• Superb sound reproduction and specifications
• Wifi Hifi browser-based remote interface
• One stereo input pair
• Ultra low noise and distortion
• High impedance low noise balanced JFET inputs
• Optimal distributed RIAA equalization
• Unique discrete trans–conductance/trans–impedance input stage
• Belleson Superpower™ regulated power

• Current mode amplification using virtual ground stages
• Optically isolated switches
• Boosted class A outputs with 70mA output
• Balanced or single ended input
• Gain remotely selectable from 30dB to 70dB
• Wide 4MHz bandwidth
• Fast, clean dynamic performance
• Low, distributed feedback

Why is Radiance so quiet, so clear and so dynamic?

In late 1982, our chief engineer built a preamplifier with a phono stage made from vacuum tubes. Over the next 40+ years he built more, some good, some not so much, in search of the ultimate performance from a vinyl playback system. As part of that work, in 2009 he designed and patented a highly respected voltage regulator, the Belleson Superpower series as used in high end audio playback and recording equipment.

Electronic phonograph amplifiers have been with us for nearly 100 years. For most of those years, audio signals have been modified before they are transcribed to their permanent form on a vinyl record. On the playback side the signals are “unmodified” to their original form while being amplified from their tiny signal levels. These tiny levels make a phono amplifier stage one of the most difficult problems in audio playback, similar to the problem of a microphone amplifier in audio recording.

A solid foundation

Belleson^® Superpower voltage regulators are known world–wide as the best regulators for audio. Radiance has a linear power supply with two levels of voltage regulation to make the most solid, quiet and stable power rail foundation possible.

The critical input stage

As is well known in analog design, the input stage is the most critical part of an amplifier. It sets the system noise floor and is responsible for most of the distortion. Inspired by Bob Cordell’s VinylTrak™, Radiance was conceived in 2021 starting with a new idea for an input stage.

A novel open loop current-output trans-conductance op amp design (we call it a “Top Amp™”) has internal local feedback to keep distortion low, while having fully balanced high impedance inputs. Low noise JFETs from specialty manufacturer Linear Systems at the input present essentially no load to a phono cartridge, allowing both resistive and capacitive input loading to be set independently. Its internal architecture also uses pseudo-Tziklai pairs combined with a folded cascode current mirror to fix all voltage nodes, avoiding voltage-variable-capacitance induced distortion.

Output current from the first stage feeds an identical op amp,  trans-impedance connected. The output of this second stage is the first node where any voltage swing occurs; all signal amplification to this point is via current gain. Thus the first two stages deliver 30dB or 50dB of gain before any appreciable voltage swing.

Stage two also includes the 2122Hz pole to roll off the RIAA boosted treble. Having this early in the amplification chain reduces higher frequency audio before it is amplified enough to cause clipping. Vinyl playback harshness can often be traced to clipping of treble frequencies (above 2kHz) because of poor internal RIAA architecture, including systems where all RIAA equalization is done in a single stage.

Stage 3 and 4

Radiance has four inverting stages, meaning the output signal polarity is same as the input. It also means all four op amps in the RIAA amplification chain are inverting. Inputs for all stages are at virtual ground, essentially static at 0V vs. inputs of non-inverting op amps, follow the signal voltage. In latter stages of a phono amplifier the signal reaches a level of volts, which can generate distortion due to common mode rejection error in non-inverting op amps. Thus inverting in stages 3 and 4 again reduces distortion.

Output current

RCA and XLR cables present a fair amount of capacitance to the output of a phono stage amplifier, generally 30pF to 80pF, sometimes more. Off the shelf monolithic op amps are prone to high frequency oscillation when driving a capacitive load. Often they are stabilized by adding a resistor between the op amp and the output connector, which increases output impedance and can slightly limit dynamic response.

The Top Amp used in Radiance has a dual driver output stage. One driver is class A complementary bipolar transistor pair, to deliver clean output current up to 5mA. The other driver is a class B complementary MOSFET pair that is activated when current approaches 5mA, and can deliver up to 80mA output current. Top Amp is fully protected by an internal sensing circuit that shuts down the output as current exceeds 80mA.

Bass response and the servo

RIAA playback requires a 20dB treble cut and a 20dB bass boost. Amplification of 70dB at 1kHz means bass amplification is 90dB due to the bass boost; 90dB equates to an amplification factor of 31,623.

Real (i.e. imperfect) op amps have DC offset voltages. This means if their inputs are set to zero volts, a DC coupled amplifier will have a non-zero DC output voltage. The majority of the offset voltage is due to mismatches in the input circuit amplified by the total circuit gain. In the case of a phono stage with 90dB gain at DC, a 1mV input offset voltage will generate 31.6V at the output, which can destroy a preamp or amplifier system and/or a speaker.

The simple, cheap solution to DC offset error is to add a blocking capacitor in series with the audio signal to block the DC and pass the audio. Unfortunately real capacitors are also imperfect. First, bass exists down to 20Hz and the lower the frequency sent through a capacitor, the higher the capacitance needed to allow it to pass without attenuation.

Higher value capacitors are typically ceramic, polypropylene or electrolytic. Their dielectric properties are non-ideal for high accuracy signals; specifically ceramic caps have voltage variable capacitance and electrolytic caps have dielectric absorption to distort low frequency signals. Polypropylene caps are much better but due to their dielectric constant they are physically large and/or expensive to get high value capacitance. A low pass filter can be optimized with a low(er) value capacitor and a high(er) value terminating resistor, trading off resistor noise for capacitor distortion and/or cost.

More insidious, putting a capacitor in series with the signal introduces frequency dependent phase shift. If done carefully it can be negligible; if not, it can phase distort low frequencies at the cross-over point.

Depending on the architecture and total gain, a blocking cap may be needed at multiple points along the signal chain. If one is used at the output, low frequency response can depend on the input impedance of the preamp or amp that follows the phono stage.

The not-so-simple, more expensive, better solution is to use a servo correction loop. The output DC voltage is sensed and a correction voltage is applied to a point early in the signal chain to null the output DC to zero. A servo loop presents its own set of challenges: it is slow and takes time after power-on to stabilize at zero; it needs large value resistors and capacitors; it affects total stage loop gain and stability; its servo amp needs to be low offset and high input impedance. A separate servo loop is required for each gain path.

Belleson developed a special hardware loop convergence accelerator to quickly stabilize the servo loop and special optically isolated analog switches  to prevent power-up transients from ever reaching the Radiance outputs.

However, IT’S WORTH THE COMPLEXITY. There is no better bass than a DC coupled signal path.

What does all this tech talk mean?

It means extremely low noise, even with gain of 70+dB for low output moving coil cartridges. It means extremely low distortion. It means treble clarity, with crisp cymbals and sweet violin. Fluid, true-life vocals and strings in the mid-range Low bass with tight kick drum. Deep, clear low strings and horns. Most importantly, it means accurate reproduction of whatever was stamped on the vinyl.