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Toroidal Transformer replaces original E-I transformer and cuts hum!
The answer, in a word, is hum! If you've replaced the stock power supply with the regulated PAT4PWR, you've already cut the hum down by a lot. If you've upgraded your phono section with PAT4LP, then you've dropped the hiss from the phono section by a lot. As each noise source is peeled away, the remaining noise source comes front and center.
The stock transformer is an EI transformer encased in metal. While the metal shield is helpful, it still allows lots of hum fields to couple into the low level circuits of the PAT-4. Then, the same gain that turns small signals (like those from the phono) into big signals also amplifies the hum. The result is that your PAT-4 isn't as quiet as it could be, especially in the phono section.
Let's get right to the point. The first picture shows the spectrum of the preamp output with
The picture below is the same arrangement, but we've replaced the original transformer with a custom designed toroidal transformer, shield, and mounting hardware
You'll notice that the PAT4X kit practically buries the hum in the phono noise floor. Of course, you still may pick up some hum on your phono leads, but in general, it's much quieter.
The PAT4X kit is in stock. The kit includes:
Download the White Paper on PAT-4 hum reduction with the PAT4X kit for more information.
Here's a system level hum reduction white paper. It's on my "Objective turntable" website.
Remember also that clean connectors minimize hum. For more information check out this link. Clean Connectors