Readers: If you'd like to build the project featured in today's post, please go to PCBWAY's Community pages--gerber file (main board); gerber for jacks board, front panel gerber, KiCAD project/pcb/schematic/library files, a B.O.M. and more are here.
You can also help out this site immensely by checking out PCBWAY using the link here. Thanks!
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This is a continuation of this previous post...where I laid out and built a Moog T904B HighPass Filter homage, based on a schematic from DIY pioneer Tom Gamble.
Revision one's design/build attempt was disappointing, since it "worked" but sounded, well, terrible.
After rewatching the Kristian Blasol video (here) I wanted to keep working. The filter in his video sounds--interesting.
Really interesting. Nasal, but also shimmery and a bit phase-shifty?
To get this filter off DogDoo Island I redesigned two of REV1's PCB's, improving ground planes, since REV1's ground and audio traces looked terrible on my oscilloscope.
Good news: after building a REV2 Moog/EFM T904B, with PCB's provided by this blog's humble sponsor, PCBWAY, then spending an entire Sunday fixing stupid mistakes and experimenting with different component values, the Moog ladder highpass VCF sounds (I think)--much better. Maybe even good.
I recall back in the day Mr. Gamble posting that his designs and kits were for experienced builders--something like that. In this case he was right! This HPF is a complex build, in terms of parts count, design complexity, and need for modification. This is not a good project for a beginner.
AN ASIDE: WORDS FOR MY SPONSOR
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Happiness is a new batch of boards from PCBWAY.... |
Shout out to Serene and the nice people at PCBWAY for sponsoring this blog. It sometimes takes me 2 or 3 revisions to get something to work; the T904B inspired HPF has been no exception.
PCBWAY has always been patient, friendly and extremely supportive as I work through design changes.
PCBWAY also has done great 3D printing for me and offers other useful services. They charge low prices and fabricate quickly. They are an asset to the DIY world; please help out this blog and check 'em out.
SMOKING HOT
What fun would DiWHY be without trying new (for me) fabrication techniques?
For the Moog/EFM T904B VCF build I purchased a $39 "Amazon's choice" 100mm x 100mm hotplate and $10 worth of solder paste.
Links for the plate and paste (hope these links still work):
TLZBK 350W soldering Station Hot Plate
Wonderway SN42 lead free no clean solder paste
I got building--gooped the paste, not being too neat about it;
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I used tweezers to put the 1206 SMD parts on top of the goop, then dropped it on the hotplate.
Please work in a well-ventilated area. This hotplate/paste/SMD process produced a lot of toxic smoke, and you shouldn't breathe its fumes.
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Baked the jacks board.... |
Marvey--what temperature to use?
This chart shows the heat curve one should follow-"IPC/JEDEC J-STD-020C impressive right?--but the plate only had set/enter switches for temperature; inputting complex functions for heating/cooling wasn't possible.
Instead I decided to heat the damn thing up, and after the parts were put in place and the solder had melted, cool things back down.
To my surprise this approach worked. At about 195C the parts magically centered themselves to solder pads and the solder flowed where it needed to go.
After the parts were set I let the plate continue to heat up to about 210C, then told the plate to climb back down to 100C, which it did, slowly.
At about 150C I removed the PCB.
Visually inspecting: in spite of my heat curve being far from ideal, it worked!
The results were good:
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"Seems working." I was chicken at first to bake on an SOIC TL071, but successfully soldered 2x TL074's using the hotplate when I soldered the main PCB's SMD parts. |
I repeated this for a few more boards.
Overall, the process was far more forgiving than anticipated. There was a solder bridge on an SOIC TL074 that I fixed with solder wick, and one 1206 resistor baked on 90 degrees opposed to where it should have, which I corrected with a soldering iron and some tweezers.
Otherwise--all good.
Overall using the hot plate and paste was far easier and less time consuming than soldering 1206 SMD parts by hand, and I imagine with a solder mask difficult SMD parts (QFN's?) are doable for DiWHYers using this inexpensive setup.
I will keep working with this hot plate for upcoming posts.
HIGH PASS DRIFTER
Long Live Moog! Long live EFM! Interesting: this is a 24db/octave high pass filter, a lot of other popular audio HPF's are 12db/octave (the Oberheim SEM's for instance--a favorite); meaning the T403B can be driven to oscillate. |
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As per a comment in the youtube video: I didn't have to match transistor Vbe's, so I didn't. |
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Testing. I used an single 3mm nylon phillips screw, standoffs, and nut to secure the jacks and mainboard to each other. |
MODS AND SODS
R6 adjusts output and in some cases (oddly) the resonance quality--try between 10K and 100K to get the sound and level you want; I started with 10K (shown) for R6 but later changed it to 50K. |
IMPROVED OUTRO
Also: since this is a highpass filter, it might be useful to change one of the modulation inputs to invert the incoming CV signal and set its bias to somewhere above ground--2.5V perhaps. Or, even better: add a modulation attenuverter to get cool inverted highpass effects.