As the entry-level class A/B stereo power .8 amplifier, the Pass Labs X150.8 features many of the same technical and sonic attributes of its larger brethren. The X150.8 features 150 watts per channel, 100K ohm input impedance and operates “higher” into Class A than its predecessors. But knowing the ear is not a microphone and the brain is not a tape recorder, Pass Labs have designed this model to bridge the gap between measured performance and outstanding subjective musical experience. Simply put, the X150.8 will compel you to listen to your entire music collection again and again.
More is More
There are times when “less is more”. Particularly if you are building little “flea watt” amplifiers, where simple Class A circuits sound exceptionally good, when the loudspeaker is easy and the musical material isn’t too complex or dynamic. Arguably, Nelson Pass pioneered the solid state genre with the 1994 single-stage “Zen Amp” and subsequent designs.
The X150.8 is intended to deliver some of the qualities qualities found in these little amplifiers, but with dramatically more power and accuracy. The formula is simple: More hardware for more power with fewer stages and lower distortion with less feedback.
In addition, The various elements of the amplifiers have to be individually adjusted to operate in the harmony that delivers the musical experience.
More Class A
The redesigned output stages of the X150.8 takes the lessons learned from the Xs amplifiers. With bigger hardware biased more deeply into the Class A operating region. In turn, the Xs design was inspired by the successes of the X.5 series of amplifiers from 2006.
Two important things emerged with the X.5 series, the first being the value of a large push-pull Class A operating envelope for low distortion and good control of a loudspeaker. Even the Class AB models of the X.5’s had large Class A bias values, so that the performance enjoyed Class A operation at ordinary listening levels.
The second element is the higher level of single-ended Class A bias current applied to the output stage, allowing arbitrary control of the values and ratios of the second and third harmonic characteristic. The subtle qualities of this approach result in a sonic signature which is unique to the X150.8. This goes back to character of previous amplifiers which have had a particularly successful sound. These amplifiers had good measurement specs – low distortion, wide bandwidth, high current, and so on, but were not “state of the art” in any particular category, simply a good balance of these qualities.
One thing they also had in common was that the distortion they did have was a balance of low order harmonics, dominantly second at low levels and dominantly third at high power.
Are we deliberately creating distortion to achieve this? No. We are optimising the remaining distortion character of an already low distortion amplifier for a superior musical character.
More Heat Sink
Part of the “more and bigger hardware” is more metal. One of the first things you will notice is that the X150.8 has new and larger heat sinks (and also more output devices). This upgrade was essential to dissipate the energy of the larger bias currents.
More Front End
Some of the biggest improvements come from the design of the “front end” circuit which contains the first two of the three stages of the amplifier – the input stage and the voltage gain stage. In the X.5 series, all the amplifiers had essentially the same front end circuit, usable interchangeably. This is not quite true of the X.8 series, where each model has subtle differences design to most perfectly complement the differences found in each output stage. Each amplifier in the X.8 series has a individual characteristic depending on the number of devices, heat sinking, supply voltage and push-pull
versus single-ended bias currents.
The output stages of each model have individual transfer curves and its their favourite feedback figures which must be complemented by the front ends. This was accomplished by using measurements to set a baseline objective performance and then adjusting the circuitry through extensive listening on six different systems.
The front ends use a mix of four each of complementary Jfet, Mosfet, and Bipolar devices by Toshiba. Pass Labs run these matched input Jfet devices undegenerated for the square-law character, followed by Bipolar cascodes and matched Common Source mode Mosfets.
They have carefully adjusted the bias and loading off the Drains of the gain devices, which selects the most appropriate load-line character and the feedback around the output stage.
The voltage gain stages of the amplifiers also sport much larger heat sinks for much higher bias currents, critical to low distortion and driving the large output stages.
The result is a front end with high stability, low distortion and noise. It has a very high input and is DC coupled. There are no compensation capacitors – in fact there are no capacitors in the circuit except across the shunt bias regulators and (obviously) the power supply.
More Power Supply
The power supplies for the X150.8 are generally larger than their predecessors. This model has a third more storage capacitance and some new CRC filtering to round out the edges of the supply ripple.
We have incorporated additional RF filtering in the AC primary circuits. The supplies still use paralleled fast/soft rectifiers and very large toroidal transformers from Plitron. The power on/off switching is new, with a 400 amp Triac taking the inrush surge and then paralleled by a high current relay. The new system has a standby draw in conformance to 1 watt requirements.
The front end circuits of the amplifiers have massively larger power supply decoupling – the ripple is now measured in microvolts. This coupled with interleaved layout techniques has reduced the output noise of the amplifiers by another 10 dB. The range between peak output and average noise floor is greater than 130 dB.