At 08:16 AM 2008-08-27, you wrote:
>Thanks for the tutorials, folks...
> > You don't want to double-terminate the cartridge.
>Ok... I just thought that would get me a lot closer to Z = 47k, since the
>combined Z would be what the cartridge actually "saw" (no?); the sum would
>be about 50k Ohms, tops. What causes the difficulty? i.e., Why not "y-"?
It sounded as if you are paralleling resistors. When resistors are
paralleled, the value goes down. The formula is
1/(1/R1+1/R2+1/R3...). So if you have an input impedance of 25 K
ohms, it is already too low, and there is no "negative resistance"
that will increase it above 25 K by paralleling. If you place the
resistor in series, you would throw away 6 dB of level and hence the
same in noise performance where you can least afford it.
Now capacitors add directly in parallel (and resistors in series). So
if you have a 30 pF input capacitance and 120 pF of cables and want a
200 pF load, adding a 50 pF capacitor will work.
>And here I thought that the capacitance (in the cabling, and entire network)
>was merely something to try to minimize (due to the LPF effect of the R-C
Yes, generally, but not in this case as the whole system needs to be
matched to be flat.
>If I were to use the 14 pF/foot Mogami 3080 cable, could I get to the target
>loading value by merely making the interconnects 10 feet long? ]: (14 x 10
>= 140 pF)
No, because the input has some capacitance so you need to subtract
that from the Mogami capacitance.
Ideally, you should have a preamp that has a variable capacitance and
variable resistance to load the cartridge.
Using the capacitance to smooth out square waves might be a good idea
however, the CBS test records were cut with ringing on the square
waves and that biased a whole generation of engineers including one
of my mentors until I saw an article (which I don't have--but I think
it was in audio) where someone did scanning electron micrographs of
the CBS test record and it showed the ringing. It would be
interesting to view one of these on an ELP laser turntable.
>Or is it actually better (for other reasons than cap and Z, alone) to use
>cabling which is as short as physically possible with a small tuning cap
>hung off the ends?
Preferred as the tolerances of everything seem that an adjustment
range would be beneficial.
Since I don't really DO this other than for my own listening
enjoyment, I would caution that my advice may not be totally in
keeping with what the people who DO discs for a living do.
In fact, I know just enough about what I don't know about discs to be
dangerous...one of the reasons I don't do discs is I know a good deal
of what I don't know and don't have time to learn or budget to
acquire what I need as I'm swamped with tapes and don't see the need
to branch out.
Richard L. Hess email: [log in to unmask]
Aurora, Ontario, Canada (905) 713 6733 1-877-TAPE-FIX
Detailed contact information: http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.