Balanced Tube Preamplifier V6.4

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V6.4 is a tube preamplifier specially designed to use in balanced audio applications. Could be used as line preamplifier as well as analog stage for DAC. This selection is done during assembly.

Balanced Tube Preamplifier V6.4 has double stage topology:

First stage is voltage differential amplifier utilizing double triode. This stage is designed to amplify only difference between “+” and “-” input and omit common input voltage.
Second stage is cathode follower with resistive load. Purpose of this stage is lowering output impedance and provide isolation of first stage from the load.

Input circuit of the board is complex design which can act as gain correction for line preamplifier or input 3-rd order filter for DAC output stage.

Power Supply Requirements:

anode supply: 300V at least 70mA
heater supply: 6,3V at least 2,5A
negative supply: 5V at least 30mA
relay supply: 5V at least 100mA

Muting circuit consist of two relays which are configured as normally closed. This means that without any supply on J6 board outputs are shorted to GND and Muted. Muting functionality is designed to use with my “HV Power Supplies” where timer circuit is built in.

Ouput connectors:

V6.4 preamplifier is designed to use two types of output connectors:

panel mount connectors directly mounted on back panel of enclosure: XLR1, XLR2, RCA1, RCA2
NS25-4 connector to use cables between connectors and board: J5 and J2

Heater of tubes:

Board should be supplied with heater supply 6,3V connected directly to J7 connector. Tubes L2 and L4 are supplied directly. Tubes L1 and L3 could be supplied in two ways:

ECC88 style – between 4 and 5 pin for E88CC, 6N2P, 6N30P
ECC82 style – between 4|5 and 9 pin for ECC81, ECC82, ECC83

This selection is done by JP1 jumpers. Please note, that V6.4 boards are predefined to supply 4|5 and 9 pin and this configuration is done by tracks on the board. To select 4 and 5 pin please cut trancs below JP1 jumpers (as shown on the board) and solder JP1. To use first configuration JP1 is not needed.

Differential gain:

with ECC81 tube – 23x
with ECC82 tube – 10x
with 6N2P tube – 30x
with E88CC tube – 20x

This gains applies only when differential signal is applied to board. When single ended signal is connected gain is 2x less.

Additional power supply J8:

This voltage is used by most configuration to power differential amplifier CCS when cathode-grid voltage is too low to do that by default. Is needed in all current output DACs configuration, but for sure could be provided for all configurations. When additional power supply is not connected to J8, connector should be shorted by jumper or R55/R53 0R resistors should be mounted!

CCS transistor Q1:

In place of Q1 transistor you can find double footprint. One for SOT23 and second for SOT223. This configuration is done for future use with stronger input tubes f.e. 6N30P. For now only BC850C should mounted. Photos above shows how to solder both transistor types.

Calculating IV resistor value:

First of all we have to choose output level of whole circuit. Standard value is 2Vrms. This give us ~2,83Vp.
Now we have to choose tube that we want to use – for example ECC81. Differential gain of proposed circuit with ECC81 is 23x.
Divide 2,83V by 23. This give us value of peak voltage on IV resistor – 123mVp.
Filter circuit could be omitted in this calculation because it has 0dB gain in passband area.
Now divide 123mV by peak value of current of your DAC.
And that’s it! Final value is value of IV resistor.

Current output DACs:

Schematics for ES9038PRO Download Schematics for ES9038PRO with 6N2P tube Download Schematics for 4 x PCM1704 Download Schematics for PCM1794A Download

Voltage output DACs:

most AKMs, WMs and Vout Sabre provides 1,4Vp output
CS4396/97/98/122 provides 1,2Vp output

Schematics for Voltage output DACs with 1.4Vp Download Schematics for Voltage output DACs with 1.2Vp Download

Line Preamplifier:

Schematics for 20dB preamplifier Download

Note for SE DACs:

Schematics for 4 x PCM1704 is valid also for 2x PCM63. In this case, signal should be connected to IN+ only. IN- inputs should be left open, but all components associated with this input should be soldered.

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