Preamplifiers — VTL TL6.5 Series II

The TL6.5 Series II Signature is a line-level preamplifier from VTL Amplifiers, whose electronics are built by hand in the USA. It is part of the Signature range and uses a hybrid layout in which vacuum tubes provide voltage gain and FET devices supply output current. There is no phono stage inside, so a separate phono preamplifier is needed for vinyl.

Key specs that matter

Connections cover three balanced XLR inputs and five single-ended RCA inputs, one balanced output pair, two single-ended RCA outputs and two further buffered tape outputs. Normal gain is 14 dB single-ended or 19.2 dB balanced, and a low-gain setting cuts this by about 6 dB to 7 dB and 13 dB. Frequency response is within +0 and -0.3 dB from 10 Hz to 200 kHz into a 5 kohm and 1000 pF load. Maximum output is 13 V at under 1 percent THD into loads above 5 kohm. Input impedance reads 112 kohm balanced (36 kohm minimum) and 54 kohm single-ended (18 kohm minimum), while output impedance is 50 ohm, rising to 500 ohm at 20 Hz. Channel separation exceeds 100 dB at 1 kHz and 80 dB at 20 kHz. The unit draws 150 W, measures 44.5 x 44.5 x 15.3 cm and weighs 20.86 kg unpacked.

Engineering and use context

A fully balanced differential circuit is used, laid out as a mirror-image for the two channels, and has only two stages - a differential gain stage built around two 12AU7/ECC82 tubes and output buffers. No global negative feedback is applied. Volume runs through a 95-step attenuator made of discrete resistors switched by relays, giving a 70 dB range. The power supply relies on current-sourced shunt regulators, a ramped soft start for B+ and filament voltages, and large energy storage with Teflon bypass capacitors. The tube gain stage is damped against microphonics, and the output is coupled through 20 microfarad capacitors. A processor loop lets any of the eight inputs be set to fixed unity gain for a surround processor, and both loops offer tape monitoring. Two programmable trigger outputs, a bi-directional RS-232 port and per-input level offsets support system control. The machined aluminium remote handles power, source, volume, mute, fade, balance and phase reverse, and the chassis combines steel with non-magnetic material.

Pairing

Because the output stage is FET-buffered and has an XLR option, the TL6.5 can drive long cables into any tube or transistor power stage whose input impedance exceeds 5 kohm. With sensitive power amplifiers the low-gain switch is the setting to try first.

Alternatives

The VTL TL5.5 Series II Signature is another line-level preamplifier from the same Signature range.