Instruct: Classify Amazon reviews into positive or negative sentiment
Query: Great Preamplifier, Great service.

When I purchased and received this unit, there was an issue with the volume knob, where it would scratch (IE, static would play over the speakers) as you change the volume, and it would cause all the sound to drift further to the left channel as you turn up the volume over 80%. I contacted Amazon and told them about the problem, and they sent me a replacement as soon as I told them there was something wrong with it. I was very impressed by the speed of the service. I received the new one about 4 days later and tested it out immediately, and it works flawlessly. Perfect for increasing volume output from my computer to my home theater system. I never expected so much for the price.