Introduction: When digital audio brought new questions
The introduction of the Compact Disc in the early 1980s changed audio reproduction significantly. Digital technology offered clear advantages, including a low noise floor, stable playback and a high level of measurable accuracy.
During the years that followed, manufacturers continued improving digital equipment. Lower distortion, lower noise levels and wider frequency response became important parts of product development.
At the same time, engineers, reviewers and listeners continued to discuss how technical measurements related to the experience of listening to music. A CD player could perform very well in laboratory measurements, while listeners could still have different opinions about its presentation.
Within this context, Philips and Marantz Japan explored several design choices that focused not only on measured performance but also on listening evaluation. One of these developments became known as Non-NFB (Non-Negative Feedback).
What is Non-NFB? A simple explanation
Negative feedback is a common technique in audio electronics. A part of the output signal is returned to the input in opposite phase, reducing distortion and improving the measurable performance of the circuit. It has been used successfully in many audio products and remains an important technique today.
During the development of the later Philips LHH models, the engineers investigated the interaction between feedback circuits and the high-frequency noise generated by digital sections. Their approach was to reduce or remove negative feedback in certain analogue stages and instead focus on the linearity, speed and stability of the circuit itself.
This required attention to many other aspects of the design, including circuit topology, component selection, power supply design and mechanical construction.
The philosophy behind Non-NFB
Non-NFB was part of a broader approach to audio development.
During the late 1980s and early 1990s, measurements remained an important part of designing high-quality audio equipment. Philips and Marantz Japan combined these measurements with extensive listening sessions during the development process.
This approach was closely connected to the work of Ken Ishiwata, who played an important role in the sound tuning of many Marantz products. His work combined technical evaluation with careful listening, reflecting the idea that measurements and listening could complement each other.
A phrase often associated with the Japanese development teams was: "You cannot talk about music through specifications alone."
This philosophy influenced many areas of the LHH designs. Signal paths were kept short, analogue stages were simplified, components were selected carefully and mechanical construction received considerable attention.
The birth of Non-NFB: The Philips LHH A700 prototype
The origins of the Non-NFB approach within the Philips LHH project are closely connected to Japanese engineer Tetsu Suzuki.
During the development of CD players such as the Philips LHH800R, Suzuki built a prototype amplifier that was used as a reference tool during listening and development work.
The amplifier was designed without conventional global negative feedback and depended on the linear behaviour and speed of the circuit itself.
The prototype was demonstrated publicly in 1993 and attracted significant interest. This eventually led to the introduction of a commercial version.
In 1994, Philips introduced the LHH A700 Power Buffer Amplifier. The term "Power Buffer" described the intended role of the amplifier within the system: transferring the signal with minimal influence from the amplification stage.
The Philips LHH A700 Power Buffer Aplifier became one of the first commercial products to apply the principles that later appeared throughout the Non-NFB LHH range.
The technical challenges of Non-NFB
Reducing or removing negative feedback placed higher demands on the circuit itself.
In conventional designs, negative feedback can reduce certain errors caused by component variations or non-linear behaviour. In a Non-NFB circuit, more of the performance depends on the quality of the circuit design, the selection of components and the stability of the surrounding electronics.
This led Philips and Marantz Japan to pay close attention to discrete analogue circuits, component matching, signal paths and power supply design.
Similar design ideas can also be found in the later work of Japanese audio designer Hideki Kato. In his own designs, Kato often emphasized short signal paths, discrete analogue stages and reducing unnecessary circuit elements between the source and the listener.
Other areas such as PCB layout, grounding and mechanical construction also received considerable attention. The later LHH models used heavy chassis constructions, copper-plated parts and dedicated power supply designs to create stable operating conditions for the audio circuits.
As a result, developing Non-NFB products required more careful engineering and a greater focus on individual parts of the design.
The foundation: Philips LHH500R and LHH800R
The first clear applications of the Non-NFB approach in Philips CD players appeared in the Philips LHH500R and later the LHH800R.
The LHH500R introduced a low-feedback analogue stage. It marked an early step in reducing the use of conventional feedback in the analogue signal path.
The LHH800R developed this concept further. The current-to-voltage conversion stage after the DAC, the fully balanced analogue output stage and dedicated high-speed Non-NFB power supplies for the analogue sections were based on the same principles.
Documentation and reviews from the period describe the LHH800R as a player with a fast, open and natural presentation. The model showed how the Non-NFB approach could be implemented in a complete CD player.
The expansion of Non-NFB: Philips LHH200R and LHH300R
After the introduction of the LHH800R, the Non-NFB approach was applied to additional models within the LHH range, including the Philips LHH200R and LHH300R.
Although positioned differently within the product range, both players shared the same design principles. Their analogue stages used Non-NFB circuits, combined with discrete components, dedicated power supplies and attention to the quality of the signal path.
The LHH300R incorporated several construction elements that were also found in the higher models, including a more advanced mechanical construction and a more extensive power supply design. It occupied a position between the LHH200R and the larger reference models.
The LHH200R and LHH300R show how the Non-NFB approach was applied across multiple CD players within the Philips and Marantz Japan collaboration.
The Philips LHH900R
The Philips LHH900R continued the development of the Non-NFB concept within the LHH series.
While earlier models mainly applied Non-NFB to the analogue sections, the LHH900R extended this approach to the digital circuitry through a dedicated high-speed Non-NFB power supply.
Additional attention was given to the construction of the player. The LHH900R used thick gold-plated copper traces, advanced grounding methods and a heavy chassis designed to reduce the influence of vibration and high-frequency noise.
