Euraudio PSS701/PSS702 specification

Welcome to the Euraudio PSS701/PSS702 specifications, in which you can read about the system requirements (application criteria), operation and technical data of the PSS701/PSS702.

The PSS701 and PSS702 are symmetrical power supplies with an SPST ultra low distortion solid state relay (SSR) and with a ground loop breaker circuit. The only difference is that PSS701 has place for one filter capacitor pair, while the PSS702 has place for two. They were primarily designed for the Euraudio LDA172hc low distortion power amplifier, but can be used for other home-made audio amplifiers as well. Please find the PSS701/PSS702c assembly instructions here, which contains information about the sizing of the filter capacitors and fuses.

PSS701/PSS702 system requirements

What kind of amplifiers can the PSS701/PSS702 be used for?

They can be used for a single amplifier channel that needs a dual or in other words symmetric (+/-) supply voltage. You need as many PSS701/PSS702's as the number of the separately grounded channels is within your amplifier chassis, for example for a dual mono amplifier, you need two PSS701/PSS702's. If you want to use them in stereo amplifiers, then be careful, because the shared grounds may create a ground loop with possible hum ingress. Note: The PSS701/PSS702 can be used for the Euraudio LDA172mc and LDA172hc low distortion, low noise audio amplifiers as well.

Up to what amplifier power can it be used?

The recommended maximum amplifier power is 250 W (speaking of class B or class AB amplifiers).

Protection board compatibility

For the SSR control, a protection board is required. PSS701/PSS702 is fully compatible with the Euraudio APBT and APB protection circuit boards. If you have to use the PSS701/PSS702 with the Euraudio APBF or Euraudio APBM, then a simple workaround is needed, which is covered in the assembly instructions of the PSS701/PSS702.

If you wish to use the PSS701/PSS702 and its ultra low distortion SSR capability with other protection boards (not with Euraudio's APBT, APB, APBF or APBM), then please contact me for the connector J92 pinout through which you may be able to connect your own protection circuits.


The  PSS701/PSS702 rectifies the two sinusoidal AC voltages connected to its input, then smooths it by using filter capacitors, thus generating the symmetric (+/-) supply voltage that is necessary for most audio amplifiers.

To avoid ground loops, the grounding of the PSS701/PSS702 has a star-like arrangement, furthermore the PSS701/PSS702 contains a so-called ground loop breaker circuit, which prevents hum from appearing through the earthing conductor, if devices with earth ground protection against electric shock are connected to your amplifier, and the chassis of your amplifier is also earthed.

The SSR circuit is controlled from a suitable protection board by a on/off voltage, but also has its own pull-up resistor. You can decide where you install the pull-up resistor. If you install it in the PSS701/PSS702 and you disconnect the protection board, then the relay is always on. If you install the pull-up resistor in the protection board and you disconnect the protection board, then the relay is always off. The latter is recommended.

The SSR circuit uses a photovoltaic optocoupler to provide a floating gate voltage. The simplified SSR circuit can be seen in the following figure:

"RLYIN" is the input of the SSR (amplifier output), "SPKHOT" is the output of the SSR (speaker hot terminal).

At the input of the rectifier bridge, there is space for a snubber RC network (R91, C91), that may be sized in order to decrease the oscillation between the leakage inductance of the transformer and the capacitance of the rectifier together with stray capacitance of the transformer. Installing the snubber network is not mandatory, and its sizing procedure is not part of the documentation. However, if you're interested, here's an article about it:

Technical data

At 25°C ambient temperature unless otherwise noted.

Power Supply

Bridge rectifier maximum input voltage: 2x50 VAC

Maximum allowed continuous output current at 60 °C ambient temperature: 10 A limited by the heatsink of the rectifier bridge)

Recommended amplifier power: max. 250 W (class B or class AB)

Size of applicable filter capacitors:  snap-in type, max. 35 mm diameter

Solid State Relay

Maximum switchable voltage: ±70 VDC; 50 VAC

Maximum continuous current: 10 A (Ta=25°C); 7 A (Ta=75°C)

Typical off-state attenuation on 4 ohms resistive load: >80 dB (1 kHz); 60 dB (20 kHz)

Maximum 100 us pulse current before turn-off signal becomes active: 80 A (non-repetitive, Tcase,switching transistors=25°C)

Turn-on time: < 1.2 ms

Turn-off time: < 400 us


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