Hybrid Class D End system   
Apr 2026

INTRODUCTION

After the incredible result obtain with the last Hybrid Class D system the evolution can only be this.
The main features are simplicity, very good sonic performances and perfect instrumental measurements.

• No opamp

• No global feedback

• No noise

• Only 1 capacitor on the signal path (ELNA Silmic II)

• Very high slew rate 

• Very low distortion until 170W on 4ohm

• Only one tube on the signal path

• Very short internal wiring

SCHEMATIC

  6Z9P - 6ZH9P - E180F (single pentode)

  ECC99 by JJ (dual triode)

5687 Jan Philips (dual triode)

5842 Raytheon (single triode)

INTERSTAGE TRANSFORMERS

In this project, I utilized the Lundahl LL2763/7mA for the first time.
After an extensive market review, this component was chosen to satisfy the following technical requirements:

• High Inductance: To ensure a good low frequency band

• Air gap designed for a minimum of 14mA to use high transconductance tubes

• DC resistance no more than 1/10 of the tube's internal resistance to minimize voltage drop, improve efficiency and obtain a good slew rate

The LL2763/7mA with primaries in serie have a primary inductance of 280H but if we connect the primaries in parallel we have 280 / 4 = 70H, air gap for 14mA and Rdc=270ohm.

This specifications are better than LL1671/20mA with 52.5H  Rdc=312ohm and than LL1689/9mA primaries in parallel with 45H  Rdc=327ohm

Ft(-3dB) = R / (2 * 3.14 * L) = (2400+ 327) / (2 * 3.14 *70) = 6.2Hz

CAPACITORS

Regarding the Ck capacitors, I have chosen the ELNA Silmic II since the Nichicon Muse series has been discontinued.

    

TUBE DRIVER MEASUREMENTS

The E180F in triode connection and the 5687 give a good low distortion < 0.3% and a perfect 20Hz - 20KHz at 0dB.
Using the E180F is necessary an attenuator after the interstage transformer to reduce the voltage gain.

OUTPUT STAGE

The TPA3255 is used in many modules with significant price differences starting from $30 but only very few modules can we consider serious products for hi-end systems.
In order to integrate theTPA3255 modules with the tube preamplifier without using an opamp it is necessary that the modules use a dual power supply for the original driver so the decoupling capacitors are oriented in the correct direction (positive connected to TPA input).
      

The best module I found for this project is the D400M P05 produced by Sylphaudio with these main specifications:

• TPA3255 in a mono PBTL with or without PFFB

• input balanced or un-balanced

• opamp with split supply design so dual power supply +12V and -12V

• 2Ω load capability

• Footprint: 130mm x 100mm x 45mm (L * W * H)

• Overcurrent /short protection

• Over voltage protection

• Overheat protection:

• Turn on-off pop suppression  

The main differences from the Fosi Fox v3 are these:

• the use of one chip for each channel in PBTL

• the use of dual power on the driver stage

• Output filter with Coilcraft SER3018H 2.56mohm instead of Sumida DEP1519 16.6mohm

• Output filter with 1uF Panasonic MKP ECWFE  instead of Wima MKS 2

• Interstage capacitor 10uF ELNA Silmic II instead of ELNA RA3

POWER SUPPLY FOR POWER STAGE

Regarding the power supply, there are numerous companies that build switching power supplies but I chose Meanwell products because I needed a reliable product that does not cause problems over time.
The first tests were done with LRS-150-36 power supplies and then I switched to the UHP-200-48 resonant design (LLC) model with also PFC.

There is a huge difference in the sound by switching to the new Mean Well UHP, many harshnesses disappear and you feel a sensation of extreme cleanliness with absolute silence in the background.

PCB

You can use this little module to connect the tube preampl. to the opamp output of D400 (use only with TPA3255 modules with dual power supply, ex. +12V / -12V or invert the interstage capacitors in the correct direction).
The first version include an attenuator about -6dB to reduce the voltage gain of the final system, to verify if it is necessary.

R1,R2,R3,R4     2700ohm 1/4W  SMD 1206
IC              2 x 4 Single row connectors 2,54mm

Here the files to produce the PCB of the op-amp connection

Here the files to produce the PCB of the op-amp connection 

CHASSIE

I will use this Hi-Fi2000 chassie.

• Slim Line 02/350 10mm BLACK front panel - 3mm aluminium covers

• Supplement for substitution of fully vented aluminium cover 3mm Slim Line 350mm