Saturday, 29 July 2017

Korg Nutube 6P1 vs 12AX7 tube: Hybrid Amplifier (3)

Korg Nutube 6P1/12AX7 Hybrid Amplifier Schematics

Schematics are implemented using Eagle CAD 6.5. Circuit is compatible with Korg NuTube 6P1 triode and 12AU7/12AX7 triode valves by changing assembly options.

Page 1 shows input preamp. The circuit consists of the following stages:
  • 1.   Input dual JFET buffer/ amplifier with gain 1 control
  • 2a. First NuTube triode amplifier with load 1 and Bias control
  • 2b. First 12AU7 triode amplifier with gain 1 control
  • 3.   Second dual JFET buffer
  • 4.   3-band equalizer (Fender tone stack)
  • 5.   Third JFET buffer
  • 6a. Second Nutbe triode amplifier with load 2 and Bias control
  • 6b. Second 12AU7 triode amplifier
  • 7.   Fourth dual JFET amplifier with Gain 2 control and Boost switch
Last stage (7) is duplicated to connect to Power Amp or Headphones/Line Out sections. This means that there are independent Gain 2 and Boost controls for Power Amp and Headphones/Line Out.

NuTube 6P1 grid Bias can be controlled via RV3 trimmer from 0 to 3.4V, optimal value is obtained between 0.4-0.5.

Both triode filaments are powered by a +5V DC/DC converter, 240R resistors control the filament current on NuTube triode.

VR1-VR2 trimmers allow to change Nutube triodes anode load. Optimal value is obtained at 0.5-0.7.

RV2 attenuation trimmer is optional, normally not connected.
Schematics page 1/3: Input preamp
Page 2 consists of the following sections:

  1. TI TPA3112D1 25W class-D Audio Power Amplifier
  2. TI TPA6111A2D 150mW headphones amplifier
  3. TI LME49723 dual opamp buffer for Headphones/Line-Out

It is very important to add a 100K pull-up on AVCC pin 14, because the +24V power-on ramp can be very abrupt (>10V/ms) since it is controlled by a switch, if this pull-up is not added, power amp will be damaged.

There are independent volume controls for Power Amp, Headphones/Line-Out.

It is recommended to turn down the corresponding volume control before switching on the Boost, since a capacitor is connected abruptly and can generate loud pops, maybe a low value resistor added in series with the switch and capacitor could avoid this.

JP1-JP2 can change the gain of the power amp, if they are not installed gain is set to maximum.

RV10 allows to set power limit on power amp.

RV8 is the power amp volume control, it can be very loud if set to maximum. A series resistor could be added to reduce loudness.

A similar thing happens to RV9 Headphones/Line Out volume control
Schematics Page 2/3: Class D 25W Power Amp, Headphones Amp, Line Out
 Page 3 consists of the following sections:
  1. +24V DC power input connector, filter and switch
  2. +24V to +5V DC/DC converter (0.5A)
  3. +24V ON LED, +5V ON LED
An external AC/DC 220VAC to 24VDC (>30W, >1.25A) external wall adapter with a 2.1 barrel DC connector (center positive) is required.

It is important to use a clean external AC/DC supply with switching frequencies out of the audible spectrum (>50kHz switching frequency recommended). A 100uH + 2x100uF input filter is used but frequencies lower than 1kHz are difficult to filter out.

The +24V to +5V DC/DC converter is used to power the heating filament for both Nutube 6P1 and 12AX7 tube and the headphones amplifier. Heater voltage for 12AX7 tubes is specified at +6.3VDC but 5V works well in starving mode.

Two +24V and +5V power on switches have been added, +24V power-on switch works as a standby control so that heater (+5V power-on switch) can be powered-on before the amp is powered. the inverse sequence should be respected when powering-off.

For NuTube 6P1 this powering on sequence is not required.

A blue/orange LED (NuTube/12AX7 use different colours) on the left shows when +24V power is active.
A green/orange LED on the right shows when +5V heater power is active.

Most of the power is dissipated by the Class-D power amp (25W), the heater + headphones amp dissipates less than 2.4W, and the preamplifier circuit 15mW, for a total of 28W max.

Schematics 3/3: +24V to +5V @ 0.5A DC/DC converter

Korg Nutube 6P1/12AX7 Hybrid Amplifier PCB layout

A simple 2-side PCB board can support both hybrid amplifier versions NuTube/12AX7 by just using different assembly options.

PCB top layer
PCB bottom layer

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