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Switching power supply for audio (revision B)



400W AC/DC Switching power supply for audio amplifiers


The SAPS-400 is an AC/DC switching power supply designed specifically to feed high-end audio amplifiers from mains, up to a total of 400W RMS (800W music). The reduction in size and weight, as compared to a linear power supply + toroidal transformer, is huge: 450g versus aprox. 4Kg for the same power level. The outputs are symmetrical, regulated and adjustable from ±35V to ±60V. It has a large capacitance, providing a very stable output even with heavy loading, for powerful bass and clear mids/highs.


Regulated, adjustable, ±35V to ±60V output, (other output voltages on demand) Aux output: ±15V 500mA Switch-selectable input ranges (100-120VAC(1) / 200-240VAC) for worldwide use Inrush limiter and soft start, fuse protected mains input. EMI filter at input Toroidal coupled output inductor for excellent cross-regulation and low radiation of EMI 12V fan output Overcurrent/short-circuit protection, latched for 2 seconds with red LED indicator Screw terminals for mains input, fast-ON for main outputs, 0.1" std. header for aux. output Green LED indicates that output is present. Dimensions: 150x89x41mm, approximately 450g



Min 100 ±35V Typ 220 50/60 4/8A


Parameter Mains input voltage(1) Mains input frequency Mains fuse rating (fast blow): 100-120AC / 200-240VAC Output voltage Max. power (220VAC in, ±60V setting, DC or 20Hz sine) Aux. output voltage Continuous auxiliary output current (each rail) Fan output voltage Fan current consumption Switching frequency Operating temperature


Max 240 ±60V

400(3) ±15 10 12 120 85 70 500

Units VAC Hz A V W (rms) V mA V mA KHz ºC

With proper voltage range switch selection. (2) Other voltages, even single-output, available on request (3) Max continuous power can be reduced slightly when operated from 100-120VAC.

WARNING: This product operates at mains voltage and has accessible live parts. It is intended to be assembled INSIDE finished equipment only and must never be exposed to inadvertent touch, that could lead to severe injury or death. It meets and exceeds all the international safety regulations: there is 8mm distance between primary and secondary, proper insulation to chassis and even triple insulation wiring in the transformer. However, as it belongs to Class I, it MUST BE EARTHED. The chassis must be connected to mains earth and use an approved mains cable, connected to an earthed mains plug. Observe extreme care during installation and NEVER touch any part of the board while it is connected to mains. Lethal voltages above 320V can be present. Once disconnected, allow at least 5 minutes so all capacitors get fully discharged and no dangerous voltages are present at any point of the circuit. If the chassis free height is below 45mm, it is mandatory to place an insulating sheet between the cover and the input bus capacitors. Replace input fuse with same type and rating only (4A/8A, see text). Always use a double-circuit input switch to disconnect both live and neutral input lines. A switched/fused IEC mains inlet is recommended, rated for at least 6A (10A in 100-120VAC mains countries) Coldamp and Elektor decline any responsibility for damages caused to persons or other equipment when any of the above rules hasn't been strictly followed.

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Switching power supply for audio (revision B)




Fig. 1: Mains input connections

In order to connect the SAPS400 power supply, only the mains input (with Earth) and output connections are required. Input: Input voltage is fed via a couple of screw terminals plus a screw for the earth wire, as shown in the figure 1. A fused/double switched IEC mains input is recommended, with a fuse rating of 4A (8A for 100-125VAC countries) See (Mains operating voltage selection section). This PSU features onboard inrush current limiter and slow start, for reliable long-term operation. WARNING: Lethal voltages are present at some points of this product. Avoid direct contact with any part of the supply during operation, and also within 5 minutes after the supply has been unplugged from mains in order to fully discharge the storage capacitors.

Fig. 2: Main output connections


Output is taken from the Fast-ONTM at the opposite side of the board. There are 2 of them for each rail (+VCC=positive rail, -VSS=negative rail) and 4 for GND. See fig. 2 for details. Note that output GND is electrically isolated from mains and Earth. At lower output voltage settings, a minimum loading may be required for the supply to operate properly. Amplifier current draw should be enough to provide this load. (see: Setting output voltage)



The SAPS-400 power supply has two onboard auxiliary outputs, with common ground (isolated from main output GND) and regulated at ±15V. This auxiliary output can deliver up to 500 mA per rail. It can be very convenient for powering circuits such as controls, crossovers, active preamplifiers, thermal managers, etc. It can be also be used to power Class-D amplifiers modulators or driver stages as well, in order to reduce the power dissipated by them in no signal conditions. Figure 3 shows where the aux. Outputs connector is located:

Fig. 3: Auxiliary outputs (upper pin in the photo corresponds to "­15V", central pin is "Aux GND", lower pin is "+15V").


