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Explanation of the DartZeel Amplifier
This schematic corresponds to a DartZeel-type power amplifier, clearly inspired by the philosophy of the Swiss models NHB-108 / NHB-458. It is not an industrial copy, but rather a discrete implementation of the DartZeel concept, with several design choices that differ from conventional amplifiers.
I will explain it by functional blocks, and at the end I will highlight what makes it “DartZeel” rather than a conventional amplifier.
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1. General DartZeel Philosophy
Before looking at the circuit:
DartZeel is based on three key ideas:
1. Current-mode input rather than voltage-mode
Internally, the amplifier operates as a transimpedance amplifier (I→V), not as a conventional voltage amplifier with heavy feedback.
2. Very low or no global feedback
Priority is given to the intrinsic linearity of each stage.
3. Extreme symmetry and carefully controlled biasing
The positive and negative halves are almost perfect mirror images.
This schematic fulfills all three principles.
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2. Input Stage (left side)
Signal source and coupling
• 1.5 µF input coupling capacitor
• 1 kΩ series resistor
This defines:
• A high-pass filter
• DC isolation
• Very low input impedance
⚠️ Important:
This is not a typical 47 kΩ or 100 kΩ input. Here the signal enters almost as current, not voltage.
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I/V conversion (transimpedance stage)
The transistor immediately after the input works as:
• A transimpedance stage
• The signal enters through the base
• This is where the amplifier stops being “conventional”.
➡️ The output voltage of this stage is proportional to the input current, not directly to the applied voltage.
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3. ±15 V Zener Bias Networks
At the top and bottom of the schematic:
• 15 V Zener diodes
• 470 KΩ resistors
• RC decoupling networks
These provide:
• Clean voltage references
• Isolation from the ±50 V main rails
• Thermal stability
DartZeel avoids active regulators here to prevent dynamic noise injection.
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4. Voltage Amplification Stage (VAS)
In the center of the schematic:
• Cascoded transistor stages
• Resistors of 27 kΩ, 56 kΩ, and 100 kΩ
• Perfect top-to-bottom symmetry
Characteristics:
• Moderate voltage gain
• Very high linearity
• Well-defined constant currents
There is no classic VAS with aggressive dominant Miller compensation.
The 100 pF capacitors only provide local stability, not heavy frequency shaping.
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5. Driver Stage
Before the output transistors:
• 22 Ω base resistors
• Discrete buffer stages
• Decoupling capacitors of 1 µF and 5.1 V Zener references
This ensures:
• Low drive impedance
• Precise base current control
• Protection against oscillation
DartZeel places special emphasis here because the output stage is not tamed by global feedback.
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6. Class-AB Biasing Without a Traditional Vbe Multiplier
Notice that there is no classic Vbe multiplier.
Instead, the design uses:
• Diodes
• 27 Ω resistors
• Zener-referenced bias networks
This creates a dynamic bias system, closer to a soft class-A region around crossover.
Result:
• Near-absence of crossover distortion
• Very fluid sonic character
• Higher idle dissipation (typical of DartZeel designs)
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7. Output Stage
Power transistors
• Complementary NPN / PNP devices
• 200 mΩ emitter resistors
• Emitter-follower configuration
Nothing exotic here;
the key is how these devices are driven, not the topology itself.
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Output network (Zobel + inductor)
• 2.2 µH output inductor
• 10 Ω series resistor
• Zobel network: 10 Ω + 100 nF
Classic, but carefully tuned for:
• Reactive loudspeaker loads
• Long speaker cables
• Stability without heavy feedback
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8. ±50 V Power Supply
• Symmetrical rails
• No heavy regulation
• Extensive local decoupling
Typical DartZeel approach:
the supply must be fast, not rigid.
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9. What This Amplifier Does NOT Have (and why it matters)
❌ It does not include:
• A classic differential input stage
• High global feedback
• Op-amps
• A traditional Vbe multiplier
• Heavy Miller compensation
All of this is intentional.
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10. Why It Sounds “DartZeel-Like”
This design delivers:
• Extremely fast transient response
• Very high micro-dynamic resolution
• A sense of “live current”
• Tight, authoritative bass without harshness
• Smooth, grain-free highs
Because:
the amplifier does not correct errors afterward; it hardly generates them in the first place.
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Short Summary
This DartZeel amplifier is:
• A transimpedance power amplifier
• With total symmetry
• Very low feedback
• Dynamic biasing
• Highly linear discrete stages
It is not designed to win THD contests at 1 kHz,
but to preserve the shape and energy of real musical signals.
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