Building a Moog T904B Inspired High Pass Filter--Using an SMD hotplate--Works!

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 my version of this filter off DogDoo Island I redesigned two of Revision 1's PCB's, improving ground planes; 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

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.

Back to the post....

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;

 

 I used tweezers to put the 1206 SMD parts on top of the goop, then dropped it on the hotplate.

Note: if you want to try out this fabrication technique, work in a well-ventilated area. This hotplate/paste/SMD process produced a lot of toxic smoke; you shouldn't breathe its fumes.

Baked the jacks board....

Marvey--what temperature to use? 

This chart shows the heat curve one should follow--"IPC/JEDEC J-STD-020C--impressive?--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 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 and put it on my bench to cool down to room temperature. I noticed that at 150C components could still be jostled and knocked out of place, so, removal of the populated, soldered PCB from the hotplate had to done carefully.

Visually inspecting after cooldown: in spite of my heat curve being far from ideal, it worked! 

 

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.

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.

One byproduct--after the 1206 bake there was a lot of leftover flux on the PCB. I used "no clean"paste, meaning this leftover flux isn't conductive and can stay where it is, but I dislike spoojim all over my PCBs. I am still trying to figure out how to best clean the no clean: flux remover, isopropyl alcohol, and other attempts haven't worked...maybe I have to leave it.

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 DiWHY'ers using this inexpensive setup.

I will keep working with this hot plate for upcoming posts.

HIGH PASS DRIFTER

OK, let's talk HP filter.

The original Moog patent design looks like this:

From this MW forum post  the EFM design resembles the Moog ladder filter patent...indeed:

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.

From here it was a VCF build like so many other VCF builds: design and revise PCB's with Kicad; send gerbers off to PCBWAY; solder SMD's first then add through-hole components...

As per a comment in the youtube video: I didn't have to match transistor Vbe's, so I didn't.

Before too long I had new PCB's, populated and ready to test.

Testing. I used an single 3mm nylon phillips screw, standoffs, and nut to secure the jacks and mainboard to each other.

Did it work first time (WFT)?  

Nope.

I spent a masochistically joyful Sunday (all pain, all day long) troubleshooting the REV2 build. 

There were a few stupid build errors, the biggest: I put .22uF caps in the resonance feedback subcircuit, where .22pF should be used.  

I had a cold solder joint in the 100mil V-- trace between main and jacks board--that didn't help.

EFM mistake? The EFM schematic showed the main frequency cutoff pot with V++ (I used +12) Vdc on one side and ground on the other. Nope. It needed V+ on one side and V- on the other--this is corrected on the PCB's uploaded to the PCBWAY community site:

Otherwise the VCF cutoff required CV values below ground....doable, but not "normal" as I see things.

I found the EFM resistor values for CV mod didn't work for me; for my rack's 0-5V CV setup I used 20K's and B50K pots as in the snippet below.... 

 

Fortunately incorporating 1206 SMD resistors made trying different values for R1/R2 easy: using a 700F soldering iron with a sharply pointed tip I heated up both ends of the tiny resistors, removed them, and soldered in something different. This was faster and easier than working with through hole resistors and posed less danger of damaging traces when changing out components.

MODS AND SODS

Besides messing with resistor values I made two semi-major modifications to the EFM schematic:

First to the jacks board I added an output buffer:

R6 adjusts output--try between 10K and 100K to get the output level you want; I started with 10K (shown) for R6 but later changed it to 47K.

 

Second: I added an audio input attenuator to the jacks board: 

 

I added the latter ("ladder"?) because in this cool Youtube video the filter sounded slightly distorted at times.

Looking at the EFM schematic I thought that high peak to peak input signals saturated the first stage of the PNP/NPN/op amp trifecta--just a guess.

To my surprise the additional input attenuator didn't only let me dial in appropriate audio levels to remediate distortion, in some cases, the attenuator changed the character of the filter's resonance.  Unexpected, but I'll take it!

 

One more tidbit: I didn't have 220K 1206 resistors in my junk box, so on the main board I used 200K for R11 and 30K for R21. I am not sure how common 220K 1206 SMD resistors are at retailers like Tayda--whatever--some resistor values aren't critical--just get them close.

IMPROVED OUTRO

Anything Moog and EFM are cool right?  

Sure, but: this EFM Moog HPF isn't a module where every setting sounds great. In general I found I had to experiment--it's an odd beast--messing around is a big part of what we do, so this is a feature, not a bug? 

If I were to continue working on this filter--I probably don't have time--I'd add a resistor in series with the V++ feed to the cutoff1 pot (current "jacks board" has a resistor for V--, but not V++).  

Why: a lot of potentiometer real estate for the frequency cutoff above 2PM was useless--turn the frequency pot above 60-70% and the audio at output was gone.   

The modification to the cutoff pot should look something like this:

Update: modified! I cut the "3" leg to Cutoff1 on the jacks board and kludged in a 12K THT resistor between it and the V++ rail. Big help! If I do a REV3 of the jacks board I will include this modification.

Also: since this is a high-pass 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. Even better: add a modulation attenuverter to get cool inverted highpass effects. 

Finally: would matching Vbe in the transistor pairs make this sound better?  A traditional LP Ladder filter doesn't have a buffer between each transistor pair, but this design does, so I am going to guess it doesn't need transistors to be matched. Just a guess. 

OK enough for now--this has been a maddingly fun project--to remain high, consider giving this filter a pass. Something like that. See ya next time.