Melcor EQs and Preamps: AE-20, GME-20, AML-27

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mjrippe

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Hey folks, I have been working on a bunch of Melcor stuff recently. I have two AML-27 preamp cards, two GME-20 EQ modules, and a single AE-20 mic pre/eq. I also finally sourced enough working original Melcor 1731 opamps to fill them all! I just wanted to document what I am doing to help share the info for future DIYers and Melcor fans. First, I uploaded a bunch of docs to the Technical Documents area here: https://groupdiy.com/threads/melcor.78858/

For some reason there was never a Melcor thread started there, although some of the files probably came from this forum. Please add any Melcor docs you may have (y)

So the most common Melcor pieces in my experience are the AML-27 preamp, which is very similar to the API 312, and the GME-20 EQ which is it's own thing. The AE-20 is a fairly rare module which combines the two. Here is a family photo:
 

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mjrippe

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For no particular reason I am starting with the AE-20 pre/eq. It has the same PCB as the GME-20, but stuffs it with an additional 1731 and a mic input transformer! Here are some pics of mine. I recapped it and replaced the impossible to find connector with a 15 pin D-Sub. The serial number is 25, all Melcor serials start at 1000.
 

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mjrippe

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The preamp in/out and eq in/out are all available on the connector, so this can be used as a pre/eq, or separate preamp and eq. Outputs are unbalanced (so far) but can drive 600 ohm loads.

My goals for this unit are:

1: Provide transformer balanced outputs for both the preamp and eq.

2: Switching for individual pre and eq or pre->eq combo.

3: Change first gain switch to 30dB, add a switchable -20dB pad to give 10, 20, 30, 40, 50 dB gain settings.

4: Add output trim controls for fine level adjustment.

5: Phantom, Phase Polarity, and Power of course. All in a nice rack case.

The AML-27/GME-20 pairs will also be racked as pre/eq channels with similar features.

That's all for now 😁
 
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mjrippe

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Before we go any further, I would like to address some common questions about Melcor modules.

Q: Are they 500 series compatible?

A: Yes-ish. They were predecsssors to the API 500 format and share many common features. (Melcor employees left to form API) The modules run off +/-15VDC and can handle 500 series +/-16VDC. The physical dimensions of the cases fit 500 series, but the faceplates do not, without modification. Also, they are lacking the card edge connector of the 500 series.

There may be some Melcor modules in existence which have the same <1.5" faceplate as API modules, but the ones I have on hand are wider. See photos.
 

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mjrippe

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Now some folks have milled down the faceplates to fit 500 series - the height and mounting holes will match perfectly. A friend had a pair like this and I added [silent:arts] pcb edge connectors to make them work in a 500 series rack.
 

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mjrippe

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Q: What the heck is that connector on the back?

A Pain in the @$$. The original 14 pin connector was made by Precision Connectors Inc. The male one on Melcor modules is part number 5040-14P. I have never been able to find the mating connector. A fellow on ebay sells one, but it is larger and will not work.

For simplicity sake, I have replaced them with 9 pin DSUB for the GME-20 and 15 pin DSUB for the AE-20.
 

mjrippe

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Q: Where can I get Melcor opamps/what opamps can I use in these?

A: Original Melcor opamps are hard to come by these days. I was very fortunate to get the ones I have. However, I can say that I have used GAR1731 as replacements in modules that I have serviced and the reports back have been great. They sound as good or better (y) In addition, almost any opamp that fits the footprint/voltage range should work in these circuits - 2520, 990, etc.

Melcor did make some other models of opamp, but I do not know if they are compatible.
 

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pucho812

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My buddy's desk has 36 of the gme-20's eq's in it and it has 8 X of the cl20 compressor/limiter modules.
His desk is an old RCA solid state desk originally built in 1968. I uploaded the main docs to the desk called the UOAB(universal operational amplifier board), the desk at it's core is passive with make upstages using the UOAB. the UOAB itself is a PCB with melter or API opamp on it and discrete regulation for power. Based on how you wire the UOAB and what transformers you use, can have a line amp, or mic pre or whatever. the preamps on the desk are huge but are fixed gain and have a pad on the output doing 0, -10, -20, -30 dB of pad. crazy stuff
 

amplexus

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Nice to see! I have racked 8ch of the MAP AM27 (which is effectively the API312, more gain than the AML27 iirc) and they are killer amps. Amazing sounding iron in that era.

