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DAC tube output stage
August 2016 - now
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.
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
UPDATE 18 Jul
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.
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).
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)
R12 100 trimmer 20 turns, to set to obtain 12.6V
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
D5,D9 Zener 10V 1W
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
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 ).
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
260VAC instead of 220VAC
260VAC 0.3A 78VA
17VAC 3.0A 41VA
8VAC 2.0A 16VA
8VAC 2.0A 16VA
if your main is 110V the primary will be 110 * (260 / 220) = 130VAC.
Obviously the secondary 260VAC will give about 260VAC / 1.18 = 220V.
Heatsink for TO220 - Thermal resistance 10 ° C/W
This pcb is available on Ebay shop jims_audio.
The output relay can be driver by the
controller of the DAC ES9038 PRO using the specific firmware called:
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.
Output stage max distortion 0.01% at 4Vrms output and 10Kohm load.
Output stage frequency response 20Hz-20KHz at 0dB.
DAC ES9038PRO + this output stage max distortion 0.0047% at 3Vrms output.
DAC ES9038PRO + this output stage max distortion 0.035% at 13Vrms output.
DAC ES9038PRO + this output stage frequency response 20Hz-20KHz at 0dB.
Here the details of heatsink with the contact to create a ground connection.
Here the details of vacuum tubes filament connections.
DAC9038PRO + this output stage