LM4780 MONOBLOCK AMPLIFIERS

I constructed four LM4780 monoblock amplifiers to replace my LM3875 based amplifier in my main system. Each monoblock contains a single LM4780 chip in parallel configuration. The circuit boards and parts were obtained as a kit from Audiosector. I chose the premium parts option which included Blackgate capacitors and MK132 precision resistors. The standard power supply configuration from Audiosector is a low capacitance design, that is, it has only 1000 microfarads (uF) of capacitance on each rail of the power supply. This is a relatively popular concept among builders of chip amps. I have since done some preliminary experiments with larger power supply capacitors (10 000 uF) and cannot hear a difference. The fact that you can use such low capacitance with these chips is a tribute to their ripple and noise rejection (Power Supply Rejection Ratio, PSRR).

picture of amplifierThe chassis was designed using Sketchup and machined on Monarch lathes and Bridgeport milling machines. The power supply cases were machined from four 6" X 6", 6061 aluminum blocks. You can see some of the pictures of the construction of power supply cases in the gallery below. For the amplifier cases, a completely different approach was taken, an internal skeleton was used that supported the outer covering of 2" X 6" extruded aluminum. The chips are mounted on a quarter inch thick copper heat spreader that is affixed to the heat sink. Because the outer cover sits against the heat spreader it is also capable of dissipating significant thermal energy. The speaker terminals are Cardas brand, the RCA connectors are from eBay, and the power supply connectors are Neutrik.

These amplifiers have been in use since circa 2007. In the future i may increase the power supply capacitance. This will require milling a space for the capacitors in the power supply chassis walls. I may also experiment with different chips or a bridged configuration. When building these amplifiers i was not aware of the importance of matching the gain of the two amplification units contained in the LM4780 die. Balanced gain ensures that the load is equally distributed across each amplification unit. When i do open these up, to balance the gain, I will check to make sure that the resistors in the feedback loops are closely matched.

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