Ribbon Microphone

Microphones / Miking

Published June 2025

Is there substance as well as style to Warm Audio’s Danish‑inspired ribbon mic?

Despite its small size, the Bang & Olufsen BM3 ribbon microphone casts a long shadow. Decades after the Danish manufacturer gave up making mics to focus on hi‑fi and lifestyle products, it remains the ultimate inspiration for current products from manufacturers such as Royer and Extinct Audio.

In the USA, Bang & Olufsen mics were imported by a company called Fenton, and the BM3 was sold under the Fen‑tone moniker. This brand name has now been revived globally by Warm Audio for a microphone very obviously inspired by the BM3.

Shine On

The Warm Audio Fen‑tone (left) is both smaller and shinier than its inspiration (right) but very much recreates the general aesthetic.Photo: Samuel Fitzgerald KayThe Warm Audio Fen‑tone closely mimics the shape of the original, to the point of including the ‘collar’ around the base even though this has no obvious function in their version. Internally, however, Warm have incorporated modern improvements, and as well as offering much greater sensitivity in passive mode, the Fen‑tone has a built‑in, fixed‑gain preamp. So, in place of the BM3’s three‑way switch, which turned it on and off and introduced a high‑pass filter, the Fen‑tone has a two‑way toggle that transforms it into an active, phantom‑powered ribbon mic.

The Fen‑tone is available individually or in stereo sets; the latter were not yet available at the time of this review, so I can’t comment on Warm Audio’s pair matching. It is supplied in a tough and practical Peli‑style plastic case, with a nice branded sock to protect it from wind blasts when not in use. However, you don’t get any sort of a shockmount, just a cheap ‘crocodile clip’ sprung holder made of black plastic, which doesn’t exactly enhance the stylish vintage aesthetic. Though well built, the Fen‑tone is noticeably smaller and lighter than either the BM3 or other modern interpretations such as the Royer R‑121 and Extinct Audio BM9, in part because the main housing is made from thinner metal than is used in those mics. The other obvious visual difference between the Fen‑tone and the other mics is that it’s a lot more shiny!

Band Politics

In a ribbon mic, the transducer is a thin strip of conductive metal suspended in a magnetic field. For this to work effectively, the ribbon needs to be corrugated. This increases its effective length, and allows the resonant frequency to be fine‑tuned by varying its tension. Bang & Olufsen exclusively used ‘pistonic’ ribbons, corrugated longitudinally for most of their length, with just a couple of horizontal pleats at either end. This has some theoretical advantages over the simpler ‘zig‑zag’ approach, although whether these translate into measurable or audible differences is debatable. However, none of the modern BM3‑derived mics that I know of have pistonic ribbons, and the Fen‑tone is no exception. Sensibly enough, Warm Audio have used pure aluminium as the ribbon material rather than the alloy in the originals, which is prone to losing its elasticity over time.

At the same time, Warm Audio have also steered clear of reproducing some of the innovations that other manufacturers have brought to the basic BM3 design. For example, the ribbon in the Royer R‑121 is slightly offset from the centre of the frame, which gives the front and back sides of the mic somewhat different tonalities. Warm Audio haven’t done this, so the Fen‑tone is a true figure‑8 mic and can be used as part of a Blumlein array, or as the Sides mic in an M‑S setup. They have also kept faith with the straightforward magnet geometry of early BM3s, rather than the more involved arrangements found in later versions and some derivatives.

Numbers Up

Warm Audio quote fairly detailed specifications for the Fen‑tone, including a frequency response that is ±3dB from 30Hz to 15kHz; within this, their plot suggests a slight presence lift around 2kHz and a smooth roll‑off above 10kHz or so. They also specify a signal‑to‑noise figure relating to use in passive mode. However, since the only source of noise in passive mics is resistive or thermal noise, the signal‑to‑noise ratio is a function of sensitivity and output impedance; and Warm’s quoted figures of ‑47dBV ref 1V/Pa and 300Ω respectively don’t quite match their claimed 87dB SNR. Moreover, the output impedance of the review mic in passive mode actually measured 600Ω rather than the specified 300Ω. This is probably because the construction uses longer wires between the ribbon motor and the transformer than are really necessary.

