Audio Precision APx525 audio analyzer review

When I got the fine folks at Audio Precision to help Jim Williams take a difficult measurement, Tom Williams, the VP of Marketing asked if I wanted to review their new APx525 audio analyzer. I didn’t feel qualified to test this advanced equipment, so I asked my friend Joe Curcio, an audiophile and application engineer at National Semiconductor to take a look at the box. I suspected Joe would be willing to do this, since I knew he owned his own personal AP System One, as well as having the latest lab equipment, the AP2722 at his lab at National Semi. If I understand it correctly, the AP2722 can measure lower distortion and noise. The APx525 is a high-end analyzer meant for R&D testing, but it has just slightly worse specs than the AP2722. So here is Joe’s review of the APx525:

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The Audio Precision APx525 review

A while back, I was contacted by Paul Rako from EDN to see if I had an interest in evaluating a new test analyzer from Audio Precision - the APx525. I know Paul well and he is well aware that I am one of those “audiophiles” who spend an irrational amount of time, energy and money on attempting to fool our ears into thinking that we are someplace we’re not.  That level of that irrationality is no better illustrated by the fact that I have invested in my own Dual Domain System One for my home lab despite the fact that I have access to a fully optioned AP2722 at work. So to Paul, I appeared to be a pretty good candidate.

After a few phone calls, Dan and Chris Gill, the nice folks at Audio Precision, agreed to meet me at my home and drop off their latest APx525 with a popular collection of options.  We quickly installed the software and drivers on my PC and then Chris went through a quick tutorial. The first big and pleasant surprise was that the Audio Precision interface card is no longer necessary - the APx525 actually connects directly to any USB port.

As Chris began going through the software, it was clear that this is not your father’s Audio Precision analyzer. The software has changed - a lot, and for the better.  With the System One / Two APWIN software, the user is presented with a battery of generators, meters, analyzers and more - not unlike a well equipped test bench. The APx525, while equally capable as the System One / Two, by contrast is all about getting to the task at hand. Once you connect your DUT (device under test) to the instrument, you set the reference levels and pick the test or tests desired and hit go. At that point, your PC begins to churn and soon your tests are completed. More importantly, all of your results are available for view and as part of a full printable report. The report output is a significant improvement over the prior models.  It is important to emphasize here that both analyzers are equally capable in providing amplitude, noise, phase, frequency, distortion and other measurements.  Both can give you almost any audio measurement imaginable as your familiarity grows with the interface.  They differ only in how they approach the task.

There is a huge upside to this, and a small downside. Being a glass half empty person, I’ll start with the downside in an illustrative way.

I enjoy the experience of driving and for me that feeling of one with the machine is significantly enhanced with a manual transmission. And although this is best enjoyed while traversing highway 9 in the Santa Cruz mountains, it is a whole different experience when inching along highway 280 at 5mph in rush hour traffic  - one’s left foot can soon become very tired. The point is that while it may be more involved to have those gear choices directly at your disposal, if the task at hand is just completing your to-do list, there is something to be said for that automatic transmission.  This is where the new user interface shines - getting to the task at hand quickly and thoroughly.

The software is not only functionally different than the APWIN software, but it is also much more refined.  You can visit Audio Precision’s web page and download a copy and run it in the demo mode. I actually did this for a while in the beginning when attempting to gain some familiarity “off line”. However, without an instrument physically connected, many of its capabilities are not functional. I would encourage Audio Precision to offer a demo version that responded as if an instrument was connected to a virtual DUT. This will be a great help to both new users and interested potential new customers.

From the Driver’s Seat

Those with experience using the System One or System Two will find the new “measurements oriented” user interface unfamiliar and perhaps a little uncomfortable. New users will find this workspace friendly and familiar, especially when framed in the standard Microsoft Windows outline. It is better to be less familiar with the earlier software so that old approaches won’t be so difficult to abandon.  This is only a minor quibble from one who has grown comfortable with the positioning of the controls in APWIN and mastered a few tricks and tweaks along the way.

As I mentioned earlier, the APWIN user interface presents a collection of state-of-the-art instruments (generators, analyzers, sweep data, meters and more) via Panels that drive the input(s) and monitor the outputs of the DUT. The generators are set to provide the levels, frequencies, filters, and other associated signal source settings. In the analyzer panel you set the channels to be monitored, ranges, bandwidth and other factors. Finally, going to the Sweep Panel (assuming that a sweep is your desired measurement), you identify the location of the desired data and what the signal source will be. Hit “Go” and your (single) measurement is executed.

