The final output stage utilizes lateral MOSFETs; four pairs are used for each channel in the 9505 and three pairs in the 9303. These devices, unlike conventional bipolar transistors do not exhibit “thermal runaway.” Thermal runaway is a phenomenon whereby a transistor heats up as it draws more current, which causes it to get hotter, and conduct more current, and so on until the device self destructs. Since the MOSFETs are inherently self protecting, no sonically degrading, complex circuitry is required to monitor and protect the devices. The lateral MOSFETs also have a linear input to output transfer function. Their connection in circuits and their operating characteristics are very similar to vacuum tubes, which is perhaps responsible for their widely recognized sonic trait of being “musical” and non-fatiguing.

Operation of the transconductance stage is a major factor in the reproduction quality of the amplifier. The number of MOSFETs used at the output stage of the 9303 and 9505 imposes sufficient capacitive load on the transconductance stage that if a conventional Class A stage were used (having intrinsically a 2:1 limit on peak-to-quiescent current) it would begin to show “stress” at the higher audio frequencies. The newly perfected DIABLO driver system (Dynamically Invariant A-B Linear Operation; patent application in progress) satisfies the current headroom requirement by smoothly and continuously varying the current transfer ratios of the two transconductance paths, under the control of the signal current itself. This implementation allows the current transfer ratio of one path to be smoothly and continuously reduced to zero while the other is smoothly and continuously increased by a factor of two. What is remarkably new here is that when this normally-limiting 2:1 value is reached there is now about 14dB of additional, perfectly linear current headroom left to drive the MOSFETs! The result is a dramatic decrease in high-frequency distortion combined with higher ultrasonic stability – the “Holy Grail” of amplifier design.

The power supply utilizes a UI style transformer with a separate primary for each channel. The transformer has a separate secondary for each channel high voltage power supply, each feeding a conventional split full wave bridge rectifier. High voltage power supply capacitance is 20,000∝ F per rail for each channel for the 9505 and 5,000 for the 9303. The third transformer secondary feeds a regulated supply for the input stage and driver circuitry. Low voltage power supply capacitance is 1,000∝ F per rail, with additional decoupling for each channel.

CALIBRATION

Common Mode Rejection:

The input common mode null is adjusted by the trim pot R1 (R101 for the left channel). The CMRR should be greater than 75dB below rated output. If the CMRR requires adjustment, feed the amplifier input with a common mode signal and adjust R1. Disconnect the power to the amplifier before removing the cover. Use a sinewave generator set to 1 volt output at 1kHz. Connect the generator signal output to the tip and ring of a 1/4" plug and ground to the sleeve. Plug this into the amplifier input. Connect an AC voltmeter to the amplifier output binding posts. Adjust R1 to give the lowest voltage output from the amplifier. For a temporary adjustment when a signal generator and voltmeter are not available, use an FM tuner and tune it to an unused station as your signal source, and connect the output to the amplifier as described above. Connect the amplifier output to a small full range speaker and adjust R1 for the lowest output from the speaker.

Bias:

The bias control establishes the quiescent Class AB output current of the amplifier. The bias should not need readjustment from the factory setting; however, if the amplifier is repaired and output devices have been changed, or if the two channels of the amplifier do not run at the same temperature, calibrating the bias is necessary. Disconnect the power to the amplifier before removing the cover. To adjust the bias, disconnect the input and speakers and remove the B+ fuse for that channel. Connect an amp meter across the now vacant fuse clips and adjust R45 (R145 for the left channel) to get a current reading of 300mA for the 9303, 400mA for the 9505.

– 10 –

Page 13 Page 15
Page 14
Image 14
Hafler 9303, 9505 manual Common Mode Rejection
Page 13 Page 15

9505, 9303 specifications

The Hafler 9303 and 9505 are both high-performance power amplifiers that have garnered a solid reputation among audiophiles and professionals in the audio industry. These amplifiers are designed to deliver high-quality sound reproduction with performance and reliability, making them suitable for various audio applications, from home theater systems to professional studio work.

The Hafler 9303 is a versatile amplifier known for its warm sound and consistent performance. With a power output of 100 watts per channel at 8 ohms and 150 watts at 4 ohms, it provides ample headroom for dynamic peaks in music without distortion. The 9303 features a classic, rugged design and is equipped with an exceptional thermal management system to prevent overheating during extended use. Its circuitry incorporates advanced feedback technology that minimizes distortion and enhances audio clarity. The amplifier also supports both balanced and unbalanced inputs, allowing for flexible integration into various audio systems.

On the other hand, the Hafler 9505 takes the performance up a notch with a powerful 200 watts per channel at 8 ohms, and it can reach 300 watts at 4 ohms. This model is ideal for driving larger speakers and can handle demanding audio setups. The 9505 incorporates Hafler's proprietary active feedback design, which maximizes damping factor and ensures accurate transient response. Its robust construction and high-quality components provide longevity and reliability, making it a favorite among professionals who require consistent performance in demanding environments.

Both amplifiers feature dual-mono construction, ensuring independent power supply for each channel, which helps in maintaining channel separation and reducing crosstalk. This design contributes to a wider soundstage and clearer audio reproduction, crucial for critical listening and professional applications. Furthermore, the amplifiers are equipped with comprehensive protection circuitry, safeguarding the components against speaker short circuits and thermal overloads.

In summary, the Hafler 9303 and 9505 power amplifiers represent an excellent choice for anyone seeking reliable and high-fidelity audio performance. With their impressive power outputs, advanced technologies, and robust designs, these amplifiers are versatile enough to cater to both home audio enthusiasts and professional sound engineers, ensuring that they deliver exceptional sonic experiences across various settings.
Top Page Image Contents