Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision
Before we get started this month, I'd like to extend AP's best wishes and hopes to our friends and customers in Japan. I have been to Japan many times over the past thirty years, most recently just last month, and I am scheduled to go back again in June. We have been in contact with our Japanese distributor TOYO since the Friday afternoon earthquake on March 11. Their main offices are located in the downtown financial district of Tokyo. They reported violent shaking within their two 9-story buildings, but fortunately no injuries or major structural damage. I'm sure you will join me in wishing everyone in Japan our best.
Here in the US, most of the company is focused on the next release of APx500 software and some accompanying new hardware due in June. I’ve also been helping our Director of Products and Technical Support, Joe Begin, prepare for a series of seminars he will be giving in China and Korea on testing Class D amplifiers, a subject that is very close to my heart. Finally, I’ve enjoyed watching a series of online videos we just posted on the website that looks at the big six audio tests, what they measure, why they’re important, and how to actually make the measurement with an analyzer. I think they will serve as an excellent primer for engineers and others just getting started with audio.
Output: Measuring Phantom Power with APx
This month's article on measuring phantom power is by Audio.TST editor Adam Liberman. It is also available as Technote 114: Measuring Phantom Power. We've also added his article on Testing Broadcast Audio Consoles, previously published in Broadcast Engineering, to the downloads area at AP.com.
Microphone preamplifiers, both professional and consumer, often supply a DC voltage back over the same wires that carry the audio, in order to power the circuitry in condenser microphones. Often, when testing microphone preamplifiers, this DC voltage is either just turned off or ignored. Therefore, the tests are not being done under actual operating conditions.
Indeed, preamplifier performance should be the same with or without the DC power applied. However, it would valuable to verify this and not to take it for granted. Additionally, it would be valuable to verify that the power is working correctly, and that the power is not being altered by the audio signal. And finally, it would be nice to be able to do all this without needing to re-cable or use additional test equipment.
We’ve constructed a test fixture that allows us to do all that, along with an APx project to go with it. The fixture is a one-of-a-kind device, but you can make one for yourself with the schematic and details that follow.
Note that attaching the generator output of an Audio Precision analyzer directly to a 48 VDC phantom powered microphone input without the fixture will not cause damage to the generator, although its low output impedance will bring the phantom voltage down to about 0.5 VDC at the microphone jack. This might be an issue if the 14 mA load on the phantom supply affects the audio power supply in the DUT (device under test). The exception is the 2700 Series of audio analyzers, when set to Balanced Floating output. In this configuration, the output transformer is ungrounded and no phantom current flows. Although we focus on the APx Series in this article, the 2700 Series can perform these phantom power measurements as well. Use the Digital Analyzer in the 2700 to make DC-only measurements.
Fig 1 Custom-built phantom power test fixture (not available for sale, but you can build your own).
Microphone preamplifiers include stand-alone devices, audio mixers, and the input circuitry incorporated into any device that incorporates or accepts a connection to a microphone. In balanced connections, found on professional and semi-professional equipment, the most common condenser microphone powering system is 48 volt phantom power.1 The fixture we’ve constructed is designed to test this type of power. Unbalanced connections, found in consumer equipment, often use “Plug-in power” (PiP) or similar variations. The principles and techniques discussed here can be adapted to it as well.
Careful attention to construction and component quality is necessary to avoid sensitivity to EMI. The accompanying projects include sections that test preamplifier noise performance with and without the fixture. If any difference in results is observed, then the cause should be rectified. If it can’t be, then the most sensitive noise measurements should be done without the test fixture.
Fig 2 Phantom power test fixture schematic.
The test fixture has three main features:
A few additional components are also present. Resistors RC compensate for the effect of load resistors RL, to maintain the rated generator output impedance. RC also serves to add additional output impedance to APx generators that do not have the desired output impedance setting. And, discharge resistors RD are added to provide a path to ground for the phantom voltage when the generator is not connected to the fixture. The value of RD may be lowered somewhat if faster discharge is desired.
Table 1 Impedance settings and RC value.
By adding a switch, you could provide a choice of different loads, or remove the load completely. For a greater degree of automation, the switch could be replaced with relays, which could easily be controlled by the APx500 sequencer using the AUX OUT port on the rear panel of APx instruments.
