My 550A Project

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Well-known member
Apr 3, 2010
Beverly, MA
So I was going to come up with some cheeky subject but I thought there will be plenty of time for that later.

At any rate, I've been slaving away for quite a while now on reproducing the "it'll never be done" 550A using standard catalog switches from Grayhill.  The frequency selection was really the tricky bit and I had to jam 104 caps in there to get it right, but it does the trick!  I'm pleased to say that I've had some great success and with the exception of chasing some small bugs and a couple of mechanical items, everything works very well!

I'll post some nice traces in a little bit, but I wanted to get some pics of the unit up.  There's still work to be done, but I'm so close I couldn't resist sharing  ;D ;D


Slightly rough front panel.  I need a new name.. TB55A isn't doing it for me.  Sick of the Kilo knobs as well, but "that's not important right now" (ala Airplane!).  Gives an idea of the operation.


The fit together.  Once I get my proper front panel, there will be a standoff between the top and bottom boards with a nylon screw on top.  That nylon screw will be the tallest part of the board (at exactly 1.49") and should protect us against any naughty neighbors who might intrude on our space and ground out any of the stuff on the filter board since it really is close to the top (side?).


Oh, and big thanks to Papa Steiger for looking over my shoulder a bit and giving me some proper nudges here and there  :D

More to come..

Wow! That looks great man. Good work. That's a whole lot of caps on there too. Looking forward to seeing your finished module!
Thanks, guys.  Been a long road for sure.

And Dan, had to use all those caps to make up for not having the custom switches.  True there are only 40 caps in the original but many get put in parallel depending upon the switch position to make different values.  Also, wanting to use the WIMAs made it even worse since they don't have as many available values as some of the others.  Recognize some of those switches? :)
Like I mentioned on FB Brian, you are now king of the Lab!  ;D

I know it was more of a personal quest and had nothing to do with being king, but my hat is off to you! Your project looks outstanding!!

Nothing like 104 caps to get you tired of soldering a 104 caps!!

Best, Jeff
Man, you said it Jeff!  This thing takes a looong time to build.  Very easy to put something in the wrong place, too.  Definitely a project for the patient and careful.

Dan, a little to early yet for listening impressions.  I've done some trials and I think it's pretty sweet!  Of course I might be slightly biased.  My next build should really be as close to the finished unit as it needs to be and I'll probably do two.  I'd like to get those out to some other more experienced ears for feedback.
[silent:arts] said:
... speechless
I will help with one thing I forgot to mention earlier. Nice job on adding the card edge for the extra 51x pins. Nice to have for alignment and easy to trim off if not needed!  ;)
jsteiger said:
I will help with one thing I forgot to mention earlier. Nice job on adding the card edge for the extra 51x pins. Nice to have for alignment and easy to trim off if not needed!  ;)

Heck yeah!  Gotta think of my 51x brethren.  ;D
And some of those sweeps right here.  Mine side-by-side with a vintage A as indicated with different gain settings.

Shelf - Shelf

50Hz - .8K - 10K

100Hz - 3K - 15K Shelf

Congratulation for this nice API eq.
your baby looks very good. :p
I hope find a kit in the white Maket soon



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