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keevhren 30th November 2012 03:48 PM

Op Amp Labs project... parts/advice request!
Hi! Long time browser, first time poster here... I don't know for certain if this is going to be in the correct sub-forum as it could fall into a few different ones but I feel as though this will be the most helpful/informative for the lot so here it goes...

Im currently gearing up to embark on what is sure to be an interesting adventure of restoring/racking up an Op Amp Labs 802 (?) Mixer. Some back story: While working at a studio in LA a few years ago, one of the in-house engineers gave me this mixer for free as he hated how bulky it was and he was pretty sure that the majority of it didnt work and was just going to get rid of it anyways. It was obviously beat up, and upon initially firing it up I did get only 3 channels to work, but figured it would be a good project to perhaps tackle down the road. About a year later, I finally got around to giving it a good once over, and with the help of a generous amount of tech spray/cleaner I got 9 of the 10 channels to pass audio cleanly via the mic pres and the echo sends as the output. However, life again got mega busy and I had to put the whole thing aside. Now Ive finally got some free time and Id like to knock this project out! Heres what my goals for the project are:

1. Id like to take the current unit (well, at least 8 of the mic pre channels) and completely remove them from the current chassis and to put them in a 2U or 3U (depending on design constraints) rack unit. Im not concerned with racking the eqs (I might rack them separately at a later date). I want this unit to be strictly a collection of mic pres.
2. The fader is obviously too large for the unit, but I would like to have two stages of gain control, so adding in an additional potentiometer in place of the pre to use as an output gain trim would be great.
3. If possible, Id like to add independent phantom power switches for every channel. I also feel that if Im doing this, I might as well add a phase switch for every channel as well?
4. Id like to use as much of the original parts as possible, but am fully willing and wanting to upgrade the unit to increase its sound quality where possible. In particular, from what Ive read about this board, there seems to be an issue with clipping the output on these units so increasing some of the capacitor values might be able to help with solving that problem if I've also read that correctly?
5. Adding PADs to each channel would also be great to have, but if its too difficult to fit in space wise, Id gladly just use inline pads with whatever mics Id be using on the channels during tracking... not essential, but would be nice.
6. Other than that, the main issue that I feel I will need help with is converting or creating a new power source for the unit. The one that comes stock with the unit is big, bulky, and seems a bit unsafe, being two prong and also having it simply exposed and not fully concealed within a casing.

Ive had a decent bit of experience with DIY related stuff having built a few seventh circle audio kits, as well as building a metropolis amplification Marshall Plexi-style kit, and have done a fair amount of simple repair jobs, trouble shooting, and things of that nature since I first got interested in recording as a teenager. I can easily take care of soldering and am comfortable with punching out a prefabricated chassis to fit with how Im going to configure the unit. Ive been scouring the net for any articles related to the project and am pretty well informed about what I need to do but am quite nervous as this will be my first truly DIY project (ie no set directions to follow and such!)
Ive found a few decent places for parts that Ill need to acquire but would love any great recommendations on places to get an empty 19 rack case, new caps, switches/buttons (for pads/phantom/etc), and things of that nature.

Here is the current rack I was looking at: 19"(W) x 5.25"(H) x 9.84"(D) - 3U Rack Mount Cabinet Equipment Enclosure. | eBay

Also as far as design, Id love to hear any input as to what people might think the best way to do this would be, and certainly any anecdotes of I did something similar, but did this and it was terrible, so do it this way instead aka things youve previously learned in doing projects. Below is a sketch of what Im envisioning for the rough layout of the face of the unit:

Switches or buttons for the phase/phantom I'm thinking will go between each pre vertically, so the switches to the left of the unit will be for that individual unit etc...

And some pics of the unit as it currently stands:

Power Supply:

I'd like to have a complete plan/design in place before moving forward and would appreciate any input/help/guidance from anyone who's got some more experience than I do! Also of note, I have an OG copy of the schematic for one of these consoles, which I'm sure will provide major help with laying this whole thing out correctly. And last but not least... I will surely be needing some new parts for this which is why I posted it in here.

Thanks in advance!

indianajo 30th November 2012 06:10 PM

That power supply cries to be put in a box. I like separate ferous steel boxes, keeps the hum out of the op amps - the thrust of the hum improvement to my disco mixer that had the power transformer 2" from the 50x gain mm phono op amp and the 120 VAC power switch 1" away. Looks like it would fit in a $2 5x8 recipe file box from Goodwill, but of course you can buy something more professional on E-bay. When I really need a fan, I cut the bezel/guard out of a PC case and weld it in. The salvage fan is free, too. But I don't recommend aluminum cases. You e-bay vendor doesn't say if his chassis is steel or aluminum. I've got chokes on the power supply lines going into my op amp ferrous box, to keep the CB radio and lamp dimmer noise out. 100 pf caps on the RCA inputs to the 50x circuits. If you can find a dead PV4, PV8 or PV1.3k amp, they have nice steel chassis, and a great fan control circuit.
Two places that don't have a good supply of stereo log slide pots- and I lucked out my unit was so bad it was hardly used, the pots were great. But I'm still looking for pots for a couple of radios, somewhere that stocks something else I need so I don't pay $10 freight for one item.
See this thread, although your op amp labs unit may not have the hiss that my unit did:

soundguybob 16th October 2014 12:45 AM

Hey @keevhren - did you ever finish this project?

JMFahey 17th October 2014 07:33 PM

I doubt so, looks a little bit too complex for a relative beginner.

Building kits is one thing, and very good to gain experience but in this case he will (or already did :( ) run with grounding problems which he'll have to solve on his own, and providing suitable power, a smaller PSU one per rack so they are independent.

I can understand fitting 1 or 2 channels inside a rackmount unit ... if they fit of course, because there's people who wants that kind of sound and control but don't need more channels, think a piano player, a lone singer with his acoustic guitar, etc. so the idea has merit, but as they say, "the devil is in the details".

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