Hybrid Class D system and new DAC Output stage     
Gen 2025

Click on Hi-End Hybrid Class D system to see this true reference hi-end system.

INTRODUCTION

This is a fully balanced and differential DAC output stage that I designed many years ago for an Italian company.
The main features are simplicity, very good sonic performances and perfect instrumental measurements.
This circuit can receive as input balanced or un-balanced signal and convert both to balanced so is perfect also like a pre-amplifier.
It can be used with many DAC chip with voltage differential outputs like CS4398, WM8741, ES9018, ES9038PRO.
I will use this circuit also to drive directly a TPA3255 chip without opamp. to create a true Hi-End Hybrid Class D system.

• Low distortion

• No feedback

• Good frequency band

• Low output impedance

• Balanced to un-balanced

• Un-balanced to balanced

• Only one tube on the signal path

• Only one capacitor on the signal path

• The 1.65V or 2.5V DC coming from a DAC chip is not a problem on input

• Zero noise

SCHEMATIC

Here the schematic of the output stage excluding power supply and output relay section

This is the new complete schematic of the output stage including power supply and output relay section.

COMPONENT LIST

R1,R3,R4,R7,R33,R35,R3,R37            82K   1W 1% PR01
R2,R15,R16,R17,R38,R39,R40,R41        470 1/8W 1% RN55C4700BB14
R5,R6,R8,R9,R67.R68,R69,R70            1K 1/8W 1% RN55C1001BB14
R20,R21,R24,R25,R44,R49,R52,R53        2k 1/8W 1% RN55D2001FB14
R18,R19,R22,R23,RR42,R43,R50,R51     220K 1/8W 1% RN55D2203FB144
R57,R58,R59,R60,R65,R66,R63,R64      100K 1/8W 1% RN55D1003FB14
R10,R27,R31,R32,R61,R62,R55,R56       33K   3W 5% PR03
R13,R28                               10K   2W 5% PR02  
R14.R29                              150K   3W 5% PR03                          
R12,R30                              470K 1/4W
R26,R54                           120-150 1/4W
R45                                   47K 1/4W
R48                                  1800 1/4W
R46                                   150 1/4W
R47                                   100 multi-turn trimmerr
R71                                  220K 1/4W
R72,R74                                1K 1/4W
R73,R11,R34                          4700 1/4W
R75                                   10K 1/4W
R76                                     0 jumpp
R77,R78,R79,R80                       220 1/2W  1%  RN60 (in series to the output capacitor, see photo)

C2,C4                                  33uF 400V   UVY2G330MHD
C6                                    100uF 400V   LGU2G101MELZ
C7                                   4700uF 25V   
C1,C8,C10,C11                         220uF 25V
CY5,CY6,CY7,CY8                       10nF 440VAC  R474I210050A1K V057
U$4,U$5,U$7,U$8,U$9,U$11              12uF 400V MKP (min. 2uF for 10Kohm load)

U43,U$6                              IRF840
U$19,U$16,D6,D8                      1N4007
Q1,Q3                                2N3819
Q2,Q4,Q5,Q6                          IRF710
T3                                   2N2905
U$15                                 LM393
B1                                   2KBP01
D5,D10                               Zener 10V 0.5W
D1,D2,D3,D4                          UF5407
U1A                                  LM393
T1                                   BC517 (invert pin E with C)

RL1                                  G6S-2 2-pole micro signal relay

Connections                          RS 718-7987  MOUSER 571-1217056-1

5 item of Heat Sink TO220 Vertical 11 Degree C/W, 2.67mm Hole, 38.1x34.92x12.7mm Aavid 513102B02500G Mouser 532-513102B25

You can add this circuit. to eliminate the bump on switch-off phase.

B1      diode bridge  DF02M
R1      2200ohm 1/4W
R2      4700ohm 1/4W
C1      10uF 35V
OK1     Opto isolator  SFH610
Q1      BC337

Connections            RS 718-7987  MOUSER 571-1217056-1

TRANSFORMER

In audio components the power transformer is very important because the quality and design change the sonic result.
If possible in my projects I use custom power trasformers with very low flux, 20% less than normal.
In this case the primary is 260V instead of normal 220V so all the secondaries are recalculated

primary
     260VAC instead of 230VAC

secondaries
     260VAC   0.2A   52VA
     15VAC     1.0A   15VA

Total 69VA

if your main is 110V the primary will be 110 * (260 / 220) = 130VAC.

PCB

This pcb will be available on Ebay shop jims_audio.

Here the files to produce the main PCB

Here the files to produce the PCB of the soft switch-off

Here the direct show link

I have used a 5814 Jan Philips to get the best sound but you can use any 12AU7/ECC82/E82CC.

A valid alternative to NOS tubes is the EH 12AU7 which is more pleasant with some tracks.

MEASUREMENTS

Output stage max distortion 0.05% at 5Vrms output and 10Kohm load.

Output stage frequency response 20Hz-20KHz at 0dB.

After the good result obtain with the Hybrid Class D System, I have decided to increase the performances using a tube pre-ampl. with differential output and two dedicated class D mono amplifier modules to exclude the opamp on the signal path.

• No opamp

• No global feedback

• No noise

• Direct connection to DAC or DSC

• Only 2 capacitors on the signal path

• Very high slew rate 

• Very low distortion until 170W on 4ohm

• Only one tube on the signal path

• Balanced or Unbalanced input signal is accepted without modification

• Very short internal wiring

Follows the simplified schematic of the Hybrid Class D system without and with the buffer to use the PFFB.

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).
If PFFB is enabled on these modules, it is necessary to add a buffer on the signal path to keep the driver output impedance always at zero.        

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

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.

About the transformer to use for the driver section I have used a custom toroidal on my specifications.
The primary and secondaries has been calculated for 260V instead of normal 230V to keep the flux very low.

primary 260V

secondaries 0,2A 260V   52VA  and  1,2A 15V  18VA

total power 70W

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 

PHOTOS AND DETAILS

The 6 big yellow capacitors are ICEL PHC 12uF 600V 37x53mm, the same used in the my loudspeaker Monitor 2023.

You can use lower values 2uF with 10Kohm load to have an ft(-3dB) about 8Hz.

The final layout will change, here has been positioned the various component modules for comparative tests between direct input without opamp and with opamp. 
I think to rotate of 180° the D400 modules to have a very short input and output cables.

This is the PCB to connect the tube preamplifier to the opamp output of on D400 module. There are 2 versions with and without the attenuator resistances.

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

In the first time all internal cables was UL1007 and UL1015 PVC Insulation Tinned Copper wires buy on Alixpress (20AWG and 14AWG).

Here the internal cables has been changed with Canare L-2T2S High performance Microphone cable shielded 0.26mm²
and 4S11G STAR QUAD Speaker cable OFC Copper 4x2.08mm², both with with insulation PE Polyethylene.

Here the last modifications to use the ultra fast diodes on board instead of the external diodes bridge with the
CLC input filter.

Follow the relay module to switch-on the amplifier with a low current button.
I have used a 22mm Waterproof Ring Illumination Black Anti Vandal Push Button Switch, Blue, 1NO1NC Latching, LED voltage 12V.

Here the file to produce this pcb.

R1 82ohm  1/4W

RLED 680ohm 1/4W in series to one pin to reduce light.

MEASUREMENTS

Below we have 2 measurements on the distortion where the exact doubling of the power when the load is halved is evident.
In these measurements is included the distortion of the vacuum tube pre-amplifier, so the complete system.