Conversion Topology
What is the Origin R2R ladder?
EverSolo describes the converter as an in house discrete resistor ladder, FPGA driven, 5+20 bit, fully differential. Each element of that description carries weight.
Discrete means the conversion network is built from individual precision resistors on the board rather than integrated into a purchased converter chip. FPGA driven means the digital filtering, mode switching and ladder sequencing are implemented in programmable logic that EverSolo controls, rather than being fixed in silicon designed by a chip vendor. Fully differential means two complementary ladders per channel, so the signal is balanced from the instant of conversion rather than being converted single ended and then split.
What does 5+20 bit mean?
It describes a segmented architecture. A pure 24 bit ladder would require resistors matched to a precision that is not manufacturable at sane cost, because the accuracy demanded of the most significant element scales with the word length. Segmentation splits the problem: an upper section handles the most significant bits, here five of them, typically thermometer coded so all elements are nominally identical and errors do not accumulate at the major carry transitions, and a lower section handles the remaining twenty in a binary weighted ladder.
Segmentation is the standard answer to the central weakness of ladder converters, which is differential nonlinearity at the large code transitions. It is why the architecture is quoted this way rather than simply as 24 bit.
Why does any of this matter against delta sigma?
Delta sigma converters trade amplitude resolution for time resolution: a coarse quantiser running very fast, with the quantisation noise shaped out of the audio band and removed by filtering. They measure extremely well and the noise shaping is mathematically elegant. The criticisms levelled at them concern what the modulator and the steep reconstruction filter do with transient material and with the noise floor’s behaviour, rather than anything visible in a steady state THD measurement.
A ladder converter reconstructs each sample directly as a voltage. Its weaknesses are different ones: resistor matching, thermal drift, and glitch energy at code transitions. It is a genuine engineering trade rather than one approach being correct. The reason to care is that the trades are audible in ways that standard measurements do not fully capture.
Filtering and the Three Modes
What distinguishes NOS, OS and BEST?
NOS runs the ladder without oversampling. No digital interpolation filter is applied, which eliminates the pre ringing that a linear phase brick wall filter produces on transients. The cost is the well known one: the inherent sinc rolloff of zero order hold toward the top of the audio band, and images that are not filtered away, both of which depend on sample rate and become far less significant with high rate material.
OS applies oversampling with digital interpolation, suppressing images and flattening the response at the cost of reintroducing the filter’s time domain behaviour. BEST is EverSolo’s own configuration, and the published material does not disclose its filter characteristics.
Frequency response is quoted as 20 Hz to 20 kHz plus or minus 0.25 dB in BEST mode specifically. The fact that the specification names a mode is itself informative, since NOS would not measure the same way.
Analog Stage and Attenuation
Why does an analog R2R volume control matter?
A digital volume control scales sample values before conversion, which discards low order bits and lowers effective resolution at reduced listening levels. At moderate attenuation in a 24 bit path this is largely academic; at substantial attenuation it is not.
The R8 uses a fully balanced stepped R2R attenuator operating in the analog domain after conversion. The full digital word is always converted, and attenuation is performed by switching precision resistors. Being stepped rather than continuous means channel balance is set by resistor matching rather than by the mechanical tracking of a potentiometer, which is where inexpensive analog volume controls usually fail at low settings.
What do the output measurements tell us?
From the balanced outputs: 5 Vrms, THD+N 0.000316 percent, which is minus 110 dB, at 1 kHz, signal to noise of 130 dB A weighted, and channel separation of minus 142 dB. The single ended outputs give 2.5 Vrms, THD+N 0.000355 percent and 127 dB signal to noise.
The separation figure is the one that reveals the architecture. Minus 142 dB is not achievable from a shared ground return and a single converter split into two phases; it indicates genuinely independent differential channels. The 3 dB difference in signal to noise between balanced and single ended outputs is the expected consequence of halving the output voltage.
Output buffering uses OPA1612 op amps, a well regarded low noise, low distortion part rather than a discrete output stage.
Clocking and Power
The internal clock and the external input
An internal 10 MHz OCXO with a PLL provides the timing reference. An oven controlled oscillator in a converter at this price is unusual; a plain temperature compensated part would be the expected choice. A 10 or 25 MHz external clock input on 50 ohms allows an outboard reference to take over.
In a ladder converter the switching instants of the resistor network define the reconstructed waveform directly, without the averaging effect that a heavily oversampled modulator provides. Timing errors therefore translate into amplitude errors at the output with less mediation than in a delta sigma design, which is a coherent argument for the external clock input being present at all.
The O-Core linear supply
EverSolo specifies a custom O-Core linear power supply rather than a switching supply. In a converter whose reference voltages set the absolute accuracy of every resistor in the ladder, supply noise maps directly onto conversion accuracy, so the choice is consistent with the rest of the design. Total consumption is given as 32 W.
Interfaces
Why eight I2S pin modes?
I2S carries clock and data on separate conductors rather than embedding clock in the data stream as S/PDIF does, which avoids clock recovery entirely. The problem is that there is no agreed pinout for carrying I2S over HDMI, and manufacturers have chosen differently, so two components that both offer I2S frequently cannot talk to each other.
Supporting eight pin configurations makes the input compatible with essentially any convention. It is an unglamorous feature that determines whether the best input on the unit is usable with your transport.
Isolation on AES/EBU
The AES/EBU input is electrically isolated, which breaks the ground path between source and converter and prevents source side ground noise from reaching the analog stages. The isolated input is capped at PCM 192 kHz 32 bit and DSD64 over DoP, as are coaxial and optical, while USB and I2S reach DSD512 and PCM 768 kHz 32 bit.
System Architecture
In a split source with a T8 or T10
The R8 is dimensioned to stack with the T8, and the architectural logic follows the chassis. A transport with no converter feeds a converter with no network stage, and each can be chosen and upgraded independently. Connection by I2S avoids clock recovery; connection by isolated AES/EBU avoids a ground path. Both options being present is the point.
In a DMP-A8 Gen 2 and AMP-F8 system
Here the R8 displaces rather than supplements. The A8 already performs conversion and preamplification, so inserting the R8 means relegating the A8 to a streaming transport and relocating both conversion and volume control into the R8, with the AMP-F8 driven from the R8 balanced outputs.
The functional exchange is specific: chip based conversion with digital domain volume becomes discrete ladder conversion with an analog stepped attenuator, while the streaming platform and app remain unchanged. Whether that trade is an improvement is a listening judgement rather than a measurement one, since the A8 is unlikely to measure worse. It is also the configuration in which adding a C10 master clock starts to make structural sense, because the digital chain has become a two box chain.
Come and Hear It
Nothing above settles whether you prefer ladder conversion, because that is not a question measurements answer. We keep EverSolo running in the showroom in Timonium and can put the R8 alongside a good delta sigma converter on your own material. Call or come by.