Is phono input cap. necessary?
wo...@teleport.com
necessary for a moving coil cartridge?
J. Szamosfalvi
: Is the cap (150pF) paralleling the 47k input load resistor on a preamp
: necessary for a moving coil cartridge?
It depends on the capacity of the cabel and the manufacturer's specs
for that specific MC cartridge. Ideally, the sum of two should equal
with the recommended value.
Kalman Rubinson
: Is the cap (150pF) paralleling the 47k input load resistor on a preamp
: necessary for a moving coil cartridge?
I have not answered your multiple postings but now I give in....
The answer is: 'It depends.' The cap performs a function with MM
pickups which will probably not be needed if you keep a 47K input
resistor. If you use a lower input-defining resistor, the value of the
capacitor will depend on the pickup and the R.
Soooo..
What pickup is it?
Kal
Hans J. Albertsson - Sun Sweden
rubi...@is2.nyu.edu (Kalman Rubinson) writes:
>wo...@teleport.com wrote:
>: Is the cap (150pF) paralleling the 47k input load resistor on a preamp
>: necessary for a moving coil cartridge?
>
much. Just leave it in place. Unless the cartridge is of the high-output type,
in which case it should PROBABLY be removed. Try it.
For moving magnet cartridges, yes, the exact load impedance is important.
The cartridge is (largely) a 4th order resonant system, the stylus
tip mass resonates with the stylus arm compliance, and the
coil inductance resonates with the cable and amp input capacitance.
Both these resonance are damped by the load resistance and by the magnetic
and mechanical losses in the stylus mount and the coil/magnet system.
You use these facts to your advantage by adjusting the capacitance to make the
two resonances combine into a well-behaved single-peak-only responce, and then
damping this resonance with the load resistance to flatten it.
This approach is often surprisingly effective, but the result is
fairly sensitive to changes in the load components.
An example: a total load capacitance of 195pf with a load resistance of
42.8Kohms has kept several samples of the Ortofon OM40 super pickup
within +- 0.5 db 20-20Khz. The OM30 ( which has similar values for all
pertinent parameters ) probably would need slightly over 200pf.
This was reported by Ingvar Ohman in the club magazine of
the Audiotechnical Society of Sweden last year.
Note: The total capacitance includes the amp input cap, the cable capacitance
and the capacitance between the wires inside the tonearm. The capacitance
in an SME type II is about 25-27 pf, RG62 cable is about 45pf/m ( 15pf/ft),
and my amp has a switch to make the input cap be 100, 200 or 300pf.
Leaving the amp at 100pf, and adding 1.5 meters of RG62, and changing
the input resistor to 150K//150K//100K, would probably do the trick.
The moving magnet is a simpler system than the moving coil, the moving coil
system has a multitude of resonances originating with the tiny wires attached
to the stylus assembly. Also, the inductance of the moving coil is low
for the ordinary low-output types. Together, these facts make compensating
the response by simple load impedance arrangements impractical.
In the early days moving coil meant lower effective mass, since
the technology for creating very small magnets and the various forms of
magnetic shunt systems didn't exist. This created the myth that MC was
inherently superior, not just easier to design at the time. This myth
unfortunately still hampers the tiny remnant of a phono cartridge producing
industry that remains.
--
Hans J. Albertsson, Sun Sweden