PVA Pre-Amplifier
PVA Pre-Amplifier
New in production are Passive Voltage Amplifiers (PVA) by Electra-Print Audio. These feature a new circuit design which loads the signal source op-amp letting it operate at its maximum ability. The passive design, but unlike other passive pre-amplifiers, the PVA delivers a higher voltage at its output than it receives from the input. Plus the output impedance remains fixed which means that the frequency response does not change as the volume is changed. There is no active circuitry in this design. Options include standard or stepped attenuators with various input and output configurations.
Op-Amp Loading Transformers in the PVAs cause Op-Amps to deliver current into an essentially pure impedance primary. The secondary then produces the equivalent voltage into hard termination which is now usable for the next stage of amplification. These nickel core Loading Transformers are only for use in low level line designs. Stock untapped transformer ratios are 1:1, 1:2 and 1:8. Plus a 1:13 design is available for tube grids
Determining Output Impedance
At times you may need to determine the output impedance from your music source. In particular, this is important if you intend to use the PVA (Passive Voltage Amplifier) and need to confirm that your sources have low output impedance. If you are testing a CD player or phono head amp you’ll need to use a test disc with 1 kHz test tone. You can also use the noise between stations on your tuner for this test.
Here is a three step process to measure the output impedance from music source: 1] play the test tone and measure the open, unloaded output signal on a good AC Voltmeter, 2] add a variable resistance (pot) across the output and adjust until the voltage reading is half of the open circuit measurement and 3] then remove the pot and measure its resistance with your standard ohmmeter. That value should be very close to the source impedance.
Last updated: Thursday July 5th, 2007 at 07:43:44 PM