Hi-End DAC tube output stage

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Hi-End DAC tube output stage
August 2016 - now

INTRODUCTION

This is a full differential balanced output stage designed by me many years ago for an Italian company.
The main features are simplicity, very good sonic performances and perfect instrumental measurements.
This circuit can also receive in input un-balanced signal and convert this to balanced so is perfect also like a pre-amplifier.
It can be used with many DAC chip with voltage differential outputs like CS4396, CS4398, WM8741, ES9018, ES9038PRO.

Low distortion
Good frequency band
Low output impedance, able to drive also headphones
balanced to un-balanced
un-balanced to balanced
only one tube on the signal path
only one capacitor on the signal path
1.65V or 2.5V DC coming from DAC chip is not a problem on input
zero noise

UPDATE 18 Jul 2017:
Lothar Diebold suggest to reduce the DAC output voltage adding a 13K (27K // 27K) resistor in series to input on this stage and changing R31,R32,R33,R34,R19,R20,R21,R22 with 2K5ohm.
Set the correct bias on tube changing the R47 and R48 to have about 135V on tube anode.

SCHEMATIC

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

This is the complete schematic of the output stage including power supply and output relay section (click to enlarge).

COMPONENT LIST

R31,R32,R33,R34,R19,R20,R21,R22     22K 1/4W 1%
R66,R67,R68,R69,R70,R71,R72,R73     2K2 1/4W 1%
R10,R11,R13,R14,R27,R28,R29,R30     470 1/4W 1%
R43,R44,R53,R54,R45,R46,R59,R60    100K 1/4W 1%
R2                                 150K 3W
R1                                  18K 2W
R4,R5                               10K 1/4W 1%
R7                                   1K 1/4W 5%
R74                                 47K 1/4W 5%
R9                                  100 1/4W 5%
R64                                  1K 1/4W 1%
R61,R62                             4K7 1/4W 5%
R63                                 5K6 1/4W 1%
R65                                220K 1/4W 1%
R3                                 470K 1/4W 1%
R6,R8                                 1   3W 5%
R11,R16,R17,R18,R25,R26,R23,R24    100K   1W 1%
R47,R48                             100 1/4W 1%
R35,R36,R37,R55,R56,R57             33K   2W 5%
R39,R40,R41,R49,R50,R51             33K   2W 5% (are necessary if you use balanced out)
R52,R38,R58,R42                   empty
R12                                 100      trimmer 20 turns, to set to obtain 12.6V

C1                               33uF 400V
C4,C5,C8                         220uF 25V
C6,C9                            100uF 25V
C10,C2                           100uF 400V
C3,C7                            1000uF 25V
CY1,CY2,CY3,CY4                  10nF 440VAC RS 335-066
U$1,U$2,U$6,U$9 = output cap     2.2uF 400V MKP
CPOWER1,2,3,4                    33uF 400V MKP Solen
B1                               4A or 6A 100V diode bridge
D1,D2,D3,D10,D11                 1N4007
D5,D9                            Zener 10V 1W
D4,D6,D7,D8                      UF5406
Q2                               2N2905A
Q1,Q3                            IRF840
Q4,Q5,Q6,Q7                      IRF820 (only Q5 and Q5 are necessary if you don't use balanced out)
IC1                              LM1086 (ADJ not fixed version) or LM317
V1,V2                            ECC82 or 12AU7
U$3,U$                           2N5459 or test the MPF102 but verify voltage on anode about 1/2 VCC
U$4                              LM311
T1                               BC337

J12,J5,J6,J7,J15,J16             RS 718-7987
J3,J4,J1,J11,J8                  RS 718-7987

RL1,RL2                          N4078-2C-12V-0.36 or N4078-2C-12V-0.2

In the anodic power supply section there is a choke to create a CLC filter in order to reduce all noise, I suggest a valid choke with 10H min, max Rdc 600ohm and a gap to support 70mA min DC current.

If the output level is too high in your environment you can add a resistor in series with the inputs, a 10K (or 2 x 22K in parallel) should be perfect if you change R31,R32,R33,R34,R19,R20,R21,R22 with 4K7 (Vout = 2350 / 12350 = 0.2x ).

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 220VAC

secondaries
     260VAC   0.3A   78VA
     17VAC     3.0A   41VA
     8VAC     2.0A   16VA
       8VAC     2.0A   16VA

Total 151VA

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

Obviously the secondary 260VAC will give about 260VAC / 1.18 = 220V.

HEATSINKS

Heatsink for TO220 - Thermal resistance 10 ° C/W

PCB

This pcb is available on Ebay shop jims_audio.

DAC INTEGRATION

The output relay can be driver by the controller of the DAC ES9038 PRO using the specific firmware called:

ES9038PRO_10a_4x20_VFD_enc_20MHz_HQ_standby_tubeout_trigger0.hex

Using this code on lcd after power-on you will see the warm-up time to wait and during mute state the output relay will short the output to ground.

It is necessary an optoisolator SFH610 and a 470ohm resistor to connect the controller to the output stage module following this following images.