Schmidlapper
Super Member
Update: Recap info with my BOM is added now, starting in posts #14. My unit is heavily modified, but the amp section follows original schematic so is applicable to a stock unit.
I'm presently in the research phase of recaping my 1984 A2801 power amp and I cannot believe how obscure this thing is on the web. I mean it is as if some powerful entity wanted to erase all traces of its existence. After looking and spending money on whole sets of Soundcraftsmen schematics I came up with zilch. So desperately one day I thought since the A2502 was similar in size and output it might teach me something by looking at its schematic, and low and behold there it was, a few reference notes on the A2801 variations. They didn't even list it under the models covered by the drawing. So I now have my correct schematic, but with it I gained an issue, the rail voltage shown is + and - 68 volts, seven amp models all sharing one circuit, all having various output wattages share the same rail voltage. I believe my A2801 to be 125 watts per channel into 8 ohms, but I have also seen 140 watts listed for it as well. Here in lays my problem with the schematic, you see my rail voltage is + and - 49 volts, not the 68 volts shown. Now I am left to wonder, is my amp wrong in some way, or do the schematics fail to show multiple rail voltages for the various power outputs? Then of course there are all sorts of contributing factors to make me question the amp, there is the sticker that says it is wired for 240 volts from the factory, but it came new with a 120 volt style power plug and also the two primary windings are wired in the expected parallel configuration, add to this many claim that this sticker was just a mistake Soundcraftsmen made for many models sent to be sold at the overseas PXs. Then there is the fact that in the mid 1990s I in a fit of modding madness converted this unit to have a seperate power supply chassis, could I have introduced a wiring error? Also the newly found schematics do show 68 volts from the power supply circuit which then magically become 70 volts at the amp section, so factory mistakes are a possibility. Then there is the whole lack of any technical discussion on this model of amp. This last part is why I am actually documenting my data research, it is my hope that I can put it on here to help a bit. This amp seems to work and sound fine, yet I now have this documented voltage disparity to account for, ignorance was so bliss. Does anyone with a A2801, know or care to measure what the rail voltage of yours is for the sake of audio archeology and to help me untwist my mystery? For that matter anyone with an A2502, MA5501, MA5502, or MA2502 who knows those rail voltages can chime in for even more data points. My greatest fear is this will just be sucked into that data black hole too.
UPDATE: +-70V is correct as stated on schematic. I was confused by my own measurement technic error, my unit is modified into two chassis, not normal.
Bill Smith
I'm presently in the research phase of recaping my 1984 A2801 power amp and I cannot believe how obscure this thing is on the web. I mean it is as if some powerful entity wanted to erase all traces of its existence. After looking and spending money on whole sets of Soundcraftsmen schematics I came up with zilch. So desperately one day I thought since the A2502 was similar in size and output it might teach me something by looking at its schematic, and low and behold there it was, a few reference notes on the A2801 variations. They didn't even list it under the models covered by the drawing. So I now have my correct schematic, but with it I gained an issue, the rail voltage shown is + and - 68 volts, seven amp models all sharing one circuit, all having various output wattages share the same rail voltage. I believe my A2801 to be 125 watts per channel into 8 ohms, but I have also seen 140 watts listed for it as well. Here in lays my problem with the schematic, you see my rail voltage is + and - 49 volts, not the 68 volts shown. Now I am left to wonder, is my amp wrong in some way, or do the schematics fail to show multiple rail voltages for the various power outputs? Then of course there are all sorts of contributing factors to make me question the amp, there is the sticker that says it is wired for 240 volts from the factory, but it came new with a 120 volt style power plug and also the two primary windings are wired in the expected parallel configuration, add to this many claim that this sticker was just a mistake Soundcraftsmen made for many models sent to be sold at the overseas PXs. Then there is the fact that in the mid 1990s I in a fit of modding madness converted this unit to have a seperate power supply chassis, could I have introduced a wiring error? Also the newly found schematics do show 68 volts from the power supply circuit which then magically become 70 volts at the amp section, so factory mistakes are a possibility. Then there is the whole lack of any technical discussion on this model of amp. This last part is why I am actually documenting my data research, it is my hope that I can put it on here to help a bit. This amp seems to work and sound fine, yet I now have this documented voltage disparity to account for, ignorance was so bliss. Does anyone with a A2801, know or care to measure what the rail voltage of yours is for the sake of audio archeology and to help me untwist my mystery? For that matter anyone with an A2502, MA5501, MA5502, or MA2502 who knows those rail voltages can chime in for even more data points. My greatest fear is this will just be sucked into that data black hole too.
UPDATE: +-70V is correct as stated on schematic. I was confused by my own measurement technic error, my unit is modified into two chassis, not normal.
Bill Smith
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