Dead End FX Inchindown Reduxe - slowing modulation speed

[slowpogo]

I built the Inchindown Reduxe, which is an Ibanez EM5 delay with modulation added.

Sounds really good, but I wish the modulation rate went slower. I like a real slow, languorous modulation, like 0.5 Hz or less. In stock form the slowest rate is like 1-2 Hz.

Looking at the schematic, RATE is the pot that controls modulation speed. My guess would be to mess with R38 to change the base rate? But I'm not sure if I'd want to raise or lower it.

I don't mind sacrificing some of the faster rate, for a lower minimum rate, if that makes sense. But if I can extend the total range, that would be preferable.

Any ideas?

Full build doc/schematic: https://drive.google.com/file/d/1Y0w6E5yRH5_b5Zk2b3HlfIm2SNVJSqwe/view

[ElectricDruid]

To slow the rate down, you'd want a larger resistor. R38 mostly limits the highest possible frequency though, not the slowest- You'd have to really increase it a lot to change the slowest rate, and then you *would* lose most of the fastest speeds. Increasing the pot value would be a better bet.

However, this is a phase shift oscillator, so I'm not sure how well that will work in truth. The other way to slow it down would be to increase the size of the caps C36/C35/C34. I note they've used 680n's, which is typically the largest value available before you get to electrolytics. If you could find some 1uF film caps that fit, that would help.

The other problem you've got is that although you're only trying to make it go 1Hz slower than it does already, these things work on multiples. Say the current lowest rate is 1Hz. You want 0.5Hz. So you need it *twice as slow*. That implies that either resistor or cap values (or a combination of both) have to be *doubled*.

[duck_arse]

how much hack do you want to do on this, slowpogo? I can show you a two range pso uses a spdt if you want to mess about with cap values.

[slowpogo]

The other way to slow it down would be to increase the size of the caps C36/C35/C34. I note they've used 680n's, which is typically the largest value available before you get to electrolytics. If you could find some 1uF film caps that fit, that would help.

The other problem you've got is that although you're only trying to make it go 1Hz slower than it does already, these things work on multiples. Say the current lowest rate is 1Hz. You want 0.5Hz. So you need it *twice as slow*. That implies that either resistor or cap values (or a combination of both) have to be *doubled*.

C36/C35/C34 are actually specified as MLCC in the doc. I'm assuming for size reasons? I do have some 1uf film caps I could try in their place. But even better, I could get some 1.5uf MLCCs in my next Mouser order. That would be more than double the value and would hopefully get me where I want to go.

[slowpogo]

how much hack do you want to do on this, slowpogo? I can show you a two range pso uses a spdt if you want to mess about with cap values.

I'm not opposed to a range switch (if I'm interpreting this correctly) but if I can get there just replacing some caps as in Electric Druid's suggestion, that would be preferable. I'll try that first and see how it goes

[slowpogo]

Update, I replaced the 680nf caps with 1uf film caps. The lowest mod rate is now about 0.8 Hz or so. I think it slowed down the whole range, so the fastest speed is not as fast, but no big deal.

I will try 1.5uf and 2.2uf as per my comment above, should hopefully do the trick.

[ElectricDruid]

Update, I replaced the 680nf caps with 1uf film caps. The lowest mod rate is now about 0.8 Hz or so. I think it slowed down the whole range, so the fastest speed is not as fast, but no big deal.

Excellent work! You could tweak R38 and make it smaller to get the highest frequencies back if you like.

The only problem is that if you make the control range *extremely* wide, it becomes a bit over-sensitive and "twitchy", where the setting you want is about 0.5mm of the pot travel and if you cough loudly it jumps to something else! Might be worth playing with though, to see how far you can go.

I will try 1.5uf and 2.2uf as per my comment above, should hopefully do the trick.

Yeah, that should do it.

[PRR]

Yes, for large change you want to up-size all the rate caps.

The phase-shift oscillator "can" be built with a 3-gang pot; but this is extremely rare. If one of three resistors is a 20:1 rheostat, the frequency change is third-root of 20, or 2.7 (instead of 20:1 with three 20:1 rheostats). True, 2.7:1 does give an octave so you can sync-in with any beat. But at very slow rates 0.2Hz is different from 0.4Hz.

[slowpogo]

Update: 1.5uf and 2.2uf definitely do the trick to get that sloooooowww modulation I like. 

1.5 uf gets down to about 0.6 Hz.
2.2 uf goes to about 0.35 Hz. At this slow rate I'm getting that barely-perceptible modulation that reminds me of an aging tape delay, sounds really great.

However at 2.2uf, the fastest rate is reduced to just 1.5 Hz. Even though I don't plan to use faster rates much, that is too slow!

So I tried reducing R38, and while that does increase the maximum mod speed (and leaves the minimum as is) it also seems to reduce the maximum possible depth. So with a 2.3k resistor in R38, the max rate goes up to about 2 Hz, but the depth is barely perceptible, even at max depth. However this depth issue only appears at the fast end of the Rate knob. With Rate at the slowest, I still get sea-sick levels of depth.

I feel like the ideal would be 1.8uf caps, with maybe 3.3k in R38. I think that would be plenty slow but leave some decent speed (and depth) up top.