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Pass Aleph 2 Power Amplifier
INTRODUCTION
The Pass Aleph 2 is a single-ended 100W design, each monobloc is biased at 300 watts. The patented Aleph circuit utilizes an active load as part of the output stage to source current into difficult speaker loads. As such, it can source 200 watts into a 4 ohm load, no other single-ended amplifier can do this.
DESCRIPTION
I am using 2 pieces of 0.2˚C/W heatsink per channel. These keep the amplifier temperature at a comfortable 55~60˚C while dissipating 300W (per channel). The variation in temperature is due to the different temperature and air flow changes. I used high performance thermal pads for all the Mosfets to keep their junction temperature well below 150˚C. So far I did not have the need to reduce the bias as the thermal stability is very good. As a further safety precaution, thermal protection comes in the form of manual resetting bimetallic switch which operates at around 70˚C.
The power supply consists of 4*15000uf@50v Nichicon capacitors from Japan bypassed with 4.7uF EVOX Rifa film caps and 500VA (+/-35vac) encapsulated toroidal transformer per channel. Each monobloc uses a high current 35A bridge rectifier. I'd like to add that I prefer the sound of large PSU caps compared to sheer capacitance values packed into small caps. They just seem to sound bigger (no pun intended) than their smaller counterparts. Actually the real reason is their lower ESR at low frequencies which provide better current delivery, but the tradeoff is a higher ESL which is reduced by bypassing with a high frequency cap. Most heavy duty amps sound muscular partly because of their massive power supply, which is hard to duplicate with a puny supply.
The input stage is built on a standard 4" by 6" pcb, linking to the output stage pcb mounted directly onto the heatsinks. Both the input MOSFETS and the output stage MOSFETS are sourced from International Rectifier. The input MOSFETS are matched to 0.005v Vgs and the output MOSFETS to 0.05v Vgs. On the input card you can see Elna Silmic electrolytics for bypass caps, not Black Gates, but reasonably priced. All signal resistors are good quality 1% precision metal film types from Multicomp, while high power resistors are low-inductance ceramic types.
Each monobloc measures about 19"W * 7"H * 16"D and weighs over 30kg. Power inlet is via a 3-pole PowerCon connector from Neutrik. I used a high current industrial electromechanical relay switch for the amplifiers. All DIN rail WAGO terminals used, including an industrial 10*38mm cartridge fuseholder, as seen beside the transformer. Fuse used is a 2A HRC aM (inductive load) cartridge type.
The balanced input signal is via a high quality Neutrik XLR connector, while the unbalanced input is via a high quality RCA socket. Speaker output is via a high current 5 way binding post. All internal wiring is teflon-sleeved silver-plated thick gauge wire. There is special attention paid to star grounding. Is there anything in this ampthat doesn't spell "High Current" ? :-)
The chassis is designed out of aluminium for anodizing. They feature modular construction. Each piece of the chassis can be taken out easily by removing just 4 screws, for easy access to modification, without need for cutting any wires of the circuit. If you look closely you will notice a connector in every cable path, to facilitate easy removal for the reasons stated above. The chassis is designed with a 2 tone color scheme and a blue LED indicator, which is quite pleasing to my eyes.
All said, I'm didn't make my choices to save money per se, but the original Aleph 2 did not use any exotic components either & still sounds fantastic. I believe quality doesn't necessarily have to come at a high price, and I wanted to get as close to the original Aleph sound as possible. After all, a good circuit is the most important key to good sound, but you may disagree...
The character of this amplifier is biased towards the warm side, more like a high powered tube amp than a transistor amp. The sound I must say is pretty transparent, with sweet treble and a warm midrange. It can be quite addictive if u like single-ended sound. Because of adequate power output and beefy output stage, it also possesses a well extended bottom end, which cannot be said of the lower powered Alephs...
PICTURES
Output stage mounted to heatsink
