OCM 800 Amplifier

Reprinted with permission from The Inner Ear Report.

With the model 800, OCM has introduced a unit which fits between their top of the line 1600 mono blocks and the model 500, both reviewed in TIER over a period of four years. For those of you not familiar with the company, here is a little background information. OCM was formed in 1990 following the acquisition of the old Belles company by the Canadian tuner manufacturer Magnum Dynalab. Headed by David Belles, the plant was located in Rochester NY and all technical planning and assembly was handled there. In 1996, the facilities were moved to Brampton, ON, Canada, a bedroom community of Toronto and integrated into the manufacturing plant of Magnum Dynalab. David Belles designed all previous models, but the 800 under review is an in-house design by Magnumıs engineers, which makes the unit an all-Canadian design, incorporating the acquired expertise of American engineering.

Appearance:

As with most components nowadays, the unit sports a black chassis and 1/2 inch thick brushed aluminum faceplate. Massive heat sinks, attached to the faceplate and the sides of the unit hint at its power. Gold plated RCAs and XLRs on the unitıs rear allow connections with preamplifiers of your choice. Five-way, gold-plated speaker terminals, a fuse holder and a detachable AC cord are located on the amp's rear panel. The unit rests on four rubber feet. It weighs in at 84 pounds (37Kg) and measures 19 inches wide, 17 inches deep and 7 inches high -- a large component by anyone's standards.

Technology:

OCM chose the classic design first implemented by David Belles in the OCM500 and later, the 1600 mono block amplifiers. Like these models, the 800 is a Class AB amplifier which operates in class A mode for low-level signals and switches to Class B operation with the surge of the signal (Class A designs are on all the time with the output devises active continually; see page 61-63 of this issue for every thing you ever wanted to know about amplifier bias). When the signalıs strength is increased and the amplifier switches to class B operation, the transition can cause a crossover distortion notch usually addressed with negative feedback (feeding back a portion of the output signal to a summing point at the input of the amplifier, which causes numerous other small problems which have to be addressed). OCM chose a design which employs MOSFET output devices instead of bipolar transistors to overcome some of the inherent problems mentioned above. MOSFETs have built-in current limiters which allow maximum current output throughput its rated parameters. DC blocking capacitors aren't used here, but a DC servo loop senses DC voltages on the output of the amplifier and feeds them back to the amplifier's input. The amplifierıs signal is channeled through a minimal number of components to eliminate colouration. The only user-accessible protection is a power line fuse.

The 800 amplifier delivers 275 watts per channel into an 8 ohm load and 550 watts into a 4 ohm load with a slew rate of 200 volts/u-second. In monaural mode the amplifier provides more than 800 watts with a slew rate of 400 volts/u-second. The amplifier is equipped with a balanced input and becomes a differentially balanced monaural amplifier when a true balanced signal is fed to the inputs (only in the mono mode as described above). The unitıs frequency response is from 0.2Hz to 100kHz -- unusually wide. The rated distortion figures are less than 0.25% (IM and THD). Crosstalk is quoted at better than 80dB, while the unit's peak current capability is a whopping 100 amps.

The Sound:

For our listening tests, we connected the OCM 800 to the Gershman Acoustics Avant Garde loudspeakers reviewed in this issue; the Wyetech Opal preamplifier and the Alchemist CD transport/DAC. Flatline speaker cables and interconnects made up the balance of the system. This amplifier has the lush, full-bodied sound of a good vacuum tube unit, but offers a lot more muscle. The Avant Gardes can handle oodles of power and the OCM really brought the speakers to life. The top frequencies are refined, lucid and sparkle when this is called for in musical program material. The midrange area is proportional with the high frequencies and really communicates "inner detail". The bass is potent and carries a lot of weight without dominating the all-round musical presentation. Compared with the OCM 500 -- one of the best amplifiers to execute control and resolution in the bass region -- the 800 falls short of the conclusive determination at the lowest end of the audible spectrum. However, the smoothness and musicality of the 800 more than makes up for that tiny nit-pick. Imaging can be considered in line with some of the best we have heard in the business, including OCMıs 1600 mono block amplifiers. The 800's front-to-back information is awesome, the horizontal range extends way beyond the edges of the loudspeakers and realistically depicts the sound stage's boundaries. Vertical imagery conjures up a stage, four feet off the floor and rising to heights well above the ceiling. We didnıt have a chance to test the 800 in the balanced monaural mode as we didnıt have another amplifier on hand. However, we did test the units reliability with loads down to about 1 ohm, and it didnıt cave in. People with unstable speaker loads rejoice, this is the amplifier for you.

Synopsis & Commentary:

This is one of the rare occasions when we feel a comparison is in order. We have used an OCM 500 in many of our reviews; it is one of our favourite auditioning amps. We think the OCM 800 amplifier accomplishes its task with a higher level of musicality than the OCM 500. In addition to that "certain touch" of sonic sophistication, it offers power to drive any loudspeaker under the sun. Its dynamics, while not inflated by any standards, realistically carry out sound pressure levels matching live rock and roll concerts. Classical music and jazz, small and large ensembles, are reproduced with the utmost ease; the amplifier seems to idle along without a hint of strain. Nevertheless, when called for, the 800 rises to the occasion and shows its dynamic range capabilities unwaveringly and with great authority. One of our panelists compared the 800 with the 500 in his system, also with Avant Garde speakers. He bought the 800 as soon as he heard it in his system (which he had been using for about a year). So, what we have here is an amplifier with greater musicality, a lot of power and sound which can fill an auditorium. Our panelists have stated that the OCM matches the Krell in stability, not in appearance, and may even sound a bit more musical, if not as technically correct. At the end of all this, it is up to the listener to make the choice, but listen you must -- this is a great product!