Melcor AML-27 restoration/racking 1RU stereo preamp
yoyoyo i got something done! i’ve been stuck on this thing and put it on thr backburner like embarassingly long ago and just went thru and got it back up and working:
Melcor Electronics Corp. AML-27 preamp cards, which were made circa 1965-1969 were sold in this backplane format, ordered or installed with fixed gain they were part of the ‘big green machine’ recording console as well as made with the RCA logo and branding for the same 1731 op amp used on these circuit boards. The 1731 uses 9 transistors. Melcor Electronics Corp/MEC still exists, and they made industrial microwaves and peltier coolers and stuff. MEC sold/ended their audio equipment division in a way that gave rise to Automated Processes, Incorporated, API. They still exist making consoles, and may even still make this format of preamp/line amp cards for all I know. It is 300mm not 500mm though, smaller than the popular 500-series lunchbox format. Anywho, API redesigned this whole thing with improved everything. They came up with the 2520 discrete op amp, which has been made along with its subsequent revisions in later years and is still the backbone of API consoles and outboard gear. The 2520 uses 11-transistors.the 400-17 and 400-20 transformers MEC used would be replaced with the API 2622 and 2623’s on their model 312 mic preamp. Or just the 2623, in the case of the 325 line amp card.
The AML-27 is a perfectly imperfect circuit design, lovely in its time, and it balances all of its various imperfections nicely… nay in an electronic sense, it absolutely juggles the metallurgy (and limitations of the transformer designs), op amp instabilities, feedback, gain, distortion, and overall resonance to make a really awesome fat sound. Compared to the API mic pre, it is much more lows and more third harmonic distortion. The distortion also becomes pretty soft clipping in the high gain settings. The API 312’s feedback lends itself to a more midrangey overall preamp sound. The whole sound comparison just boils down to sounding like the late 60’s vs the API sounding like early 70’s circuits, i dont have much better way to describe it. I read on the now-renamed gearslutz that Melcor circuits are all over the Frank Zappa and the Mothers of Invention album Uncle Meat.
My pair of cards was likely parted out from one of these old MEC backplanes around 6-8 years ago. I have serial numbers 012 and 936, but I have no idea their history before me!
For my part, I removed a PCB card kit build in 2010 that I took apart to rack these up, so everything is sort of prototype quality to get them usable and make sure they work right. It has way too many screw holes and scratched up paint, sharpie, and all kind of mess but so far I recapped the electrolytics, matched the feedback and built Grayhill 1073 12-position rotary switches with 1% metal film resistors for the stepped gain control. It has Bourns output attenuators, 110% shielded pure copper twist wire on all the audio path connections with quad 26ga +/- conductor wires. DC supply is on solid 22ga copper throughout, but retains the original Melcor card power LC filter instead of the diodes that were common on the later API mic pre designs that were based on this. I really paid a lot of attention to grounding, shielding, and cable routing, but turns out I did a terrible job back when I first hooked up the audio lines and need to redo it while spacing the cards out evenly in the chassis.
I got these cards stable up to 65dB of gain! :D I have a plan to get the 70dB stable without having to flip 2 switches, but it will have to wait until I can change to a multigang rotary switch.
So far I’ve fixed the input impedance at 600-Ohms, but I installed switches that will allow for the 3-way 300-, 600-, 1200-Ohm input strapping; i still have to wire that up. There is also already functioning 180° phase reverse, -18dB pad, and 48v phantom power on the input line. In the spirit of the 60’s prototype feel, I just went with directly wired mini toggle bat switches.
The gain and output attenuation knobs are a repro RCA skirted pointer knob scaled down to an appropriate size. The chassis is a 1U Middle Atlantic 8” deep steel puppy I got at Randolph & Rice in 2009 or 10 for the kit build. RIP Randolph & Rice Electronics, I miss them every time I build a thing… I am probably going to paint this with the vintage Melcor logo and the logo silhouette repeated as the eye slit in a stencil and possibly rattle-can likeness of the Big Bad of Tolkien lore, Melkor, aka Morgoth. AFAIK nobody else has seized on the company having a Tolkien themed name in arting up any mic pre builds… does anybody have any good Melkor art they’d want hidden in my rack??
So next I have to polish up my racking Valley People mic preamps project, and build an RCA tube console mic preamp pair from scratch of cobbled together vintage parts and new components. I’m also going to turn an Olsen Color Organ deathtrap box into a multiband filter/EQ, and make a clone Audimation Corp. EQ in an original Audimation EQ rack chassis. haha my work is cut out for me… but the mic pre’s will fill out my setup with a pair or two of every decade of noteworthy and/or Nashville linked sounds. for the early 60’s: something like what was originally installed at RCA Studio B, for the late 60’s: these Melcors, for the 70’s: API 2098 console pre’s modded out to the max for what is in RCA studio B now, the 80’s: Valley People/1979 Loft Audio 800 Console MPA5 made hy Valley People in Nashville, for the 90’s and 00’s: i feel like everyone was using protools and mbox interfaces and stuff that kinda didnt have much of a sound so im throwing in MOTU and RME transformerless preamps, and for the 2010’s: Miktek MPA-201, assembled in Nashville version of a rack pair of 1073 mic preamps using correct NOS transistors with 6 AMI audio transformers, dual metering, and lit relay switches for all the controls. 70dB of clean gain. 🔥🔥 I can pretty much make a recording sound like any of those decades with what I have, or combine it all for a mix with all this richness and character of some sick sounding mic and preamp combinations to do all the hard work up front!
