Current Amplification
A recent review of the CH
Precision P1 phono preamplifier ($48,000) states “Current amplification
circuits produce the best signal/noise ratios [for moving coil cartridges]”.
Their spec is 138 dBu [1 volt reference]
with a 1 ohm source and 22 kHz bandwidth.
A current preamplifier responds to the current output
of the source and has near zero input impedance*. A voltage preamplifier
has a high input impedance and responds to the voltage of the source.
In a voltage amplifier the principal noise sources
are the source resistance, the input transistor, and the feedback resistor
to ground. In a current amplifier there is no resistor to ground; the
source resistance is the feedback resistor. So one noise source is eliminated
and the theoretical noise is lower. A side affect which may be beneficial
is that the gain is inversely proportional to the source resistance,
so the lower the source resistance the higher the gain. Since in general
the low output MC cartridges also have low resistance, that is self
compensating.
Is it low noise?
The DB Systems DB4B head amp, introduced in 1976,
specs 86 dB A Weighted [1 mV reference] That is 146 dBu, or about
143 with a 22 kHz bandwidth. So it is 5 dB better, using conventional
voltage amplification. BTW the noise of a 1 ohm resistor is 154.4.
Note: This calculation doesn't take into account
the RIAA curve which would give the DB4B a few dB more advantage.
I emailed CH Precision on their website and got
an error message.
*Speaking in terms of operational amplifiers, the
+ input is grounded and a resistor is connected from the output to the
– input which is the summing node. The cartridge has one side grounded
and the other is connected to the summing node which is essentially
zero impedance and has no voltage associated with it.
A current amplifier output has a very high impedance
output but that is a different story.
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