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Author Topic: ModTone Atomic Phaser trimpot
Double D

Posts: 195
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Post ModTone Atomic Phaser trimpot
on: October 4, 2012, 10:27
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Hey gang. I just got a really good deal on a ModTone Atomic Phaser. It's a pretty cool box, good swirl, reasonable build quality, although it lends a pretty serious upper-mid eq bump to your signal. While writing/videoing/photoing the unit for a blog post https://inspireformation.blogspot.ca/2012/09/the-modtone-atomic-phaser.html I opened up the unit (pain in the ass getting at the board, 'cause all the jacks and switches are mounted to it) and found an internal trimpot. There is no documentation of this feature that I can find, so I'm not entirely sure what it's doing. Full clockwise or anti-clockwise shuts the phasing down, and in the middle of the sweep you get a fairly abrupt change from bassy to really trebly phasing. It's clearly not a tone control as such, although it does affect tone. In the end I set it where I found it; taming the upper mid issue also de-juiced the phasing, and I have tone controls on my guitars. 😉
Any ideas what this thing is actually doing? The internet has a right to know!

Digital-
Larry

Posts: 192
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Post Re: ModTone Atomic Phaser trimpot
on: October 6, 2012, 07:40
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I haven't checked your schematic yet, but it's possible this pot is to adjust for variations in FET ON resistance, or whatever element is actually being used for the variable phase shift.

[Edit: It's not the variations in ON resistance, but rather variations in the gate to source voltage at which the FET begins to turn on, that the trimpot is likely meant to handle. There is a wide range of this value, known as Vgs(ON), for any given FET part number. You want all the phase shift stages to track together as the LFO voltage wiggles the gate voltage. Manufacturers and DIY'ers will often hand sort FETs into different Vgs(ON) ranges, but then the LFO voltage needs to be biased into the correct range - thus the trimpot.]

Here's someone's description of a Maestro Phaser which includes a FET bias trimpot.

https://www.wingspreadrecords.com/maestro_tech.html

Geofex has a thorough discussion here:

https://www.geofex.com/article_folders/fetmatch/fetmatch.htm

Double D

Posts: 195
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Post Re: ModTone Atomic Phaser trimpot
on: October 6, 2012, 10:09
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Geez, some of this stuff makes my head spin a little...starts sounding altogether too much like physics class, but I think I get it. It would explain the slightly woozy sound quality generated when the trimpot's just slightly altered and it shutting right off with minimal further tweaks. So here's my verdict: don't mess around with the trimpot in your Atomic Phaser, unless you'd prefer it sounded bad.
Thanks a lot D. Larry!

Digital-
Larry

Posts: 192
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Post Re: ModTone Atomic Phaser trimpot
on: October 6, 2012, 12:56
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Dont know if this will help...

Each FET is operating as a voltage controlled resistor. If you had a pot in place of the FET and could twist all the pots rhythmically and in sync you'd have a manual phase shifter.

But there is a "dead range" of voltage input to the FET where the resistance doesn't change. This varies widely from FET to FET. There is also some variation in how the resistance reacts to the voltage. So, you try to sort out groups of FETs that act mostly the same and then compensate for their particular offset using the trim pot. This way, the LFO sweep does not go into the dead range.

pinkjimiph-
oton

Posts: 7
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Post Re: ModTone Atomic Phaser trimpot
on: July 19, 2013, 17:00
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it's the bias, it sets the operating range of the phaser. somewhere around half is where you want it,
without a scope, plug it into a fairly loud amp and adjust it until you get the most phasing with the least distortion.
you may find some sweet spots that sound better than others... if you find a setting that sounds perfect and isn't distorting, cool, just leave it there and you're good!

touch my what?!?!?!?!?!?

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