Pass XOno Clone

Table of Contents



With the very good results from my Aleph P clone I start thinking about a complete differential audio chain. The output signal of my MC preamplifier (MC step-up transformer and phono preamplifier from M. Cotter) was not very high and I have to use the maximum gain and nearly the maximum volume at my Aleph P to hear with my normal level. After reading about a couple of very successfully DIY projects and some excellent test results from different HiFi magazines I decided to build a Pass XOno clone.

Description of the Pass XOno Clone (Rev. 0)


I have made nearly no changes to the circuit of the preamplifier. In a discussion on diyAudio you can find a description of the differences of the Ono and XOno design. The member t024484 wrote at 2005/03/17 :


In the very first mail of this thread, KDT asked what the differences are between the ONO and the XONO.
Here it is

  1. Each channel has now its own set of power supply regulators by means of two times 7815 giving +/- 30Volt instead of one single regulator with MosFets for both channels
  2. Power Cap’s on the +/- 30Volt lines have been increased from 1000uF to 3300uF//150nF
  3. C10 has been changed from 10uF into 220uF//47nF
  4. C1 and C41, both 220uF, have been changed to one single 1000 uF
  5. on the unregulated side the four 1000uf Cap’s have been replaced by four 10.000uF Cap’s
  6. C16 330pF, connected to Q14, has been deleted

That’s it

These are the main changes I made to the original design in the service manual.

Instead of the 2SC1844 and 2SA991 I use BC550C and BC560C. You can find the complete circuit on an A4 and an A3 sheet, on the first the moving coil preamplifier and on the second the moving magnet preamp with the equalisation and the differential output drivers.

Because of the results with the Wima MKP4 coupling caps in my Aleph P clone I used them also for C7, C9 and C38 (10μF/160V). The double JFET Q5 is a BL type (2SK389BL) and the four 2SK170GR FET’s at the input of the MC preamplifier (Q10…Q13) you have to match. You can find a lot of informations in the following threads : 13981, 10326 and 60297 The coupling caps C10, C19 and C37 are Panasonic FC with a 47nF polypropylene capacitor in parallel. All resistors are standard unselected 1% metal film types. In the RIAA equalisation I use 1% glimmer caps (C4 and C5) and a measured polypropylene capacitor (C6).

There are switchable resistors and capacitors at the input of the moving magnet preamplifier. You can find also one user selectable resistor Rx and capacitor Cx. For them I use pins of a socket for an integrated circuit.

The photo shows one channel of my Pass XOno Clone preamplifier.

Pass XOno Clone Rev. 0

Pass XOno Clone Rev. 0

Power Supply (Rev. 0)


I had no schematics from the power supply of the Pass XOno, but I found enough informations in the thread diyAudio to build a similar supply. The Pass XOno use an active regulation with two positive 15V voltage regulators (7815/7915). The output voltage is ±30V. To reach this the two 15V regulators have a resistive devider at their reference input. In the original Ono design you can find a capacity multiplier with MOSFET’s similar to the power supply of the Aleph P. I prefer the variable voltage regulators LM317 and LM337, which I use in my power supply for the Ono. This regulators have the best operating point when they have a current greater than 30mA, therefore I use the two resistors R2 and R3 (see also TNT-Audio).

In the symmetrical power supply I use for both voltages an own bridge rectifier (full wave rectifier). After the recitifier follows a CRC filter with 2×10mF capacitors and a 3.3Ω resistor to reduce the voltage ripple. The output connector has two ground pins, one of them is to connect the ground with earth (two antiparallel diode – one half of a bridge rectifier – and a 5.6Ω/5W resistor in parallel). A second connector is an output for the unregulated power, you can use them for LED’s and for the muting relay.

Power Supply with Regulators

Power Supply with Regulators



The first prototype consists of two XOno boards, two power supplies and two standard transformers (2×30V, 50VA). At the picture below you see on the right side the transformers and the power supplies and on the left side you find the XOno boards. The phono inputs are in the back, I use RG174 to connect the cinch connectors with the board. In the front you see the symmetrical XLR output connectors.

First Running Prototype of the Pass XOno Clone

First Running Prototype of the Pass XOno Clone

Afterwards I wired both grounds to earth with a bridge rectifier and two 5.6Ω/5W resistors.

I used the XOno with a Thorens TD126 MKIII / SME Series III / Benz Micro ACE L2 at that time. I brought the Benz at the same time into operation as my XOno (I destroid my old Denon system during the work at my XOno / Thorens) and so I can’t say anything about the XOno itself, but the whole audio chain is very impressive. I have around 250 record discs (most of them Jazz) and I hear them like the first time. To build the XOno is in my opinion a way forward for most audio systems.

