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DIY Synthrotek ECHO - too much feedback?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author DIY Synthrotek ECHO - too much feedback?
ub1k
I've built a Synthrotek ECHO. It was quite a simple one, especially for someone who has built the Equinox II Reverb in the past.
However, while following the "testing procedure" in the build guide, I noticed that something was wrong. If I set all knobs to noon as suggested, I get growing feedback that quickly becomes unbearable. It looks as if the PT2399 starts oscillating too easily.

There are some minor differences between my build and the official schematic:

2 x 200K -> 200K
1 x 300K -> 330K

I happened to not have the right resistors around, so I thought these would be close enough, since they seem to concern mostly the IN/OUT buffers (except for the 300K, but how could it cause feedback?)

Schematic: http://www.synthrotek.com/wp-content/uploads/2014/07/Synthrotek-ECHO-S chematic.png

I found at least one other case of someone complaining about feedback in a Synthrotek DLY module (not the ECHO).
Any clue on what may be going on or how to do further troubleshooting? I double-checked that the resistors around the feedback pot are correct and I'm out of ideas. seriously, i just don't get it
SoundPool
I don't know that the differences would be big enough to cause such extreme feedback, but my understanding with PT2399 is that the production consistency/quality is not very good. Where did you source the chip, and do you have more than one so you could try swapping them to see if it helps?
ub1k
SoundPool wrote:
I don't know that the differences would be big enough to cause such extreme feedback, but my understanding with PT2399 is that the production consistency/quality is not very good. Where did you source the chip, and do you have more than one so you could try swapping them to see if it helps?


Thanks for your answer. I have more than one, so I tried to swap it and the behavior is the same. I don't recall where exactly I bought them, probably Aliexpress? I thought they might be counterfeit, but from what I found online, they look legit.
sduck
I just this evening reinstalled my DIY echo in my case after not using it since I built it maybe 2 years ago. It's still just as quirky as I remember, in both good and bad ways. I don't have the same runaway feedback at the settings you describe though. What I noticed is that there's some kind of weird interaction between the mix control and the feedback control - if you have the mix up too high, it goes into runaway feedback too soon - if you back off the mix control to maybe 7-8 then you get better feedback performance. So it's possible that that resistor sub you did may actually be causing this, although I'm not sure, and I don't really understand what you did.
SoundPool
I also get what sduck describes, but a bit less extreme. I have a feeling that the circuit is FAR from optimized/stable seeing as Synthrotek's business model to me seems to have been taking very common circuits almost direct from a datasheet or from guitar pedals and porting them to Eurorack when they aren't just outright using other people's designs. Not to turn this into some sort of trash talk situation rather than trying to solve your problem, but when a company seems just as focused on producing t-shirts and goofy merch as they are on actual circuits I suppose we should expect to get what we pay for...

If the circuit is touchy enough that your substitutions makes a difference I wonder if it could explain the instability difference between other built units - is the circuit so touchy that resistor tolerances could eliminate or accent the bleed effect between the knobs? Also if you want to try some alternate 2399's, ones I have ordered from MusikDing come marked as Tested, but they are about the price per piece you can get a lot of 10 for on AliExpress.
ub1k
Quote:
If the circuit is touchy enough that your substitutions makes a difference I wonder if it could explain the instability difference between other built units - is the circuit so touchy that resistor tolerances could eliminate or accent the bleed effect between the knobs? Also if you want to try some alternate 2399's, ones I have ordered from MusikDing come marked as Tested, but they are about the price per piece you can get a lot of 10 for on AliExpress.


Funnily enough, I actually ordered a PT2399 from MusikDing yesterday. It seemed like the only "reputable" seller I could find. I also ordered the exact resistors in the BOM from Arrow (16 cent order with free delivery, YAY!)
Let's wait a few days and see how it goes. If it doesn't work, then I'll mail Synthrotek.
ub1k
OK, I just got a PT2399 from MusikDing on the mail, as well as the 2x200K and 1x300K resistors. If anything changed, then it must have been for the worse. I have the exact same feedback problem with the pots at noon.
At this point, I will e-mail Synthrotek. But if someone has any tips on how to further troubleshoot this, I would be very thankful.
jimfowler
I’ve built two of these recently and had no issues...lemme take a look at mine when I get home from vacation.

EDIT - sorry, it's their DLY module that I built. That one works just fine.
ub1k
Here are some photos of the build:




https://imgur.com/vVHc0JC
https://imgur.com/83XxBAJ

The underside is a little messy because some pads came out while replacing the resistors. In any case, the issue was already there before.[/img]
JohnLRice
sduck wrote:
What I noticed is that there's some kind of weird interaction between the mix control and the feedback control - if you have the mix up too high, it goes into runaway feedback too soon - if you back off the mix control to maybe 7-8 then you get better feedback performance.
I concur with the good Sir Sduck. It can be a useful and interesting module sometimes, but I often end up pulling back out of an active setup because of the non-standard control interaction.
euromorcego
JohnLRice wrote:
sduck wrote:
What I noticed is that there's some kind of weird interaction between the mix control and the feedback control - if you have the mix up too high, it goes into runaway feedback too soon - if you back off the mix control to maybe 7-8 then you get better feedback performance.
I concur with the good Sir Sduck. It can be a useful and interesting module sometimes, but I often end up pulling back out of an active setup because of the non-standard control interaction.

this is true ... the schematics are a bit weird, the mix isn't really a mix control but more an additional feedback.
basicbasic
I fixed one of these for a friend and there is a resistor that runs from the mix pot to the feedback insert - R11. If you remove this it behaves a little more conventionally from memory.
ub1k
Quote:
I fixed one of these for a friend and there is a resistor that runs from the mix pot to the feedback insert - R11. If you remove this it behaves a little more conventionally from memory.


Thanks! I tried that out and indeed it seems to get better. But the feedback is still much higher than what you see e.g. on videos on YouTube. I cannot increase the feedback without lowering the mix a lot (and vice-versa).
ub1k
OK, I contacted Synthrotek but unfortunately they weren't of much help. They say they suspect there's something wrong with the feedback mechanism, which isn't exactly news to me.

Does anyone have any suggestions on how to go about troubleshooting this thing? I would like to make the module usable, if not using the original schematic at least through some sort of workaround.

Suggestions will be very appreciated! help
basicbasic
Replacing R9 with higher value (or ideally a trimmer) will let you can tame the feedback, but MIX will still be a factor in how much feedback there is so you will always need to use it and Feedback pots to dial in the overall amount.

Also, R24 sets the scale for the rate knob so if you want to remove the dead spot at the end of it's travel try something around ~280k.
ub1k
I FIGURED IT OUT! w00t

It turns out that I was using 33 and 39nF capacitors for C10 and C16, respectively, instead of 3.3 and 3.9nF. I must have miscalculated the values since they were rated in uF when I bought them online. I replaced them and the module now sounds perfect. It's peanut butter jelly time!

Thanks a lot to everyone who has replied to this thread. I hope it will be of help to others in the same situation.
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