Schu-tone DM-3

[MBFX]

Build Rating

5.00 star(s)

Behold, the Schu-Tone version of the DM-3! This bad boy can do 600ms of delay, boingy sproingy slapback, and everything in between.

The stacked boards work really well, and using tantalum capacitors made everything fit really well. I had to use a too-large film capacitor on the top board, though... ran out of the right MLCCs. Board layout is clean and sensible, with just enough room around the IC sockets. I used 1/4W resistors, and they work, but I'd probably buy 1/8W if I did it again.

This was a satisfying build. It sounds really good! Mine is a bit quiet, though - anyone else noticing some volume drop when engaged? I have a big bag of 4558s, should I try some others?

EDIT: The gain problem was caused by the Chanzon JRC4558 I used. IC1 needs a good-quality IC for the pedal to work. I used not that. I found a TL072 in my stash, dropped it in, and the problem was fixed. It sounds awesome (but brighter) with the TL072, so that seems to be a viable part substitution if need be! Clock noise is also gone, as having the right amount of gain at IC1 made biasing by ear easy. I look forward to reporting back when my shipment of nice Japanese 4558s comes in, and posting a demo video. Should I A/B the different ICs?

EDIT 2: I have somehow broken my footswitch. Don't do that

 

[blackhatboojum]

If you’re overall output volume is quieter than you like, you can change that by turning up the BBD gain trimmer. Keep in mind that increasing the gain into the second BBD will also affect your feedback control.

 

[MBFX]

If you’re overall output volume is quieter than you like, you can change that by turning up the BBD gain trimmer. Keep in mind that increasing the gain into the second BBD will also affect your feedback control.

I have the gain trimmer at about the max it can go, before it just makes a little sputtering sound and stops outputting anything. There's a sweet spot in a little bit of the range of the pot travel where it doesn't do that, but it doesn't seem to really affect the volume of the initial note too much; it just seems to affect the repeats. Too high or too low and it just gives me the dry signal. I'll see if a friend of mine has an oscilloscope I can use.

I tried biasing it by ear according to the instructions, and I fed it a 200Hz sine wave via my DAW. The bias trimmers are tied together, so moving one around affects where the balanced spot is where the tone is purest.

One thing I'm not entirely sure about is the one part substitution I made. I didn't have and couldn't find 20K trimpots for a reasonable price, so I used 25K trimpots. @blackhatboojum , should I get 20Ks and remove the 25Ks?

 

[blackhatboojum]

I have the gain trimmer at about the max it can go, before it just makes a little sputtering sound and stops outputting anything. There's a sweet spot in a little bit of the range of the pot travel where it doesn't do that, but it doesn't seem to really affect the volume of the initial note too much; it just seems to affect the repeats. Too high or too low and it just gives me the dry signal. I'll see if a friend of mine has an oscilloscope I can use.

I tried biasing it by ear according to the instructions, and I fed it a 200Hz sine wave via my DAW. The bias trimmers are tied together, so moving one around affects where the balanced spot is where the tone is purest.

One thing I'm not entirely sure about is the one part substitution I made. I didn't have and couldn't find 20K trimpots for a reasonable price, so I used 25K trimpots. @blackhatboojum , should I get 20Ks and remove the 25Ks?

It sounds like you’re confusing which trimpots do what. Each bias trimpot controls the bias voltage to its respective BBD chip. They are not tied together. If you adjust the bias too far one way or the other, yes… the signal will sputter out like you described. The gain trimmer is just a signal attenuator essentially. It’s there to attenuate the output from the first BBD into the input of the second BBD, because too hot of signal will overload the second delay chip. From the picture you posted, your gain trimmer looks to be just a little over halfway up. You can probably bump that up to somewhere between 2/3 and 3/4 of the way up. I don’t think switching to 20k trimpots will make a whole lot of difference.

 

[MBFX]

"Tied together" was my (poor) attempt at explaining what was happening. Thank you for bearing with me, and for your explanation! It helps.

With both bias trimpots at about noon, the 200Hz sine wave sounds pure, using my headphones as a reference. If I move one of the bias trimpots out of that range to where I hear the two signals phasing, I can adjust the other and get it back to where it works. There are a couple "good" positions possible, and they appear to be with the trimpots in nearly identical positions. Given my rudimentary understanding of how biasing op-amps works, I feel like this makes sense. I can get the right "ratio" a couple different ways that factor down the same. Does that make sense, or am I way off target?

I had the gain where it was because any higher the repeats started to get pretty dirty. I realized that I am used to a Carbon Copy Bright and delay plugins, so I looked up some DM-3 demos and it seems like it should be a little dirty. I bumped up the gain trimpot, but I'm still below unity gain with the pedal engaged. Dan messaged me to check the resistors in the feedback network, and they all appear to be correct. I double-checked that I was reading them right, and I am confident that I am. I also tested each component before populating it into the board and soldered components 5 at a time, so hypothetically we should be good. I am increasingly suspicious of the probably-counterfeit 4558 I used. I ordered some nicer Japanese ones from StompBoxParts, and I will try one and report back.

Regarding the 4558 - am I correct that the initial gain boost happens here, and if that gain boost is too low, the pedal will output below unity?

As far as the clock goes, I definitely have clock noise, but I'm not sure if eliminating it completely is a thing with my equipment. I have tried the entire sweep of the trimpot. At one end, the noise is at idle, increasing in frequency as I turn the pot up until it stops doing that. From that point until abbout halfway up, the repeat has a "shhh" sound at the end as it cuts off. The "shhhh" gets quieter and shorter as I continue to adjust the trimpot up. About 2/3 up, the repeat has a very small "eeee" where the "shhh" was. The "eeee" is high frequency, probably around 8K. It's most noticeable at low volume. Adjusting the trimpot above 2/3 makes the "eeee" get louder and longer. I can't seem to get it silent; there's either a "shhhhh" or an "eeeee". A multiturn trimpot would probably help, as would a visual indicator. I am probably going to use this as an excuse to buy my first o-scope. Maybe my wife won't notice the new gadget on my desk.

EDIT: after lots of messing with it, I have found that I can actually set the BBD gain really high! I can get the repeat louder than unity, but the initial note is quiet, so the problem is in the initial gain stage.

EDIT 2: I found a Texas Instruments TL072 in my stash of semiconductors, stuck it in, and boom, unity gain. Be choosy with IC1 - a cheap IC that works in a distortion pedal may not work here.

 

[blackhatboojum]

EDIT: after lots of messing with it, I have found that I can actually set the BBD gain really high! I can get the repeat louder than unity, but the initial note is quiet, so the problem is in the initial gain stage.

EDIT 2: I found a Texas Instruments TL072 in my stash of semiconductors, stuck it in, and boom, unity gain. Be choosy with IC1 - a cheap IC that works in a distortion pedal may not work here.

Nice! Glad you sussed it out and found the source of the problem.