M4 | Replica of British Sugden SF60 Kit | Audiophile-Grade Amplifier Board | Classic Sounding | Outperforms LM3886 & TDA7293

Note: Many have reported issues such as operating point drift or other problems after swapping op-amps. In fact, the use of AD827 and LME49720 is not recommended, as these two op-amps require relatively low feedback resistor values. Directly replacing with dual op-amps from the OPA or MUSES series will work without any issues. Examples include the OPA2604, OPA2132, OPA1612, MUSES8920, MUSES02, OP275, and TL1057, all of which are fully compatible and can serve as drop-in replacements.

The resistor marked with “?” on the board has a value of 3.9 kΩ.

Finished board weight: approx. 205 g (net weight)
Finished board + capacitors + Toshiba heat sink: approx. 797 g
Finished board + capacitors + ON Semiconductor heat sink: approx. 767 g

The M4 circuit is derived from a replication of the British Sugden SF-60. This British circuit is truly remarkable, reproducing musical melodies that captivate and immerse the listener completely. Its pure and natural sound quality is thoroughly embodied in the circuit design. Compared to the Audiolab 8000A, it clearly surpasses it by two levels, especially in the layering and refinement of the mid-to-high frequencies.

The performance of this amplifier far exceeds that of amplifiers using the NEC UPC2581V as a driver transistor. Its midrange, in particular, is much more listenable and comfortable. More importantly, it allows for the replacement of the input op-amp, offering the enjoyment and satisfaction of tailoring the sound signature and performing upgrades. If sufficient heat sinking is provided, the unit can even be adjusted for pure Class A output! The product includes complete overcurrent protection, delayed startup protection, and output DC protection to ensure the safety of your valuable speakers.

Purchase options include either a component kit or a finished board. The main heat sink must be purchased separately: thickened version ($14).

Power Supply Voltage: AC22VX2 to AC36VX2
PCB Dimensions: 153 x 128 mm
Output Power: 150W + 150W (4Ω), 100W + 100W (8Ω)
Optional Power Transistors:

• ON Semiconductor complementary pair NJW0281/NJW0302

• Toshiba 2SC5200/2SA1943
  Frequency Response: 10 Hz – 40 kHz
  Included Accessories: Mica insulators, mounting screws, fixing brass spacers

Notes on Transistor Selection:
The ON Semiconductor transistors offer excellent technical performance with well-layered sound, while the Toshiba transistors deliver warm, captivating tonality and outstanding bass.

Purchase Note:
Both the kit and the finished board do not include the two 10000 µF main filter capacitors, which must be purchased separately.

Key Components:

• Double-sided immersion gold PCB

• German WIMA MKS coupling capacitors

• Military-grade 718 series resistors with copper leads

• Japanese Omron relay

• Imported original transistors

• Japanese Nippon Chemi-Con (Black Gate) capacitors

• Sanken rectifier bridge

Adjustment:
This board is very simple to adjust. You only need to measure and set the quiescent current of the output transistors.

Copper Leads of Resistors: Noticeably thicker and sturdier than standard resistors, comparable to those used in DALE brand resistors.
Temperature Coefficient: 25-50 pp
Power Rating: 1/4W
Tolerance: 1% (indicated by 5 color bands)