In practice, though, the Fen‑tone is both more sensitive than many passive ribbon mics, and not at all noisy in everyday use. And if passive mode is too quiet, the built‑in ‘Warm Lifter’ delivers the promised 26dB gain boost. Unlike the active stages found in some cheap ribbon mics, it does so without swamping the signal in additional hiss.

A natural point of comparison for the Fen‑tone was my own Extinct Audio BM9 ribbon, another modern take on the BM3 design. Subjectively, the two sounded very similar through the midrange, and on sources such as male vocals, you’d be hard put to tell them apart. I sometimes thought I could detect a slightly gritty quality to the upper midrange on the Fen‑tone that wasn’t present on the BM9; depending on the voice, this either gave it a welcome edge that helped it cut through, or slightly negated the appealing smoothness of the ribbon tone. Either way, it was marginal, and you could easily drop in on a vocal track recorded with the other mic without anyone noticing.

However, clear differences emerged when I tested the two mics as overheads on a drum kit. The BM9 was audibly more extended above 10kHz, and the bass drum sounded less weighty on the Fen‑tone. EQ matching suggested that the Warm Audio mic was at least 5dB down at 50Hz compared with the BM9. More scientific measurements later confirmed this, and also revealed that the Royer R‑121 and the original B&O BM3 both fall somewhere inbetween the two in terms of bass response. Impressively, the Fen‑tone’s measured response matched that of the BM3 extremely closely between about 100Hz and 10kHz — a band that covers most of the frequencies that matter for most sources! The slightly shy bass response could be partly down to the tension of the ribbon, but it could also be that the relatively high output impedance in passive mode loads the mic preamp, as it improved a little in active mode.

Warm Audio haven’t just replicated the look and feel of the Bang & Olufsen BM3. They’ve also successfully matched the sound.

Warm Or Cold?

With the Fen‑tone, then, Warm Audio haven’t just replicated the look and feel of the Bang & Olufsen BM3. They’ve also successfully matched the sound, whilst offering the sensitivity we have come to expect from modern mics and the convenience of active circuitry. And in a sense, that’s exactly what you’d expect, because Warm Audio’s specialism is modernising classic designs and making them more widely available.

However, we’ve also come to expect Warm Audio’s recreations to be more affordable than the alternatives, and that’s less obviously true of the Fen‑tone. For example, the aforementioned Extinct Audio BM9 is only a little more expensive, and whereas the Fen‑tone is made in China to a good standard, the BM9 is built in the UK to an exceptional standard. Across the pond, the Royer R‑10, Alder H44S and Samar AL95 are high‑quality ribbon mics made in the US, and all are less costly than the Fen‑tone, albeit that none has the same B&O‑derived visual aesthetic. (If you love that look, you could even buy an original B&O BM3 and have it upgraded with modern magnets for around the same price.)

The main thing that all of these alternatives lack compared with the Fen‑tone is the switchable active preamp. This in itself is quite an unusual feature — most ribbon mics are either active or passive, but not both — and does its job very well. A good‑quality inline preamp such as Warm’s own Warm Lifter will typically set you back at least £100, so this definitely adds value. In my tests, I found that the sensitivity in passive mode was healthy enough that I rarely needed or wanted to use active mode, but if you don’t have good high‑gain preamps, or you want to use long cable runs, it could make all the difference.

The Warm Audio price advantage is less clear‑cut with the Fen‑tone than with most of their products, then, but it undoubtedly represents another neat exercise in vintage cloning from the company who lead the way in this field.

Summary

The Fen‑tone closely matches the look, feel and sound of the classic B&O BM3, with the useful option of an active preamp built in, but faces some strong competition.