Moving Coil Phono Step Up box for Newcomb TR-91/TR-100 Mic Transformers MC SUT

Description

Here's an empty moving coil phono step up transformer box for use with your Newcomb TR-91 or TR-100 9 pin input transformers. The Newcomb transformers have split primaries and this box allows you to use them either in series or parallel for lower or higher gain. The TR-91 has a step up ratio of approx 1:15 on low gain, 1:30 on high, and the TR-100 is 1:10 and 1:20. Assuming your MM phono preamp has a standard input resistance of 47k ohms, the TR-91 will load the cartridge with approximately 200 ohms on low gain, 50 ohms on high, and the TR-100 is 500 ohms and 125 ohms. On the high gain setting, this box does put the primaries in parallel rather than just using the center tap, in order to use the full amount of copper possible in both settings.
The ground switch selects how the aluminum box is earthed for shielding. You can attach it to the single left hand audio shield in the "L" position, to both shields (meaning the shields are also joined together) in "L+R" or put it in the middle to separate all three grounds from one another. In the middle position you should run a separate wire from the SUT's ground post to your phono preamp ground post to be sure the box is actually shielded. In general you don't have to worry too much about the ground switch, just wire up the box however it's convenient to do so, and put the switch in whatever position gives you the lowest noise.
This box is priced a little higher than the other empty transformers boxes I sell because of the difficulty and expense of finding good quality 9 pin sockets. I only use NOS Amphenol sockets for these, never used ones. This is a newly made box. It has been tested and is guaranteed to work, assuming your transformers are also working fine. Let me know if you have any questions. Thanks!