After reading Stereophile's reposting of their 1993 review of the Acurus DIA-100 (Direct Input Amp) Integrated Amplifier (rated at 100 wpc in 8 ohms, 150 wpc in 4 ohms), I saw a posting on Audiogon and bought it on a whim. The amp was purchased and shipped to Hawaii for $300, relatively cheap since shipping was 1/3 the cost.
As a note, I have gone thru a number of amp/receivers thru the years and still have a lot of them around. Still running around the house are a 1976 Yamaha CR-1040 80wpc receiver, 1976 Sony TA-8650 V-FET 80wpc integrated amp, Sony STR-V7 150wpc receiver, 1995 Sony TA-F808ES MOS-FET 110wpc integrated amp, a 2002 Denon AVR-3802 110wpc receiver, Marantz PM7200 105wpc amp and an Onkyo TX-NR905 130wpc AV receiver.
I have been driving a pair of Polk LSi15s and now a Sonicapped RTA-12C (as well as a set of Sony SS-M9 speakers designed by ex-Polk engineer Dan Anagnos). The Polks present a difficult load to some of the amps, especially with the Denon 3802. While rated at 110wpc, the amp easily and audibly clips at moderate volumes, even when only two channels are driven. Most of the older stuff have no problems driving the Polks, but audibly they left something to be desired. Most of the time, I would use the V-FET amp which is noted for their smoothness but they are not known to handle low impedances very well. My second choice was the 808ES, a MOS-FET amp which can drive low impedance speakers but the sound felt flat.
Two days after shipping from Virginia, the DIA-100 was surprisingly dropped off by the U.S. Postal Service (UPS/Fedex was to dang expensive... $150 to ship). The amplifier was designed by Mondial and sold by the Acurus nameplate, a lower model line. It has some unique design features.
One thing is that it uses a passive pre-amplifier stage. This means the signal from your device is used to drive the amp, there is absolutely no signal processing involved. The only options you have is a volume (actually an attenuator), balance and a input selector. Since there is no active electronics, there is no phono stage built-in. The advantage is the signal from your source device is as clean as it can be. The disadvantage is that with normal passive designs, many sources cannot deliver a strong enough signal to really drive the amp to full power. To compensate, Acurus made their amp about 20dB more sensitive than normal.
Looking on the front panel, you have four knobs, the attenuator (volume), balance, source selector and tape monitor selector) and a power switch. Unfortunately, a poorly implemented design in the power switch left most of the amps with a flickering power LED which cannot be remedied.
The back panel includes a pair of binding posts on each side for the speakers. The posts can accept bare wire, spades or banana plugs. The inputs are lined up along the top center. The power cord is user replaceable standard IEC three prong.
Internally, the amp has a large (750kv?) toroidal transformer in the front center with two large caps in the rear. The amplifier circuitry are along the left and right sides. The preamp circuits are on the back panel. The front knobs use long rods to connect to the rear pre-amp to keep circuit paths short. It appears that the amplifier uses all discrete circuits, none of the IC stuff so common now. The components are well built and looks nearly military spec type of stuff.
Hooked up the amplifier to the Polk RTA-12Cs using Kimber Cables. The source device used was a Denon DVD3930CI player which uses Burr Brown 1794 DACs. Since the amp did not have any digital inputs, the player was connected via analog cables.
The first thing i noticed was that the amp tends to be a wee bit bright. It was not harsh but the treble seems a bit elevated over all the other amps i had. The top end of the amp was very smooth. The second thing was the amp had a lot of bottom end strength. The bass was solid and went down low. A good thing since there are no bass/treble controls on the amp. I was really impressed with the smoothness of the sound. This was easily one of the smoothest amps i heard. It made the RTA-12Cs sound a tad bright but matched really well the my LSI15s, which I felt was a bit weak in the high end extension. Depth was good but not great, width was very good. Imaging was very focused and stable.
The biggest impression this amp made on me was its sheer strength. The amplifer could drive the LSi15 and RTA-12Cs to extremely loud levels, a LOT louder than what the Denon 3802 and the Sony 8650 could do. it was not rock level volumes but it was more than enough than any "normal" person would want (party animals look elsewhere).
I wish the amp had remote control capability (the replacement model DIA-150 does) but other than the wee bit of brightness added, there was nothing else to fault the amp on. It impressed me so much, i am looking for either the 150wpc DIA-150 or the seperates (L10 pre-amp/A250 250wpc amp).
Oh yeah, one last thing, if you are looking for Acurus units, be forewarned that the company is no longer in existance. Klipsch bought them out several years ago and discontnued the line in 2007. Still, it is a worthwhile investment and good prices can be found.