RStAudio XOno 2019

this project / page is under construction


Current status of the project (2020/03/25) :
  – XOno circuit diagram created
  – XOno PCB design created
  – Controller circuit diagram created
  – Controller PCB design created
  – 2 XOno circuit board supplied (prototyping)
  – XOno prototypes assembled
  – Controller circuit board supplied (prototyping)
  – first version of the documentation prepared
  – Software created for the Controller Board
  – unregulated power supply successfully put into operation


Table of Contents



Since 2008 I provide printed circuit boards for a Pass Labs XOno replica and over the years unexpectedly many have been built. Unfortunately the gold dust – the outstanding 2SK170 JFETs from Toshiba – are running out. The supplier I always recommended can’t offer any more. So it becomes difficult to impossible to build an original XOno.

At the end of 2018 I decided to build a MC-prepreamplifier based on the XOno topology with the 2SK2145 JFET’s from Toshiba as a plug-in card for my DPV1 phono preamplifier. It should serve as a test setup for a new XOno to be designed. I put this plug-in card into operation at the beginning of 2019 with great success and thus nothing stood in the way of a redesign of the XOno.

The design should still be as close as possible to the original XOno design, but with a few useful improvements from the design of the Pass Labs XP-15 and my own experience of many years. I have written the following points into my personal specifications:

  • Optionally assembled with 2SK170 / LSK170 or 2SK2145.
  • Reference voltage source instead of the simple LED for the current sources of the RIAA amplifier.
  • Modern design of the inverter (XP-15).
  • Optionally assembled with DIP switches or relays.
  • Optional AC or DC coupling at the outputs of the RIAA stage and the inverter.
  • DC power supply input with CLC filter.
  • Discrete power supply regulation with a modified Jung regulator.
  • Extension Board with additional Equalization Characteristics (Columbia, Decca, EMI & Teldec)
  • Microcontroller PCB for the relay operation.
  • Use of mainly through-hole components.

Description of the XOno Hardware


The design goal was to develop a XOno with available components. Therefore you can replace the original 2SK170 (gold dust) with the currently available 2SK2145. The 2SK2145 is a double JFET in a 5 pin SOT case. The design of this phono preamplifier is nearly identical to the original, with some useful changes and improvements from the XP-15 design and my own many years of experience.

MC Input Stage

The circuit topology of the MC stage corresponds to the XOno, only the setting of the gain I have moved to the output of this stage – as in the XP-15 design. This seemed to me to be more practical than changing the amplification factor in the original. Both stages of the MC preamplifier are based on the JFET’s mentioned above. You can build the stage with five 2SK170 or with a corresponding amount of 2SK2145. When using the double JFET’s, this results in a significant reduction of the noise due to the double number of JFET’s connected in parallel. However, the operating points must be adapted to the number of transistors – other resistance values.

4× 2SK2145GR in the Input of the MC Stage

4× 2SK2145GR in the Input of the MC Stage

At the input of the MC stage there are 8 switchable resistors for the adaptation to the used pickup. In the original XOno these resistors are switched by a DIP switch on the PCB. This can also be realized with this PCB. For those who want it more comfortable, relays can also be soldered to the PCB. In combination with the optional microcontroller board, the resistors can be selected by rotary encoder and display.

The 3 amplification factors of the MC stage are also set either at a DIP switch on the circuit board or with relays via microcontroller.

MM Phono Preamplifier

In this stage you can use either 2SK170BL or 2SK2145BL for the input differential amplifier. Otherwise no adjustments have to be made to other components. There is also a significant improvement in the reference voltage of the current sources compared to the original XOno. I don’t use a red LED anymore but a TL431 reference voltage diode. This means that the operating point is no longer dependent on the lot of a component used. The TL431 has a specified output voltage of 2.5V in the circuit topology used here. Otherwise this amplifier corresponds to the original XOno design.

It is possible to switch 3 capacitors and 1 resistor at the input of the MM amplifier. The same applies here: either DIP switches or relays are used in combination with the optional microcontroller board.


The inverter is located at the output of the MM stage and is used to generate the inverted signal for a balanced output signal. I decided to take a more modern design of Pass Labs – the IRFD9110 in the original will get extremely hot. The current inverter behaves much more pleasantly here and has at least the same sound level.

Servo Controller

In the original XOno the MM amplifier and the inverter are AC coupled – there are 10μF foil capacitors at the output of both stages and at the input of the inverter. You can build these XOno in this way and stay with the original, the corresponding space for the large foil capacitors is available.

However, there is also the possibility to omit these coupling capacitors if both servo-controllers are mounted. Thus the outputs of the stages are kept electronically DC-free. I used this circuit concept for the first time with great success in my DPV1.

