LM-503PA Monoblock power amplifier - Single Ended 845, Drivers 310A - 1x24W

The LM-503PA power block uses a single-ended 845 tube, which operates in pure Class A for a power of 1x24W.



- In March 2017, the LM-503PA received a Red Dot Award, a German award recognized as the Oscar for International Industrial Design. View the article in PDF.
- The Haute-Fidélité magazine N ° 224 of April / May 2017 awards the "Reference" award to the LM-503PA in its test bench entitled "The triode waltz".
Read the article in PDF.


The LM-503PA mono blocs has been created from an original design of Western Electric. It uses a single ended 845 output stage with Full Music 300B drivers. Pre-amplifier stage use a special Line Magnetic tube, the 310A.

The rectifier tube is a 5U3C and the power line is filtered by two special chokes. User bias controls for 300B and 845 are used for bias adjustment with a specific vu meter to allow you to monitor bias with ease.

- Single ended design
- Pure class A
- Point-to-point wiring
- High quality components

Input stage: 1x 310A
Rectifier: 1x 5U3C
Drivers: 1x 300B
Power stage:  1x 845  "Single-ended" and Class A amplifier


Class A amplifier
Power: 1x24W RMS
THD: 1% (1kHz)
Signal/Noise ratio: 87dB (Weighted A)
Frequency response: 15 à 35.000 Hz (-1.5 dB)
Sensitivity: 1000 mV
Input Impedance: 100kΩ
Load impedance for HP to connect: 4/8/16Ω


Bias control is accessible on top of the amplifier panel with true ampere meter.
Selector NFB min/max
Analog inputs: 1x XLR et 1 RCA
Load impedance for HP to connect: 4/8/16Ω
SoftStart and time-delay circuit. (30 seconds)
Two EI output transformers with wide bandwidth are applied  

Metal and removable tube guard cage


Tubes Basics           Common Replacements
5U3C                        2748, 5931
310A                        WE310A
300B                        300B, 300A, 4300B
845                          845 A/B/C

Included Accessories

2x Fuses
1x Remote control
1x Detachable power cord
1x Manual

Dimensions (L x H x P): 287 x 279 x 574 mm
Weight: 30 Kg

Finish: Silver