The amplifier is powered by a unipolar power source that can be a computer’s PSU (via an adapter for a Molex connector) or the included 12V/3A power adapter.
But how can this tiny box be a power amplifier? It has little resemblance to grownup models in size, weight, connectors and electrical parameters. Every owner of hi-fi equipment knows that an amplifier is a large and very heavy unit!
The fact is the Scythe SDA-1000 is not a simple analogue, but a digital amplifier (Class D) which heart is a Yamaha YDA138 chip.
Digital amplifiers differ from ordinary ones with high efficiency in the first place. Widespread amplifiers from the combined amplification class AB have an efficiency of 50% and higher only when the output power is close to maximum. It means that under ordinary conditions over half of the power received from the mains is dissipated by such amplifiers uselessly as heat. Class B amplifiers can achieve an efficiency of 75% but produce significant distortions at low output power. Class D amplifiers amplify not the sound proper but a modulated pure tone whose frequency is many times higher than the top of the audio range. This helps reduce the half-open period of the transistors and minimize useless heat dissipation without compromising the quality of the input signal. The YDA138’s efficiency is lower than 75% only at an output power of 1W or less. Such a high efficiency is achieved by using the output transistors in switching mode – they are either open (letting the current pass almost without loss) or shut.
You can learn more about the specifics of various amplification classes in Analog Devices’ material and by the following link: http://sound.westhost.com/articles/pwm.htm. Right now let’s make out what the main chip of the Scythe KamaBay Amp can do.
The Yamaha YDA138 is an integrated Class D amplifier with pulse-width modulation of the carrier frequency of 500MHz whose special feature is the ability to operate without low-pass output filters. The manufacturer calls this a “Pure Pulse Direct Speaker Drive Circuit”.
Each of the amplifier’s two channels is responsible for two bridged outputs providing high output power at low voltage. However, the bridge design increases the minimum permissible load impedance (it equals 8 Ohms for this chip). Oddly enough, this fact is not mentioned in the end product documentation. When I perused the chip documentation more, I found out that it could work at a load impedance of 4 Ohms but the outputs of the left and right channels must be connected in parallel for that, transforming the amplifier into a monophonic one.
The chip also incorporates an analog headphone amplifier (2x50mW, class AB). Its wattage rating is more than enough for most headphones with an impedance of 32 Ohms and higher. Unfortunately, the amplifier does not use a bridge circuit and does not incorporate internal voltage converters. Therefore it requires blocking electrolytic capacitors at the output.
I also want to note that the chip allows setting one of four gain settings for both the integrated amplifiers.
I never had to turn the volume control by more than half-way, so the manufacturer seems to have chosen the maximum gain setting. This is hardly a reasonable solution because the output signal of most sources is higher than 0.5V and as high as 2V with modern sound cards.
More apprehensions arise when you look inside the amplifier. On one hand, everything is assembled neatly, but the input cables are not screened. They are connected to the PCB via connectors while the other cables are soldered in. A number of electrolytic capacitors from obscure firms are employed – even where Yamaha recommends using multilayer ceramic capacitors and where film capacitors might even be used to achieve higher sound quality. The chip is located on the reverse side of the PCB and has no contact with the casing. In other words, it dissipates its heat through the PCB itself.
Summing up this section of the review I want to show you the specifications of the YDA138 chip that are absolutely the same as the specifications of the end product called Scythe KamaBay Amp SDA-1000.