DOD 201 phasor mods, slower?

[intrentnet7]

Anyone tried or know how to mod this to make the speed go reeeeally slow?

I don't ever put the speed knob above 2 o'clock but I often wish I could make it go a lot slower than it does at the lowest point. So I'd like to make the current 2 o'clock position roughly the max speed and the lowest speed setting to be a heck of a lot slower.

So, how can I accomplish this? Change of pot value? change a few resistors? Is it possible? is it too complex that I shouldn't even bother?

FYI I'm using this for my bass rig as it was the best sounding phase (IMO) for bass. I have a bad stone, small stone, phase 95, phase 100, and Digitech SP-7 stereo phaser. I'm building a guitar pcb Dr. Phybes (90 w/ a script switch) for a friend and will be building myself a madbean smoothie (phase 45) in the near future. So if you're going to suggest using a different phase effect, don't bother cause this one is staying whether I can mod it or not  ...unless the smoothie beats it of course. 

[zombie_rock123]

As far as I know the 201 and 45 (smoothie) are only a couple of components apart. Maybe cap values?

I think if you up the 3.3uf cap im the LFO section itll slow it down. And if thats wrong then the fastest way to get the right answer on the internet is post the wrong one

[intrentnet7]

Here's a schematic with the phase 45 differences I found on the googles if that helps anyone to help me. I understand very little of what's going on in there but I'm sure someone else will understand it.

[reddesert]

Here is a thread from DIYSB that talks about the DOD Phasor 201 / MXR Phase 45 differences, and has a link to a more-readable version of the schematic. http://www.diystompboxes.com/smfforum/index.php?topic=116726.msg1081976#msg1081976

Some of the differences are purely in what component values were standard at the time of building, like 0.05 vs 0.047 uf caps.

The 3.9M vs 3.3M resistor that Mark Hammer is talking about modifies the width of the LFO speed, not the depth, AFAIK. To slow down the LFO, my guess would be to increase the value of the large cap in the LFO section (C8 on Smoothie schematic), the speed pot, or the resistor in series with the speed pot (R25 on Smoothie).

[midwayfair]

Locate the LFO on the schematic. If you aren't sure what this sort of LFO looks like in general, you can generally assume that it's the op amp stage with the speed pot attached. However, I'm going to walk you through figuring out what, precisely, the components MUST be doing, so you can locate the parts in this and any other low frequency oscillator.

Let's think a little about what a low frequency oscillator is.

Oscillator: This means it has a cycle. Well, what's making it cycle? It's not like our audio signal section does stuff on its own. If you look closely, though, you can see that an LFO is built in such a way that some signal from the output goes back to the input. It's a positive feedback loop. You can get noises in the audio path by feeding the output back to the input. Same thing's happening here. What's the input? It's actually the natural noise of the power supply and the op amp.

Low frequency: This is where you might start thinking about what makes it tick. Frequency is how often something happens. If it's low enough, you can't hear it.

Maybe play around with the fact that you can make squealing noises when you feed the output to the input. What happens if you ... filter out some of those high frequencies with a low-pass filter? (You should actually experiment with this, because it's fun to make squealing noises.) The frequency gets lower. You know those seagull noises you can make with the guitar knobs when playing with something like a fuzz factory, or when you plug into a wah pedal backwards? This is all part of the same effect on a feedback loop.

Back to the LFO. Since it's a positive feedback loop, it has to operate on some principals. We have some wave with a frequency. Each time the wave cycles, it gets fed back through the loop. So it has to complete a cycle before it generates some feedback. We know if the frequency is high enough, we can hear it.

The previous experiment tells us that we can alter the pitch of the feedback signal by filtering out some frequencies. We need a low-pass filter to do that. What we want, though, is REALLY LOW frequencies. We want to cycle maybe a couple times per second, down to maybe once every few seconds. Maybe even slower. Once every 10 seconds? How slow can we go? One cycle per second is a single Hertz. Audio stuff is 20-20000 Hertz. We want way lower.

So we're looking for a pretty extreme low pass filter, then. The formula for an RC filter is the reciprocal of 2 * Pi * R * C, where R is the resistance in ohms and C is the capacitance in Farads. You can do the math by hand or use a calculator. Some back of the napkin math tells me that if I want 1Hz:

1 = 1/2piRC
RC = 1/2PI

So I just need some combination of Ohms and Farads that multiple out to 0.15. 1uF and 150K work, those are pretty normal values. Otherwise I'd need something like 1.5M and 100nF, or 10uF and 15K.

Anyway, we're looking for a capacitor and a fairly large resistor forming an RC low-pass filter to control the speed. The resistor's going to be variable to get different frequencies. That's our speed pot. We don't normally see potentiometers bigger than 1M, so we can pretty much assume that the capacitor's going to be bigger than 1uF -- if it were smaller, we'd need a gigantic pot to get to the moderate speed of 1Hz.

Now, I said at the beginning of the post that you could pretty much assume that you're looking around the speed pot.

Here's what you should do as homework.

Look at the schematic of a low pass filter if you don't know what one looks like off the top of your head. Match it up to the parts that connect to the speed pot. Can you see what part is doing what for a low pass filter?

Get out your calculator. What is the oscillator frequency when the speed pot is set to its smallest value? (Look VERY carefully at the schematic, and make sure you've accounted for every part!) What is the frequency when it's set to its largest value?

What happens when you increase the capacitance in a low pass filter? What happens when you increase the resistance?

What parts can you change in a low pass filter if you want the frequency to be lower?

Make a chart, or even a spreadsheet, and write down the values when you change the resistance or capacitance. What do you notice about how much capacitance or resistance you need to change the number of cycles per second? (Hint: it is not linear. WHAT ELSE in the circuit in question is not linear? Look closely at the smoothie schematic, look up a definition if you aren't sure.)

Now: What parts in the MXR schematic, specifically, could be changed to achieve the goal that you originally sought?

Now here's where you're going to need some direct data. What is the actual value of the speed pot when it's at your favorite setting? What would the speed pot need to be set at to get to the speed you want? Can you buy a potentiometer that does what you need? If not, what else in the circuit could you change that achieves your goal? Is it easier or harder to change one of the parts than the other?

This is a lot to absorb, I know. And for what is, in the end, a fairly simple change to the circuit. However, learn it once, and you won't have to think about it for more than a minute if you want to do something similar in the future.

[intrentnet7]

Math? Homework? 
I wasn't expecting this. ha ha.

Alright. Challenge accepted. My weekend is now booked. 

Thanks for your help!

[intrentnet7]

So, I'm an idiot and forgot that the DT Hardwire SP-7 has a tap tempo mode for the speed knob. I got that sucker to slow way down. It doesn't sound as nice as the DOD 201 (probably a digital vs analog thing). I also found that it sounds better for my bass setup in the 4 stage setting rather than the 2 stage setting. Anyway, it's keeping me satisfied until I can build a slower version smoothie using the advice from midwayfair.