How to read an amplifier measurement report

Every Demograf Audio amplifier ships with its own measurement report, and a page of graphs can look like a wall of lines. This guide explains the common charts in plain words, using a published report on a Tantal headphone output as the worked example. It also sets out what a report cannot tell you before you choose.

What a measurement report is for

A measurement report records how one unit behaved on a test bench: how evenly it passes the audio band, how much voltage and current it can deliver, how well it keeps the two channels apart and what it adds to the signal. Measurements answer different questions from listening impressions. They will not tell you whether you will enjoy the sound, but they do show how an output is likely to behave with the headphones you already own.

Our example is a report by Reference Audio Analyzer (RAA), a website that measures headphones, amplifiers and other audio equipment on its own test bench. RAA tests every amplifier as a "black box", without regard to price or reputation. It labels the unit "DIY Tantal" under the maker Demograf, and its report does not say when the unit was built, which revision it is or how it was configured, so we cannot confirm that it matches the Tantal in today's catalogue. RAA tested the unbalanced headphone output only; the electrostatic and loudspeaker outputs were not measured.

Here are RAA's main figures, with a note on what each one tells a buyer.

The charts one by one

Frequency response and output impedance

A frequency-response graph shows level in decibels (dB) on the vertical axis against frequency on the horizontal axis, from deep bass on the left to treble on the right. A flat line means every frequency leaves the amplifier at the same level. RAA draws its reference curve with a 10 kΩ test load, which takes almost no current, so the curve shows the amplifier on its own.

In the example, the response is flat through the midrange. It falls gently in the deep bass and by several dB above about 7 kHz, with its lowest point near 12 kHz. RAA's figure for the band from 40 Hz to 15 kHz is −5.6 to +0.9 dB.

The same chart carries a second line: the output impedance, measured in ohms (Ω). This is the resistance the amplifier's output presents to whatever is plugged into it. For headphones it is often the more important number. The output impedance and the headphone's own impedance form a voltage divider, so the share of the signal that reaches the headphone depends on both; where a headphone's impedance changes with frequency, the level it receives changes with it. At an average of 36.7 Ω, the Tantal headphone output is high, as is common for tube output stages, and far higher than the fraction of an ohm many solid-state headphone amplifiers have.

How the response changes with your headphones

Because of that divider, RAA repeats the response test with different loads and reports how far each curve moves from the unloaded one. Read these numbers as "how much this output retunes the headphone".

Planar magnetic headphones and many in-ear models with a single dynamic driver have an impedance that is nearly constant with frequency, and RAA simulates them with plain resistors. The lower the load, the larger the change, mainly as a rise in the treble: 16 Ω gives a 4.5 dB deviation from the unloaded response, 32 Ω gives 3.3 dB, 64 Ω 2.1 dB, 126 Ω 1.3 dB, 256 Ω 0.7 dB and 512 Ω 0.4 dB.

A dynamic headphone's impedance usually peaks at its bass resonance. With the Grado GH4, which RAA uses because its resonance is pronounced, the response deviates by 4.3 dB, as a lift around that resonance. Multi-driver balanced-armature in-ear monitors swing widely in impedance, so the effect is large: about 16 dB with the Campfire Audio Andromeda, as a rise in the upper treble, and about 9 dB with the Fearless Audio S8F Freedom. RAA does not call such changes a defect; in its words, they are a matter of taste, like extra equalisation or a tone adjustment that either evens out or colours the final sound.

Voltage and current against load

This graph plots the voltage the output can deliver against headphone impedance, here from 10 Ω to about 1 kΩ. Voltage is given in volts RMS and sometimes in dBV, decibels relative to 1 volt. In the example the output reaches 7.2 V RMS into high impedances. Into low impedances it is limited by current, 84.4 mA in its linear range, so the available voltage falls as the load goes down.

The dashed lines are RAA's reference levels for over-ear (99 dB/mW) and in-ear (112 dB/mW) headphones at 107 dB SPL peaks. A figure in dB/mW is a headphone's sensitivity: the sound level it produces from one milliwatt of power. Where the amplifier's curve sits above a dashed line, it can reach that loudness. On RAA's graph the Tantal reaches the reference for typical full-size headphones and for in-ear models at every load from 16 Ω up, and for high-impedance headphones, which need voltage more than current, 7.2 V leaves a generous margin.

Crosstalk between channels

Crosstalk is the part of one channel's signal that leaks into the other, given in dB below the signal; the larger the negative number, the better the separation. At 1 kHz the example measures −25 to −34 dB, depending on the load. RAA treats values worse than −40 dB as audible in the stereo image, which then moves towards mono or, more commonly, widens and blurs the placement of sources. The curve runs at about −55 dB in the bass and rises with frequency; in the top octave it comes within a few dB of the signal. RAA's phase graph puts most of the treble crosstalk close to the out-of-phase zone, which it associates with a wider, less sharply focused stage.