The Power Supply

The original XOno PCB gets its DC voltages from the external power supply which is located in a separate housing. Also in my design such a distribution was and is intended. The unregulated DC voltages are then regulated on the board of the XOno to the required operating voltages. Pass Labs used 15V voltage regulators with raised reference point (7815 & 7915) to regulate the ±30V operating voltage. In my old XOno I used variable voltage regulators (LM317 & LM337).

In the current design I use modified Jung regulators. I have used them for the first time in my DHA and am amazed by the audiophile and control performance of these regulators. I also use a C-L-C filtering with current compensated chokes and a total capacity of 60000μF on the printed circuit board in front of this regulators.

Photo’s of built XOno’s

In the picture below you can see a finished XOno. Here it is the variant with the capacitor coupling and without relays – thus finally the classical XOno construction. As coupling capacitors I used types of Aeon, which I still had in stock. Comparable types were also used in the XOno.

RStAudio XOno 2019 with 2SK2145 and AC coupling, without relay

RStAudio XOno 2019 with 2SK2145 and AC coupling, without relay

The next picture shows the version with 2SK2145, servo-controller and relay.

RStAudio XOno 2019 with 2SK2145, DC Coupling and Relay

RStAudio XOno 2019 with 2SK2145, DC Coupling and Relay

Microcontroller Board


As mentioned above, there is an option to use a microcontroller board to switch the relays – which take over the function of the DIP switches – on the XOno board. A display with 2 rows and 20 columns as well as a rotary encoder are available for operation.

Power Supply


An audio preamplifier and of course especially a phono preamplifier depends in a very special way also on the quality of the power supply used. Over the years I have gathered a lot of experience in this field and my circuits have become more and more complex, but also better and better. The RStAudio XOno 2019 got a power supply where I implemented all my knowledge at the time of development. About the last part of the power supply, which is located on the XOno circuit board itself, I have already written above. At this point it deals with the generation of the unregulated DC voltages.

230V/AC DC Filter

Nowadays there are more and more disturbances on the 230V/AC grid. It is therefore essential that these disturbances do not affect the DC supply of the audio circuits. As a first protection mechanism all my power supplies have a power socket with integrated filter from the company Schurter. Another filter with Wima X2 and Y2 capacitors is following, followed by a DC filter. This filter ensures that any DC voltage components in the 230V/AC supply do not drive the transformer into saturation. There is enough space for larger electrolytic capacitor values – the rule of thumb is that the DC filter needs 10000μF per 100VA.

On the following picture you can see this circuit with the two filters. The circuit board is connected between the mains socket and the primary winding of the transformer.

230V/AC DC Power Supply Filter

230V/AC DC Power Supply Filter

Unregulated Power Supply for the Audio Circuits

This circuit board is used to generate the unregulated symmetrical DC voltage for an audio channel. As you can see on the photo below it is not only the usual circuit consisting of a rectifier and a capacitor.

With the input – on the photo below left – two secondary windings of the transformer are connected. Directly following this, two snubber networks can be found which strongly dampen the oscillation tendencies of the oscillating circuit of the transformer’s inductance and the junction capacitance of the diodes. The bridge rectifier with discrete Ultra-Fast Soft-Recovery diodes follows. The next part of the circuit then consists of the C-L-C filtering with 4× 10000μF capacitors and a current compensated double choke. This is followed by a capacitance multiplier which removes any residual AC voltage components (ripple) from the DC voltage. The ground is connected to earth via a diode-resistor network. An extremely good, but not yet regulated, DC voltage is available at the output.

The output voltage is of course dependent on the ratio of the transformer and the AC voltage currently available at the socket (230V/AC ±10%).

One channel of the analog unregulated Power Supply

One channel of the analog unregulated Power Supply

Unregulated Power Supply for the Digital Circuits

The power supply for the digital circuitry, i.e. finally for the controller, is of rather modest design. There is also a snubber network and a ground-earth connection, but otherwise there is only a classic unregulated power supply consisting of a rectifier and charging capacitor. Even if the electrolytic capacitor was chosen here very large. For the controller, which has no direct connection to the audio circuits, this power supply is more than sufficient.

Unregulated Power Supply of the Controller

Unregulated Power Supply of the Controller

Printed Circuit Boards


At the moment there are no printed circuit boards to buy. The PCB’s shown below are the prototypes under construction and testing. I only sell PCB’s again when I am sure that everything is working properly and I can provide the buyer with assembly instructions.

Top Side of the RStAudio XOno 2019 PCB

Top Side of the RStAudio XOno 2019 PCB

Top Side of the RStAudio XOno Controller PCB

Top Side of the RStAudio XOno Controller PCB