Welcome to the next part of our microphone guide.

In the previous guides, we looked at moving-coil dynamic microphones, ribbon microphones and condenser microphones. This time we are looking at a microphone design which sits within the condenser family but uses a different type of internal electronics: the valve microphone (or tube microphone in American terminology).

Valve microphones are often associated with a rich, smooth and sometimes slightly coloured sound. Many classic studio microphones which have enriched countless vocal and instrument recordings belong to this category.

Valve microphones are often associated with a rich, smooth and sometimes slightly coloured sound. Many classic studio microphones which have enriched countless vocal and instrument recordings belong to this category.

But first, let’s look at what makes a valve microphone different from a standard solid-state condenser microphone.

Before we start: a valve microphone is still a condenser microphone

The term ‘valve microphone’ describes the electronics inside the microphone rather than a separate transducer principle. The microphone still uses a condenser capsule; the difference lies in the electronics which follow it.

Sound moves the diaphragm which changes the capacitance between the diaphragm and backplate:

sound wave → diaphragm moves → capacitance changes → electrical signal is produced

Instead of using a solid-state circuit to buffer and amplify the signal from the capsule, a valve microphone uses a valve as part of its internal electronics.

A little history

Valve microphones became particularly important during the middle of the twentieth century when valve technology was standard in recording, broadcast and telecommunications equipment.

Indeed, many classic studio microphones are valve microphones. The Neumann U47, introduced in 1949 and already discussed in our condenser microphone guide, is perhaps one of the most famous examples.

Another is the Neumann M49, introduced in 1951 and used extensively for vocals, orchestral recording and other studio applications. The M49 can even be seen in photographs from the sessions for Miles Davis’s Kind of Blue.

The AKG C12, introduced in the 1950s, became another influential valve condenser design, followed by microphones such as the Telefunken ELA M 251.

During the 1960s, solid-state electronics increasingly replaced valve circuits in microphone design. Transistors, particularly FETs, allowed microphones to become smaller, generate less heat and operate without the comparatively elaborate power supplies required by valve microphones. They were also generally more robust and consistent.

Nevertheless, valve microphones remained highly valued and many manufacturers continue to produce both modern valve microphones and recreations of classic designs.

How does a valve microphone work?

The first part works just like a condenser microphone: a thin diaphragm and fixed backplate form a capacitor. Sound reaching the microphone moves the diaphragm which changes the distance between the diaphragm and backplate. This change in distance causes the capacitance to change, producing an electrical signal.

The signal coming from the capsule is electrically very small and has an extremely high impedance, so it cannot simply be sent down a long microphone cable without help from additional electronics.

This is where the valve becomes important: it forms part of the impedance-conversion and amplification stage inside the microphone.

The overall process is:

sound wave → diaphragm moves → capacitance changes → very small electrical signal → valve electronics buffer/amplify the signal → microphone output

It is the condenser capsule which performs the actual sound-to-electricity conversion, while the valve electronics buffer and amplify the signal so it can be passed on to the microphone preamplifier, mixing desk or audio interface.

What does this construction mean when recording?

Valve circuits behave differently from modern solid-state electronics. Depending on the circuit design and operating level, they can introduce characteristic forms of harmonic distortion which may sound musically pleasing.

The result is a sound which is often described as:

• warm

• rich

• smooth

• full

• slightly compressed

• harmonically complex

However, these descriptions are a) subjective and b) different valve microphones can sound very different from each other, even when they use the same type of valve. The sound is also determined by more than the valve alone: the capsule, transformer, circuit design, power supply and other components all influence the final result.

Harmonic distortion and saturation

As briefly mentioned, one reason valve microphones can sound distinctive is the way their electronics generate harmonic distortion. When a valve circuit is driven harder, it can move gradually towards saturation rather than reaching a sudden hard clipping point.

This saturation can introduce additional harmonics into the signal which may make the sound seem fuller or richer without sounding obviously distorted. Think of this change in character as a gentle change in tone and density.

Valve microphone or solid-state condenser?

Since both designs use the same transducer principle, the difference lies in the electronics which follow the capsule. A solid-state condenser microphone generally uses transistor circuitry, often based around FETs, while a valve condenser uses a valve.

Neither design is automatically better. A solid-state microphone can be extremely transparent, detailed and low-noise, while a valve microphone may introduce more tonal colour and harmonic character. The choice therefore depends on the source and on the sound required from the recording.

Advantages and disadvantages

Where would we use a valve microphone?

Valve microphones are particularly common in situations where the character of the microphone becomes part of the recording.

Typical applications include:

• vocals

• voice-over

• acoustic guitar

• piano

• strings

• brass

• room recording

• orchestral recording

• drum room microphones

• acoustic ensembles

Vocals might perhaps be the most famous application since valve microphones can help create a sound which feels full and present without requiring an aggressive high-frequency boost.

However, always listen to the sound as the choice depends heavily on the overall sound of the singer, the room and the particular microphone.

It often happens that a valve microphone which sounds excellent on one voice may be completely unsuitable for another.

Valve microphones are particularly useful when the character of the microphone itself becomes part of the recording.

Power supplies

Valve microphones are often easy to recognise from their dedicated external power supply (PSU). The microphone normally connects to the PSU using a multi-pin cable. The PSU provides the voltages required by the valve circuit and the capsule polarisation voltage. On many multi-pattern valve microphones it also provides the polar-pattern control.

The PSU provides an audio output, commonly via a standard 3-pin XLR connector on modern systems, which can be connected to the microphone preamplifier, mixing desk or audio interface.

So the signal path often looks like this:

microphone → multi-pin cable → PSU → XLR cable → preamplifier / mixing desk / audio interface

Warm-up time

Vacuum tubes, or valves, need to reach their normal operating temperature before the circuit reaches stable operating conditions. This is true for a valve guitar amplifier as well as for a valve microphone. Hence, allow valve microphones some warm-up time after they have been switched on.

The exact warm-up time depends on the microphone and manufacturer, but it is good practice to allow the microphone to stabilise before an important recording session. In the same spirit, after finishing the recording and switching the microphone off, allow it some time to cool before packing or moving it.

Some well-known valve microphones

Microphone	Typical use
	Vocals, instruments and general studio recording
	Vocals, orchestral recording and room microphones
	Vocals, strings, piano and orchestral recording
	Vocals and acoustic instruments
	Modern pop and vocal recording
	Vocals and general studio recording

I hope this introduction to valve microphones was useful. If there are any questions, drop me an email.

In the next episode, we can look at piezoelectric microphone design and its applications.