Audio Precision System compatible USB adapter

The older Audio Precision System instruments are very capable audio analyzers, especially for analog work. However they are missing modern PC connectivity and the control software 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 AP System One, System Two, System Two Cascade, or ATS-2 on modern Microsoft Windows systems.

The AP 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 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.

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 good performance a USB 2.0 High-Speed (480 Mbit/s) or USB 3.0 connector should be used.


The Audio Precision compatible USB adapter is tested with APWIN 2.24 (Build 1390) and ATS_1.5.

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

ATS-2 is still supported by AP and does work with the USB adapter from AP with ATS control software version 1.6.

Any Audio Precision hardware that is supported by APWIN 2.24 or ATS 1.5 will function with the adapter:

Supported Windows systems:

Ordering information

The price of the adapter is 290 € excluding 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 or to order a 2 meter USB extension cable.

Software Installation

S1/S2 files, 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 APWIN 2.24 or the ATS 1.5 control software, if it is not already installed, using an Administrator account.

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).

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

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 program, 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:

Optional: Windows XP Hibernation.
The original Apwin drivers prevent Windows XP from entering hibernation mode.
The driver files are in the Windows directory (typically C:\Windows):

As they are not needed with the new apio.dll you can delete them to enable hibernation mode.

Optional: Verify downloads with the Win-SFV checksum file.

Optional: Download the usbtreeview utility to check, that the USB adapter is correctly recognized.

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

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

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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.
ATS_1.5 download link.
The ATS-2 control software version 1.5, which supports ATS-2.
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