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Audio Research M300 MkII monoblocks, to de-mod or not?

Posted by triode3 (M) on May 21, 2025 at 03:00:51

Hello, I know this is the amp/preamp asylum, but this question is about the front end, and has nothing to do with the sixteen 6550 output tubes in the amps :)

I have come into possession of a pair of Audio Research M300 MkII amps, which were owned by a well known audio equipment manufacturer. They do not recall who did this (e.g. ARC or one of their own techs), but the amps were modified. I am really on the fence about performing a (relatively simple) de-modification. I would like to hear opinions on this matter.

Another small bit of background info, after receiving these amps I had a few realizations: Two amps are much bigger than one. My listening room is not as large as I thought. While I love tube amps in winter, I like to run SS in summer and the M300s are slightly heavy (again, space becomes a small issue). This is all to say, not 100% sure I will keep them forever.

Ok, finally, the mod: If you look at the schematic, the input goes to a gain control (potentiometer on "Mk I", stepped resistor network on Mk II) and then immediately is split through a phase inverter and the rest of the amplifier is balanced. Someone removed the unbalanced RCA input, removed the gain control and then installed (yes, drilled two holes in the chassis) a XRL Balanced connection in the gain hole. They then bypassed the phase inverter by simply jumping over it on the circuit board and soldering the inverting input from the XLR to the output of the phase inverter.

Physically, this conversion was not done poorly, other than two new holes in the rear around the gain hole. I could convert this back without issue. Since none of my gear is balanced, a friend donated me an older preamp with balanced output so I can "play" with these monsters right away. While I have nothing against balanced signals, the loaner preamp is not my speed (it is totally focused on A/V 7.1 dts, dolby, insert awesome tech acronym here, etc, and I am a 2 channel person).

Having said all of that, I now find myself in the position of:

- Purchase a preamp with balanced outputs that is more my speed

- De-mod the M300 MkII (install volume pot, install RCA, move solder connection on PCB).

I suspect the de-mod might also increase any resale value, although it seems resale value on these is reaching rock bottom as of late.

Opinions wanted.

Thanks for reading.

Edits: 05/21/25

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