Passive or active preamplifier: TVC, resistor attenuators and tube line stages

Every system needs a way to choose a source and set the volume. Whether a passive attenuator or an active preamplifier with its own gain and power supply should do that job is one of the oldest debates in hi-fi. This guide explains how passive resistor and transformer volume controls and active tube and solid-state line stages work, how gain and impedance decide which suits a system, and why a phono stage is a separate question.

What does a preamplifier do?

A preamplifier sits between the sources and the power amplifier. Its basic jobs are to select a source and to set the level. An active preamplifier can also add gain, and its output stage can drive long interconnects and amplifiers with a low input impedance.

The key point is level. A CD player or DAC typically delivers about 2 V at full scale, and many power amplifiers reach full output with an input of around 1 V or less. In most digital systems the preamplifier therefore has to reduce the level, not raise it. That is why a passive preamplifier, which can only attenuate, works in so many systems.

How does a passive preamp work?

A passive preamplifier has no gain stage and, in its simplest form, no power supply. It consists of an input selector and one of two kinds of attenuator.

Resistor attenuators

A potentiometer or a stepped attenuator is a voltage divider: part of the signal is dropped across one resistance and the rest is passed on. It is simple and inexpensive, but its impedances change with the setting. The source sees a load between the attenuator’s full resistance (at minimum volume) and that resistance in parallel with the amplifier’s input impedance (at full volume). The amplifier, in turn, sees an output impedance that rises from almost zero at full volume to a peak of a quarter of the potentiometer’s value near the middle of its travel, at about −6 dB. A 100 kΩ potentiometer can therefore present up to 25 kΩ to the interconnect and the amplifier, which is high enough to interact with cable capacitance. A stepped attenuator behaves the same way, with better channel matching.

Transformer and autoformer volume controls (TVC)

A transformer volume control sets the level by selecting taps on a transformer winding with a stepped switch. A true transformer has separate primary and secondary windings; an autoformer uses one tapped winding. Both are commonly called TVCs. The important difference from a resistor is that a transformer transforms impedance as well as voltage, by the square of the turns ratio. At −12 dB the voltage ratio is 4:1, so the amplifier sees the source’s output impedance divided by 16, while the source sees the amplifier’s input impedance multiplied by 16. As the volume goes down, the output impedance falls and the load on the source becomes lighter, the opposite of a potentiometer.

The trade-offs are in the transformer itself. Its bandwidth, its behaviour at high level in the bass, its winding resistance and its shielding all matter. It still adds no gain, it has a fixed number of steps, and it wants a source with a reasonably low output impedance, because the transformer’s own inductance loads the source at low frequencies.

Transformer volume control vs potentiometer

• Output impedance. A potentiometer’s rises towards the middle of its range; a TVC’s falls as the volume is turned down.
• Load on the source. A potentiometer loads the source with its own resistance at every setting; a TVC presents a lighter load at lower volume.
• Channel balance. Potentiometers can track unevenly between channels near the bottom of their range; stepped attenuators and TVCs are matched step by step.

Owners moving from a resistor passive to a TVC commonly report fuller dynamics and bass at low volume, which the impedances above partly explain.

Active preamplifiers: tube and solid-state line stages

An active preamplifier, or line stage, adds amplification and an output buffer after the volume control. Its low output impedance drives long cables and amplifiers with a low input impedance without loss of bass or treble, and its gain helps sources with a low output level. A solid-state line stage can be very quiet and neutral. A tube line stage brings a character commonly described as fuller and more three-dimensional.

The cost is that every active stage adds its own noise, distortion and colour. If a system already has more gain than it needs, an active preamplifier adds more, and the volume control ends up near the bottom of its range, where noise is more audible and potentiometers track least well.

Gain structure: how much gain does your system need?

Gain structure is the chain of gains and levels from source to loudspeaker. Start with three numbers: the source’s output level, the amplifier’s input sensitivity (the input voltage that gives full power) and the loudspeaker’s sensitivity.

• A DAC with a 2 V output feeding an amplifier rated for full power at 1 V already has about 6 dB more level than it needs. A passive preamplifier will do.
• With loudspeakers of 95 dB and above, normal listening uses less than 1 W, which strengthens the case for a passive.
• A source with an output of 0.5 V feeding an amplifier that needs 1.5 V for full power is about 10 dB short. It needs an active stage, or a source with a higher output.

Signs of too much gain are audible hiss, a volume control that never passes nine o’clock and poor channel balance at low settings.

Impedance: source, cable and amplifier

A long-standing rule of thumb is that an input impedance should be at least ten times the output impedance driving it. With a passive preamplifier, the output impedance the amplifier sees is the source’s output impedance plus whatever the attenuator adds, and that is the number to check.

