Tube Sound Balanced Precision Line preamplifier VLP 620B

The last creation of  the  ‘tubeSound‘  is VLP62oB.

Projected as a full tube, fully balanced line amplifier, the VLP620B sports also an all tube phono stage, which

supports an MC cartridge section.  The MC input is via a Sowter transformer, and was also configured in a

balanced mode.

The front panel displays a big volume dial, equipped with a small red led, the power switch also having a red light

ring to show the on-off status, and two huge VU-meters lighted from the back with a warm dimmed light; Vu-meter

screens are totally invisible when the power is off, as being maksed by the smoked dark  acrylic cover-plate.

The balanced configuration was achieved thanks to high quality  input/output transformers provided by the Brittish

manufacturer Sowter, as well as the step-up transformers for the MC input stage used in the phono section.

The phono section is realized with passive RIAA technique, using an E183CC mu follower  for the first stage,

and a E188CC used in SRPP configuration for the second one. The RIAA curve was achieved with a 0.5 dB precision.


The power supply: two separate transformers were used for this line amplifier. One transformer for the audio section, and one for the auxiliary functions: VU-meter precision amplifiers, on-off circuit, vigh voltage and filament PSU delay circuits, audio programme selector and volume motor controller.


On the back panel there are disposed, as follows:

- two separate line balanced inputs

- one phono MC  balanced input

- one balanced output

- one  unbalanced input

- one balnced by-pass input

- a switch for phono  ground lifting

here there are some details of the back and front panels:



Line section ( load 600 Ohm; midband gain 13 dB )

a – Sensitivity

- for an input of 0,35 Volt results 1,55 Volt output

- for an input of 1,55 Volt results an maximum output of 7 Volt before clipping occurs

b – Rise time: <1 micro second at 1kHz

c – Frequency response: 15 Hz – 50 Khz +/- 1 dB

Phono section ( gain 78 dB – with Sowter 9750 MC 1:10 input transformer connected)

a) Frequency response: 15Hz – 50 Khz +/- 1 dB

b) RIAA curve precision +/- 0.5 dB

c) sensitivity: 0,5 Volt/ 500 Ohm

The provided data was measured with Tektronix 465B oscilloscope and Klark Teknik DN6000 Spectrum Analyzer.

Here are some evidence of the measurment:

- on the left is the time response, and on the right is the sinus signal before clipping

The upper picture shows the monitored output – on the Klark Teknik spectrum analyzer front screen -  of the preamplifier output when a 0,5 Volt  signal sweep was injected at the phono input trough a inverted RIAA stage, thus showing the RIAA  accuracy.


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