Whitestone Audio Instruments, USA


Dear P331 owners,

We have developed and tested a new version of the firmware that runs our P331 Tube Loading Amplifier. The firmware in the P331 runs all the controls and signal routing through the unit. 

It was never (and still isn’t) our intention to continually update this firmware. However, after having the P331 in the market for a little over two years now, there has been valuable feedback from our users we felt we’d be able to act upon. If we are capable of improving the P331 through a firmware update, we feel compelled to do so. 

This firmware is not user-upgradable. There are specialized programming tools and software needed to perform these updates and a full re-calibration is required following the update.

There has only been one prior firmware update since we began shipping P331s. This V2 update added the LINK functionality.

  • Serial numbers 00001 – 00019 were shipped with V1 firmware installed (No LINK function)
  • Serial numbers 00020 – 00108 were shipped with V2 firmware installed (LINK function added)
  • Serial numbers 00109 and later are shipped with this latest V3 firmware installed


The new P331 V3 Firmware update addresses two main items we’ve received consistent feedback on. All other P331 features have been unchanged.

  1. The ability to save the last used LINK and MODE settings in memory. This allows you shut down the P331 and power it back up with the last used settings. For instance, if you are in LINK with OPEN selected on the MODE indicator, your unit will power back up exactly as you left off.
  1. We addressed feedback suggesting our XFORMER settings may be a bit too subtle. While subtlety is important to us (and the new XFORMER settings are still “mastering appropriate”) we experimented with what we could do with our internal signal routing to perhaps enhance things a bit. We like what we came up with and deemed it to be a worthy update.


For P331 owners within the United States:  We are offering to perform a full service to your P331 at no charge. You will, however, need to cover all insured shipping charges. 

For P331 owners outside the United States:  We will work to help facilitate the upgrade through your local distributor and provide technical assistance, however, since this is a voluntary upgrade we cannot require our international distributors to offer this service at no charge.  You will need to work with your local distributor to arrange price of the upgrade.

The full service includes:

  1. Update to V3 firmware
  2. Make internal component updates to match current spec
  3. Full calibration

After service your P331 will be returned to you performing as new.

NOTE: If it is determined new tubes are required, we will need to charge $149USD for a new pair of tested low-noise 6SN7 tubes with matched and balanced triodes.

Please fill in the form below to begin the process and we will contact you to coordinate your service request.