What is also very impressive is that you hear nearly no noise with the Pass XOno clone. I use an input resistor of 499Ω and the -4dB gain jumper JP1 activated with my Benz ACE L2. Even with a fully opened volume at my Aleph P and no record disc playing there is nearly nothing to hear.

Rev. 2 – Preamp & Power Supply


After the successful commissioning of the prototype I made little changes at the PCB layout (Rev. 1). During the whole time with the prototype I thought about another sectioning of the electronic and at least I came to the conclusion to make a bigger redesign before I start working on the case. Here are the changes I made :

  • room for bigger output capacitor on the audio board
  • regulation of the power supply direct on the audio board
  • sectioning of the power supply in rectification and regulation
  • rectification, DC filtering and earthing of both channels on one PCB

I want to place the case with the power supply (the transformers) in a bigger distance from the audio unit and therefore I need longer cables for the DC power between both units. But I don’t like the idea to send the regulated power over this longer outside connection. As an advantage of the revision 2 there is only the unregulated voltage going through this connections.

Apart from that there are no further modifications between the circuits of revision 0 and 2. As I noticed above I divided the power supply in two parts and you can find the regulation (LM317/337 voltage regulators with input and output capacitors) on the audio board. I also spend some more room for the 10μF coupling capacitors, so you can use several types of capacitors. I spend also a small parallel capacitor for this three caps – I got very good results with 100nF Vishay MKP1837. Furthermore there are three test points on the board – TP1 is connected with ground and between TP2 and TP3 you can measure the voltage at the resistor R28 (nominal value VR28 = 0.35V).

At least I have used cheaper capacitors from Mundorf – I want to know if there is a significant difference to the Wima’s. I have also matched Q1 and Q2.

Pass XOno Clone Rev. 2

Pass XOno Clone Rev. 2

Basically there are no variations at the circuit of the power supply, but I change the integrated bridge rectifier to four HFA08TB60 ultrafast, soft recovery diodes. Both channels with the earth connection and the 230Vac DC filter are placed on a single PCB now.

Power Supply Rev. 2

Power Supply Rev. 2

During the operation of the prototype I noticed that the XOno only reach a stable operation point after more than 45 minutes (see also the article about the XOno at the German magazine LP Magazin, 2006/03, side 18). Because of this my XOno is always connected with power and I need no muting circuit – therefore I leave it out.

Prototype of the Pass XOno Clone Rev. 2

Prototype of the Pass XOno Clone Rev. 2

Building the Case


I’ve set both audio boards in the case ALG 36-44-30 from Thel Audio-World. Front and back of the case are milled and marked at the Schaeffer AG. I use for the wiring of the audio signals Van den Hul D101 cable. For the cinch connectors I took WBT 0210 AG Nextgen for the MC and WBT 0210 CU Nextgen for the MM input. At the output you find XLR connectors from Neutrik. The connector for the DC power supply is an industrial type from Hirschmann.

In the meantime (the prototype exists now for more than 1½ years) I’ve started with the design of my VV4 preamplifier and the XOno has become a part of the concept of this preamp. Therefore the XOno gets now the unregulated power supply from the Control Unit of the VV4 (the concept of the unregulated power supply unit is the same).

Pass XOno Clone Rev. 2 inside the Case

Pass XOno Clone Rev. 2 inside the Case

Pass XOno Rev. 2 from the back side

Pass XOno Rev. 2 from the back side

Setting the Operation Point


To get an operational point near the needed ones I have trimmed R25 directly after I switched the power on. You have to measure the voltage between TP2 and TP3 and adjust it with R25 to 350mV. After that I have waited more than one day so that all semiconductor devices are in a stable condition. Thereafter I adjust the operational point again and get a very stable result (see below).

In the following table you find all relevant operational point of my two XOno boards.

Arbeitspunkte / Operational Points
linker Kanal
Left Channel
rechter Kanal
Right Channel
UR28349.9 mV350.2 mVR25 (OP1)
UR4574.7 mV77.3 mVIQ10 = 3.40 mA / 3.51 mA
UR4676.5 mV79.4 mVIQ11 = 3.48 mA / 3.61 mA
UR4776.0 mV81.4 mVIQ12 = 3.45 mA / 3.70 mA
UR4876.9 mV80.6 mVIQ13 = 3.50 mA / 3.66 mA

It must be pointed out that the operation point strongly depends on the temperature. A tiny airflow in the room – e.g. a little movement of your body – changes the setting.

On my board you find some parts which are originally intended to use for an additional adjustment of the operation points – R4a, R59a, JP3, JP4 and JP5. They based upon a discussion on about a way to use the XOno without the output coupling caps. It doesn’t work as described, the operation points are too temperature dependend. The only serious way is the use of servo regulators.

Hints and Changes


Below you find some hints and changes for the assembly of the XOno. They are the results of the long time experience and feedback I collected over the years.

