Silver Blues
Well-known member
- Build Rating
Recently I needed some of those breadboard adapters for the plastic-shaft 9mm pots from Madbean, but felt silly paying the shipping cost just for those. I had been wanting to try the VFE Mini-Mu so I grabbed a kit at the same time and managed to build and box it yesterday:
I was very lucky to have most of the parts for this build in my stash already, including, miraculously, the optocouplers. A long time ago I purchased some random vactrols from The Electronic Goldmine, thinking they might be useful someday, that in a great stroke of fortune turned out to actually be NSL-32s. I then spent some time matching them and chose the two that matched the most closely that also had the right resistance characteristics that the circuit seemed to like. I read a lot about how to best tune the vactrols in the Mutron circuit and decided I would change the 100K stopper resistor that limited the current going to the vactrols to 4K7, given the particular resistance characteristics of the vactrols I chose. This turned out to be partway a good choice, as the sweet spot of the trimmer to my ears was almost (but, crucially, not quite) to the maximum (?) rotation of the trimmer (you can see in the picture above). Sourcing some of the weirder resistor values was a pain in the ass and now I have 14x 1/8 W 14K3 and 14x 1/8 W 65K resistors that I have no idea what to do with. I managed to find an OPA134 and substituted an LM324 for the specified TLE2074 like I did in my Triumvirate and it works just fine.
The enclosure is the August Special Run colour from Love My Switches called "Razzle Dazzle", which turned out to be quite a bit darker than I was expecting; in hindsight, I should have gone for white or otherwise light-coloured labels. Printing was done by Spencer at AmplifyFun. All the text, including the big Mu, is rendered in my (probably) favourite font, High Tower Text. The accent circles and lines are a nod to the original Musitronics logo. Switches are short-bat Taiways from LMS (lovely things), knobs are black Tall Boys from LMS, the LED bezel is a "Warp Pipe" from LMS (very cool) with a cyan Electronic Goldmine LED.
I'm giving this build 4 stars mostly because of the unreasonably large number of esoteric parts it requires. I was lucky that I bought multiples of all the parts needed for the bypass/power daughterboard when I made the Triumvirate because I would not have wanted to deal with that again. This plus the vactrols, 2.5mm-pitch 100 nF MLCCs, unusual-value 1/8W resistors, and the bougie op-amps is a bit of a headache in terms of parts sourcing. The board itself is also quite tight, although soldering the switches and pots was not as big of a hassle as I thought it would be. There's also an error in the build document, you only need 4x of the 2.5mm 100 nF MLCCs, not 5x. It took me a while to dial in the trimpot and I believe a multi-turn pot would have helped here, because moving the wiper just a hair in either direction made a dramatic difference to the sound. I can't help but feel like I landed on the right (???) value purely by luck. The rest of the assembly though was quite easy and enjoyable.
So how does it sound? This circuit has a lot of tunability, but this is a double-edged sword. The high-pass option is completely useless for bass and the bandpass option is only usable at very specific settings of the other controls. The low-pass sounds great, though, and I don't mind keeping that switch parked there indefinitely. I also found the "down" position of the Drive switch to be largely useless so that one will stay at "Up" for the foreseeable future. However, you can get a nice range of filter responses out of the combination of the Gain, Range and Peak controls, and it was fun to see what would happen with different combinations of settings. The thing has quite a lot of level on tap and easily clips my recording interface if you set the resonance too high. It's a nice, smooth filter, but I think that I like my envelope filters to be a little bit "synthier"/deeper "bwow" than this circuit can give, though that's not its fault at all. I think it sounds good and it will be fun to play with.
I was very lucky to have most of the parts for this build in my stash already, including, miraculously, the optocouplers. A long time ago I purchased some random vactrols from The Electronic Goldmine, thinking they might be useful someday, that in a great stroke of fortune turned out to actually be NSL-32s. I then spent some time matching them and chose the two that matched the most closely that also had the right resistance characteristics that the circuit seemed to like. I read a lot about how to best tune the vactrols in the Mutron circuit and decided I would change the 100K stopper resistor that limited the current going to the vactrols to 4K7, given the particular resistance characteristics of the vactrols I chose. This turned out to be partway a good choice, as the sweet spot of the trimmer to my ears was almost (but, crucially, not quite) to the maximum (?) rotation of the trimmer (you can see in the picture above). Sourcing some of the weirder resistor values was a pain in the ass and now I have 14x 1/8 W 14K3 and 14x 1/8 W 65K resistors that I have no idea what to do with. I managed to find an OPA134 and substituted an LM324 for the specified TLE2074 like I did in my Triumvirate and it works just fine.
The enclosure is the August Special Run colour from Love My Switches called "Razzle Dazzle", which turned out to be quite a bit darker than I was expecting; in hindsight, I should have gone for white or otherwise light-coloured labels. Printing was done by Spencer at AmplifyFun. All the text, including the big Mu, is rendered in my (probably) favourite font, High Tower Text. The accent circles and lines are a nod to the original Musitronics logo. Switches are short-bat Taiways from LMS (lovely things), knobs are black Tall Boys from LMS, the LED bezel is a "Warp Pipe" from LMS (very cool) with a cyan Electronic Goldmine LED.
I'm giving this build 4 stars mostly because of the unreasonably large number of esoteric parts it requires. I was lucky that I bought multiples of all the parts needed for the bypass/power daughterboard when I made the Triumvirate because I would not have wanted to deal with that again. This plus the vactrols, 2.5mm-pitch 100 nF MLCCs, unusual-value 1/8W resistors, and the bougie op-amps is a bit of a headache in terms of parts sourcing. The board itself is also quite tight, although soldering the switches and pots was not as big of a hassle as I thought it would be. There's also an error in the build document, you only need 4x of the 2.5mm 100 nF MLCCs, not 5x. It took me a while to dial in the trimpot and I believe a multi-turn pot would have helped here, because moving the wiper just a hair in either direction made a dramatic difference to the sound. I can't help but feel like I landed on the right (???) value purely by luck. The rest of the assembly though was quite easy and enjoyable.
So how does it sound? This circuit has a lot of tunability, but this is a double-edged sword. The high-pass option is completely useless for bass and the bandpass option is only usable at very specific settings of the other controls. The low-pass sounds great, though, and I don't mind keeping that switch parked there indefinitely. I also found the "down" position of the Drive switch to be largely useless so that one will stay at "Up" for the foreseeable future. However, you can get a nice range of filter responses out of the combination of the Gain, Range and Peak controls, and it was fun to see what would happen with different combinations of settings. The thing has quite a lot of level on tap and easily clips my recording interface if you set the resonance too high. It's a nice, smooth filter, but I think that I like my envelope filters to be a little bit "synthier"/deeper "bwow" than this circuit can give, though that's not its fault at all. I think it sounds good and it will be fun to play with.
