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Output power 85 Wrms @ 8 Ohms. 110 Wrms @ 4 Ohms.
Frequency response 8.3 Hz - 28.5 KHz (-1dB)
Input Sensitivity 650 mV
Midband gain 30.8 dB
THD Less than 0.01%
S/N Ratio 122 dB
The Denon PMA-850 integrated amplifier showcases a sophisticated circuit design aimed at delivering high-fidelity sound with minimal distortion. During the late 1970s and early 1980s, Denon focused heavily on technological advancements in audio engineering, and the PMA-850 was a prime example of these efforts. Here’s a breakdown of its circuit design highlights:
1. High-Current Output Stage
• The PMA-850 uses a high-current output stage that’s designed to handle demanding speaker loads, delivering powerful sound without distortion even at high volumes. It employs a push-pull Class AB amplifier circuit, which provides high efficiency while keeping distortion low, balancing the best qualities of Class A and Class B designs.
• This output stage design enables the PMA-850 to drive speakers with varying impedance levels, providing a clean, dynamic sound with good bass response and control.
2. Dual Power Supply
• The amplifier is equipped with a dual power supply system, separating the power needed for the preamplifier and the power amplifier sections. This configuration reduces interference between sections, keeping the signal as pure as possible. It also allows each section to draw power independently, avoiding sudden voltage drops and maintaining stable operation.
• A large, shielded transformer with separate windings for each power rail further enhances stability and minimizes noise.
3. Direct-Coupled Circuitry
• Denon employed a direct-coupled (DC) circuit in the PMA-850, eliminating the use of capacitors in the signal path from the input to the output. This design reduces phase shifts and distortion, especially at low frequencies, preserving the integrity of the audio signal.
• This direct-coupling approach also allows the amplifier to achieve a wider frequency response, which contributes to the unit’s well-regarded transparency and detail in playback.
4. Low-Noise Transistors and Components
• Denon used high-quality, low-noise transistors and resistors throughout the circuitry, aiming to reduce thermal noise and achieve a cleaner signal. The selection of components in the signal path is also optimized for minimal distortion and long-lasting performance, enhancing the overall clarity and warmth of the sound.
5. Precision Volume Control
• The volume potentiometer in the PMA-850 is designed to maintain channel balance even at low levels, which can be challenging in traditional potentiometers. This precision control allows for more accurate channel matching, resulting in a consistent stereo image and balanced sound across different volume levels.
6. Short Signal Path Layout
• The internal layout minimizes the length of signal paths by placing components strategically close to one another. This helps reduce potential interference and crosstalk, keeping the signal cleaner and preserving audio clarity. The logical, compact layout is also designed for easier maintenance.
Overall Circuit Design Philosophy:
The Denon PMA-850’s circuit design embodies a philosophy of purity and precision, with a focus on achieving a transparent, uncolored sound. By combining high-current capabilities with low-noise components and a well-thought-out power supply, Denon created an amplifier that was both powerful and refined, capable of delivering music with depth, warmth, and detail. This engineering approach has made the PMA-850 a lasting favorite among vintage audio enthusiasts.
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