Transformerless Vari Mu Compressor build thread

[Script]

Can anyone confirm that the BJT alternative to the sidechain transformer works ok?

I had intended to go BJT in the Diode 8 simply for cost reasons, but then never did -- also cos I had run out of real estate in the case.
Heikki's dual comp does without SC input xfrm. But THR is implemented differently there.

 

[C H]

The 107N is 1.5dB down at 300Hz. Data sheet doesn't say what happens in the three octaves below that. Should we care? Doesn't this mean there's a high pass filter in the sidechain whether we choose it or not?

 

[JMan]

There is a HPF at around 20Hz by default

 

[C H]

Indeed, but doesn't the transformer also impose a high pass at a much higher frequency?

 

[thelivingroom]

The 107N is 1.5dB down at 300Hz. Data sheet doesn't say what happens in the three octaves below that. Should we care? Doesn't this mean there's a high pass filter in the sidechain whether we choose it or not?

The 107N and 107V data sheets show the frequency plot. Somewhere in my messages Heikki recommended the 107V over the 107N.

 

[JMan]

Indeed, but doesn't the transformer also impose a high pass at a much higher frequency?

I do see your reasoning here and I can't speak to it on a technical level. Just going by ear, my unit does seem to be responding as I would expect well down into the low frequencies when I do not have the added HPF engaged. But I haven't verified by running any sweeps or the like.

 

[C H]

The 107N and 107V data sheets show the frequency plot. Somewhere in my messages Heikki recommended the 107V over the 107N.

View attachment 146213

View attachment 146214

Ah OK, that's not too bad. Still, wouldn't bjts be better (and a lot cheaper)?
Does the transformer step up, or down? The pinouts on schematic and data sheet don't match.

 

[scott2000]

wouldn't bjts be better (and a lot cheaper)?

I'm not having any issues with the Hammonds. Can use any transformer too obviously..
from Heikki when I was talking with him on the subject
"Resistance from R35 & R37 and the sidechain transformer inductance form a high pass filter for the sidechain. This is done on purpose. In my opinion in a backward acting vari-mu compressor there should always be a high pass filter on the sidechain. I think I have written more about this on groupdiy"

Better? Who knows.. Cheaper?
Would be cheaper to replace R6 on the sidechain boards with a jumper... if you wanted faster attack times... But then we're getting into maybe more subjective design choices.. Mods...
but again, Heikki's view..
"In my experience when limiting/compressing for effect 2 ms is about the fastest attack needed. With faster attack times you would want to make sure the tubes are in near perfect balance. Because when the “thump” gets to be high enough frequency it is hard to keep it out from the sidechain and from making that overcompression effect which can be seen in the Manley example, even though the “thump” itself might be inaudible. In my opinion ultra fast tube compressor should have a feedforward topology like the Gyraf vari-mu. Feedforward vari-mu compressors have their own unique problems and probably thats why feedforward topology is rarely used in vari-mu compressors. "


These units are laughingly good for what they cost. When I was in Bob Katz studio testing it along side some other very impressive units, it had zero issues being right at home and was preferable to some. And this was before the mastering version being made. Of course that's program dependent and the Leapwing plugin bested all the hardware he had on certain material...

Does the transformer step up, or down

In the compressor they step up
"With 107N the output levels where compression starts will shift few dB up"

The pinouts on schematic and data sheet don't match.

I think it was an error that was never corrected. Pin 1 on transformer goes to the square pad on the pcb.

 

[thelivingroom]

I finished mine yesterday, then listened to many genres of program material through it on numerous settings for hours while cleaning up my build mess and setting up to shoot photos of it, and share the same sentiment as @JMan with feeling like a kid on Christmas. This thing rips, is extremely quiet at idle, and I’m surprised that the thread isn’t 100+ pages by now. It’s affordable, easy to follow, easy to calibrate, original, modern, and was a pure joy for me to build. It sounds so good running it transformerless that in hindsight I wish I’d have omitted the input trans option and saved some $$ and wiring time.

It’s been a long time since I’ve hardwired an entire build without one breakaway connector, so I was very happy to fire it up and have every voltage test point measure spot on. A +4dBu sweep at unity gain was dead flat from 20Hz to 25kHz on both channels. The tube balance calibration measured less than 1mV on each channel (-60 and -62dBu) with the NOS GE 6SK7 steel tubes. I also bought extra bundles of NOS RCA (also steel), Sylvania JAN, and Hit-Ray (both glass), but didn’t feel the urge to swap out the GE’s. The AC-DC power adapter is a TDK-Lambda DTM65PW360C that I bought a couple of new on eBay. The datasheet for it listed the model of the Kycon connector, so sourcing the correct chassis power inlet was easy. I can also confirm that you can use any brightness LED that you want for the GR meters and it doesn’t affect the metering; just adjust the 4.7k resistor values to taste (R4, 7, 10, 13, 16, and 19). I used super bright and dumbed them way down, with the added benefit of drawing less current from the PSU. I sent the 25.3-ish volt power for them out to a breadboard and worked all that out beforehand. Each color change ended up with a different value for them to all match in brightness, as expected.

