Saturday, February 02, 2008

The Yamaha P2100 Amplifier

The P2100 was introduced in 1977, about 10 years before the P2075, and it's interesting to look at the differences that there are in the circuits of these two amplifiers.

The 2075 is about a 50 W/Channel stereo amplifier. It includes fully balanced inputs thanks to a pair of JRC op-amps, as well as monolithic chip driver and amplifier stages. While you may sniff at the chip amp design, they did a lot of things better in the 2075 than in the 2100. The inputs are truly balanced, many of the audio signal resistors are metal film, copious amounts of bypass capacitors are used throughout the design, and the output stage is heavily protected. Also, the 2075 relies on a lot of ceramic capacitors used as bypass caps.

On the other hand the P2100, while it has a completely discrete audio path, has a single ended input, and almost no bypass capacitors to speak of. The power supply rails go through three different boards with no local capacitor storage at all. While the resistors are mostly cheap carbon composite the capacitors are all polystyrene or tantalum. There's also almost no output protection to speak of. A couple of thermal cut off switches attached to the heat sinks are all you get.

Both amps use several cemented resistors especially near the output stages.

So, the question is what to do, what to do. First, the 15,000uF/86V screw terminal storage caps in the power supply are completely unavailable today. I'm going to have to find suitable substitutes. This probably means designing a new power supply board. If I'm going to do that, I might as well throw in some discrete, zero-recovery diodes, and large value bypass caps.

Next, the cement and carbon comp. resistors will have to go. So will every single electrolytic in the unit.

Last, if I get really ambitious, I'll design a new input driver board to turn the input into a truly balanced design. I'll probably use the design from the 2075 to stay true to the Yamaha spirit.

More when I start ordering parts. :)

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