The player combined the developments introduced in earlier models and represented the most extensive implementation of the Non-NFB approach within the Philips LHH CD player range.
The LHH900R remains one of the clearest examples of the technical collaboration between Philips and Marantz Japan during this period.
Non-NFB in power amplification: Philips LHH A700 and LHH A200
The Non-NFB approach was also applied outside the CD player range.
The Philips LHH A700, based on T. Suzuki’s original prototype amplifier, introduced the same design principles in a power amplifier. The later LHH A200 followed the same direction in a more compact model.
Both amplifiers used a feedback-free MOS-FET output stage with a wide bandwidth and high slew rate. The design focused on reproducing the incoming signal with as little influence from the amplifier as possible.
The LHH A700 and LHH A200 show that Non-NFB was not limited to a single product, but became part of a wider design approach within the Philips and Marantz Japan collaboration.
Technical insight: How does a Non-NFB circuit work?
In conventional analogue circuits, negative feedback is used to improve linearity. A small portion of the output signal is fed back to the input in opposite phase. The difference between the original input signal and the corrected feedback signal is known as the error signal. The amplifier uses this error signal to reduce non-linearity, lower harmonic distortion and stabilize gain.
The effectiveness of negative feedback can be explained by the difference between open-loop and closed-loop behaviour.
An amplifier without feedback operates in open-loop mode. Its performance is determined entirely by the intrinsic characteristics of the circuit: transistor linearity, bias conditions, power supply stability, component matching and the physical layout of the circuit.
When negative feedback is applied, the amplifier operates in closed-loop mode. The feedback loop continuously compares the output with the input and corrects deviations. This can significantly reduce measured distortion and improve consistency between individual units.
For this reason, negative feedback became one of the most important techniques in modern audio design and is used in the majority of high-performance amplifiers, DACs and analogue stages.
The engineers behind the Philips and Marantz Japan Non-NFB designs focused on the limitations that could occur when applying feedback in very wide-bandwidth, high-speed audio circuits. Their documentation described concerns about the interaction between feedback loops and high-frequency noise from digital circuits, as well as the response of the feedback loop to rapidly changing musical signals.
Their approach was therefore not to correct errors after they occurred, but to reduce the creation of those errors in the first place.
This required a circuit with very high intrinsic performance. In practice, this led to:
- Fully discrete analogue stages instead of integrated operational amplifiers.
- Very wide open-loop bandwidth.
- High slew rate.
- Carefully selected and matched components.
- Short signal paths.
- Symmetrical circuit layouts.
- Highly stable and low-noise power supplies.
- Careful PCB layout and grounding.
In the Philips LHH A700 power amplifier, for example, the Non-NFB MOS-FET output stage achieved an open-loop bandwidth of approximately 500 kHz and a slew rate of 200 V/μs. These values were considered important because they allowed the amplifier to respond to rapid changes in the audio signal without relying on a correction loop.
The same design philosophy was later applied to the analogue stages of the LHH800R, LHH200R, LHH300R and LHH900R CD players. In the LHH900R, the approach was extended further through a dedicated high-speed Non-NFB power supply for the digital circuitry.
From an engineering perspective, Non-NFB does not mean that feedback is inherently inferior. It represents a different design strategy. A conventional feedback amplifier starts with a circuit that can be improved through correction, while a Non-NFB circuit requires the circuit itself to achieve the required performance before any correction is applied.
Both approaches can result in excellent audio performance. The Non-NFB philosophy of Philips and Marantz Japan is interesting because it shows how a small group of engineers explored a different solution during a period of rapid development in digital audio.
Measurements and listening
The development of Non-NFB took place during a period in which digital audio technology was developing rapidly.
Technical measurements remained an essential part of audio design. Negative feedback continued to be widely used because it allowed engineers to achieve low distortion, stable circuits and predictable performance. Many highly regarded audio products were, and still are, based on these principles.
Philips and Marantz Japan approached the subject from a broader perspective. Alongside technical measurements, listening sessions played an important role during the development of the LHH products.
Ryuichi Sawada, who was responsible for Sound Quality Control at Marantz Japan, was closely involved in these evaluations. Listening sessions were carried out under controlled conditions, often over longer periods of time. According to accounts from that period, some sessions were held during evenings and weekends, when disturbances from the electrical power grid were believed to be lower.
For the development team, listening was not a replacement for technical measurements, but an additional method of evaluating the final result.
The sound of Non-NFB
The sound of the Non-NFB LHH models has been described in many reviews and listening reports from the 1990s.
Common descriptions include a fast and open presentation, a spacious soundstage, natural voices and a relaxed character during longer listening sessions. Reviews of models such as the LHH800R, LHH300R and LHH900R often mention the combination of detail and ease in the presentation.
These descriptions reflect the experiences of reviewers and listeners from that period. They also correspond with the design goals described by Philips and Marantz Japan, where signal speed, simplified analogue circuits and the reduction of unwanted influences within the audio path received significant attention.
The legacy of the Non-NFB philosophy
The Non-NFB approach was developed during a specific period in the history of digital audio.
Many of the technical challenges from that period are addressed differently in modern digital equipment, and today's audio designers have access to technologies and measurement techniques that were not available during the development of the LHH series.
The later Philips LHH models remain interesting because they document a particular approach to digital audio design. The LHH500R, LHH800R, LHH200R, LHH300R, LHH900R and the LHH A-series show how Philips and Marantz Japan explored the relationship between circuit design, power supply construction, mechanical design and listening evaluation.
The story of Non-NFB is therefore not only about the absence of negative feedback. It also shows how different engineering choices were explored during the early years of high-end digital audio, and how these ideas became part of the history of the Philips LHH series.