I’ve been wanting to design a front panel inspired by a vintage HH Scott integrated amp that I read about many years ago in a tube amp magazine, with a gold faceplate and slide switches, and this project felt like the perfect fit. I did a fair amount of CAD work to lay out the symmetry and figure out the side chain mounting panel (to both conceal the hardware and buy myself some space for where I wanted to mount High-Low switches), GR PCB locations, power switch plate, and case layout. I wanted the GR LED’s to be on the same centerline as the slide and rotary switches, so that meant bending an offset on the ones for Channel 2. I drew the LED with the offset in CAD, then printed it and taped it to a wood block with a cavity for the LED to sit in and bent them. It worked like a charm when installing the meter PCB’s. I clamped the face of the front panel to a piece of 4mm aluminum I had laying around, placed all the LED’s in the PCB’s, mounted the boards to the standoffs, used a hex driver to reach in and push each down until they hit the 4mm panel, then soldered them. To clean up the look of the gap through the power switch hole and keep any light emitted from the backside of the meters from shining through it, I cut a scrap piece of tool box drawer liner, used a leather punch to make the holes for it to slide over the standoffs, cut a slit in it, then affixed it to the front panel with E6000. In case of future surgery or potential initial troubleshooting with everything hardwired, I wired it with everything coming up from the bottom to the controls so the front panel can be unbolted and rotated down flat on the table.

Thank you for the boards @scott2000, Heikki for the wonderful project, and everyone else who contributed useful information in this thread!

 

[JMan]

Unreal. As usual, you've raised the bar on clean, attractive design work!

 

[thelivingroom]

Unreal. As usual, you've raised the bar on clean, attractive design work!

Thanks bud! It’s an aspect of these projects that I really enjoy more and more. I probably benefit from not being wealthy and there usually being months between steps to take my time, think about it, and make changes haha.

 

[inf0]

Stunning attention to detail ! Indeed raising the bar ! Love it

 

[JMan]

Thanks bud! It’s an aspect of these projects that I really enjoy more and more. I probably benefit from not being wealthy and there usually being months between steps to take my time, think about it, and make changes haha.

I hear you on this. I started this project at least three years ago. I always seem to build this way - a big push forward followed by a long period of no progress, mostly because like you I don't have the resources to do it all at once. In some ways, though, I like it, because it gives plenty of time to mull over the details and plan out the little things. But my main takeaway is that I spent three+ years on my unit and the wiring still isn't anywhere near as beautiful as yours!

EDIT to add: Am I correct in my assumption that you use solid core wire for most of your wiring? Some of those bends just seem too clean for stranded.

 

[TimG]

That’s gorgeous. Grew up with a Fisher 400.
The transformers (you would skip) are UTM?

 

[scott2000]

 

[thelivingroom]

I hear you on this. I started this project at least three years ago. I always seem to build this way - a big push forward followed by a long period of no progress, mostly because like you I don't have the resources to do it all at once. In some ways, though, I like it, because it gives plenty of time to mull over the details and plan out the little things. But my main takeaway is that I spent three+ years on my unit and the wiring still isn't anywhere near as beautiful as yours!

EDIT to add: Am I correct in my assumption that you use solid core wire for most of your wiring? Some of those bends just seem too clean for stranded.

I build in a similar manner. First I’ll read an entire thread and get the BOM entered into my personal template that also has a vendor list section at the bottom with total cost for each vendor, ordered checkbox for each, grand total for the project, purchased total, and total of what’s left. Then I’ll get the boards and all the small stuff that gets stuffed first and see all these checkboxes X’d out for purchased and installed, only to realize that I was thinking “I’m close, just need this and that”, when the reality is that it was all the cheap stuff and I still have 75% of the cost left to go from the three vendors left haha. That’s the boat I’m in with the Reddish EQ’s that are coming up next (my Nu VariMu case and front panel are currently on order from Frank - woohoo!). I looked at the spreadsheet yesterday and “all I need” are the Grayhills, iron, and metalwork.

