Mark or anyone- Ross Phaser mods (Mark Hammer Ross mods)

[jimbob]

I can figure out how to do the vibrato mod, but Im still unclear how to do the auto wah mod.

mark-

The cap mod can also be applied to all the caps, making the unit a 4-pole lowpass filter that automatically wahs up and down.

How would I connect this?

[Mark Hammer]

Simple.  Take all the .0033uf caps in each of the phase-shift sections and tie one end to ground.  The end that is tied to either pins 5/7 or 10/12 should be left where they are.  It's the other end you're interested in.  This is shown for two stages in the Tonepad pdf file.  Just do it for all 4 stages.

This will turn it into a 4-pole lowpass filter.  In combination with the Regen capabilities, that should produce some interesting sounds.  Note that it will be most intense when the vibrato mod is used.  That is, if you still have some clean signal mixed in with the filtered signal, the filtering won't seem as pronounced as it would with filter-only.  Note as well, that this should not produce any pitch warble, the way that the vibrato mode does when you have some all-pass (phase-shift) stages in there.

If you go here - http://www.synthdiy.com/files/1/20011030/ssm2040.pdf - you will see the datasheet for the chip that formed the basis of the first phasefilters, made by John Blacet (and described in DEVICE, which you can find at my site around page 11 or so).  Go to page 3 of the datasheet, and you will see some different configurations of the SSM2040 gain cells, used to make different sorts of filters.  You'll see that where you direct the cap we've been discussing will determine whether the filter shaves off highs, or lows, or just adds some phase-shift (allpass mode).  

You will note that the Ross Phaser has a 27k input resistor between the "-" pin of each section (pin 4/13) and the output of the previous section (pin 8/9).  Look at the SSM2040 datasheet (page 3, figure 5, specifically), and you'll see that if the output of the previous stage comes through the cap, and NOT through the 27k, you get a highpass section.

Okay, what good is this to you?  Well, if all the caps are directed to ground, you'll get a lowpass function only.  If at least one of the sections is transformed into a highpass filter, you'll get a bandpass function.

Logic and tradition would suggest that if you have 4 assignable sections, that you make 2 of them highpass and 2 lowpass, so as to create a bandpass filter with symmetrical slopes on each side of the passband.  But let's set tradition aside for the moment.  One of the things that people don't like about swept bandpass filters is that you lose the bass too often, resulting in a reedy, thin sound.  So, what if we make 3 of the sections lowpass, and just ONE of them (the first or last one) highpass?  On paper (well, on virtual paper, since are here, aren't we), this should produce an interesting tone with less of a "bass-penalty" to pay.

Note that to create such a highpass section, you don't have to re-route the cap, as you would for the allpass-to-lowpass conversion.  Rather, you simply lift the end of the 27k resistor that is tied to the cap and pin 9 (or .

Once you consider the sorts of filters that can be made with a quartet of OTA gain cells, such as found in either the Ross Phaser or the SSM2040 (or CEM3320), all sorts of tonal possibilities make themselves apparent.  Whether they sound appealing using a simple LFO to modulate them is another thing, but at least they are easily produced so you can see if it interests you or not.

[jimbob]

Awsome! Thanks Mark. Going to give it a try.

[jimbob]

So from what I understand I want to place a spst switch between the 27k and the 1 off the IC 1 for the vibrato to work? I guess I jusr drill 2 holes in that area and break the copper lead there and place 2 wires, one going from one hole to the spst and another wire from the other hole to the other lug of the spst?

[Mark Hammer]

What I would (and what I usually) do is take the 27k resistor off the board and use the two existing pads for the wires going to the switch.  The 27k resistor is soldered directly to the switch, and shrink wrap is used to both insulate the exposed lead as well as provide some rigidity to this "flying" component, so that the joint doesn't break.  If you can score a 1/8W resistor, use that since it will stick out less and permit more convenient tucking away of connecting wires to the switch.  Of course, if your intent is to use a stompswitch for vibrato/phaser selection (though quite frankly, given the number of things/options encompassed in the non-vibrato setting, it probably deserves to be called "dry-lift"), there is more than enough room to attach the resistor.

[jimbob]

Thanks Mark! I get it. That actually makes more sense. Its going to be a toggle switch so no need for a footswitch.