DOD 670 Flanger schem - weird switching

[Mark Hammer]

Somebody at the former AMPAGE (now Music Electronics Forum) posted a PDF factory schematic from the DOD 670 Flanger today.  Not sure how to link to the document itself, so here's the thread link: http://music-electronics-forum.com/showthread.php?p=46492#poststop

The 670 uses one of the most unusual switching arrangements I've ever seen.  The circuit uses an NE571 compander chip for noise reduction.  It also has a status LED connected to the output of the LFO that blinks in sync with the sweep.  Folks familiar with the 570/571 will know that pins 1 and 16 are where one connects an averaging cap to ground to set the attack/decay time constants for the compression and expansion.  The cap on pin 1 should match the cap on pin 16 to achieve a perfect restoration of original signal dynamics.



What DOD has done with this circuit is use a SPDT stompswitch to connect either the status LED to ground (permitting it to conduct when in effect mode) OR pin 16 of the expandor half of the 571 to ground, effectively eliminating any control current to turn the gain cell on and cancelling the expanded delay signal which would normally be mixed in with the dry signal to produce flanging.  Note, that since the status LED has its ground connection lifted, it stops conducting and stops flashing.  So status LED switching and effects switching done with a single lousy SPDT stomp.  Neat, huh?

Note that, assuming it works nicely and painlessly, this same sort of logic can also be applied to a variety of circuits that feed a time-delayed signal through a compander chip.  What comes to mind here is the DM-2/DM-3 series of delay pedals. 

[slacker]

Cool idea.

I guess you could also do something similar with delay/modulation pedals that don't use a compander by grounding the wet signal somewhere between the delay chip and the mixer section. You'd just need to find somewhere that was only carrying AC.
For example on the Rebote 2.5 you could ground the wiper of the level pot.

[Rodgre]

Very interesting...  I'm a long-time fan of the 670. I haven't fired mine up in a year or so, and if I recall, the transformer was always buzzing against the chassis. I'm thinking now that I should replace it. Mark, looking at the schematic, is there anything specific I would need to look for to replace the power transformer?

Roger

[foxfire]

i got one of these the this week. i like it. it's way too big but it has some nice sounds. i have found it to be a bit more useful/warmer than my ibanez FL9. anyway i got it for under $50 shipped so it was more than worth it. everything worked but the led. so i cracked it open and saw that it was just a broken wire so i re soldered it. it works but isn't all that bright. i'm guessing/figuring that since it worked without the led then the led isn't all that integral, i should be able to lower that 3.3k just above the led in the picture Mark posted. it's easy enough to just try it, so i will. either way it's a nice pedal