• Figure 1 - APx-525 Control Panel

By contrast, the APx525 greets you with a Workspace with several regions (Figure 1). The most functionally central is the Measurement Navigator, where you set reference levels and select the measurements desired. The Graphs Panel, with its clever Selector (Filmstrip) is where the test results are displayed. New to the user will be the Monitors Panel, where you are provided a scope, FFT, and other instruments to display the input signal in real time. There is also the familiar collective of Microsoft Windows features like menu bars, toolbars, and status bars. The software also provides the standard file management tools like copy, paste, rename, and delete when appropriate.

The center ring, however, is the Measurement Navigator and the Selector. The Measurement Navigator is a Windows Explorer type of interface with menus and sub-menus arranged in a folder and tree like manner. Beyond the “project” branch is the Signal Path branch, from which the setup and measurements are defined. The top (non-removable) branches inform the system about the connections to the DUT (signal path setup) and signal references (Reference levels). Next is the collection of test measurement branches, where you select the tests to be performed. Specific test groups such as SNR (signal to noise ratio), THD+N (total harmonic distortion plus noise), crosstalk, frequncy response, can be selected and then further defined with sub-menus from each measurement.  The associated signal generator will appear to the right on the Measurements Settings panel when that test group is highlighted. Essential generators settings are kept simple and presented on the main layer, with extended functionality available but off-screen in deeper menu layers.

For example, if you will be selecting the Frequency Response measurement, you can then check (or uncheck) in the sub-menu the level, gain, relative level and deviation. The Measurement Settings panel to the right will display the associated generator settings, such as start and stop frequency, signal level and EQ (equalization). The “Advanced Settings” button is there if you need more control. I most commonly used the Stepped Frequency Sweep measurement, which offers sub-menu choices of level, gain, relative level, deviation, phase, THD ratio, THD level, THD+N level, THD+N ratio, Distortion Product Ratio and Distortion Product Level. 

You can then instruct the APx525 to run the single test group, or select additional test groups and then summon it to run the complete sequence. Hit go and take a sip of your coffee. Using your PC’s engine, the APx525 will begin to execute the full battery of tests you have selected. Once completed, all of the test results will be available in the Selector Filmstrip below the Graph Panel.  When you select any test result from the Filmstrip, it will be displayed in a larger view in the Graph Panel.  While viewing any measurement, you can set the pass/fail criteria either from a table of values or simply by placing markers on the measurement itself.

Up to this point I had not mentioned one of the most significant new additions, the report generator. At the bottom of each “project” tree, you will find the report branch available for selection. If you select the report tab in the measurement navigator panel, a report showing all of the test data will pop up for your review, including a pass-fail summary for each measurement. You can review the report on-screen or print it out in a PDF file. I normally save to PDF and send the report summary to my customer. The report can be personalized with your company header and even further refined in formant using Microsoft Windows. From my perspective, this batch test and report summary is one of the main improvements over APWIN. The usefulness of a complete report summary one click away cannot be overstated. This is especially true when your goal is to characterize the complete performance portfolio of a device and present the information to your customer. 

While the Measurement Navigator is clearly the center stage, another useful feature is the Monitors panel. The Monitors panel displays signals from the DUT in the form of a scope, FFT (fast-Fourier transform) analyzer and tabular data, among others. The responsiveness of these monitors is very dependent on the processing power of the associated PC and in my case, their slow response time made them very difficult to use, especially when wanting to see the impact of making changes in the DUT that otherwise would have been instantaneous using an external DSO (digital storage oscilloscope). The Monitors panel is nice to have, and as your PC power improves, so will its usefulness. However, I would encourage Audio Precision to add physical outputs to the next release of the APx500 to match the System One & Two.

It is important to note that while not an expert, I am reasonably well versed with the APWIN based System One and System Two analyzers. In contrast, my time with the APx525 is less than 20 hours and therefore the more I use it, the more I discover about both its capabilities and its personality. Accordingly, this review speaks only to my experiences to date, clearly only a fraction of its capabilities. As I go through this process and when compared to my experience with the System  One & Two, there is one perspective that must be shared.  While in reality the APx525 is a very well-executed piece of lab gear, for me it goes a little beyond that. It is something that I look forward to using, in a word, fun.  After eight hours of “play” the wow factor has yet to diminish, and while I am sure it will, for now I find myself looking forward to running analyses and especially to providing a full report.   

Test Drives

To get an idea of the APx525 flexibility, I have included a few projects using gear I had available at the time. Note that these samples are purely illustrative of some of only a small segment of its capabilities. Covering its extensive range of capabilities would require more time and word count than budgeted. I suggest that you download the manual from AP’s web page to get a much better understanding of the capabilities of this instrument.

