FiiO Warmer R2R DAC REVIEW – The Heat Is On

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The $349 FiiO Warmer R2R DAC was provided by the company for my review – and I thank them for that. You can get it here. The R2R explanation in this article is borrowed from Kazi’s FiiO K13 review.

The $349 FiiO Warmer R2R DAC is a very attractive looking and good sounding bare-bones tube DAC that is unique in this combination at this price.

PROS

Unique marriage of sound quality and looks
Attractive design and impeccable build
Great real VU meters

CONS

Output power at 3 V (balanced) is low and should be 4 V
Not as warm sounding as expected from a tube amp
Flimsy on/off button
Backlight of VU meters cannot be dimmed or switched off

Introduction

FiiO have been big on retro designs lately. We remember their CP13 Cassette player, their DM13 BT portable CD player, their TT13 turnable, not to forget, their excellent K17 desktop DAC/amp combo…and a few more. Their latest very interesting endeavours were two R2R DACs, the FiiO K11 and K13, which are desirable for two reasons: they are very good and very reasonably priced. Traditionally, R2R DACs have been $$$$ as the technology is expensive.

If you are not familiar with the R2R technology, you can catch up on it behind this spoiler.

R2R Explained

An R2R ladder DAC translates digital values into analog voltages using a precisely arranged network of resistors. The “ladder” gets its name from the repeating sequence of two resistor values, R and 2R, that form a balanced structure. Each bit in the digital word controls a small switch, connecting a node in the ladder either to a reference voltage or to ground. When the switches operate together, the voltage drops across the resistors combine in proportion to the binary weight of each bit, creating a smooth analog output that mirrors the digital input.

R2R Network Diagram — A 4-bit R2R network.

What makes this design clever is that it avoids the need for exponentially scaled resistors, which are nearly impossible to match accurately in production. Instead, it relies on identical resistor pairs, which are easier to manufacture consistently. That said, the performance of an R2R DAC still depends on extremely precise resistor matching. Even a fraction of a percent variation can introduce non-linearity or subtle distortion. These imperfections are often quantified as Integral Non-linearity (INL) and Differential Non-linearity (DNL). To achieve high accuracy, premium R2R DACs use laser-trimmed or thin-film resistors with very tight tolerances.

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And now there is the FiiO Warmer, a DAC with a tube pre-amp that shares its R2R module with the K13, but does not feature the K13’s amp part (or any amp). It was not designed with any amp combination in mind. I saw it on pictures paired with the K17, but that was coincidental. Use your fantasy for combinations..

What strikes everybody, and that’s the retro part, are these gigantic VU (“Volume Unit”) meters, that will particularly appeal to us older audiophiles. VU meters delivered the optimal appeal of power amps in the 1980s. Luxman and Onkyo had such dinos (Onkyo still got a current one) … and they were fantastic. Even if you don’t have a VU meter handy, play Electric Light Orchstra’s Mr Blue Sky (YouTube) and imagine how these little needles jump around.

But, VU meters on a DAC? That’s certainly something new (for me).

R2R vs. Delta Sigma and the DAC Chip Zombie Myth

In our previous lives, our daily bread has been the so-called ΔΣ DACs (if that’s Greek to you, I translate: Delta-Sigma DACs. These use oversampling and noise-shaping to convert the digital signal to analog. You are well aware of ESS Sabre and AKM chips, which are assigned characteristic sonic qualities by the reviewing echo chamber…which is a bunch of stirred-up nonsense…as these chips only perform data handling.

The sound is determined by the output stage, which is individually designed for each DAC by the audio engineers (unless it is a System on Chip, that is DAC and AMP on one small logic board). The DAC chip zombie myth makes reviewing just a little easier.

ΔΣ DACs have the reputation of being cost effective, they achieve hi res and tend to sound neutral and detailed, with excellent instrument separation; but they can sound “digital” and not natural, as criticized by the vinyl crowd. No surprise that essentially all CD players operate with ΔΣ DACs.

R2R converters are far more direct in their approach. They convert every bit of data in real time without relying on heavy digital filtering, resulting in almost no latency and a very “immediate” sound. However, they tend to measure poorly [hello SINAD believers and audiosciencereviews.com dogmatists, you can tune out now!] and in general show higher distortion and noise floor than their delta-sigma counterparts.

R2R DACS may sound more “analog” and warm compare to ΔΣ [hence “FiiO Warmer”], providing a more immersive feel, at the cost of detail resolution. The technology is generally more expensive than ΔΣ and quality varies widely.

Basic Specifications FiiO Warmer

Color: Black, Silver
Dimensions: About 223.5x213x66.8mm (incl. feet)
Weight: About 2865g
Display: Mechanical pointer VU meter
DAC: Fully differential true 24Bit R2R resistor array
Amplifier stage tube: JJ ELECTRONIC E88CC*4
Linear power supply: 46W toroidal audio linear transformer
USB decoding: PCM 384KHz/32bit; DSD256
Coaxial decoding: PCM 192kHz/ 24bit
Optical decoding: PCM 96kHz/ 24bit
RCA output: L+R≥1.8Vrms+1.8Vrms (1kHz@10KS)
XLR output: L+R≥3.8Vrms+3.8Vrms (1kHz@10KS)
Frequency response: 20Hz-45KHz, attenuation < 2.4dB
SNR: ≥114dB (A-weighted)
Noise floor: ≤7uV (A-weighted)
THD+N: ≤0.053% (1kHz/-7dB@10k2)
Tested at: $373 USD
Product Page: fiio.com
Purchase Link: FiiO Audio Store

You find the Warmer’s extended parameters on the FiiO page.

