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.
email me here