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Notes
on Cartridge loading (certain information and figures courtesy Tom Evans Audio, maker of the
Groove phono preamp)
The following discussion applies to electronic loading (i.e. connection to a phono preamp), not electro-mechanical loading (i.e. connection to a step-up transformer).
Quick summary to skip the details:
Loading resistor in preamp/phono stage = approx. 10-20 x (moving coil cartridge coil impedance).
Loading value for step-up transformer = approx 1-5 x (moving coil cartridge coil impedance)
There are exceptions, for example, Denon 103 cartridges.
| Cartridge | Internal Impedance (Ohm) | Loading (Ohm) | Step-Up (Ohm) | Remark |
| Lyra: Lydian, Helikon, etc. | 5.5 | 100 | 4-10 | Lyra is less sensitive to loading |
| Koetsu (all) | 5 | 100 | 10-15 | ˇˇ |
| Denon 103 family | 14-40 | 100 | 20-40 | ˇˇ |
Please e-mail Extremephono your ideal combination or experience so the above table can be updated to help others. Thank you.
In
phono preamplification all aspects require careful consideration. Cartridge
loading has a major influence on reproduction.
If there is non-optimal impedance matching between a cartridge and the
input stage of the preamplifier, optimal reproduction is impossible.
A
loading resistance is required because electro-acoustic transducers must be
damped to avoid ringing, overshoot and other negative effects.
The loading also influences the preamplifier's noise level, the frequency
response, etc.
Designing
for low noise at low impedances is a real challenge. To avoid this challenge many preamplifier designers just use
a high input resistance and thereby force the cartridge to work into the wrong
load. Some moving coil (=MC)
cartridge manufacturers specify a loading resistance of 47kOhm. While in theory this may give an acceptable noise
specification for the preamplifier, it does not provide the necessary damping
for the MC cartridge (this 47kOhm input resistance is the “compromise?
setting that was decided upon for loading MM cartridges. It is a theoretical
figure and it only is the correct termination for a few (but by far not for all)
MM cartridges. It is certainly not correct for MC cartridges most MC cartridges
perform best with a load of between 20 Ohm and a few hundred Ohm).
Fig. 1 shows the effect of varying the resistive load
on an MC cartridge. The potential deviation from a linear response by a wrong
loading resistance can be massive.
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Fig. 2 shows various combinations of resistance and
capacitance loading. With this feature the response of cartridges can be
linearized.
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Preamplifiers having
just fixed input resistances that do not allow variable resistance and
capacitance loading are unable to extract the full performance from cartridges.
Despite that, almost all preamplifiers lack this important feature.
This is one of the reasons why many preamplifiers work acceptably with
one or two cartridges but do not provide satisfactory performance with other
cartridge makes, leaving the listener at the mercy of the fixed input loading of
the preamplifier. Thus performance is far from optimal.
The
elevating top-end caused by improper loading can sometimes be perceived as 'air'
or high-frequency extension, but in reality, it is actually odd-harmonic high
frequency distortion. In a very
well balanced reproduction system, it will sound edgy and bright.
One
common mistake is to add mechanical damping to the turntable/tonearm system
without first considering electrical optimization to address the brightness and
high-frequency. The problem
with this approach is that the transient and decay of musical notes are affected
by mechanical over-damping.