Physical Things

In the box…

①—————————————– WARMER R2R*1
②—————————————-AC power cable*1
③——————————————— USB cable*1
④————————————— Spare fuse tube*1
⑤————————————– Quick start guide*1
⑥—————————————– Warranty card*1

Build is impeccable, the VU meters are almost a deal maker, the on-off button is a bit flimsy. Sadly, the VU meters’s backlight cannot be dimmed or switched off.

Functionality and Operation

The Warmer is a plain-Jane DAC for the purist.

It does

connect to balanced and single-ended amplifier circuits
accepts a variety of sources per Bluetooth (phone, dap), USB (computer), and coaxial/optical (CD player)
have the ability to upsample to 384 kHz

It does not

work as amp or pre-amp
feature selectable filters
have a remote or app

Interface

Front Panel

Minimalistic arrangement: on/off button on the left, rotary knob for inputs on the right (optical, coax, USB). BUT: a monstrous VU meter panel in the centre occupying most of the front panel. The VU meters are real, they are physical, moving needles. A pity that the bright backlight cannot be dimmed or switched off.

Back Panel: I/O

As usual, the rear features the inputs (USB, coax, optical) and the outputs (single-ended RCA; balanced XLR), the power port and two toggle switches: one for upsampling, the other for alternating between USB 2.0 and 1.0 (1.0 for legacy source devices).

Oversampling

The Warmer can upsample a digital input signal up to 385 kHz. This means it creates non-existing data points and interpolates between them, a bit like hedging at the stock market. This creates improved resolution…but at a price as we will see in the next chapter.

TL;FR: the purist will stick to the original sample rate.

Sound

I tested the Warmer with desktop headphone devices: the Burson Funk via the single-ended RCA connection and with FiiO’s K17’s internal amp through balanced (with an XLR to 4.4 mm pentaconn cable) and AB’ed to the K17’s internal DAC. An M4 Mac Mini was the music source in all cases, and I listened with Sennheiser HD 550 and HD 600 headphones and the NICEHCK Rockies earphone.s

As mentioned, the Warmer offers an oversampling switch in the back that upsamples the signal from my 44.1 kHz by the factor of 8 to 384 kHz (for USB input only; upsampling is max 192 kHz at 24-bit depth in optical and coax modes). This means, in OS mode, the DAC creates non-existing data points and extrapolates between them. The difference is very obvious to my ear.

My standard usage is the NOS [“No OverSampling”] mode, as I prefer the original sound. The Warmer plays…warm (but not as warm as a typical tube amp) and relaxed compared to the K17’s ΔΣ DAC, and its output stage has less power. At balanced 3 V output power, you have to turn the amp’s volume up compared to the K17’s internal DAC. A balanced output power of 4 V would be typical for a tube amp, and for matching the K17’s ΔΣ DAC output power. In single-ended mode, the Warmer’s output is even smaller. I really had to turn the Burson up quite a bit more than with the EarMen Tradutto or SMSL DO200 MKII DACs.

In comparison, the K17’s ΔΣ DAC sounds punchier, narrower, deeper, denser, edgier, with harsher notes and, in analogy to a digital photo, overpixelated. Everything is more robust with a bigger drive. Voices are weightier and everything is right into your face/ears. The Warmer plays smoother, wider, finer, more fragile, softer around the edges, less bassy, with more rounded notes (=sacrifices resolution)

If you switch the Warmer to OS (with USB input), the focus increases, image becomes more distant, the stage widens, and the notes become better defined/sharper, from a tighter bass to a cleaner midrange, which adds clarity but also a clinical component.

Going back to NOS, the music comes closer, bass is rounder with more rumble, the notes are broader and less defined, but the sound is more organic with more soul. I therefore happily sacrifice technicalities for authenticity. I only use OS for podcasts as it adds definition to the voices and therefore clarity.

The Warmer’s downside – if it is a downside – is that it has limited resolution compared to more expensive DACs. For example, the Questyle CMA Twelve Master plays a class above in terms of resolution and imaging, particularly in spatial reconstruction, where the Warmer lacks depth and width in comparison. The Questyle also has weightier notes and better midrange transparency.

Concluding Remarks

R2R DACs have traditionally been expensive but FiiO have lowered the ladder [excuse the pun] with their K11, K13, and Warmer models. Considering their affordability and uniqueness, it is certainly worth trying one of these out. The Warmer does not only have a warm sound [excuse the flat joke again] but also a great optical appeal for older audiophiles like me, whose monstrous 1980s power amps don’t fit in their permanent-care homes. Just kidding.

As they say: the last shirt has no pockets.

Until next time…keep on listening!