Zalman ZM400B PSU

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For a complete rundown of testing equipment and procedures, please refer to the article SPCR's Revised PSU Testing System. (It was modified again since first published; there is now a exhaust vent 80mm fan, and the PSU Load Tester's fans are running at 5V, not 12.)

Measure temperature of ambient air, case, and PSU exhaust
Digital readout thermometer. There are several in the lab that are used. (Like DigiDoc.) They measure within ~1°C of each other, which is good enough for our purposes. Powered by the PSU being tested. (1~2W power draw.)
Measure voltages across fans and DC output line
Heath / Zenith SM-2320 and Circuit-Test DMR2208 multimeters. These are ordinary multimeters that have been compared against a much more expensive lab instrument and comes very close (within 2%) on readings of 0~20VDC. Plenty good enough for our purposes.
Load PSU to specific DC output power loads for each voltage line

DBS-2100 PSU Load Tester. Made specifically for testing computer power supplies, it consists of a large bank of high power precision resistors along with an extensive selection of switches on the front panel calibrated in Amps (current) and grouped into 6 voltage lines: +5, +12, -12V, +3.3, -5, +5SR. Leads from the PSU plug into the front panel, and there are taps for taking voltage readings for the 3.3V, 5V and 12V lines. It is capable of loading a PSU to over 600W DC power output.

Measure AC power, power factor (PF), VA, AC line voltage
Kill-A-Watt Power Meter. An inexpensive consumer power meter with very good accuracy and a host of useful functions.
Measure noise in dBA from 1 meter distance B&K model 2204 sound level meter. This professional caliber SLM dates back to 1978, weighs over 10 pounds, and is completely analog in design. It has a dynamic range that spans over 140 dB. The unit's absolute sensitivity reaches below 0 dBA. A quiet environment is a prerequisite to low noise testing; the lab has been measured down to 16~17 dBA at night. A 12 dBA adjacent room is also available for PSUs that are quieter.
Thermal environment directly related to the power delivered. Custom Thermal Simulation Box. A NEW ADDITION to the lab, it ensures that the heat generated by the PSU during testing actually creates the thermal ambient of its working environment. So the temperature seen by the PSU is directly tied to how much power it generates. This is probably the most thermally realistic testing rig for PC power supplies used by any testing lab. Please see SPCR's Revised PSU Testing System for full details!

Normally, the voltage line to the fan is accessible from the inside, but not on this unit. The wiring was just too packed around the fan controller circuit. So, an alternate method was used. A pin on a wire was used to penetrate the insulation of the positive lead at the fan grill ° much like a hypodermic needle. It allowed the (+) voltage to the fan to be accessed; the (-) voltage could be tapped at any ground terminal of the PSU so that the fan voltage could be monitored.

The testing was conducted in the "sound lab", a 20' x 10' x 8'(ceiling) carpeted den with heavy drapes on windows across one of the short walls. Acoustics are fairly well damped. Ambient conditions during testing were 22°C and 15 dBA, with input of 120VAC at 60 Hz.

Zalman ZM400B-APS
DC Output (W)
AC Input (W)
Case Temp (°C)
PSU Exhaust (°C)
Fan Voltage
Noise (dBA/1m)


A NOTE on Voltage Regulation

VR is an easy test for any good PSU to pass as long as it is operating within its power output limits. Many will be within 1% accurate. The variances commonly found in reports from motherboard voltage sensors (such as provided by Motherboard Monitor) are not at all useful to test power supplies. The problem is that those readings are the sum of interactions between the motherboard, its power circuitry, the connectors and cables between the board and the PSU, and the PSU itself.

The output voltage of the PSU must be monitored in isolation from external influences while it is doing work (delivering current). The only practical way to do this is to use a voltmeter (multimeter) to check the voltage across the terminals to which the power is being delivered.

In SPCR's VR testing, a multimeter is connected to each of the voltage lines for several minutes. The voltage reading is monitored continuously while the loads on each and all the lines is varied, and the peaks and valleys recorded manually.

1. VOLTAGE REGULATION is excellent, within -/+2% on all lines in any nominal combination of loads. The low and high voltage seen on each of the main lines is shown:

  • +12V: 11.7 to 12.3
  • +5V: 4.75 to 5.2
  • +3.3V: 3.3 to 3.5

2. AC : DC Conversion EFFICIENCY is fairly high in the mid-70s. While the maximum of 76% reached does not match the Seasonic Super series models or the recently reviewed Enermax Noisetaker 475, it is higher than most PSUs.

3. POWER OUTPUT: The unit ran with very good stability at all output levels. Even at 400W output, it ran for a full 5 minutes without overheating or overload.

4. POWER FACTOR is excellent as expected for an Active PFC unit, staying at around 0.97 throughout the testing.

5. FAN VOLTAGE: The start voltage is ~4.7V. In the thermal simulation box, the fan voltage follows the temperature fairly closely. The fan speed-up curve is not very steep, risingly only a little over 3V as the temperature rose from 30°C to 40°C. Surprisingly, the maximum voltage did not reach past 10V, even at full power.

6. NOISE was measured at 1 meter from the exhaust grill, with both the SLM and the PSU at least 1 meter from walls. All other noise sources (inlcuding the fans in the test setup) were turned off during measurements and listening.

Subjectively, the Zalman ZM400B-APS is very quiet at startup and at low power. By the time the 150W mark is reached, however, it is probably loud enough to become the primary noise source in a very quiet PC. The noise continues rising steadily all the way to the maximum output.

The fan sounds a bit clickety at low speed. It is not as clattery as the fans on the recently reviewed Enermax NoiseTaker 475, however. This bearing or commutator firing noise fades beneath the usual whoosh of wind turbulence as the fan speeds up. There is no high frequency component from the fans or the electronics. (Coil noise is often the result of interactions between components, however, so the absence of this noise in the lab does not ensure its absence when the PSU is connected to PC components.)

A CAUTION: The noise-to-power performance achieved here is specific to prevailing test temperatures. In other words, if your ambient temperature is 35°C and your case temperature measures 45°C, you can expect higher noise, regardless of the electrical load.


The Zalman ZM400B-APS is a solid all-around performer that is very quiet at lower power levels and moderately quiet beyondt. While none of the measured parameters is outstanding, they are all very good. The current capability of the 12V line is high enough to handle all but the most loaded systems, and the high efficiency ensures the PSU stays reasonably cool throughout.

The stability even at full power output with the exhaust temperature at 49°C tends to substantiate Zalman's claim that the unit is capable of safe operation to 50°C.

Those who seek the lowest noise will want very good case airflow to minimize the amount of heat this PSU sucks in from the hot CPU area, because the fan controller tends to spin up the fan at a relatively low temperature (and power) level, especially considering the high temperature capability of the unt..

Much thanks to Zalman USA and to Sharka Corp. for the Zalman ZM400B-APS review sample and for their kind support.

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Zalman power supplies are also available at Silicon Acoustics and EndPCNoise.

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