I'd feel more comfortable poking around in there if it weren't for the mains plug... :erm:


Re: Adding a LED to old EHX Graphic EQ?

Thu Oct 01, 2020 6:52 pm

Hey, I know that blog! I don't know if Tim frequents this site though.

You can power the LED from a dropping resistor connected to +VS, and your bypass switch will actuate the ground connection. I would use something like a 2K7 for dropping resistor on that 13.5 volt rail, though anything between 1K and 10K should set you straight, depending on what you get for LED (with some of the hyper efficient water clears, my corneas benefit from something a little taller in the resistor department).

For an output volume, you could probably just replace that 33K ground reference resistor at the output with a 50K pot and connect the output line to the wiper instead of C20.

Re: Adding a LED to old EHX Graphic EQ?

Fri Oct 02, 2020 7:53 am

Wow! I’ll try this out and report back. :!!!:

Re: Adding a LED to old EHX Graphic EQ?

Fri Oct 02, 2020 9:11 am

I'm happy to answer questions, my earlier post was a bit vague.

You probably will need to replace the foot switch to support this, or install a millenium bypass circuit to pull on the LED.
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