For the last year or so, my Audio Research PH5 tube phono preamp had a hiss and high noise floor in the left channel that really bugged me. I brushed it off at first, since the channel imbalance was very slight. Over the last eight months the hiss became louder and louder. So much so, that I stopped making Youtube videos because it would be noticeable and quite annoying. Tonight, I decided to open it up and investigate the cause of the problem.

Audio Research ships the PH5 with the tubes removed which must be installed by the customer or audio shop before using. The tubes are matched at the factory and labeled V1 through V4, which match the socket labels on the main board of the preamp.

To open the PH5, there’s 14 screws that need to be removed from the top and sides. I lifted the cover and started out testing the four Electro-Harmonix 6922 tubes. I have a Bravo Audio headphone that is based on a single 6922 tube, which can easily be removed and swapped out for testing other tubes. I started with the V1 tube, pulling it out of the PH5 and installing it into the Bravo Audio headphone amp. As soon as the tube warmed up, I knew something was wrong – the left channel was faint and very distorted.

I set the V1 tube aside and proceeded to test the remaining tubes V2 through V4. All of them tested fine, both channels sounding strong, clean and balanced.

Since the Bravo Audio headphone amp came standard with an Electro Harmonix 6922EH tube, the exact brand that is used in the PH5, I installed it into the V1 socket and powered up the preamp. After a 45 second warm up, the phono pre amp came to life and the hiss was gone! All this time, I was listening to  my preamp with a bad tube!

I couldn’t believe the difference. The sound stage was wide and deep with pinpoint accuracy. The separation between instruments was very pronounced and low level details popped out of a deep black background.

Audio Research keeps a record of the tube specifications that were used in each preamp; referenced by the serial #. You can order a new set of matched and tested replacement tubes by calling them and giving them your serial #. I’m going to call them tomorrow to find out pricing and availability. For now, I can make due using my spare 6922 tube from my headphone amp.

Click here to hear audio samples of the repaired preamp, posted in the forum.