Fig 3 The APx sequencer Aux Control Out step controls the AUX OUT port on APx analyzers.
Note that CB and RL form an RC circuit, which will roll off low frequencies. The value chosen for CB is large enough to keep response extremely flat at 20 Hz, and to not interact with the RC filter internal to most microphone preamps to block the phantom power from reaching the input stage.
When measuring microphone preamplifiers, the source impedance of the audio generator must be carefully set, or else the noise measurements will be invalid. This is for two reasons:
1) the thermal noise produced across the source impedance determines the lowest noise level that can be achieved, and
2) input circuit noise performance is affected by source impedance.
The standard source impedance with which balanced microphone preamplifiers are tested is 150 Ω, so at a minimum, this source impedance should be used to produce or verify specifications. However, an increasing number of condenser microphones, especially those with ultra-low self-noise (3 – 14 dBSPL), have very low output impedances on the order of 25 to 50 Ω. Therefore, testing preamplifiers using a 50 Ω source impedance can give additional real-world performance data.
Unbalanced microphone inputs are tested with either a 150 or 600 Ω source. With semi-pro gear, such as portable digital recorders, unbalanced inputs are normally tested at 150 Ω, while consumer grade gear such as computer sound cards are often tested at 600 Ω. If the preamplifier under test is to be used with a dedicated built-in microphone, then the source impedance should be set to mimic the actual operating conditions.
Measuring 48 Volt Phantom Power
Phantom power is fed onto the audio conductors through two 6.8 kΩ resistors, which limit its current and prevent it from loading down the attached microphone. The APx project that accompanies this article conducts a number of tests to verify that it is working correctly.
The phantom power with no load should measure 48.0 VDC (± 4.0 VDC). Some budget devices are labeled as capable of supplying phantom power, but use non-standard voltages that do not meet the IEC P48 phantom specification. These devices often derive microphone power from an available power rail inside the unit, and do not have a dedicated 48 volt supply.
Below are the results of testing two different microphone preamps, using the APx project we created. With the APx526, 585, or 586, we can see all four channels of interest at once. A slightly different version of the project is provided for the two-channel APx525 family and the APx515.
The phantom voltage in Figures 4 and 5 is measured at a spare microphone input channel. Even though we are measuring through the 6.8 kΩ resistors, the high input impedance of the analyzer ensures that the voltage drop across them will be negligible. If the DUT has only one microphone input channel, then this measurement can be made on it by removing the test fixture or removing the load. As can be seen, the first preamplifier meets specifications IEC, but the second one doesn’t.
Fig 4 Preamplifier #1: Phantom measured at a spare microphone input channel (no load).
Fig 5 Preamplifier #2: Phantom measured at a spare microphone input channel (no load).
We now add a 10 mA load by connecting the test fixture. The display shows the phantom voltage measured as above, along with the voltages measured at the test fixture on the balanced audio lines of the preamplifier input channel under test. When drawing 10 mA from the phantom supply, the voltage measured on the audio lines (after the drop across the 6.8 kΩ phantom power resistors) should be 14.0 VDC.
Fig 6 Preamplifier #1: Phantom measured at the spare microphone input channel (red, bar 2), and phantom measured on the audio conductors of the input channel under test (brown and green, bars 3 & 4).
Fig 7 Preamplifier #2: Phantom measured at the spare microphone input channel (red, bar 2), and phantom measured on the audio conductors of the input channel under test (brown and green, bars 3 & 4).
In this test, the first DUT passes, but the second one fails badly. We can see that not only is the voltage low overall, but the phantom supply itself (measured at the unused spare microphone input) has dropped from 37.44 to 17.96 VDC under the 10 mA load. This test also lets us check that both pin 2 (HI) and pin 3 (LO) of the audio cable are supplying the same voltage. A difference between them might indicate a fault in one of the 6.8 kΩ resistors or its associated signal path.
Additional measurements in the project allow us to check if there are any changes in the phantom voltage with or without audio signal, and if there are any changes in audio performance when the phantom power is turned on or off. The projects use a gain setting of 70 dB, with the intent that this will make differences more visible than at a lower setting. It is not designed to be a comprehensive microphone preamplifier audio performance test, which would require that tests be conducted over a full range of gain settings.
