Since last Fall, my system has been a PS Audio PWD DAC directly driving a pair of Emotiva XPA-1 amps (1,000 watts rms per channel into 4 ohms), then into Magneplanar 1.7’s, reportedly 86 db efficient.
My amps are fully internally balanced, as is my PWD, and they are connected balanced. The amps have 32 db of gain, 3db more than the more usual 29 db. The DAC ‘s volume control operates in the digital domain, with bit truncation starting at volume settings of 50 or lower. I’ve wished for a little less amp gain so I could run by DAC/preamp a little “hotter” than just borderline bit truncation.
I use a Macbook Air via USB to my DAC for hi-res files, an Airport Express through Toslink for streaming Spotify and for accessing my network stored collection of CDs, and my TV (with associated Apple TV and Blu-Ray) connected via Coax.
My listening room is approximately 20’ x 20’, open at one end with 18’ ceilings. At least 7-8,000 cubic feet, a big room to fill.
While I’ve been very happy with this setup, I wanted to re-explore tubes, having owned Audio Research and BAT components in the past.
I had concerns about adequate power in my setting, with inefficient speakers in a large room, so I did a little research.
With my listening position 11’ back, I enjoy primarily acoustic jazz at about 83 db listening levels, measured with a Radio Shack SPL meter. And I’d like to have the ability to play as much as 10 db over that, or at about 93db.
Using the online power requirements calculator at Crown, I learned that for 83 db SPL at 3.5 meters back in basically a non- reflective room, I needed 6 watts. Doubling that to provide 3 db of amplifier headroom requires 12 watts, and multiplying that by 10 for the additional power necessary to reach 93 db then requires 120 watts – right where the M125 sits, conservatively! BTW, my calculations show the M125 has about 29 db of gain, more usual than my Emotivas.
I talked this thinking through with Bob Latino, who recommended the M-125 over the ST120 in my situation. He also drives Maggies in his system, and found the M125 works well for him. I found Bob patient, knowledgeable and experienced.
So, I ordered the M125s, and tubes (KT-120s and 12bh7) from Jim McShane.
My experience with unpacking and building the amps was as others have described, with the only glitches having been self-imposed!
I spent the following time building, totaling about 10.5 hrs per amp over a period of three days (some steps done simultaneously):
Prep and paint transformers .5 hr
Polish chassis with stainless polish and NEVR-DULL (highly recommended!) .5 hr
Assemble driver board 1.5 hr
Mount chassis components, including transformers 2 hrs
Wire chassis 4 hrs
Mount driver board and check wiring 2 hrs
Holding my breath, I fired an amp up carefully following Bob’s instructions, and was pleased that everything worked as expected! No hum, no bias problems, just that great glow from the tubes!
Pix and listening impressions to come - Stay tuned!
Last edited by audiobill on Fri Mar 28, 2014 7:32 am; edited 3 times in total (Reason for editing : Update 3/26)