Altec Passive EQ's

[bendymusic]

After years of gear lust I finally came up with 4 sweet Altec 61a (9061a) Passive eq's. Two are NOS and look like they came off the assembly line yesterday, the chrome is perfect. What they are: 2 bands of passive equalizers with 40, 100, bypass, and 3k, 5k, 10k, 15k. +12db, -16db. Everything is switched. There is 14db of insertion loss so I decided to use the JLM dINgo kit for interfacing. More on that later. This thread is a collection of postings of my progress over the last 3 months, hey I want to get these done as fast as possible but I gotta work too.. trying to learn some patience.

[bendymusic]

The first one is always the hardest. When drilling large holes in thin aluminum it is helpful to screw the panel onto a block of wood to prevent warpage. In this case I had an extra screw in the center. Then I used the nibbler tool, PITA.

[bendymusic]

The square hole is complete, now just a little filing... and drill two mounting holes... First eq now is mounted! Next I have to punch a couple holes for connectors and mount the JLM buffer PCB that I stuffed a couple months before. Then wiring and testing. Why did I think this would be easy?

[bendymusic]

Took some doing and an incorrect order from Markertek but I finally got the right connectors (XLR PCB mount) on the i/o board.

Then drilling the guide holes for the rear panel. And then the chassis punch.

[bendymusic]

Lots of room, need a slightly smaller punch after I figure out the exact fit. For the prototype this works well to have a little extra slop. The i/o board is mounted to the rear panel via XLR connectors and side panel via 2 small screws. There are a couple pieces of rubber between the bottom of the circuit board and the side of the case.

[bendymusic]

Here you can see the IO mounted and the box fitted. The fit is very tight, no extra room to spare between the i/o board and the eq module. Not my preference, but it seems okay.. Next mounted the power connector and such (5 pin XLR).

[bendymusic]

Got the IO up and running. Using a PowerOne to supply +/- 18volts. Am having some issues with the JLM dINgo, for one its trying to do too much on one circuit board (too many options), and another the schematic reads like an auto wiring diagram. Also, it came with the wrong IC's!! So we had to drive all over LA to find something that would work and ended up with some scratchy old 353's, but they worked. I have a little debate with my friend who is helping me with the circuit, he wants to make it unbalanced so as to simplify the circuit, and I want balanced IO. I just think it will interface with the rest of the world more readily, but its not ideal from other POV's. Always there is an option for a transformer, but thats really a can of worms, #1 not much room, and #2 will change the sonics of the equalizer.

[bendymusic]

I decided to box up the power supply, I hate the 5 pin XLR's and am wondering why I got talked into using them..

[bendymusic]

Okay, a little testing.. First image shows 1k with the 353's, some noise and ringing. (thats about -70db overall noise)

Second image is with 5532's.. The noise floor is within the testing noise floor error (not using my best A/D converter and adding ~20db makeup gain). Very low noise unit and little distortion. Still have some circuit tweaking to do however....considering some more "audiophile" ic's and discreet as well, not sure if it will make a noticeable difference in the application however.

[bendymusic]

(sorry for the smallish images, not sure why they are coming up like that).. I did some extensive messing around with the prototype and am totally pleased with the results. First I played a lifeless (mono) mix into the thing, boosted 40 and 15k, all the way up, WOW :cool:, I can't get a plugin to do this. The q gets steeper as you boost, but it doesnt get overwhelming maxed out, its like the transients just punch more, hard to describe, its like they increase the clarity as well as the frequency. In cut, the eq is very broad and almost subtle. Then I tried it on a guitar with a dark sounding DI, brought out the high end sweetly with great transients.

[EMRR]

Cool, yes, these are cool. To avoid confusing anyone I'd point out these are not 500 series modules, since they sorta look like they might be.

I run mine without any sort of dedicated make-up amp. I drive them harder and ride the fader higher. Here's some curves:

http://farm4.static.flickr.com/3037/2696523034_50424b8f9d_b.jpg
http://farm4.static.flickr.com/3033/2696522870_07847a2fb8_b.jpg

[bendymusic]

You da man!! Nice graphs there. Not many know about these things, and thats kinda okay with me. ;) So how are you dealing with the source load impedance, are you using a transformer, or just connecting straight in? Check out how perfect those curves are! Wonder why there is that stair stepping on the LF though. It will be interesting to see how mine compare with the buffer amp, my buddy John and I still have some tweaking to do, and the board accepts DOA's too, so there are loads of options. And I can always ditch that whole idea too. Thanks for posting these graphs, excellent.

[EMRR]

The stair stepping is a product of my pink noise generator, they are really smooth. These units are really unaffected by source and load. There's an Altec paper about EQ loading for various Altec filter types that can be found at the Altec Heritage site. I did try some Edcor 600:10K's for passive step up on the output side, and that worked fine too. Felt I didn't need them in the end.

[pan60]

:D

nice EMRR!

[bendymusic]

Marked the square hole with a scribe for the other units. The panels are centered so they can be used for a left or right facing eq's.

[bendymusic]

Screwed the four panels together for drilling. I get impatient. Also, an idea I should have thought of sooner, the blue tape helps keep the face of the panel from getting all scratched up when using the nibble tool.

[bendymusic]

Here are the second pair, new old stock. The chrome is perfect! Too bad all that chrome is on the inside of the unit though.

[bendymusic]

Here are all four eq's and the power supply in a temporary tray. Still tons of work to do, but most everything else is internal: PCB stuffing, PCB mounting, power connectors, internal wiring, power cable wiring, power indication on each unit, testing, mods, etc..

[EMRR]

Lookin' good. The blue tape is nice for this.

[bendymusic]

Thanks!

Yeah my two favorite things in the shop are now blue tape and the hot-glue gun. I was concerned at first that the tape would gum up my cheap nibbler tool, but it turned out to be only a minor issue. Overall they look pretty good, well, the two newish looking ones look better than the other two. I have to be careful how I go about projects like this, to break them into tasks that I can complete in a single afternoon. Otherwise I might get impatient and screw it up. Fortunately, I just ordered that Pico M/S Matrix kit so I will have more distractions. ;)

&e

[pan60]

looking nice! :D