this project / page is under construction
!!! Please Don’t Ask for Schematics !!!
As long as Nelson Pass don’t publish this schematics, I also won’t give it away. I don’t want to compromise the commercial success of these wonderful power amplifier.
Table of Contents
- Input Board with UGS5 Module
- Output Board
- Power Supply
- Inrush Current Limiter and Remote On/Off
- First Start-Up
- Changes to the Original Design
Since a long time I have the circuit diagrams of the X.5 and XA.5 power amplifier series from Pass Labs available. I had not enough amplifiers for my active RQM system and so it was obvious to select a system out of that series. The amplifier must be able to drive my Quad ESL and as a consequence the performance characteristics have to match the needs of my electrostatic loadspeakers. My choice was the XA30.5. It is a stereo power amplifier which I wanted to use again per channel for my stacked Quads.
The XA30.5 is a push-pull class AB amplifier with an output power of around 100W/8Ω per channel. But in the released data’s from the supplier the published output data is only 30W/8Ω. It is the maximum output power of this amplifier in class A mode. At least this is an amplifier with a very high quiescent current. Additionally the output stage has a circuit part which lets the output run in single ended class A mode at very low output power levels.
The input stage of this amplifier is a so called UGS module. This power amplifier has the famous SUSY (super symmetrical) circuit topology which is most meaningful with symmetrical signals. As a consequence this output has a bridge topology – the loadspeaker is connected between 2 inversely phased active outputs. In this way also the speaker benefits from the special feedback topology of the SUSY circuit.
The super symmetrical circuit topology is described in the following patent
Amplifier With Gain Stages Coupled for Differential Error Correction
Nelson S. Pass
US 5376899, Feb. 4, 1994
On this board the input circuit with a selectable amplification factor (24dB or 30dB) and the bridgeable AC coupling, the UGS module and the bias circuits for the bridged output modules are placed.
Input Board with UGS5 Module
With the 2 potentiometers in the middle of the board the absolute offset against ground and the differential offset between both active outputs are adjusted. The other 2 potentiometers are used to set the bias of both output modules.
The output board with 20 power MOSFET’s is directly mounted on the heat sink. You can see at the structure of this PCB both parts of the bridge topology – each with 10 MOSFET’s. The power resistors left and right beneath the board are parts of the single ended class A circuits for the power output. At the left side of the heat sink a thermal circuit breaker is placed, it shut down the amplifier at a heat sink temperature above 75°C.
Push-Pull AB Output Stage of One Channel
In the final installation I have not used the simple circuit with the power resistors for the single ended class A section. It is a more complex circuit which is also used in the newer power amplifiers from Pass Labs.
This technology you can use in nearly every push-pull power amplifier. You can find a very good introduction in the book
Audio Power Amplifier Design Handbook – 5th ed.
in the chapter 11 Class-XDTM: Crossover Displacement Technology.
The power supply determine, particularly with power amplifiers, the audiophile quality of a device. Also for this amplifier I decided to use a dual mono construction – with the exception of the transformer. This means that every channel has its own power supply. The transformer has separate secondary windings for the channels.
As in all my devices common in the last years I use a DC filter in the primary supply line of the transformer. How such a filter have to configure I describe in details at Tips & Tricks. Here I use 4× 22000µF/25V electrolytic capacitors – I have a bigger amount of them available.
I used a classical construction of the power supply. The first parts behind the secondary windings are fuses – the transformer is not short-circuit proof, followed by snubber networks to damp the resonant circuit build of the junction capacitance of the bridge rectifier diodes and the inductive reactance of the secondary winding coil. The next circuit is the discrete bridge rectifier build with ultra fast soft recovery diodes from International Rectifier (HFA25PB60). The last circuit is a CRC filter with 4× 47000µF and 2× 0,22Ω/25W. I only can use a CRC filter in this design because I’m sure that I don’t leave the class A region of this amplifier with my speakers – in the class B region we get a modulation of the music signal at the resistors.
With toroidal transformer above about 400VA you get very high inrush currents so that a normal automatic circuit breaker at your house switched off when you try to bring the amplifier into operation. This is the reason why you shall take care about this currents. The board I used here I have also in operation since years in my Hypex and Aleph J amplifiers.
Additional to the inrush current limited two other circuits parts are on the board. With the first part the amplifier can switched on with an external DC voltage and the second part delivers such a DC voltage with a little time delay to a next device.
Board with Inrush Current Limiter and Remote On/Off
The circuit diagrams and a description of this board you find at the page of my Hypex Class D power amplifier.
I made the first start-up – without back plane – at the 2014/03/23. The first impressions are rather disillusioning and in comparison with the Aleph J the XA30.5 sounds worse with my speakers. Four days later I had the first light bulb moment and two additional days later it was clear what I can expect from this amplifier. The XA30.5 needs definitely some time to reach its audiophile performance.
In the meantime I rank the comparison with the Aleph J like the comparison between the preamplifiers X0.2 and XP-30. With the Aleph J I heard a couple of years very satisfied music and it is without doubt an excellent power amplifier. But if you compare it directly with the XA30.5 – and at that time I only had one stereo power amplifier available and it means that I ran 2 Quads in parallel per channel – it is very clear which amplifier I prefer. As the preamplifier the audiophile resolution and the richness of details of the XA30.5 are simply indescribable and I’m not able to find the right words for it.
It is always amazing for me whichever progress is still possible.
First Start-Up of One Stereo Power Amplifier
Of course I made changes compared to the original design of the XA30.5 – as always in my clones. I have plans from 2007 available and I refer to them here.
The biggest modifications I made at the power supply. In fact the complete supply is my own construction and I used only the most important values – output voltage and current – from the XA30.5 schematics.
- The original amplifier has only one power supply for both channels, I use a dual mono construction with the exception of the transformer (see above).
- The overall capacity of the original power supply is 180000µF, I have 376000µF in place.
- I use a CRC bank, the original has only capacitors – the reasons for it I explain above.
- I use a 230V/AC DC filter on the primary side of the transformer.
- The original design use a simple ǸTC as an inrush current limiter, I switch off the current limiting elements (no permanent source of heat).
- I have snubber networks at all secondary windings to damp high frequency resonances at the DC voltage.
I also made changes to the audio circuits.
- The AC input coupling capacitors are bridgeable.
- The operation point of both JFET differential amplifiers in the UGS module are optimized.
- I use a Hawksford cascode in the UGS5 module. This lowers once again the distortion.
- At the output of the UGS5 I used Toshiba MOSFET’s like in the newest designs from Pass Labs.
- For the single ended class A operation I don’t use the simple resistor circuit.
- I increase the single ended class A output power in comparison to the original design.