DOD FX20 Phasor--Schemo? etc.

[Ben N]

I have one of the old mono 2-knobbers (Speed & Regeneration), and it is a great sounding phaser, but I find it too noisy and and possibly a bit of a tone-sucker when bypassed.  I realize that some noise comes with the OTA territory, but I wonder if (a) anybody has modded one of these, for lower noise etc, (b) before I ruin my eyes and suck down too many Advils tracing it, does anyone have a schematic, and (c) how close is this to, say, a Small Stone, and how applicable are various mods such as the near-Univibe mod.

A quick peek under the hood reveals the following semis: four CA3094Es, one HCF4007UBE (SGS brand), two TL022CPs, a J113 and what appears to be a 2N5089.

BTW, is there a difference between the 4007 UBE and UBEY?  I could only find a datasheet for the latter.  The UBE is apparently obsolete.

Thanks,
Ben

[Mark Hammer]

Someone posted a schem, and I managed to snag it.  This is for a FX-20C, which is the 4-stage unit, not the FX-20B, which is the 6-stage unit.   Both of these are based around the LM13600, rather than the CA3094, however one might expect the component values around the OTAs to be equivalent or very close.  Certainly when Ross fakd a Small Stone using 13600s, all the resistive components were identical.

I won't likely be able to send it before tomorrow, but send me a PM with an e-mail address and I'll get it out to you soon, unless someone can get their copy faster.

The unit is in the same territory as the Small Stone and Ross phaser, which means that it will do/accommodate the mixed-cap-value Uni-Vibe conversion just fine.  Note that regen on a Uni-Vibe positively sucks (which is why you never see regen on a pedal explicitly intended to be a Uni-Vibe clone).

I'm not sure that anything more CAN be done to reduce noise in this unit, beyond swapping one of the TL022s for something with lower noise.  The TL022 is generally used for low current consumption in control circuitry, rather than the audio path.  So, changing the one usd for the buffer and output mixer stage might help, but changing the one used for the LFO obviously won't do anything for the noise levels.  Personally, I don't find my FX-20B to be noisy, but you may be using yours in a context where the noise just shows up more.

For DOD switching purposes, my understanding is that a 4007 is a 4007 is a 4007.  It is being used as a CMOS chip, and not in linear mode.  (see http://ampage.org/hammer/files/dodswitch.gif for a more complte description of the switching circuitry inside)  Although the 2N5089 is normally designated as a low-noise transistor, in this instance it is simply lighting up an LED, not playing any role in the audio.  The J113 is being used for the switch, and is selected for its low resistance when in an "on" state.

[Dreverb]

Hello, any idea about how to mod this pedal to sound more univibe??
Thanks anyway

[reddesert]

This is a very old thread.  But anyway, the DOD FX20 phasor schematic can be found at http://www.americaspedal.net/fx20b/fx20bschem_corr.jpg.

Start with reading RGK's write up on how phasers work at http://www.geofex.com/Article_Folders/phasers/phase.html .  The Univibe, as discussed there, uses four transistor phase shift stages.  Unlike many phasers, the univibe uses different values of the capacitors in each stage, so each shifts at different frequencies. For example, these are C5, C9, C12, C15 in the Neovibe schematic at GGG: http://www.generalguitargadgets.com/pdf/univibeupdate.pdf

The DOD FX20 is an OTA phaser like the Small Stone described on RGK's page, so getting it to sound like a Univibe may be a struggle. However, might as well start with the phase shift capacitors. These are the small caps marked ".0068" (0.0068 uF, or 6.8 nF) between output and input of the six OTA stages marked U2, U3, U4 on the FX20 schematic at http://www.americaspedal.net/fx20b/fx20bschem_corr.jpg .

[Dreverb]

Thanks so much.


I tried some configurations but ended changing C5 to C8 for the classic vibe caps ( (0.15u, 0.22u, 470p, 4n7,... ) and leaving c3 an C4 stock.
Sounds better to me.. has it any sense??? Any suggestions? Also, any idea to make the speed as little faster?? maybe R42 with a lower one??
Thanks anyway. All the best.