Boss DD3 mods! Wiring clipping pot into self oscillating momentary switch?!

[Tome]

Hey! I've posted a few times in regards to this mod project.. thanks to those that have helped. This really is my final query.. I've attached a drawing of what I'm aiming to do overall.. mainly need assistance with wiring in a pot that allows me to mix in dirt/clipping with the oscillating momentary switch that's connected to the feedback pot of the DD3. What do you all think of how I plan to do it??

[nocentelli]

The lofi switch will allow you to clip the repeat signal as it "enters" the feedback pot via lug3 by diverting peaks in the signal to ground (lug 1): This is a standard way of distorting guitar signal like in an OD pedal.

The pot you have added will allow you to set how much more feedback/repeats you'll get when you engage the oscillation switch (or how rapidly it runs into self-osc): However, the diodes you have added in that loop (which I assume is anti-parallel pair) are in series with the signal. This will not clip the peaks of the signal to ground, but I suspect will have a gating effect as only signal that is higher than the forward voltage will pass.

If you want to control the amount of clipping, I would suggest leaving out the diodes in the oscillation loop and instead replace one of the blue wires that connects the analog and lofi switches with another pot (maybe another 50k) to adjust the amount of clipped signal you send to ground. However, you might find that the three options of the switch (no clipping/some LED clipping/lots of diode clipping) give enough variation without needing a pot.

[Tome]

My thinking was to have the pot offer clipping independent if the switches so if I'm using the pedal clean, the momentary switch/oscillation can be clipped/distorted when engaged then returned to clean delay when disengaged. What do you think?

Although I do like the idea that pot could offer more repeats and control how quickly it self oscillates!

[nocentelli]

My thinking was to have the pot offer clipping independent if the switches so if I'm using the pedal clean, the momentary switch/oscillation can be clipped/distorted when engaged then returned to clean delay when disengaged. What do you think?

First off, that sort of series clipping diode arrangement produces what i believe is known as "crossover" distortion, which is not used in pedals very often, possibly because of the gating effect on small signals. There's a thread here about it if you want to read up -

https://www.diystompboxes.com/smfforum/index.php?topic=69274.0

I do like the idea that pot could offer more repeats and control how quickly it self oscillates!

Without the diodes, that's exactly what it can do: The exact way it reacts will depend up the settings of both the feedback pot and the extra "self-oscillation" pot. If you look at the schematic below, the usual feedback pot is essentially just a volume pot controlling the signal level of repeats fed back into the delay chip.



The delay chip produces a single sample/copy of the input signal every 300ms or whatever and spits that copy out to the feedback pot AND the pedal output in parallel. If the feedback pot is set to zero, only the original copy is heard (i.e. one repeat) because none is fed back into the chip. At higher settings, the increased amount of delayed signal fed back into the chip is heard as progressively more repeats. When max'ed out, the high signal level will lead to infinite repeats, or if the gain structure around the chip is sufficiently high, it will actually drive the chip hard enough to self oscillate even without any input signal.

The extra pot in conjunction with the momentary switch allows you to connect another resistance path in parallel with the pot lugs 2/3 of the FB pot, reducing the effective resistance of the path from lug 3 to 2 and boosting the number of repeats. So if your FB pot halfway up gives you 5 audible repeats, setting the self-osc pot to halfway might then give you ten repeats (which will fade out rapidly when you release the switch). If you crank the self-osc pot to full (i.e. no resistance), the momentary switch will just short feedback pot lugs 2/3 and you'll get instance self-oscillation. Between those two extremes, you should be able to set exactly how fast the self-oscillation builds - Does the DD3 self-oscillate easily at max feedback?

[Tome]

Brilliant. I think ill leave the diodes out and have the additional pot operate as you've described. However, i did want that pot to offer something a little more by way of effecting the signal.. even if its minor.. something that can contribute to subtle signal degradation.. any suggestions?

If i left it simply as an additional pot, i'd wire it up the same as i've drawn it minus the diodes..?

Yeah it does oscillate max out.. although its not dramatic.

Thanks heaps for your help btw. 

[nocentelli]

There are not many ways to seriously mess with the repeats without adding too many extra parts to what already seems to be a fairly busy modification - Is it a full rehouse? You could add in the series diodes as you've drawn (i assume low vf ones might be best, e.g germanium or Schottky) and if it's rubbish you can just short them with a wire. Or if it's horrible in a good way that you might want from time to time, add a single throw switch to short them or not.

[Tome]

Yep, it's a full rehouse. I'll give it a go as I've drawn it and remove the diodes if it's too much.

Ill post the results in a few weeks time once it's all assembled.

One last related query, I'm planning on replacing the boss momentary switch with the same type of two pronged momentary switch I'll be using fir the self oscillation. I fairly certain this is correct.

[nocentelli]

Yep, i think it just makes monetary contact between ground and a node in the the flipflop circuit.

[Tome]

Hey - So thought I'd post an update.:.  things haven't gone totally to plan.

The hi cut (capacitors), low cut (resistors), analog switches (capacitors) work.

Issues: The lofi (diodes) switch isn't doing anything. The pot is behaving like a Level switch. The 'self oscillation momentary switch isn't self oscillating.

Any ideas would be very welcome!!

[Tome]

Bump!! Really having difficulties with this mod.. if anyone can help that would incredible. The self oscillating momentary switch connected to lugs 2 and 3 of the feedback pot isn't doing anything. If tried short circuiting uses a piece of wire and still no results.. any ideas? Is there another point I should try??

[nocentelli]

Are these "established" mods, known to work? First off, try using a piece of wire to physically bridge lug 2 and 3 of the feedback/repeats pot. If it doesn't self-oscillate, it's not going to do so with a momentary switch. If it does, your momentary is wired wrong.

If it doesn't, you can probably make it s/o, perhaps by shorting the 47k resistor (R56 in the Boss schematic here - https://www.manualslib.com/download/869345/Roland-Boss-Dd-3a.html): I must state i cannot guarantee this will not harm your device.

The clipping diode effect is probably quite subtle, but it is definitely noticeable on a PT2399 DIY delay like the king dubby and different diodes have different sounds, although i emphasise that this is more obvious in the character of the self-oscillation sound which you haven't got yet.

[Tome]

I'm trying to install a range of mods that have been reported to work.. although I'm aiming to install the whole range!

I did try physically bridging and that didn't work. I then went looking on the main board and discovered what seems to be r56 by accident. There are a bunch of others that also sounds similar. There's also another connections whereby the repeats get quieter but thinner which is cool. I was thinking about wiring each of these two options on either side of a po sot I can move between each. Thoughts?

[nocentelli]

I did try physically bridging and that didn't work. I then went looking on the main board and discovered what seems to be r56 by accident.

You could try wiring the momentary switch in series with the 50k pot across R56 - If shorting it causes oscillation, the pot may well allow you to set how quickly this builds when the momentary is engaged. 

There's also another connections whereby the repeats get quieter but thinner which is cool. I was thinking about wiring each of these two options on either side of a po sot I can move between each. Thoughts?

You could use a DPDT switch to decide which two "circuit bending points" the momentary switch and pot will short, yes.

I made a PT2399 delay with a momentary footswitch that could either trigger self-oscillation OR apply the delay only when held down for "dub echo" effects: I couldn't envisage using both at once, so a small toggle switch allowed both functions from the one footswitch.

[Tome]

Again, thanks for taking the time to respond!

I've drawn a diagram (in my own style) to capture your recommendations.. what do you think?

Cheers