1. Customer unit characterization

I provide a restoration service for selected premium vintage audio equipment. As part of the service, the unit is analyzed upon arrival and the report is presented to the customer with a menu of recommendations for remediation. Once the work is completed, a final analysis, using the same project file, is run and provided to the customer as an outgoing certification. The APx525 is ideal for this purpose, providing the customer with a detailed quantitative analysis of their individual unit, including all of performance metrics compared against the original spec with pass/fail highlighted.  The performance certification is highly valued by customers who will be retaining the equipment or offering it up for sale. Since this is a frequent task, I have created a library of pre-defined projects that I use for a specific equipment type (amplifier, preamplifier, processor, etc).  

2. RIAA (Recording Industry Association of America) preamp design characterization

I had authored an application tech note (AN-1651) for National Semiconductor using their ultra-low distortion op amp (LME49710). The tech note describes several applications including a RIAA phono pre-amplifier. The preamp performance was originally characterized using our System 2722 and so I wanted to run another analysis, but this time using the APx525.

• Figure 2 - APx-525 Project Execution in Progress

Figure 2 shows a screen capture of the analysis in progress, including the Measurement Navigator setup, the Monitors panel, and the Graph Panel for this project.  This is another example of the APx525 software at its best - where you require a full characterization of the device or circuit. The final report or any if the individual test results can be captured and included in the tech note. This can be done initially to characterize the first design iteration and again after any changes or improvements have been made.

3. Vacuum tube driver stage design

Recently I wanted to optimize a push-pull tube amplifier driver stage consisting of a cascode gain stage followed by a split-load phase inverter. This design had originally been designed using PSpice and then built and optimized using a System Two with APWIN. I wanted to repeat this design process but this time using the APx525 to optimize the design. The goal is to characterize circuit performance variations in response to circuit operating point changes and to understand the comparative impact of these changes.  When used for this purpose, the APx525 limitations surfaced. I ran each analysis and appended the subsequent runs as circuit parameters were changed. 

• Figure 3 - APx-525 Comparative Analysis

Each subsequent measurement appears as the same color, making them difficult to differentiate (Figure 3). Although it is possible to label each measurement, as the number of measurements increased, labeling loses its usefulness.  APWIN based instruments provide each appended measurement with its own color and associated label below - this is a preferable approach. I spoke with Tom at AP and he informs me that this is planned for a future revision.

Conclusion:

The APx525 is an outstanding instrument, easily capable of bearing the Audio Precision insignia. It no longer requires the proprietary Audio Precision interface, evolving instead to USB - a welcome change and great improvement. Its revised software / user interface is dramatically different and improved over APWIN, with a direct focus on executing measurements vs. setting instruments and analyzers.  The designers need to be complimented in that the improvement in productivity has compromised neither flexibility nor capability. The top menus have been thoughtfully designed to provide the most common selections, while deeper menus are there to provide you with added power when needed.

The new batch test capability is a huge productivity enhancement, especially when combined with the ease of use afforded by the software interface. New users will quickly be engaged in collecting data, rather than be challenged with a moderate learning curve. Experienced AP users may initially feel like they have lost capability, but will soon realize that not only is it retained, but they will soon grow to appreciate the additional productivity of this approach.

The response of the instrument is very dependent on the power of the associated PC. Slower machines will result in perceptible lag time - not a big issue, but nonetheless real. With my system (Dell with Intel Core 2 Duo @ 2GHz & 1G RAM), it was barely noticeable except in the Monitors Panel. To that end, an external connection would be a welcome addition.

The other main value adder is the report generator. Once the desired measurement data has been selected and the run executed, the system delivers a custom and comprehensive time stamped performance report. This report can be used as the central document to discuss initial findings, propose recommended changes for improvement, and report on and identify the impact of performance enhancements. It can identify performance issues at the sub-system level so that you can easily pinpoint areas of the design that require attention. The only change that I would request would be to allow for multiple colors for each appended run.  This omission is noticeable only in comparison with the significant capability that this unit offers.

For me, with both my System One and the APx525 instruments sitting on the bench, and with a backlog of work, which instrument did I consistently select to use? The answer was the APx525. And so, if forced to choose only one to live with, it would be the APx525 - that automatic transmission is still fun to drive and just gets the job done.

Joe Curcio is an audio enthusiast with over 21 years of experience as an analog engineer and systems integrator. He has provided consultation services to several high-end audio companies and published white papers, app notes and magazine articles. Currently he is serving as Principal Field Application Engineer (strategic account assignment) with National Semiconductor, Santa Clara CA.