BAS Message
June 2016

1.  John S. Allen has asked for help in locating the attendee at the May meeting (MBT) who was taking photos with a digital SLR camera. I would like to use some of his photos in the meeting summary. He told me at the meeting that I could, but I didn't get his name. Thanks.

2.  DB Systems manufactures separate phono preamps for moving magnet (DB-8) and moving coil (DB-8HG MC) cartridges, each optimized for its type. I see that it is common in the marketplace for inexpensive preamps to be switchable MM/MC which is a convenience for the user. The noise performance they specify is superb for either type. But I wonder if there's a compromise. They measure noise with shorted input which is valid for MC but not for MM. The source impedance of an MM cartridge can be 20,000 ohms at 5 kHz. If the input has high current noise (which doesn't show up on shorted input tests) that can translate to a lot of input voltage noise. If I had one of these on the bench I could test with a cartridge source and compare with ours.

But I don't so I looked up the chip used by one of the preamp manufacturers (listed in Stereophile's Recommended Components), ADA4897, and it has 2.8 pA/rt Hz current noise. This compares with the chip we use which has .6 pA. My Shure M97XE cartridge is 1550 ohms and 650 mH. Crunching the numbers I get that that preamp is 13 dB noisier than ours at 5 kHz.

So the specs are true but misleading in actual application.

If you are buying a switchable MM/MC preamp you should ask the manufacturer whether there are separate front ends for the two cartridge types. Switchable gain is not enough.

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updated 9/15/16