I appreciate the compliment! Don’t sell yourself short; yours looks amazing. When I saw it my thought was that mine was about to feel quite basic compared to that masterpiece. Yours necessitated more wiring than mine, so it’s naturally going to give the illusion of being less clean. My Orange 86 and TG compressor wiring doesn’t appear as nice to me as this or my D-LA2A, even though the thought process and care were the same.

The hookup wire is actually all stranded, PTFE coated from Apex Jr. in 22 and 18 gauge. The following is my procedure and thought process for how I get to this kind of end result now, as it’s something I’ve worked on improving with each build. The first thing I do is lay out every color of wire I have, including any pre-twisted remnants left over from previous projects that I may be able to use, with 22 and 18 gauge separated on the table. I always have 10 solid colors and 8 or so striped in 22 gauge, and 10 solid colors in 18 gauge. Then I get the schematic, build guide, and/or in this case the pages where Heikki drew all the connections, and jot down each run on a legal pad, in addition to any bells and whistles I’ve added. After that I think about the incoming power to the PSU board and any givens (example: on my Orange 86, the power trans 6.3vAC taps for filament were orange and blue, yellow and white taps were for B+, and red and black taps for primary), write them down, and pull those colors out of the pool and place them to the side. Green is always for anything I’m running to star ground, so that gets pulled. Then I think about the color palette of the boards, parts, and false bottom panels that I’ve started doing lately and go from there with picking colors for the power out of the PSU, because it runs all over the place. For this project all 0v power is grey, with only the different +/- voltage wires changing colors to differentiate them. I maintain this thought throughout so I don’t run out of color options (example: on this one CV-0 are both the same color, with only CV changing colors between channels 1 and 2. Same for gain, side chain input, etc.). I do this until everything is chosen and written down. When all of the hardware gets installed in the case I’ll visualize it completed, consider future disassembly, start estimating where and how long the runs will be, add at least a foot (they get shorter with twisting) or even more if I know it’s going to get cut and jumped a few times (like power), then start twisting and placing off to the side. I use a drill and the Model 303 vice in my Panavise base for twisting. It goes quickly, doesn’t kill my hands, is nice and tight, and looks more even than when I did it by hand. Hooking up to the front panel is always last for me, so I start from the back of the case and work my way forward, with power and I/O XLR’s first, then go from there until everything is in and all front panel wires are hanging out the front of the case. After all of the controls are installed in the front panel, I’ll lay it down flat on its own and install any wiring I can that’s separate from what’s exiting the case (on this one SC HPF switch to SC board, High-Low switch, Link switch, etc.). Then I either hold the front panel up to the case or loosely install it, and start the case to front panel wire trimming and installing, on this one starting from right (power) and working my way left. Tip: I use flat head machine screws to temporarily hold the front panel up so I don’t mar it around the mounting holes, as I probably removed this one 30 times while trimming wires and soldering before final mounting. For the bends on the hookup wire, I start by using three fingers and pulling along the run several times to get it bending in the direction I want and shorten the radius more with each pull (think tubing roller), then to finish it off I’ll do the final bend over a screwdriver, finger, etc. With the nice twists done in the previous step it stays in form. For the Belden 8451 2C shielded, I use a heat gun to coax it and hold it in form until it cools. Some of all of this I’ve described may deviate from project to project upon necessity, but is the general flow for me. I apologize if this was a bit long-winded or too “101” in spots, but I always see you helping people on here and wanted to return the favor, and hope you or anyone else may benefit from some of it!

 

[inf0]

I swear to god , I bought the PTFE wire from Apex when I randomly found about him through google and I've been cherishing it like gold ( even though the prices are ridiculously low compared to other vendors ) . I couldn't have finished my p2p LA2A without it , such a nice wire!

 

[thelivingroom]

That’s gorgeous. Grew up with a Fisher 400.
The transformers (you would skip) are UTM?

Thank you!

Yeah, I would skip them if I could go back in time. It’s not the transformers’ fault. They sound fine and the frequency response measured identical enough to when they’re out. The only difference was the slightest very high end .2dBu dip after 15kHz that you’d never notice. My turnoff is that there’s almost a 1dBu insertion loss, so it’s not a true “A/B” switch comparison without having to make a controls adjustment. It’s also a testament to how good this unit sounds without them in.

 

[thelivingroom]

I swear to god , I bought the PTFE wire from Apex when I randomly found about him through google and I've been cherishing it like gold ( even though the prices are ridiculously low compared to other vendors ) . I couldn't have finished my p2p LA2A without it , such a nice wire!

I started buying it from him probably about 8 years ago, and haven’t looked back. I’ll check my stock before the next wave of projects and order 20’ of whatever I need, then throw in a few extra striped of his choice to add to my collection.