Live from the show Floor
Today at the AES Europe Convention in Hammersmith, London, Audio Precision unveiled its latest audio analyzer: the APx515.
The APx515 is a production test audio analyzer: it's fast, has a small footprint, and is very low cost—all without sacrificing AP performance or functionality. It uses the same software as the APx525 and APx585 Families, making it very easy to share projects between R&D and production, and then to compare results.
Reaction has been very positive here at the show. We created a demo that incorporates a start/stop footswitch, pass/fail light pole, barcode scanner, and infrared interface—all controlled by the APx software without any programming. Several engineers agreed with our proposition that being able to change tests without a developer on hand, and knowing that everyone involved with a product can trust their results, has immense value.
To mark the occasion, press and customers (plus a couple of new friends) joined AP in a champagne toast.
AP staff, customers and members of the press enjoy a “515”. Note the production test theme.
Read on below for more information about the APx515 and its place in the APx line of best-in-class audio analyzers. AP Sales Partners have demo gear, so if you'd like to see the APx515 in action, don't hesitate to call.
Output: AP introduces new production test audio analyzer
You can read more about the APx515 on the AP website at http://ap.com/products/apx515 or by downloading the datasheet. You may also be interested in the latest AP Applied, which looks at calculating the ROI of audio production test.
For a top level view, here's a photo and the official press release:
AUDIO PRECISION REDEFINES BEST IN CLASS FOR PRODUCTION TEST
AES Convention, London, May 24, 2010: Audio Precision, the recognized standard in audio test and measurement, today introduced the APx515, a new two‑channel audio analyzer optimized for production test, starting at only $6200 in the US.
Despite its low cost, the APx515 still has excellent performance, with a typical THD+N of -106 dB, 24‑bit FFTs, and 192k digital I/O. With respect to the other instruments in the APx Series, it sits just below the APx525, whose lower THD+N and chip-level connectivity options make it more appropriate for R&D.
Comprehensive production test in 3 seconds, easy automation AND low cost
Trusted results between vendors, designers, and manufacturers
The right instrument for the job
“AP is already the recognized standard in the R&D world,” says Bruce Hofer, co‑founder and head analog engineer at Audio Precision. “APx515 adds another best-in-class analyzer, this time at the production test price point. So now, manufacturers can rely on AP on the factory floor, the same as they do on the engineer’s bench.”
Price & Availability
Sound Advice: AP Knowledge Base
Measuring Output Impedance with APx500
Measuring the output impedance of line and power amplifiers, microphones, and other audio equipment can be easily done with the new APx Output Impedance Measurement Utility, created by AP's Director of Technical Support Joe Begin. It joins our other impedance measurement utilities for APx, the Loudspeaker Output Impedance Measurement Utility, and the Damping Factor Measurement Utility.
The output impedance of a device can easily be calculated by measuring its output voltage with and without a known load attached. Consider the loaded and unloaded circuits below, where RO represents the amplifier’s output impedance, RL represents its rated load impedance, VNL is the voltage measured with no load, and VL is the voltage measured under load. Note that for the purposes of this measurement, RO is assumed to be totally resistive, and RL is a non-inductive load resistor.
Fig 1 Schematic of test setup.
The load resistance acts as a voltage divider, such that
The above equation can be rearranged to yield
The amplifier’s output impedance can vary with frequency, and therefore it may be desirable to measure output impedance as a function of frequency. This can be done by conducting sweeps of the loaded and unloaded amplifier circuit and calculating the impedance at each step in the sweep.
For the APx525 Family audio analyzers (including the APx520, 521, 525 and 526), the selectable 300 Ω and 600 Ω input termination resistors or an external resistor may be used as the load. The APx585 and 586 instruments require use of an external load.
For accurate results, the ratio of the load resistance to the actual output impedance of the device must not be excessive. For most line level audio devices, a value of 600 Ω or 1 kΩ works well.
This technique is not suitable for measuring the extremely low (milliohm level) output impedance at the pins of a typical op-amp. Most op-amps can not drive loads of less than 600 Ω, and using a 600 Ω load would lead to highly inaccurate results. In typical audio devices, however, there is often a 47 or 100 Ω isolation resistor after an op-amp’s output, so making measurements at the output jacks is not a problem.
