Tube or solid-state output stage: Reference DAC, Hyperion, Charon

A tube DAC is almost never a converter in which tubes do the converting. The step from numbers to an analogue signal happens in a silicon chip. The tubes, when there are any, sit after it, in the output stage that turns the chip’s raw output into a signal an amplifier can use. This guide explains what that stage does, what changes when it uses tubes instead of transistors, and why a tube buffer in a master clock is a different thing altogether.

What comes out of a DAC chip?

A DAC chip receives a stream of numbers and produces an analogue quantity that follows them. Depending on the chip, that quantity is a current or a voltage, and in neither case is it ready to be sent down a cable to an amplifier.

Current-output chips and I/V conversion

The classic multibit chips, among them the Philips TDA1540 and TDA1541 and the Burr-Brown PCM63 and PCM1704, deliver a current. Inside the chip each bit switches a weighted current, and the sum of those currents is the signal. Many modern delta-sigma chips also have current outputs; others carry an amplifier on the chip and deliver a voltage directly.

A current has to be converted into a voltage before it can drive anything. This is I/V (current-to-voltage) conversion, and there are several ways to do it:

• A resistor. The current flows through a resistor and develops a voltage across it. It is the simplest method, but the chip’s output works properly only into an almost constant voltage, so the resistor has to be small. The voltage it produces is small too, and needs gain afterwards.
• An op-amp. An op-amp wired as a current-to-voltage converter holds the chip’s output at virtual ground and produces a full-size voltage. It is compact and measures well; how it sounds depends heavily on the op-amp chosen and on the layout.
• A transformer. A transformer can perform the conversion and, at the same time, provide galvanic isolation and a balanced output.
• A tube stage. The small voltage developed across a resistor is amplified by a tube, or the tube stage is arranged to take the current more directly.

Filtering: what has to be removed

The raw output of any converter carries energy above the audio band. In an oversampling converter a digital filter has already removed most of it, but a delta-sigma chip leaves shaped noise at high frequencies, and an analogue low-pass filter is still needed. In a non-oversampling (NOS) converter there is no digital filter at all, so mirror images of the audio band appear above 22.05 kHz for CD material. The designer either filters them gently in the analogue stage or relies on the limited bandwidth of the rest of the system. Our guide to NOS multibit DAC chips explains where those images come from.

What does a DAC output stage do?

Whatever performs the I/V conversion, the circuit between the chip and the output socket has several jobs:

• Gain. It brings the signal up to line level. Two volts at full scale is the usual output of a CD source, and it is what Demograf Audio converters deliver as standard.
• Buffering. It separates the sensitive conversion stage from what follows, so that the cable and the amplifier input do not load the chip.
• Drive. It has to supply current into the capacitance of the interconnect and into the input impedance of the amplifier or preamplifier. A low output impedance makes the result less dependent on cable length and on the next input.
• DC blocking. The output must carry no DC. A tube stage always needs a coupling capacitor or an output transformer here, because its anode sits at a high DC voltage.
• Filtering and balance. The output stage often contains the analogue low-pass filter, and in a balanced design it provides the two opposite-phase legs of an XLR output.

Tube or solid-state output stage: what changes?

Neither type is correct in itself. Each has properties that follow from its devices, and listeners describe each in fairly consistent terms.

The question “tube DAC or solid-state DAC?” is therefore really a question about the system. If a tube amplifier already brings plenty of colour, a solid-state output stage keeps the converter out of the way. If the system sounds lean or sharp, a tube stage is the usual way to add body. Long interconnects and low amplifier input impedances favour a solid-state stage, or a tube stage designed for low output impedance.

Tube buffer or tube output stage?

“Tube buffer DAC” is used loosely. Strictly, a buffer adds no voltage gain: it is a stage with a gain close to one, often a cathode follower, placed after a solid-state circuit to lower the output impedance or to add a tube’s character. A tube output stage may provide gain as well, and can take over part or all of the I/V conversion. When a product is called a tube DAC, it is worth asking which of these it is, and whether the tubes are in the signal path or only in the power supply.

What does the rectifier tube do?

