Class A Preamplifier

Petrúngaro Transformer Microphone Preamplifier Model NV-II

This device was designed based on the structure of pure Class “A” amplifier; this is, a one-phase design, not counter-phase, therefore avoiding crossover distortion, found in most preamplifiers that use operational amplifying circuits to handle audio signals.

This characteristic of design (Class “A”), related to the fact that audio signals pass through only six transistors, makes this equipment a one-of-a-kind in reference to its signal to noise ratio (without a doubt, the best in reference to its noise floor).

The NV-II is fitted in a 1U rack mountable chassis, and features two channels. The signal connects to the circuit passing through a transformer with permalloy — mumetal core, electrostatic screen and double primary winding. Combining these two windings enables different input impedance options (Hi and Low), thus, offering the possibility to choose between different sound “colors”, thanks to the difference in the magnetic transfer of such transformer, making this characteristic a unique variable at the moment to approach the desired sound, not only for transparency, but for color and musical character, too.

Even though these terms cannot be technically defined, when the moment comes to find a specific sound, these are the psychoacoustic appreciations that define the election of the preamplifier to be used and its setup.

Going back to the audio path, the signal passes through an input trim with 5dB steps, which is linked to an overload detector with very high impedance, indicating if the input signal reaches saturation level at any time.

For signal handling, the equipment has an output level control, that together with the trim control and the impedance (z) switch, provide three setting variables to find the desired sound. Also, it features a phantom power switch and a phase- reverse switch.

Finally, the signal comes out passing through an excellent bifilar winding transformer with flat response at up to 80 KHz, and with grain-oriented core, that makes it possible to develop output levels up to 30 dBm without distortion.

Enjoy it…


Daniel Petrúngaro

Power Supply Unit

Petrungaro Transformer Microphone Preamplifier Power Supply

The power supply unit consists on two different blocks, one for phantom power, and the other for audio.
It is capable of feeding up to five NV-II units.
As it is an external circuit, “hum” noise does not exist.
This is an essential condition, considering the high amplification and sensitivity levels.

In addition, the unit is designed to provide a large regulation margin, which allows it to work safely in improper voltage situations, without the need of an external stabilizer.
Also, it features surge protection, making it an excellent and reliable power station.

Technical Specifications

Microphone Input

Balanced, earth-fee, RF isolated, electrostatically screenered. Astatically wound transformer with mumetal core.
The sensitivity is adjustable in 5 dB steps between -70 and -20 dBm for 0dBm output, referred to 600 ohm terminating impedance. The input impedance is 300 ohms or 1200 ohms selected by a switch on the front panel (Hi / Lo Z). The double coil input transformer allows to select this two different input impedances.

Line Input

Balanced, earth-fee, RF isolated, electrostatically screenered. Astatically wound transformer with mumetal core.
The sensitivity is adjustable in 5 dB steps between -20 and +10 dBm for 0dBm output, referred to 600 ohm source un-terminated. The input impedance is 10 Kohms line bridging from 600 ohms or lower source.

Noise

Between -80 and -40 dBm sensitivity the equivalent input noise is better than -125 dBm referred to 600 ohms input impedance, terminated with 600 ohms resistance.
Between -35 and +10 dBm sensitivity the noise measured at the amplifier output does not exceed -83 dBm. RMS values, 20 Hz to 20 kHz bandwith unweighted.

Output

The preamplifier output is balanced and floating and is designed to feed a 600 ohm load.
The maximum output level is +26 dBm into
600 ohms or +20 dBm into 150 ohms.

Distortion

Measured at -60 dBm sensitivity (microphone input) and -20 dBm sensitivity (line input)
+20 dBm - 1 kHz < 0.01%
+25 dBm - 1 kHz < 0.03%

Frequency Response

Measured from 200 ohms source into 1200 ohm microphone input at -60 dBm sensitivity or from 600 ohm source into 10Kohms line input at -20 dBm.
Output at 0 dBm into 600 ohms load:
+- 0.5 dB 15 Hz - 25 kHz
-3 dB 7 Hz - 40 kHz