Output GND

+VCC out


FAN output


The SAPS-400 power supply has very high efficiency that makes cooling very easy. However, in cases where extreme operating conditions are feasible, or during max. continuous power tests, forced cooling is recommended. For these applications, this PSU has an onboard FAN output (12VDC fixed voltage, 120mA max). The approximate location of the connector is signalled in figure 3. Please observe polarity (marked in the board itself).

Place a wire between these points for 100-120VAC mains operation



SAPS-400 power supply can operate from a broad range of AC input voltages (100 to 240VAC in two ranges, 100-120VAC and 200240VAC). This PSU is set up to work with 200-240VAC input by default when it is shipped. If your country's mains voltage is between 100 and 120VAC, a wire must be connected from points A to B in the board (near the primary 200V capacitors). Note that maximum continuous power capability may be reduced slightly with this setting.

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Switching power supply for audio (revision B)




Fig. 5: Selection and adjustment of output voltage

SAPS-400 power supply, as standard, has a main output that can be adjusted in two ranges, between ±35V and ±60V by means of a small onboard trimmer potentiometer, see fig. 5. This figure also shows where the voltage selection jumpers are. The first range (jumper near the input side) allows adjustment from ±35V to ±54V approx. The second range (jumper near the output side) allows for ±53 to ±60V approx. Carefully turn the potentiometer while monitoring +VCC. The reading may take some time to get stable. There is a small positive temperature coefficient, so measure the final voltage after a couple of minutes from power on. Never exceed +60V to avoid danger of overvoltage in the output capacitors.


When the output voltage is lower than around ±55V, the supply requires a minimal load (30mA/rail, approx.) to operate correctly. There are no large ballast resistors onboard; this is done deliberately in order to avoid wasting power: once the amplifier is connected, its Voltage range selection jumper: idle current will be enough to provide this minimal load in most -Top position: ±35 to ±54V. cases. -Bottom position: ±53 to ±60V. If nothing is connected, the supply will stop operation after 1 minute or so (thermal protection of the startup circuit). It will take up to 3 Fine adjustment potentiometer (turn minutes for the PSU to start again. clockwise and gently to increase Vout) The best way to check whether the supply has the required load is by monitoring the voltage present in J8 jumper (placed at the left of the join-points for 100-120VAC selection): it should be between 11V and 11.8V with no signal at the amplifier's input, indicating that it has started properly. Use a header with insulated wires for this purpose. Don't touch them while measuring at J8. It is not isolated from mains input!




(All dimensions are in mm)


Total height is 41mm max due to components (transformer). Use M3 screws to fix the board to a chassis in order to provide enough cooling for continuous high power usage. The bottom has threaded inserts as shown in the photo above.

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Switching power supply for audio (revision B)




Startup and stop behaviours:

Startup with no load (+/-56V setting)

Startup with 320W DC load (+/-56V setting)

Power down with Coldamp BP4078 load connected (+/-58V setting)

It can be seen from these figures that operation is perfectly stable and that no overshoot is produced at start up or power down in all the load range.

Ripple voltage:

Ouput ripple voltage with 120W DC load (AC coupling) VCC only

Output ripple voltage with 320W DC load (AC coupling) both rails shown

The ripple voltage is very low. With a resistive DC load, it is smaller than 300mVp-p at full load.

Waveforms with 400W class-d amplifier module:

410W Output on 3.4ohm load, 1KHz. VCC rail shown.

410W Output on 3.4ohm load, 20Hz. VCC rail shown

As can be seen, the PSU is perfectly capable of continuous >400W output from DC to >20KHz, including very low frequency audio signals (20Hz). Note that, when using class-D amplifiers, a phenomena called "bus pumping" can occur with very large and low frequency signals (such as 410W/20Hz, shown at the right). This is not a problem but, anyway, it can be improved by adding some more capacitance at the output (up to 6800uF/rail). When two channels are connected, bus pumping can be automatically cancelled by inverting the input to one of them, and also the speaker polarity. This way one channel will cancel the bus pumping caused by the other.

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