Replacements for the JFET’s

It becomes more and more difficult to find the needed JFET’s for the XOno. Normally the only chance is ebay or a comparable platform. But the situations is not totally hopeless and in the following list you find replacements without any compromise. By the way you can use also a BL type for Q15.

Q10…Q13, Q152SK170GR2SK370GRToshiba
LSK170ALinear Systems
LSK170BLinear Systems
Q52SK389BL2× 2SK170BLToshiba
2× 2SK370BLToshiba
2× LSK170BLinear Systems
LSK389BLinear Systems

Apparently Toshiba had stopped the production of the low noise JFET’s and as a result the listed replacements are also not so easy to get. However the chance become bigger if you can select between different types. Certainly you have to match the single JFET’s for the replacement of the double JFET.

I have heard now several times about fake 2SK389BL and therefore my advice is not to buy some from dubious sources. As a replacement 2 matched 2SK170BL are obvious. How you have to assemble them at the PCB you can see at the photo below.

2×2SK170BL as a replacement for a 2SK389BL

2×2SK170BL as a replacement for a 2SK389BL

The left JFET faces us with its flat side and the middle pin is bent forward. On the right JFET the flat side is facing away from us. So the JFET’s are rotated 180° to each other in the PCB.

D5 LED Reference Voltage

In the thread Help wanted,aleph (cl) ono problem on I’ve heard for the first time of a problem with my XOno clone. Shortly thereafter another DIYer contacted me and told me his very similar problem which we can solve after a couple of emails. Below you find a description and a solution for it.

The two current sources with Q16 and Q17 have a common reference voltage source build with D5. This LED gets its operating voltage via the 6.8kΩ resistor R5. If I calcutate the dissipation loss of this resistor I get

    \[ P_{R_5} = \frac{(\mbox{60V}-U_{\mbox{LED}})^2}{R_5} = \frac{(\mbox{60V}-\mbox{1,8V})^2}{\mbox{6,8k}\Omega{}}\approx{}500\mbox{mW} \]

This dissipation loss can be too high regarding the resistor type you have in use.

In the thread you also read about an instable voltage of the LED after some heating period. This depends in the same way on the type of LED you use and how much current the diode can have. The LED current of my clone is around ID5≈8mA.

The simplest solution for both problems is to higher the resistor value of R5 to 15kΩ. This lowers the dissipation loss of the resistor and decrease the current of the LED at the same time (ID5≈4mA and PR5≈230mW).

The second alternative is to use the circuit of the Pearl II. Here R5 is not connected to the positive power supply voltage but with ground. The value of the resistor is 4.75kΩ. With this we get a LED current of ID5≈6mA and a dissipation loss of PR5≈170mW. This solution however demanded a minor change of the PCB.

The first solution is included in the actual bill of materials and in the schematic above.

Current Sources Q16 and Q17

In the schematic you find remarks that the voltage across the resistors R53 and R67 of the current sources build with the bipolar transistors Q16 and Q17 is approximately 1V. With this voltage you get a current of I=2.1mA for the current source Q16 and I=6.7mA for the ones with Q17.

You can calculate the voltage of the resistors with

    \[ U_R=U_{D5}-U_{BE} \]

If you have a LED voltage of e.g. 1.8V the voltage of the resistors is above 1V. There are two possibilities to adjust the current sources towards the original values:

  • search for a LED with ULED≈1.6V
  • adapt the resistor values to the actual LED voltage

I want to show here the calculations for both current sources for the second solution.

Let us assume that the voltage at the resistors is 1.2V. With this the resulting currents are IQ16=2.5mA and IQ17=8mA with the original resistor values. Particularly the deviation of the second current source is considerable. To get the default current of the original circuit the resistor values of R53 and R67 have to be adjusted.

    \begin{eqnarray*} \mbox{R53} & = & \frac{\mbox{1,2V}}{\mbox{6,7mA}}\approx \mbox{180}\Omega\\ \mbox{R67} & = & \frac{\mbox{1,2V}}{\mbox{2,1mA}}\approx \mbox{560}\Omega \end{eqnarray*}

What you see here is that it is not needed to match a lot of LED’s for getting the correct current. All you have to do is to measure the real voltage of the LED you have in use and calculate the needed resistor values. At least you get the original current much faster and cheaper.


For the assemby of new XOno PCB’s for my VV5 preamp I used standard LED’s from the distributor Reichelt (order number LED 3MM ST RT) and standard values for the resistors R53 and R67. At the first bringing into operation the voltage at both resistors on both boards I measured a constant value of 1.3V. Herefrom follows the following values for a correct current:

    \begin{eqnarray*} \mbox{R53} & = & \frac{\mbox{1,3V}}{\mbox{6,7mA}}\approx \mbox{195}\Omega = \mbox{390}\Omega\,||\,\mbox{390}\Omega\\ \mbox{R67} & = & \frac{\mbox{1,3V}}{\mbox{2,1mA}}\approx \mbox{619}\Omega \approx  \mbox{680}\Omega\,||\,\mbox{6,8k}\Omega \end{eqnarray*}

If you don’t want to have a parallel connection of two resistors you can also use R53=200Ω and R67=620Ω out of the E96 series which are more than sufficient enough accurate.

