Sharing my experience with building a 440 envelope filter. Maybe it will help others understand problems with their build and save some frustration. As always, YMMV but this was my experience.
Using an Xvive VTl5c4/2 from SBE, the tip from RajahMoore about changing C5 to 10uf is mandatory to get anything useful out of the circuit. I would recommend substituting a 5k trim pot and a 500ohm resister in series as a replacement for R9. That will allow tuning to get the best response. The best value for you is going to be different depending on your pickups.
In my build there was too much distortion; part is due to the diode to ground in the envelope section (sounds like a maxed out distortion+) somehow interacting with the filter section. From other build reports I’ve read from others more knowledgeable than I, this appears to be from some sort of ‘cross talk’ within the chip. If you use a separate chip for the envelope filter this distortion is greatly reduced.
There is also a significant amount of ripple that sounds like bad distortion. With C5 at any value less than 10uf the ripple/distortion is unacceptable IMHO.
I was able to get some usable ‘quack’ from the circuit, but with C5 at 10uf the decay slope is way too long and has a very rapid, stuttering and not pleasing end to the decay.
Although NOS vintage VTL5c4/2’s are becoming exceedingly difficult to find, after a 30 minute search on Google I was able to locate an electronics parts supplier (not a broker) that had four NOS VACTEC brand VTL54c/2’s on hand for less than the price of the Xvives from SBE.
Received the NOS parts today. Put R5 & C5 back to stock, swapped in the NOS VTL5c4/2 and I can report the Xvive does not work the same as the originals. With the VACTEC brand in place, the ripple distortion is significantly reduced (the circuit even worked ok with C5 omitted!), the decay is much more ‘natural’ BUT the overall effect is much more subtle than what I got with the tweaked circuit using the Xvive. The Xvive and VACTOL are not even close in performance.
I’m not an EE so can’t test/verify but I’m inclined to believe that the spec sheet from Xvive for their VTL5c4/2 is made up, or the mfg is stamping some other product as VTL5c4/2. I’m can’t say that all Xvive are like this, but the 3 I tried all gave same poor results compared to the NOS parts.
For the reults described above I used an 1983 Ibanez with stock humbuckers. When I plugged in a 2024 Fender ‘American professional 2’ Stratocaster with stock (I.e. weak) single coil pickups I had no effect (just clean tone) with the Xvive unless I really beat on the strings and at that there was only brief stuttering. With the NOS VACTEC I had barely any noticeable effect but at least it didn’t stutter. Putting a preamp in front helped.
Bottom line - if want an original 440 effect sound, you’re going to have to track down an original VTL5c4/2. I assume Perkins, EG&G or Excelitas brand of VTL5c4/2 will give similar results to the VACTOL brand but I cannot confirm that.
If you like a challenge, use the Xvive part and see if you can modify the envelope shaping section of the circuit to make it work better. If you’re just looking for a decent envelope filter then I’d recommend building something different (I built the mxr120 and am pleased with it but that is also another circuit where the manufacturer of the chips is the difference between success and failure - if you go that route read all the build reports you can find for what chips to use and what mods will get you unity gain)
-Rob