Cable capacitance completes the picture. Interconnects commonly measure about 50–150 pF per metre, and together with the output impedance they form a low-pass filter. With 25 kΩ driving 2 m of cable at 100 pF per metre, the treble starts to fall at about 32 kHz, close enough to the audio band to soften it slightly; with 5 m of the same cable the corner drops to about 13 kHz, which is clearly audible. With a TVC, or with an active stage of a few hundred Ω, the same cables put the corner far above the audio band.

The amplifier’s input impedance matters too. Many tube amplifiers have 47–100 kΩ, which is an easy load. Some solid-state amplifiers have 10 kΩ, which loads a resistor attenuator and a tube source much more heavily.

When does a passive preamp work, and when does it not?

As a rule, a passive preamplifier, and a TVC in particular, works well with sources of 2 V or more from a low output impedance, as most modern DACs are, short interconnects, amplifiers with an input impedance of tens of kΩ, and sensitive loudspeakers. An active stage is the safer choice when a source has a low output, when the amplifiers stand far from the sources (monoblocks beside the loudspeakers, for example), when one preamplifier must drive several amplifiers or a subwoofer in parallel, or when the amplifier’s input impedance is low.

Phono stages: a separate need

A turntable cartridge produces far less signal than a line source: a moving-magnet (MM) cartridge typically a few mV, a moving-coil (MC) cartridge often a few tenths of a mV. Records are also cut with the RIAA curve, which reduces the bass and raises the treble; playback must reverse it, by roughly 20 dB at each end of the audio band relative to 1 kHz. A phono stage provides the gain, commonly 35–45 dB for MM and 55–70 dB for MC, the RIAA equalisation and the correct load for the cartridge.

No line preamplifier can replace it: the phono stage comes first, and its line-level output goes to the preamplifier or to an integrated amplifier. For MC cartridges the extra gain comes either from more active gain or from a step-up transformer (SUT) ahead of an MM-level stage. Our article Vinyl forever covers cartridge alignment and loading.

Volume controls and preamplifiers in the Demograf Audio range

Hector is a preamplifier in two versions, tube and solid-state, built on a transformer volume control on a Swiss stepped mechanism in a wooden chassis, with an Elma rotary input selector; the passive-only version needs no mains connection at all. Its TVC is inspired by the Stevens & Billington TX-103 passive volume control. Hector is built as a passive TVC, a relay-switched TVC with remote control, an active solid-state or tube preamplifier (on EL8, AL4 or ML6 tubes), or a dual-mode unit that switches between passive and active.

Tantal Senior, our 30 W single-ended amplifier on 4E27 tubes, has a transformer volume control built in: the level is set by a transformer attenuator on an Elma stepped switch, and the circuit has no negative feedback. A DAC can therefore connect to it directly.

Our other tube amplifiers, from Solaria to Ether, have an ALPS potentiometer as standard, with a Demograf Audio stepped attenuator as an option. They can also be ordered with the Demograf Audio Swiss-stepped pre-amp, which the catalogue describes as similar to the Stevens & Billington TX-103, and with a variable gain control module. Our DACs, the Reference Tube DAC, Hyperion and Prometheus, have a 2 V output with 4 V as an option, and can be fitted with an ALPS volume control or the Swiss-stepped pre-amp to drive a power amplifier directly.

Ulixes is our phono stage for MM or MC cartridges: one chassis and a solid-state circuit as standard. A tube version (a pentode first, for gain and low noise, then a triode, with a kenotron-rectified supply), a two-chassis build and MM/MC inputs in one unit are options; a step-up transformer can be added for MC.

Which Demograf Audio piece fits

• A DAC of 2 V or more, a short cable and a power amplifier without its own volume control: Hector as a passive TVC, or as a relay-switched TVC if you want remote control.
• Low-output sources, long cables or several amplifiers: Hector as an active solid-state or tube preamplifier, or as a dual-mode unit, so you can compare passive and active in your own room.
• One source and one amplifier: Tantal Senior with its built-in TVC, or a Demograf Audio DAC fitted with a volume control.
• Vinyl: Ulixes, with a step-up transformer for MC, into Hector or an integrated amplifier.

Every piece is made by Dmitriy Gubchenko. Each is made to order, and Hector and Ulixes each ship with their own measurement report. Our guide to power triodes for single-ended amplifiers covers amplifier matching, and our glossary of hi-fi terms explains the other words used here. Tell us your sources, amplifier and cable lengths, and we will suggest whether a passive or an active stage fits.