An interesting case is measurement of microphone output impedance. Substituting for the typical DUT is a power amplifier, an acoustic chamber housing a speaker and the microphone under test, and a phantom power supply (if required by the mic). A 1.2 kΩ load resistor is chosen to match the load impedance that a professional microphone is designed to drive. For some microphones, a test head is available that replaces the screw-on capsule and allows the audio analyzer’s generator to make a direct electrical connection. When measuring unbalanced condenser computer microphones, it will be necessary to supply the microphone with plug-in power on its signal lead.
For accurate results, the speaker must be able to cover the frequency range being measured, and the chamber must not introduce reflections and standing waves that null out the signal. Figure 2 shows the measured output impedance of a dynamic microphone. In this case, a small powered monitor speaker was used to create the acoustic signal, and the speaker and microphone were housed in a small wooden box. This poor (especially at low frequencies) acoustic environment, probably explains the erratic fluctuations in the impedance curve at frequencies below about 70 Hz.
Fig 2 Impedance plot of a dynamic microphone.
Power amplifier output impedance should be measured using the smallest possible load, in order to obtain the most accurate results. This is usually 4 or 8 Ω for consumer amps, and 2 or 4 Ω for professional amps. For hookup details, see the knowledge base article Measuring Amplifier Damping Factor with APx500. Damping factor is really just a measurement of output impedance, stated in the form DF = RL / RO.
APx500 Output Impedance Measurement Utility
Fig 3 APx Output Impedance Measurement Utility.
The APx Output Impedance Measurement Utility simplifies the procedure above by calculating and graphing the results. When the utility is run, it opens the APx500 software if it isn't already running.
The Load Project button on the front panel allows you to load an APx500 project file, and defaults to the included Output_Impedance.approjx project. The Measurement selector allows you to choose the measurement from among the valid possibilities in the currently loaded project. An output impedance measurement may be based on any of the measurements used to conduct a level versus frequency measurement, (i.e. Frequency Response, Continuous Sweep, Stepped Frequency Sweep, or Acoustic Response). For measuring the output impedance of a microphone, the Acoustic Response measurement works best, because you can use fractional octave smoothing to smooth the data, and multiple averages to improve signal to noise ratio.
Now, set the load resistance. For the APx525 Family, either set the Termination Load control as desired to use the internal load resistors, or fill in the R-ext text box with the external load resistance that will be attached (the Termination Load control will automatically be disabled in this case). For the APx585 Family, fill in R-ext to reflect the external load that will be used.
Connect the APx generator outputs to the DUT, and connect the DUT outputs to the APx analyzer inputs. Go to the selected measurement in the APx navigator and set the signal level as appropriate. It should be well above the noise floor of the device, but below the clipping level.
Click Start on the utility to begin the measurement. If you are using an APx525 Family instrument without an external resistor, the utility automatically runs the selected measurement twice: once with the maximum input termination of 100 kΩ (unbalanced input) or 200 kΩ (balanced input), and once with the input termination switched to the 300 Ω or 600 Ω Termination Load as selected above. If you are using an APx585 or 586, or an APx525 Family instrument with an external load, you will be prompted to remove the load resistor for the first measurement and insert it for the second measurement. When the measurements are complete, the utility displays the measured output impedance versus frequency (Figure 4).
Fig 4 APx Output Impedance Measurement Utility results.
With an APx525 Family instrument, it’s easy to check the utility. Simply use an external loopback cable between the analog output and the analog input (either unbalanced or balanced), and start the measurement. The results should be equal to the output impedance set in the APx Signal Path Setup.
Fig 5 APx525 Print Results dialog.
For a hard copy of the results, click the Print Results button. This opens a Print Results dialog window, which includes various test details and a note field where you can add an annotation, if desired. Click the Print Window button to print this results window to the system default printer, or the Save Image button, to save an image of the widow to a graphics file. The Export Data button allows you to save the data in tab-delimited text format, which can be opened by typical spreadsheet programs like Microsoft Excel.
APx Output Impedance Measurement Utility
Test Results: AP News & Events
AP introduces the APx515: Press release
©2010 Audio Precision Inc.