Ti Kan's β22 Amplifier and σ22 Power Supply

April 3, 2007

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Also see the official β22 amplifier, σ22 power supply and ε22 backplane board websites.

Brawn and finesse taken to a new height

The β22 is a high-end stereo amplifier for headphones and speakers. I designed this to be a top-tier amplifier, as a community service project for the DIY audio hobbyist. It is capable of driving any dynamic headphone and extract the optimum performance from the best of them. And it's powerful enough to serve as a superb low-power speaker amplifier in a moderate sized room.

Featuring an all-discrete, fully complementary topology with dynamic cascode and pure class A operation in every stage, the β22 employs low noise JFETs in the input stage, high-gain wideband BJTs in the VAS stage, and high-current power MOSFETs in the output stage. Every detail is carefully considered for the utmost in linearity even before application of global negative feedback. And only a moderate amount of feedback is used to reduce output impedance, extend the bandwidth and maintain overall stability.

The β22 is designed to be flexible, and can be built in 2-channel passive ground, 3-channel active ground and 4-channel fully balanced configurations. An optional ε22 backplane board may be used for a 3-channel active ground configuration, and makes for minimum internal wiring, improved shielding, optimum grounding, and improved serviceability.

The companion power supply, σ22, is a dual-rail tracking regulated wideband design, also with an all-discrete complementary topology. It has very low noise, low output impedance and excellent line and load regulation characteristics. It also features MOSFETs for very high current output capability and reliable operation.

I am now offering professionally-manufactured β22 and σ22 printed circuit boards and some related parts for sale at very reasonable prices. A limited quantity of ε22 boards is also available.

For full details please visit the official β22 amplifier, σ22 power supply and ε22 backplane board websites.

This unit was my own build. I use it daily and it serves as the demonstrator/showpiece in meets and gatherings.

Configuration

These units are configured as follows:

RMAA Test Results

RightMark Audio Analyzer software, running on a Toshiba 2.8GHz Celeron laptop computer via an M-Audio Firewire Audiophile mobile interface running in 32-bit, 96KHz mode.

This test provides data and graphs of frequency response, noise, dynamic range, total harmonic distortion, intermodulation distortion and stereo crosstalk performance.
Even though the β22's measured results are excellent, it can be seen from the loopback graphs that the β22 added almost negligible distortion to the M-Audio Firewire Audiophile's baseline. Hence, the distortion measured is predominently the sound interface's performance, not that of the β22 amplifier.

The slight rolloff at the low end in the frequency response graph is due to the M-Audio's coupling capacitor. The β22's actual frequency response is flat down to 0Hz. Likewise, the high-end rolloff in the graph is due to the M-Audio's anti-aliasing filters. β22's actual response extends to 2.5MHz (-3dB) in the tested configuration.

The β22's RMAA results remain essentially unchanged whether its outputs are loaded with 330Ω, 33Ω or 8Ω. Many amplifiers' distortion performance deteriorate significantly with lower impedance loads, and the stereo crosstalk behavior degrades. There is no such problem here.

These results amply illustrate the benefits of β22's high-current class A MOSFET output stage, three-channel active ground topology and the use of dynamic cascoding in every stage. See the Technical highlights section for details.

Other Test Results

These were measured with a Wavetek 4MHz sweep function generator, a Tektronix TDS2014B 100MHz digital storage oscilloscope, a Protek 6510 100MHz analog oscilloscope and a Fluke 95 50MHz digital ScopeMeter.

Oscilloscope waveforms

The oscillogram waveforms of the β22 amplifier are shown below. All input waveforms are produced by a Wavetek 188 4MHz sweep function generator.

The following is the 100KHz square wave response as tested with a Tektronix TDS2014B 100MHz digital storage oscilloscope. The top trace is the input and the bottom trace is the output. The rise and fall times as well as the peak-to-peak output amplitude is shown on the right hand side of the display.



Below are additional waveforms as measured with a Protek 6510 100MHz analog oscilloscope. In all graphs except the Lissajous waveform, the top trace is the input and the bottom is the output.

The square wave graphs show that there is minimum slewing and ringing at the leading and falling edges. The 100KHz sine, triangle, and Lissajous graphs also show very small amount of phase shift between the input and output. Within the 20Hz to 20KHz audio band, there is no measurable phase shift.


1KHz square wave

10KHz square wave

100KHz square wave

100KHz sine wave

100KHz triangle wave

100KHz Lissajous




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