I have a combination of 2520s (Fabio, Whistle Rock, Red Dots), and JH990’s in mine and they all sound fabulous. Always great to see some MAP/Melcor love.
 

mjrippe

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My buddy's desk has 36 of the gme-20's eq's in it and it has 8 X of the cl20 compressor/limiter modules.
His desk is an old RCA solid state desk originally built in 1968. I uploaded the main docs to the desk called the UOAB(universal operational amplifier board), the desk at it's core is passive with make upstages using the UOAB. the UOAB itself is a PCB with melter or API opamp on it and discrete regulation for power. Based on how you wire the UOAB and what transformers you use, can have a line amp, or mic pre or whatever. the preamps on the desk are huge but are fixed gain and have a pad on the output doing 0, -10, -20, -30 dB of pad. crazy stuff
Hey Pucho, Yes! I have the UOAB docs you shared here. I was not going to get into the whole Melcor/RCA thing in this thread but I am aware of it. I had some RCA boards with RCA labelled opamps and the last four Melcor 1731 I bought were on UOAB boards! In fact, if your buddy wants the boards please let me know.

Also, if you have any docs on the CL-20 please add those to the Tech Docs section (y)
 

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mjrippe

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Nice to see! I have racked 8ch of the MAP AM27 (which is effectively the API312, more gain than the AML27 iirc) and they are killer amps. Amazing sounding iron in that era.

I have a combination of 2520s (Fabio, Whistle Rock, Red Dots), and JH990’s in mine and they all sound fabulous. Always great to see some MAP/Melcor love.
Awesome, thanks for sharing! Feel free to ad a pic or two to the thread (y)
 

mjrippe

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DOA DOAs! Some dead opamps I have saved. Three slightly different typefaces on the Melcors. Note the triangle MEC inspection stamp and engraved codes on the bottoms of some - not all opamps have these.
 

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zmix

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Before we go any further, I would like to address some common questions about Melcor modules.

Q: Are they 500 series compatible?

A: Yes-ish. They were predecsssors to the API 500 format and share many common features. (Melcor employees left to form API) The modules run off +/-15VDC and can handle 500 series +/-16VDC. The physical dimensions of the cases fit 500 series, but the faceplates do not, without modification. Also, they are lacking the card edge connector of the 500 series.

There may be some Melcor modules in existence which have the same <1.5" faceplate as API modules, but the ones I have on hand are wider. See photos.
The card edge pinouts are completely different from a 500 series module.. *do not* slot them in a 500 frame,,!

melcor_AML-27_info_sheet.jpgAM-27 Melcor.jpg
 

pucho812

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Hey Pucho, Yes! I have the UOAB docs you shared here. I was not going to get into the whole Melcor/RCA thing in this thread but I am aware of it. I had some RCA boards with RCA labelled opamps and the last four Melcor 1731 I bought were on UOAB boards! In fact, if your buddy wants the boards please let me know.

Also, if you have any docs on the CL-20 please add those to the Tech Docs section (y)
Sadly I do not have an cl-20 docs, they were not with the desk. When I first started to tech it, it was down the street from me in a private studio. It now lives in nashville. the desk when I first touched it had a hum that we tracked down to the +/-15V supply for the melcor eq's and comps. Installed a new supply consisting of 2 X PSU wired in series to make a +/-15VDC supply. good stuff. the desk sonic is huge even without having a dedicated stereo bus
 

mjrippe

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The card edge pinouts are completely different from a 500 series module.. *do not* slot them in a 500 frame,,!
Ah, yes! Thank you for clarifying that! I was actually talking about the GME-20 and AE-20 but failed to make that plain. Don't ever assume that similar looking items will be compatible (y)

And the MAP is not always equal to the Melcor. ;) There is a whole other story behind that relationship, again beyond the scope of this thread.
 

mjrippe

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Ok, been busy with other things but I wanted to share an interesting way to mount these modules in a 1RU case! I buy mine from Par-Metal (Par-Metal) and they are modular with aluminum front and back panels and steel top, bottom, and sides. Because the height of the Melcors is close to the 1.75" faceplate height, I decided to try something new.

First I took two 1.5" threaded standoffs and drilled a hole in the center. This was then threaded for a #4-40 bolt. (Photo 1)

Next I measured the faceplate and the width of the module and cut out a piece of the faceplate to fit. (Photo 2)

Then, drill holes to attach the module to the faceplate and mounting posts. (Photo 3)
 

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mjrippe

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So the top and bottom panels have a lip at the front to provide rigidity. Once again the modules are a bit too thick for that, so out comes the Dremel. (Photo A)

Next we carefully locate two holes that will allow bolts to hold the mounting posts in place. Yes, that is a rusty used screw in (Photo B)!

And finally, attach the sides so it looks like a completed rack. (Photo C - But of course we have a lot left to do!)

More soon!
 

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