There are any number of ways to build a proper DAC. Think single-ended or balanced topology, ΔΣ chips, R2R chips or discrete, FPGA-based, DSD --> low-pass filter, all of them terminating in an output stage of op-amps, transistors or valves with our without output transformers. Add various memory buffers, upsamplers, sample-rate converters or the zero-sampling NOS approach. Serve up Redbook, hi-rez, PCM, DSD or both. Go fancy on a CNC-milled chassis, stick to a wooden box or keep it simple with bent sheet metal. And that's not the end. In short, design options are legion. To pick just one solution from each group seems somewhat limiting with so much good stuff available. Yet truth be told, properly implemented, each school of thought can deliver fine outcomes.

What about the AMR DP-777SE? What kind of an animal is it? For starters, AMR's lucky number seven is a dual-mono single-ended affair with two separate DACs plus a valved output stage to seal the deal with a harmonically rich finish. Tubes make the unit flexible to some extent. Throw in selectable conversion methods. Physically, the DP-777SE is on the bigger side, measuring 45 x 37 x 12cm WxDxH and weighing 11.5kg. Frequency response is 20Hz–20kHz, A-weighted S/NR is >100 dB, THD+N is <0.3%. Dynamic range and channel separation are the same >90dB. Finish is black or silver and switchable 115/230V AC makes for one universal model. Power consumption in standby is below 1 watts and below 60 watts in operation. The build is tank-like to communicate seriousness of purpose.

It's a full-size enclosure made from aluminium. The front sports a big monochromatic display of blue digits against black for high legibility. There's a dim/off option. The readout includes filter choice, bit rate, volume, digital input and jitter lock. The last feature is subject to AMR's proprietary GMT system clock tracker. Once 'J' blinks, GMT is in search mode. As soon as lock is established, 'J' displays solid. A bit below the display, five buttons control power, ±input and ±volume. Once pressed, each button fires up a nice blue light ring. The rear is crowded with two analog RCA inputs and one each RCA/XLR outputs to make for direct-coupled preamplifier use. In DAC-only mode, the volume control can be mechanically bypassed with a small switch near the analog inputs. Digital inputs are more than generous with 2 each AES/EBU, BNC, Toslink and coax and 1 x USB type B. That's nine digital inputs in total!

AES/EBU and BNC are different by adding a nearby switch to match impedance. The most important aspect is that these particular inputs are tube driven, subject to AMR's proprietary VDi circuitry. This is a rare sight. Moving on, the rear panel is also equipped with the 115/230V selector and an IEC inlet with spare fuse. The aluminium top sports a rather large logo and six acrylic windows on certain innards, especially the valves as the key players. Four rubber footers in round aluminium rings couple to terra firma. Each footer ends in four smaller tabs which prevent wobbles even on rough surfaces. This machine is very well made. It looks honest, serious and was a pleasure to operate. Each operational option is accessible from the small metal remote. Over the few months I had the deck, everything worked without any hiccup under quite extensive use. A peek under the hood shows quite simple innards. For that one undoes four rear screws which then allow the cover to slide off. Easy. This reveals a larger motherboard and two smaller ones, one of which houses AMR's proprietary USB transceiver. Rather than an OEM Amanero or equivalent, AMR roll their own based on XMOS and similar to what's in their iFi range. The digital board is unique. It contains two very different converter circuits bearing the HD Gemini Engine name.

Gemini could suggest very similar converters but that's really not the case. One is based on a 32-bit chip meant for files of high sample rates. The other sports a quad of vintage 16-bit R2R UDA1305AT chips presumably made by Philips and closely related to their famous TDA1543A crown jewel. To increase S/NR, each channel gets two of them, hence the quad-core marketing slogan. This converter is dedicated to Redbook resolution and only supports 44.1/48kHz fare at 16 bits. An FPGA handles glue logic, buffering, DSP and ISM aka intelligent system memory tasks. To combat ringing for the HD module, there are user-selectable filters spanning apodizing, minimum phase, traditional and organic. The classic module gets two bit-perfect options, one with analog filtering. Options galore. The analogue section looks as though it was built by an enthusiast for enthusiasts. In large part it's really a power supply. Each channel has its own AMR transformer. The same bespoke approach goes for crucial signal-path capacitors. I couldn't identify their origin but it's no exaggeration to put them in the boutique category.

The output stage is based on one NOS GE5670 JAN (joint army/navy) double triode per channel. There's an interesting story behind it. The very similar 6922 gained enormous popularity over the years whereas the 5670 with its slightly different pin-out flew under the radar to patiently await discovery. Not only does that make it affordable and quite available, its JAN version is more rugged. It's said to sound very good especially for the money and the only thing necessary is a simple pin adapter which of course is included. One half of each 5670 amplifies the signal, the other buffers it. Schottky diodes show up for rectification and filtering. There are no solid-state devices in the signal path, neither op-amps nor transistors, just valves and capacitors. Let me repeat that all of it looked and felt very music-oriented and solid. One of the most surprising things were the tubed BNC inputs with their single NOS 6H11P double triode. With very well-written S/PDIF explanations on their website, there's no need to dive deeper here. The takeaway seems to be perfect square-wave performance from valve-buffered S/PDIF. Or as AMR put it, some analogue problems need analogue solutions. Finally, the volume control is a chip-based stepped attenuator with switched resistors in the analog domain, again executed without op-amps.