Adjustment of R25

Before the first bringing into operation of the board the variable resistor R25 should set to a reasonable value. Otherwise it is possible that the current through the output stage is so high that one of the 33Ω resistors can be damaged. A value of

    \[ \mbox{R25}\,||\,\mbox{R25P} = \mbox{800}\Omega \]

measured in the circuit was allways a good starting point for me.

LM317/337 Voltage Regulators

I very often get the question how to adjust the voltage regulators IC100 and IC101 before the first bringing into operation. To answer this questions we have to look at the mathematics behind this regulators which you can find in the data sheets. For both voltage regulators the informations are the same and therefore I do the calculations below only for the positive regulator LM317 (IC100).

The output voltage of IC100 is calculated with

    \[ U_a = U_{ref} \left( 1+\frac{\mbox{P100}}{\mbox{R100}}\right) + I_{Adj}\cdot\mbox{P100} \]

Because IAdj is relatively small we can neglect the second term. The value for Uref = 1,25V you can find in the data sheet. To be on the savety side it is better to first adjust the voltage below the norminal value and during the commissionig set the voltage to the correct value of ±30V. To get an output voltage of e.g. 25V you have to adjust the variable resistor P100 to the following value :

    \[ \mbox{P100} = \left(\frac{U_a}{U_{ref}}-1\right)\cdot\mbox{R100}\approx\mbox{5,1k}\Omega \]

The calculation for the voltage regulator IC101 is the same. If both variable resistors P100 and P101 are set to a value of approximately 5kΩ – which is half of the value of both potentiometers – you get an output voltage of around ±25V.

MC Input Capacitor

With 2 of my moving coil pick-ups I heard a kind of crackle – not a hum – from my loudspeaker. From one of them it was so quiet that the operating noise of the record disc covers this crackle, from the other pick-up I could hear it even in the break between 2 compositions. I could influence the loudness of this noise with the amplification factor of the MC input stage, so it was obvious that it was produced in the input stage of the MC amplifier.

This noise was a periodical oscillation and such a behaviour must have something to do with capacitors and inductors. As a consequence I start thinking about the input capacitor C29. It exists an excellent FAQ about everything around pick-ups from Arlt van den Hul which you can find at his homepage. In it he described that a MC pick-up don’t need a capacitor as a load, however even capacitors above 100nF don’t have a noticable influence because of the very low coil inductance.

I confirmed this statement with my own tests. Therefore I connected capacitors in parallel to the 100pF input capacitor with a cinch adaptor – starting with 220pF – and finally I installed a 1nF Wima FKP2. The noise is totally gone and there isn’t any oscillation left. The MC input signal with a tone arm lifted up is nearly as quite as a line signal at my VV5 preamplifier. There is also a tonal difference to the situation before, the pick-ups are slightly more open.

Resistors and Coupling Capacitors

Although I have used Dale resistors in the XOno of the VV4 I wouldn’t advice them today. I have assembled the complete VV5 with normal metal film resistors. It is better to use the saved money for the coupling capacitors at the output. From my own experience I can advice the Obbligato Gold Premium, but they are too big for my PCB’s. I know from friends that the Clarity Caps ESA 250V bring very good results (see below).

XOno Installations


Once in a while it appears that I not only sell boards for the XOno but I build a complete phono preamplifier. How this XOno’s looking like I want to show here.

XOno for H.F.

This XOno has a couple of features in the power supply. Some circuits from my VV5 preamplifier you find as additional PCB’s in this installation. I used HI-FI 2000 chassis.

In the power supply chassis you find two toroidal transformers and the power supply board. Electrically behind the PSU board connected is an additional PCB with cap multipliers.

XOno Power Supply with Cap Multipliers

XOno Power Supply with Cap Multipliers

At the voltage input of the XOno preamplifier are CLC-filters for the DC voltages integrated. The LM317/337 voltage regulators on the XOno boards are replaced with discrete voltage regulator circuits. The type of coupling capacitors at the outputs are Clarity Caps ESA 250V. Furthermore you can see that – on demand of the owner – Dale resistors are used.

Pass XOno Clone with Discrete Voltage Regulators

Pass XOno Clone with Discrete Voltage Regulators

On the last photo you see both chassis from behind with all the connectors.

Pass XOno Clone and Power Supply from Behind

Pass XOno Clone and Power Supply from Behind

The cap multipliers and discrete voltage regulators are circuits from the actual preamplifiers of Pass Labs and therefore I don’t share the schematics with anyone.