Note: Technote 114 includes further information on running the project, and has the wiring diagram for the special cable used to measure phantom voltage on the spare microphone input.
1Other balanced audio powering schemes include 12 and 24 volt phantom, T-power or AB power (obsolete), and AES 42‑DPP 10 volt digital audio phantom.
Sound Advice: Measuring PSRR with AP2700
By Audio Precision sales representative Steve Peterson. This article describes measurement of PSRR using the capacitor coupling method. An alternate method, using transformer coupling, is described in Technote 106: Measuring Power Supply Rejection Ratio (PSRR). AP's Director of Products and Technical Support, Joe Begin, will be demonstrating this method as part of the upcoming AP seminars in China and South Korea (see Events below).
Power Supply Rejection Ratio (PSRR) is a measure of a device’s ability to reject noise from the supply used to power it. It is defined as the ratio of the change in supply voltage to the corresponding change in output voltage of the device. It is often desirable to measure PSRR over a range of frequencies and to produce a spectrum plot of PSRR versus test signal frequency. PSRR is often expressed in dB, where ΔVin is the change in voltage input and ΔVout is the change in voltage output (see equation below). However, due to lack of standardization, the ratio is sometimes inverted, and the value in dB is sometimes expressed as a negative number. Some datasheets also refer to AC (as opposed to DC) PSRR measurements as kSVR.
PSRR measurements are typically made for ICs and other functional assemblies. To measure such a device’s power supply rejection, we need to insert an AC signal (ΔVin) onto the DC voltage from a power supply and examine the device’s output (ΔVout) for the presence of the signal. The AC signal generator, in this case an AP 2700 Series instrument, can be coupled to the power supply rail with either a capacitor or a transformer.
Capacitive coupling is simpler to do, and is discussed below. Its major limitation is that because the generator is loaded down by the output impedance of the DC power supply, it is necessary to insert a series resistor into the power rail. You need to be careful not to increase the output voltage or decrease the load so much that the maximum output current of the generator is exceeded. You also need to verify that excessive load is not causing distortion to increase, which would invalidate the readings. If you need to supply larger amounts of current, you can use the transformer coupling method instead, which is discussed in Technote 106: Measuring Power Supply Rejection Ratio (PSRR).
The graph below shows PSRR versus frequency for a class D amplifier used in mobile devices, with appended traces for different rail voltages. You may download the .at27 test file and sweep file that we used to make it from our website.
RSVR is chosen to maintain an adequate voltage at the node with CSVR, and to prevent excessive load on the generator. However, if it is too high, it will reduce the “stiffness” of the DC supply (its ability to maintain a constant voltage) at the device under test. A good compromise is 20 Ω—this will cause a 6 dB drop in generator output when its source impedance is set to 20 Ω.
DC blocking capacitor CSVR prevents the AP 2722’s generator from loading down the power supply. Since CSVR and RSVR form an RC filter, CSVR should be chosen so that the lowest frequency of interest is not rolled off too much. Choosing a 330 µF electrolytic capacitor for CSVR, along with a 20 Ω resistor for RSVR, creates a high-pass filter with a 24 Hz corner frequency. The regulation feature in AP2700 will compensate for this roll-off when we make the measurements.
The Analog Generator is set up as shown below:
Analyzer channel A is connected across the power-supply pins of the DUT, to measure the incoming AC stimulus. Channel B is connected across the DUT’s speaker terminals, to measure any change in output. Since the DUT in this case is a class D amplifier, an AP AUX-0025 Switching Amplifier Measurement Filter is inserted before the analyzer inputs to minimize the high-frequency switching frequency.
The Analog Analyzer Bandpass function meter is used to measure the amplitude of the ripple created by the analog generator.
The Digital Analyzer Crosstalk function is used to find the ratio between the Channel A stimulus and the Channel B DUT output. The crosstalk measurement utilizes a 1/13th octave bandpass filter to eliminate broadband noise from the reading.
Regulation is used at each step to keep the ripple magnitude constant at all frequencies.
Set the Sweep panel as shown. In this example, we use a 1/6th octave sweep table, but you can use whatever interval (number of steps) that you want.
Ground the inputs to the DUT. Turn on the DC power supply and set it for the correct DC voltage. Then, run the sweep and watch the plots being created.
Test Results: AP News & Events
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