Melcor AML-27 Mic Preamp (Gerbers)

[zzzzz]

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]

edit: this one has been verified / confirmed working by forum member @shaggy . thank you!

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.

 

[tk@halmi]

That is lovely!

 

[shaggy]

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.

View attachment 140314

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.

Appreciate all your work on these! I received 10 boards the other day from PCBWay (arrived within a week, even w/ the budget shipping option), and am currently stuffing a pair to experiment with my collection of mystery DOAs and non-stock txfrs. Will update my progress.
(If you do further revisions, accommodation on the board for phantom power feed would be handy.)
-thanks!

 

[shaggy]

I finished putting together a pair of zzzzz's Melcor boards tonight. MAP 5002 DOAs, Reichenbach inputs, Tamura (ex-TEAC mixer) OPTs.
Still have some testing to do, but very happy with them so far.

Update: Removed the Zobel network after fooling with it for awhile. HF response is much improved, with only slight overshoot from the input txfr.

I have some notes on these Reichenbach input txfrs (from their previous life in a 1974 Cetec console) that suggest the Faraday shield should be connected to the junction/center tap of the two primaries, not to signal ground. Any ideas on this?

Update 2: These DOAs are disappointingly noisy at higher gain. I'm wondering how much of that is to be expected, given the external gain control works by decreasing NFB. So far at anything over 40-45dB gain they're almost unusable.

Update 3: This afternoon I made up a powered I/O connector panel for the pair and hooked them up for a proper listen, and just about blew out my speakers. A tremendous amount of gain is needed to be able to hear the noise I mentioned earlier. At normal levels they sound good, and are surprisingly quiet.

Thanks again to zzzzz for these boards.

 

[jonn9000]

Hey ZZZZ, impressive work you shared here, massive thank you on behalf of the community!

I’m starting the process of racking my aml27 in a more serious manner, and I’m considering doing a “go between” pcb for PAD, Phase, 48v.

Any interest in collaborating on this one?

 

[zzzzz]

Hey ZZZZ, impressive work you shared here, massive thank you on behalf of the community!

I’m starting the process of racking my aml27 in a more serious manner, and I’m considering doing a “go between” pcb for PAD, Phase, 48v.

Any interest in collaborating on this one?

Amazing collection! I have about half a dozen AML27 cards, including a weird diy 2-slot lunchbox someone made.

I ended up making a 500 series layout and have transplanted the guts of mine into those for more versatility and the addition of phantom, phase, pad, etc all on the front panel. Don’t have any great photos but attached is one of some of the prototypes (bottom unit is not an AML27 card, but has a Beyer peanut IT not unlike the Melcor branded ones and a Profile 4804). Jorgen Schou input transformers on a pair sound amazing.

I do have a mini / compact “go between” style PCB project using some surface mount parts that I can send you that would probably be a good fit. Shoot me a message.

 

[shaggy]

Hey ZZZZ, impressive work you shared here, massive thank you on behalf of the community!

I’m starting the process of racking my aml27 in a more serious manner, and I’m considering doing a “go between” pcb for PAD, Phase, 48v.

Any interest in collaborating on this one?

Hi John. Thanks for the kinds words, though I feel I'm pretty much a newbie in this area, and still very much learning as I go .
Like you, I've thought about an aux. Pad/Pol/Phantom PCB, and as you might know, JLM does make them. As I'm not a PCB designer myself, that's the way I'd go. On the other hand, I see you have many more boards to rack than I do(!)
I'd suggest PM'ing zzzzz to see if he's interested. And btw, let me know if you have any spare boards for sale! cheers

 

[shaggy]

Amazing collection! I have about half a dozen AML27 cards, including a weird diy 2-slot lunchbox someone made.

I ended up making a 500 series layout and have transplanted the guts of mine into those for more versatility and the addition of phantom, phase, pad, etc all on the front panel. Don’t have any great photos but attached is one of some of the prototypes (bottom unit is not an AML27 card, but has a Beyer peanut IT not unlike the Melcor branded ones and a Profile 4804). Jorgen Schou input transformers on a pair sound amazing.

I do have a mini / compact “go between” style PCB project using some surface mount parts that I can send you that would probably be a good fit. Shoot me a message.

'Weird diy two-slot lunchbox'..
Lol, maybe I was reading your mind the other day when I started on this: