Melcor AML-27 Mic Preamp (Gerbers)

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since @jsteiger / CAPI has released the MEC1731 DOA in addition to replica Melcor input and output transformers, it only made sense to post this goofy project I did last summer. Will anyone use this? Who knows! But it's free / open-source for you.

These are gerbers for a replica of the Melcor AML-27 microphone preamp. We have several of these and love them.

I've been slowly transplanting the components from mine into a 500 series board that I made (still prototyping... more on that later), but it seems like a waste to just toss out old Melcor racks, so you can use this board to build you own and have so many preamps. I could make an EAGLE footprint for just the card / edge connector too if anyone is interested.

Keep in mind this is NOT the same pinout as 500 series.

Schematic attached is a direct trace from my units -- more info on those here (some different resistor values, trim resistor info, etc).

Loading resistor “R5” is on the board but not on the schematic (see attached addendum). Original AML-27 cards did not use this, but it’s there if you want it because a couple of mine had been modded for it.

These gerbers are to JLCPCB's specs, but I can't see why they wouldn't work elsewhere.

One other note that I've found is that the mounting bolt spacing for the original Melcor input transformers is very inconsistent unit-to-unit, so you might have to file / drill one of the holes out a little...



For authenticity (although idk why you would want this in 2024), you can unzip and delete some of the gerbers (soldermask and all top layers) to essentially make a single-sided "replica" board with exposed traces just like the originals. If you want to be EVEN MORE AUTHENTIC..... just remove the top screen print layer and it'll look just like the real deal

edit 11/2024: someone informed me that PCBWay didn't like the gerber files even though other fabs approved them, so I went back and made a minor dimension adjustment and ran PCBWay's specific design rule checks to make sure they will work. That new version is attached here as "V1.2"

SPECIALIZED PARTS:

• Input Transformer footprint is spec'ed for an original Melcor 400-20 or alternatively the L bracket they used when they switched over to using the Beyer "peanut" xformers. CAPI's new MEC-400-20 *should* fit the hole spacing (1" point to point), but their bracket will not without modification because it's 0.85" point to point.
  • One other note that I've found is that the mounting bolt spacing for the original Melcor input transformers is very inconsistent unit-to-unit, so you might have to file / drill one of the holes out a little...

• Output Transformer footprint was for the original Melcor 400-17, which happens to be the same footprint as Jensen 123, API 2503, etc etc and CAPI's new MEC-400-17 is also that footprint so hey that's perfect.

• Discrete Opampshould be a 1731 but you could obviously use others. As mentioned above, the original 1731 on the AML27 utilized a trim resistor. Various options:
  • CAPI's new MEC1731 (assembled+potted for $75 USD or DIY kit for $17.99)
  • gar1731 resources here on gdiy
  • FiveFish Audio's DOA-17 DIY kit is basically a 1731 ($22 USD)
  • Tom Hilbe what is advertised as an assembled 1731 ($59 USD)
  • ...or vintage, and there are a few other DIY versions from the past that are still kicking around

• PCB Guide Handle original part was # 10036 by Vero Electronics Ltd. UK.
  • Vero currently makes a new part (“PCB Handle Vero Type A”) with identical dimensions which comes in various colors and it should fit. Exact part numbers for various colors are on the data sheet but the black color like the original is “21-0246” and is available on their website from the UK ($6ish for a ten pack? not bad). There are some NOS yellow ones in the USA on eBay as of this writing but they ain't cheap: https://www.ebay.com/itm/195142507591

EDIT 8/11/24: attached original schematic of the later MAP ABL-27 line amp which has helpful, applicable information on calculating gain trim and hf rolloff (R2 and C3 on my AML-27 trace)

 

[zzzzz]

Here is another version of the AML-27 card that I made last year, as well. It is generally the same scheme but has some modern component footprints in addition to holes sized for Mill Max pins for a socketed DOA option, proper offboard gain pot, etc.



PCBWay link for ordering: https://www.pcbway.com/project/shareproject/AML27_Preamp_With_Mods_3c15cfa0.html

Gerbers and messy schematic attached. Gerbers pass design rule checks for both PCBWay and JLCPCB but are otherwise unverified.

Some kinda weird things happened with label placement on the silkscreen after PCBWay spit out the mockup: labels for pot "LUG 1", some components, etc. Not make-or-break crucial and maybe I will fix someday if I have time.