"That's great. Can you get it in one of the little boxes?" Engineer's Thumb #3

[midwayfair]

A friend of mine came over to try out a couple pedals, and he was casting around for a really super sustain-y compressor. I had made him a 3-knob Afterlife (the one with Van Gogh artwork: http://www.madbeanpedals.com/forum/index.php?topic=5589.0), but he didn't want "subtle" this time around. So of course, I had him try one of my favorite compressors, the Engineer's Thumb, and then he reveals that he's got a space about the length of two 1590As end-to-end on a single slat of his pedal train.

So he says, "That's great. Can you get it in one of the little boxes?"

I laugh and show him the size of the layout in my second one (http://jonpattonmusic.files.wordpress.com/2012/05/engineers-thumb-2-guts.jpg?w=457), which was shrunken considerably from both my first attempt and Merlin's original board.

Then I go to print out the layout, and I notice that it's only 14x20 perfboard rows. One of those rows was just a ground line on the bottom, and another row at the top was easily shaved off simply by rearranging a couple resistors.

CHALLENGE ACCEPTED.

The board already had some jumpers (unavoidable with this circuit, it seems), but this time I ended up with one fewer under the board, which was nice. I decided to shave a couple columns off, too, which meant more 1/8W resistors, but what the heck, in for a penny in for a pound and all that.

I checked with Merlin that he was okay with me making this for someone for money, and he gave the thumbs up. (This will not be my only thumb pun, YOU HAVE BEEN WARNED.)

Lordy, this was not my smoothest build. I made a horrible mistake the first time around by trying to sub a 5532 in the audio op amp, because I only had two TL072s in the parts box and really didn't want to use them both if I could help it. This was just pure foolishness. Merlin Blencowe does not under-engineer anything, and in this case, it not only turns out that TL072 is THE best choice for the op amp, but that very few dual op amps will actually WORK thanks to the required input impedence and biasing (1M for a biasing resistor is pretty huge .. the 5532 probably wants something more along the lines of, oh, a couple 47K). The TL072 performs much better than the normal "upgrade" of an OPA2134 here, which had too much output and kicks into too much compression early on the knob. (Most other chips wouldn't work or created distortion.)

So there followed a massive amount of frustration that ate up my Saturday -- unexpressible how awful it is to have all my voltages and connections correct and have no idea what can still be wrong. Even worse, one of the OP amps was dodgy and had previously been desoldered, and it turns out was blown. I rebuilt from scratch using sockets so I could do some experimenting. I still didn't know that the 5532 didn't work, and since that's the first thing I plugged into the audio path, I again started to go insane trying to figure out how it couldn't be working. Fortunately, I decided to sleep on it, woke up with an inkling that it HAD to be a bad op amp or something, dug a couple TL062s out of the parts bin, popped them in, and it worked beautifully. Pulled the blown TL072 from the other circuit and soldered in a socket, plugged in a TL062 and that one worked too. So YAY. Now *I* can have a mini thumb, too. In the end, since I was building this one for a friend, I started opening up some boxes and managed to find two TL072s in sockets from older builds before I stopped using sockets all the time, just to make sure he got the quietest available build.

So after that novel, here it is, Engineer's Thumb, bite-sized.



No, a yin-yang symbol has nothing to do with Sherlock Holmes. He specifically asked for it. I'm not really sure why, but the customer's always correct, right? :-/

By the way, looking back at my first perf layout for this pedal, done freehand and which was almost too big for a 1590B: http://jonpattonmusic.files.wordpress.com/2012/05/engineers-thumb-guts.jpg
... Yeah.

[pickdropper]

Super cool, Jon.  Always feels good to squeeze a circuit into a 1590A.

[midwayfair]

Super cool, Jon.  Always feels good to squeeze a circuit into a 1590A.

Why thank you! Though your last build is making me feel like a chump!

[raulduke]

Nice build mate.

Really nice vero layout for the compressor too.

[dbharris]

Wow, this is just stunning, the finish and the build quality!!!

[nzCdog]

Haha.  That circuit would be hard enought to squeeze into a 1590A on a double sided pcboard, let alone vero.   Looks great