AES paper describing the 1731 opamp
Melcor
- Thread starter mjrippe
- Start date
Help Support GroupDIY Audio Forum:
Melcor Module Pinouts
GME-20 Schematic
ATP-24 Manual (no schematic)
1731 Schematic #2
1731 Schematic #3
Traced my AML-27 cards and drew up a schematic at higher resolution.
I have also laid out a reproduction PCB for these and will follow up with that eventually. My cards have some modifications so I used photos from Reverb and eBay listings plus the Barnett / Jeff Goodman trace above for value reference.
edit 2024: I have another AML-27 card which is a different PCB layout than my other cards (those ones are the same as the Barnett / Jeff Goodman trace). This card is definitely factory stock with no mods, including 1731, 400-17 output xformer, and 400-20 black rectangle input xformer. It has the most "vibe" out of all of the AML-27 cards I own! Very warm and punchy, way moreso than an API 312.
The circuit trace is the same, but the physical layout is different and it incorporates what appears to be a factory original 10K RV6 Clarostat gain pot and series resistor in place of what is otherwise the "R2" shunt arm resistor on both my original attached trace and the Barnett/Goodman trace (10K on my other cards, 7K5 on the Barnett/Goodman schematic)
The pot is wired as follows:
R3, which is otherwise the gain resistor, is 150R to ground/0v (AML-27 card pin 14)
R4 trim resistor on this card is 3K9. My other cards have 2K2 and the Barnett/Goodman trace notes this as 3K or 4K7.
C1 in the zobel network after the input xformer in all of my cards regardless of board revision is 1200p, rather than the 470p in the Barnett/Goodman schematic.
I have also laid out a reproduction PCB for these and will follow up with that eventually. My cards have some modifications so I used photos from Reverb and eBay listings plus the Barnett / Jeff Goodman trace above for value reference.
edit 2024: I have another AML-27 card which is a different PCB layout than my other cards (those ones are the same as the Barnett / Jeff Goodman trace). This card is definitely factory stock with no mods, including 1731, 400-17 output xformer, and 400-20 black rectangle input xformer. It has the most "vibe" out of all of the AML-27 cards I own! Very warm and punchy, way moreso than an API 312.
The circuit trace is the same, but the physical layout is different and it incorporates what appears to be a factory original 10K RV6 Clarostat gain pot and series resistor in place of what is otherwise the "R2" shunt arm resistor on both my original attached trace and the Barnett/Goodman trace (10K on my other cards, 7K5 on the Barnett/Goodman schematic)
The pot is wired as follows:
- lug 3 is n/c,
- lug 2 is a wire coming from the DOA Input- trace (AML-27 card pin 10),
- lug 1 in series with a 1K2 resistor goes to the DOA Output pin trace (AML-27 card pin 12).
R3, which is otherwise the gain resistor, is 150R to ground/0v (AML-27 card pin 14)
R4 trim resistor on this card is 3K9. My other cards have 2K2 and the Barnett/Goodman trace notes this as 3K or 4K7.
C1 in the zobel network after the input xformer in all of my cards regardless of board revision is 1200p, rather than the 470p in the Barnett/Goodman schematic.
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mrbutterskin
Member
- Joined
- Jun 4, 2012
- Messages
- 20
I'm working on racking some Melcor A24s for a friend so I've traced out the circuit to understand these modules better. I'm uploading the circuit I drew here.
Note these are the A24, not the ATP24. They are not the same.
Note these are the A24, not the ATP24. They are not the same.
TheseGoToEleven
Well-known member
- Joined
- Sep 4, 2020
- Messages
- 49
Melcor 1731 op amp schematic and BOM, based on my teardown post. This also agrees with hand-drawn schematic #3 above.
gar381
Well-known member
Very nice update guys to the work that Peter C and I did many years ago !! congrates 
