|
What about Impedance?
Impedance: There
are 2 types of impedance to be concerned about with
Moving coil
Cartridges: Internal Impedance and
Loaded Impedance.
Cartridge
Manufacturers are not consistent
with how they specify loaded impedance.
With
Step up transformers, we boost the
voltage, since transformers are voltage changing devices.
There
is also a secondary effect of the
internal impedance being the square of the step-up ratio.
So,
for example, if a transformer has a step
up ratio of 1:10, then the internal impedance of the transformer would
be 100 (10 x 10). So
the reflected impedance back to the
cartridge would be 47,000 ohms (using the RIAA standard for a standard
MM phono
input), divided by the internal impedance of the transformer. So
for this example, you take 47,000/100 = 470 ohms
impedance at the
cartridge. So, the manufacturer
sometimes advertises 470 and sometimes advertises less than that, if
they
choose to use a multiplier.
But
it is not that
simple. The voltage ratio and impedance ratio changes
depending
on the
input voltage, which I why I spec the units I build using an input
signal of
0.5mV and the standard of 1kHz frequency, and measure them
myself.
Usually the step up ratios at that voltage are lower than what the
manufacturer
specifies, which means that the impedance is higher than advertised for
the
input. We are talking about extremely low voltages.
There
is a lot of confusion about how the
loading is specified. Some
cartridge
manufacturers try to take the guess work out of it and put in some kind
of
multiplier, while others base their loading assuming a head amplifier
will be
used to
boost the signal. For example, Koetsu
recommends anything between 5 and 100k. Other manufacturers
just
specify
a minimum, like >100 Ohms. The problem is that we
don’t
really know
whether the manufacturer has provided loading information based on the
internal
impedance or the loaded impedance of the cartridge, or considered
whether we would
be using a SUT or head amp. We do know
from considerable testing, that you don't want to load a MC cartridge
using a
SUT to match the internal impedance, or you will
“choke-out” the cartridge and
it will sound muddy.
Just
how close do you want to approach the
internal impedance before this happens, and what happens as you
approach this
ratio? Is resistor loading
necessary?
The
deal on impedance is that it is not that
important with SUTs. Voltage match is
what is critical. Having said that, once
you make the voltage match, you should look at the reflected impedance. You
normally want it to be about 10-times the internal
impedance of the cartridge. Now that is
not a hard number, it could be 8-times and be fine, and sometimes
4-times can
sound great. The important part is
that
more headroom is better. A lot depends
on the characteristics of the phono preamp.
Typically if you have enough headroom with impedance, the natural
characteristics of the cartridge will be apparent. So, if you
have a "bright" sounding cartridge, you may want to have a lower
impedance at the cartridge, but if you have the impedance too low, it
may sound muddy and flat.
I no longer recommend adding resistors. There
is considerable controversy over this and there is a lot of discussion
elsewhere. The signal is so low
coming from
the MC cartridge, that is the one place I would not want to waste any
signal. I work really hard to ensure that the
output of the transformer has as short of a signal path as possible.
That is the most critical part of the setup, where you have
already reduced the current coming from the cartridge to gain extra
voltage.
I
have read several articles that discuss
loading of cartridges. I took it a step
further and experimented with different loads on several different test
cartridges. I both measured the
response
using a signal generator and dual trace oscilloscope and also conducted
many
listening tests. What I found was really
interesting. Basically if you are
just
looking at the test equipment, you can tune it pretty accurately, but
the real
test is in the listening. The biggest
challenge is in gathering the correct information on the cartridge you
are
using. The manufacturer can
tell you the
impedance and the output voltage, but is frequently wrong with telling
you what
loading impedance is required. To test
that, I used several of my transformers with different cartridges and
instead
of hard wiring resistors to the input or output of the transformer, I
built a set of resistor tees
(2
female to 1 male) and inserted them on the inputs and/or outputs of the
transformers. I
then wired several resistors onto male
plugs and inserted them into the tee on the output to try different
resistances. Starting with high
output
values and changing them to lower and lower values, I found that there
was a
very slight improvement as the resistance came down until there was a
major
change for the worse when it reached a certain point.
This
point usually turns out to be about 8-12 times
the output of the cartridges I tested but again it was cartridge and
internal impedance dependent. Now,
I am a purist and don't really want to put anything
into a circuit
unless it improves it.
I
have also mounted lugs on the outputs where
you can attach resistors as an after-market thing, but they introduced
noise, so
I stopped doing it. The output of a
transformer is the most critical part of the circuit and you have to be
very
careful with wiring between there and the preamp to eliminate noise.
In
most cases where I have tried to load
using resistors and tuned them in, I tried an A-B test afterwards both
with and
without any resistors and did not notice the difference.
In
my opinion, you usually do not gain
anything by adding resistors, and only stand to degrade the sound if
not done
properly.
So,
to make a long story short, I don't
recommend messing with loading resistors.
For
more reading pleasure, check out these links:
http://www.vinylengine.com/step-ups-and-mc-cartridges.shtml
http://www.theanalogdept.com/sut.htm
The formulas used on
theanalogdept.com site above seem to fall into line with our
experiences with listening tests. Email me with your cartridge
and phono preamp and I will run the calculations for you and recommend
a step up ratio for you to use.
bob@ec.rr.com
|