RAA adds two cautions: a rise at high frequencies usually points to capacitance in the headphone socket's contacts, and crosstalk depends heavily on contact resistance, the headphone cable and the impedances involved. So this graph gives a general picture, not a fixed property of your system.

Distortion and noise

Distortion charts show what the amplifier adds to the signal. Harmonic distortion appears as extra tones at whole-number multiples of the original frequency; intermodulation tests feed in two tones and look for new tones made by their mixing. RAA publishes distortion against output level for nine loads, with harmonic spectra and intermodulation tests, but gives no single summary figure for this unit. As is typical of tube output stages, its graphs show distortion rising with level towards full output.

A noise figure describes the hiss an output produces with no music playing, usually as a voltage or as a ratio in dB to a reference level; it is commonly described as most noticeable with sensitive in-ear monitors. The measurement article we draw on here quotes no noise figure for this unit.

How to compare two reports

Two reports can be compared only where they measured the same thing in the same way. Check these points before you set numbers side by side:

• The same output. A headphone output, a loudspeaker output and a balanced or unbalanced socket are different circuits. The example covers the unbalanced headphone output only.
• The same band. A figure over 40 Hz–15 kHz is not the same as one over 20 Hz–20 kHz. Read the conditions next to each number.
• The same loads. Output voltage, response deviation and crosstalk all change with the load, so compare 32 Ω with 32 Ω, not with "no load".
• The same reference. Loudness margins depend on the reference levels used, such as RAA's 99 dB/mW and 112 dB/mW lines.
• The same frequency for crosstalk. A single figure at 1 kHz says little about the treble, where the example's crosstalk rises.
• Similar cables and sockets. As RAA notes, crosstalk depends on contacts, cable and impedances, so small differences between benches are to be expected.

Compare the graphs as well as the summary figures. Two outputs with the same average output impedance can still shape a headphone differently if their curves differ across the band.

What a report does not tell you

A report describes one unit, on one bench, on one day. It does not tell you:

• How you will like the sound. RAA itself treats a headphone-dependent response as a matter of taste, not a fault.
• That a different unit measures the same. For the RAA example, the build date, revision and configuration are not known.
• How the outputs it skipped behave. Here, the electrostatic and loudspeaker outputs were not measured.
• How your cable and socket affect the result. Crosstalk in particular depends on them.
• How the figures change over time. Tubes wear with use; owners and technicians commonly replace them when listening or measurement shows wear, not on a schedule.

What ships with an amplifier

Each Demograf Audio unit is made to order and ships with its own measurement report. Made by Dmitriy Gubchenko. The report comes before the shipment: a 50% queue deposit reserves the build slot, and the balance is due after you receive the measurement report, before shipping. That gives you time to read it with this guide in hand and to ask about anything you do not understand. If a particular measurement matters to you, ask about it when you discuss the configuration.

On tube models such as Tantal, the output tubes are selected and matched in pairs, and a reserve matched pair can be ordered with the amplifier. Every unit is packed for long-distance transport in heavy-duty flight cases and a wooden crate. Shipping and insurance are paid by the buyer and shown as a separate line in your quotation.

Which piece fits

• Tantal uses vintage 3C24 output triodes with tantalum anodes. On RAA's measurements, its headphone output is a natural match for high-impedance dynamic headphones (roughly 250 Ω and above): the response changes by less than 1 dB and the voltage is ample. Low-impedance planar and dynamic headphones play loudly enough, with a treble lift of a few dB that depends on their impedance; multi-armature in-ear monitors are a poor match if you want to hear them as their makers tuned them. Tantal also has a STAX-type output for electrostatic headphones as standard.
• Tantal Senior uses vintage 4E27 pentodes with tantalum anodes, each pair measured and matched for emission and distortion. The RAA report does not cover it.
• Eurybia (2A3 triodes) and Solaria (45 triodes) are low-power single-ended designs, 2 W per channel. The 6B4G is an octal-base relative of the 2A3; in owner reviews in our Journal, a 6B4G amplifier made the Sennheiser HD 800 fuller and darker.

Every tube amplifier in the range has a 6.3 mm and a 4-pin XLR headphone output beside its loudspeaker terminals, and the solid-state Hades and Bellerophon add a built-in DAC. Each model's own report, not the Tantal example, is the one to read for that model. All headphone-capable models are listed on the headphone amplifiers page.