Audio Precision System compatible USB adapter

The Audio Precision System One and Two instruments are very capable audio analyzers, especially for analog work. However they are missing modern PC connectivity and the control software APWIN does not run on modern Windows systems.

This APIB (Audio Precision Interface Bus) compatible USB adapter provides a simple and cost effective way to use your older System One, System Two, or System Two Cascade models on modern Microsoft Windows systems.

With this USB adapter, the APWIN control software and Visual BASIC scripts runs flawlessly – without a virtual machine or emulator – on modern Windows systems.

The USB adapter works by replacing the APWIN dynamic link library for low level bus communication.

The new library redirects the low level bus communication to the standard Microsoft WinUSB driver, which is built-in on newer Windows systems.

By replacing the proprietary APWIN driver with the standard WinUSB driver, the application runs flawlessly on modern Windows version.

The communication speed is slightly slower than with the original Audio Precision bus driver. This is due to the communication protocol which is not designed for USB and the conservative timings.

For best performance a modern USB 2.0 High-Speed (480 Mbit/s) capable host controller should be used.


The Audio Precision compatible USB adapter is tested with APWIN 2.24 (Build 1390).

Older versions of the APWIN control software may work too, but are not tested. Newer APWIN version do not work.

All Audio Precision instrument hardware that is supported by APWIN 2.24 will function with the adapter:

The following operation systems are supported:

Ordering information

The price of the adapter is 300 € excluding taxes and shipping.

Please send email with your shipping address to if you want to order, or if you need information.

For US customers: please contact Duke Aguiar at Duke is a very helpful former AP engineer with lot of experience in servicing and troubleshooting AP systems.

Payment can be done via Paypal or bank transfer and should be done before shipping the adapter.

If the adapter does not meet the expectation it can be returned within four weeks.

The standard USB cable length is 2 meters. It is possible to change the cable length to 3 or 5 meters for a small charge.

EU customers without a VAT-ID number (UID or Umsatzsteueridentifikationsnummer in German) need to pay 20% taxes.

Customers outside of the EU are responsible for payment of possible import taxes and custom duties.

Software Installation

All installation files with manuals and self check procedures can be downloaded from For installation of the adapter you only need the files which are linked in the instructions below.

First: Install the Audio Precision APWIN 2.24 control software, if it is not already installed, using an Administrator account. Adjust User Account Control (UAC) to the lowest level, install and return to the previous level. Some Anti-virus programs may prevent Installshield from finishing the installation on newer Windows systems. If the installation hangs, reboot Windows in Safe Mode and restart the APWIN software setup. Installing different AP software packages (like ATS-2 and APWIN) can cause trouble because of shared environment variables. Typically the Icons for the measurement scripts (like "Analog Frequency Response") do not work as the PATH is incorrect. If you see blurred fonts try the program compatibility options. Right click on APWIN.EXE and click on "Properties". Set the "Overwrite high DPI scaling behavior" to "Application" (Do not set the "Run this program in compatibility mode" option).

Second: Download the new low level bus communication DLL apio.dll from Overwrite the original apio.dll in the APWIN installation directory with the apio.dll from the zip file (make a backup of the original apio.dll first). The apio.dll is typically installed in "C:\Program Files\Audio Precision\Apwin200" or "C:\Program Files (x86)\Audio Precision\Apwin200".

Third: Install the Microsoft WinUSB driver.

If you are using Microsoft Windows 8 or newer, the Microsoft WinUSB driver is already preinstalled and you are done.

For Windows 7, Windows Vista and Windows XP, download the USB driver installation utility from For manual USB driver installation, the USB driver files can be downloaded from Extract and start the USB driver installation utility. A wizard will guide you through the USB driver installation. Click the pictures below for step-by-step instructions.

Optional: Install the Microsoft WinHlp32 help if you are using Windows Vista or newer and want to view help files. Details are here: How to run Help Files on Windows Vista / 7 / 8 / 8.1 / 10

The Windows 10 Installer for WinHlp32 is officially not supported. You can download it here:

You can verify all downloads with the Win-SFV checksum file.

Finally: Connect your AP System to the USB adapter and switch it on. AWPIN should use the WinUSB driver and detect your AP System as usual.

If you want to start APWIN in DEMO mode, do not connect your AP System and click on the "Ignore" button on the first message box.

Please sign on the newsletter below to stay up to date!

Get notified about software updates

Email is sent only for new product releases, articles or important updates.
Your email address is not passed on to third parties and only used for the newsletter dispatch. Unsubscribe any time.

Hardware Details

The USB adapter electronics was designed from the beginning to fit into a high-quality all metal DB-25 backshell connector. The metal housing in conjunction with the cable shield provides uninterrupted shielding.

The PCB is standard two layer with 0.15 mm trace width and 0.6 mm vias. Because of the small size of only 23 mm x 35 mm the components had to be placed on both sides. The layout was done carefully with separate ground return paths for the different parts. Actually the layout is on the limit because of the number of traces and vias.

The heart of the adapter is the Cypress FX2 microcontroller, which is capable of USB 2.0 High-Speed (480 MBit/s) communication and provides a fast programmable I/O interface in a small housing.

ESD diodes on the USB D+ and D- lines improve ruggedness. The Audio Precision control bus signal lines are protected against ESD by the R-C filters in conjunction with the robust output buffers. The metall shell too helps by providing a low impedance path to earth.

The interface to the Audio Precision bus is implemented with 5 Volt TTL compatible buffers. One buffer is used for the bidirectional 8-bit data bus and a second buffer is used for the bus control and status signals. The bus signals are heavily R-C filtered to limit signal rise time and reflections on the bus.

The DB-25 connector is directly soldered on the PCB. The gold plated, machined pins guarantee long lasting low contact resistance.

Details of the Audio Precision interface bus can be found in the service manual for Sytem One. The bus termination has changed many times and not every detail of the protocol is documented.


Discontinued Audio Precision Models
A survey of the discontinued Audio Precison models on the site. Unfortunatly a lot of information and downloads have beed discarded with the new webpage layout.
APWIN 2.24 download link
The latest Audio Precision control software, which supports the older System One and Two models.
USB adapter thread on
A rather long thread with lot of details about the USB adapter on the diyAudio site. Many bugs in the software were fixed with the help of diyAudio members.
LinkedIn, Classic AP Audio Systems Group LA
The Classic AP Audio Systems Group LA on LinkedIn with helpful and experienced users. This has replace the older LinkedIn, Classic AP Audio Systems Group