A rectifier tube (kenotron) does not touch the signal. It turns mains AC into DC for the high-voltage supply of a tube stage. Many listeners nevertheless report that the choice between a tube and a solid-state rectifier changes the sound, which is why the rectifier is often offered as an option of its own.

The output stage in a NOS multibit DAC

In a NOS converter the output stage matters even more than usual. With no digital filter in front of it, it receives the image frequencies along with the music, so its behaviour well above 20 kHz affects what reaches the amplifier. The multibit chips used in NOS designs are current-output parts, so the I/V stage shapes much of the result. That is why converters built on the TDA1541 or the PCM1704 are so often discussed in terms of their output stage rather than the chip alone.

Not every classic chip is multibit. The Philips TDA1547 is a vintage DAC chip of a different design, and the TDA1540 is a 14-bit chip. In each case the chip sets the starting point, and the output stage decides how much of its character reaches the loudspeakers.

Why a clock buffer is a different thing

A master clock can also have a tube stage, but that stage is not an output stage in the sense used above: no music passes through it. A master clock produces a timing signal, a steady train of edges at one frequency, which tells a transport and a converter when to act. Its buffer shapes that signal and drives it down a BNC cable to the device that receives it.

What matters there is timing. Small variations in the timing of the edges are called jitter, and in a converter they become small errors in the moment each sample is converted. A clock buffer is judged by how steadily and cleanly it delivers its edges, not by gain, harmonic profile or drive into an amplifier input. The reasons for choosing a tube or a solid-state buffer in a clock are therefore different from the reasons for choosing one in a DAC, and the two choices are independent of each other.

Questions to ask before you choose

• What follows the converter? Note the input impedance of your amplifier or preamplifier and the length of the interconnect.
• How much colour does the system already have? A tube amplifier with a tube-output converter adds two layers of the same character, which some listeners want and others do not.
• Are you comfortable with tubes? A tube stage needs warm-up and, over the years, replacement tubes.
• What outputs do you need? A balanced XLR output or a higher output level should be agreed before the build.

Which Demograf Audio piece fits

• The Reference Tube DAC is one chassis built in five versions: Calliope (TDA1540), Persephone (TDA1541), Aphrodite (the vintage TDA1547), Artemis (PCM63) and Katerina (PCM1704). It takes 16-bit / 44.1 kHz PCM over S/PDIF coaxial, with no upsampling and no DSD. The stock output stage is solid-state. The optional tube output stage uses 6Zh4, EF14, EL8, AL4 or ML6, with a 5Ts4M rectifier (5Y3 optional); in Dmitriy Gubchenko’s voicing it takes the digital edge off the signal. Output is 2 V on RCA, with XLR optional, and a ground lift switch is fitted.
• Hyperion is built on the Cirrus Logic CS4397, a multibit delta-sigma chip, with S/PDIF coaxial and USB (Amanero) inputs, up to 24-bit / 192 kHz and DSD256 over USB. Stock output is solid-state; a tube stage on 6Zh4, EF14, EL8, AL4 or ML6 can be fitted for those who want more body and air around voices. The rectifier is solid-state, or a 5Ts4M or 5Y3 kenotron. Output is 2 V, with 4 V optional.
• Prometheus is the programmable-logic converter: PCM is converted to DSD and passed through a logic-based window filter, so there is no LC output filter. Its recommended output stage is solid-state; a tube stage on AL4, EL8, EF14 or ML6 is available.
• Charon is the external master clock. Its circuit is solid-state as standard, with a tube stage on 6Zh4, EF14, EL8 or ML6 available. It has a galvanically isolated multi-transformer linear supply and BNC outputs, and its clock frequency is set for your transport.

The output tubes are selected NOS (new old stock) parts. When delivering to countries that have imposed sanctions, we do not install sanctioned components; we fit vintage and modern equivalents instead.

Every converter is made to order in Kazakhstan by Dmitriy Gubchenko. Options such as the output stage and the rectifier are chosen at order time, and each unit ships with its own measurement report. If you are unsure whether a tube or a solid-state stage suits your system, write to us with your amplifier, loudspeakers and the music you play most.