These are the first images of Jensen JT-347-AXT MC transformers installed in a Conrad Johnson Premier 15 Series 2 phono preamplifier. These are wired in 1:12 gain, for 21 dB. With the transformers installed, the total gain of the Premier 15 is 75 dB. The CJ was quiet without the Jensens, but now noise is inaudible.
This mod elevates the Premier 15 to an entirely new level of performance and sounds awesome. It is almost as if the stock ’15 is starved for signal with less than 1 mv moving coil cartridges. The C-J seems to love the added input voltage. Everything from the low bass to the ultrasonic enjoys increased dynamic impact, greater depth of tone and richness, and higher resolution without any electronic artifacts going along for the ride. Instead of 0.6 mv (Clearaudio, Koetsus, Shelter), the effective input voltage is now 7.2 mv. With an input overload spec of 125 mv, there is not a problem with the higher input voltage. Assuming a 50% overload margin, the maximum “safe” cartridge output would be approximately 5 mv.
The output level with 75 dB of gain is more than sufficient to use a .2mv cartridge, an amplifier with an input sensitivity under 2v, and a low-gain or passive preamplifier.
Note the caps behind the blue DIP switches. The caps are part of RC networks, and allow loading for 5, 10, 20, and 40 ohm coil DCR cartridges. With all switches off, almost any other cartridge can be accommodated. (The resistors are not visible due to the camera angle.)
A different view. Note the tight twisting of the transformer leads to minimize hum and noise.
View from the bottom. There is plenty of room.
I have a problem. It’s a numbers thing. The C-J was awarded a 10 LP rating in the review, but adding the step-up transformers made a instantly identifiable – and large – improvement in the performance in every parameter. Since the rating system is based on enjoyment and involvement instead of absolute performance, the 10 LP rating will not be revised based on this modification because the stock unit earned that high achievement. But…if you ever hear what is shown on this page, you’ll wonder if the review’s rating was correct. Look at it this way: In the shot-put event in the Olympics, all the countries compete, and the person who puts the shot the farthest wins the gold medal. Then Superman shows up. Get the picture?
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Click here to go to the CJ review on 10 Audio.