At 350 watts per channel class A/B, the X350.8 is the largest and most powerful stereo power amplifier of Pass Labs line-up. Because Pass Labs care more about how holistic design affects the musical experience and relatively less about how parts affect specifications, you can expect superb and accurate representation of the recording performance and venue, as well as the distinctive layering of instruments captured faithfully.
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 I pioneered the solid state genre with the 1994 single-stage “Zen Amp” and subsequent designs.
The X.8 series 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 X.8 series takes the lessons learned from the Xs amplifiers – 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 X.8 series, but 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 smaller amplifier models, the X150.8,
X250.8, XA30.8, XA60.8 and XA100.8 have 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. These have been discontinued, but we
had the wisdom to have a large inventory of these superior parts. We run these matched input Jfet devices undegenerated for the square-law
character, followed by Bipolar cascodes and matched Common Source mode Mosfets.
We 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 amplifier circuit except across the shunt bias regulators and (obviously) the power supply.
More Power Supply
The power supplies for the X.8 series are generally larger than their predecessors. The models X150.8, X250.8, XA30.8, XA60.8, and XA100.8 have a third more storage capacitance and some new CRC filtering to round out the edges of the supply ripple.
They 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 stand-by 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.
So there you have it. Pass Laboratories celebrates its twenty-second year by presenting the some of the best amplifiers they have ever made. If you
have heard the qualities of the highly acclaimed Xs series of amplifiers you will have had a taste of what they offer – amplifiers that bridge the gap between measured performance and subjective experience: Amplifiers that invite you into the music
Gain (dB): 26
Full Pwr @ 26 dB gain (V): 2.65
Low Frequency Response: 1.5 Hz
High Frequency Response: 100 KHz
Power Output /ch (8 ohm): 350
Distortion (1 KHz, full power): 1%
Input Impedance (SE & BAL Kohms): 50/100
Damping Factor: 150
Output Noise (uV): 200
Current Draw @ Idle (Amps): 4.6
Power Consumption (Watts): 550
Temperature (deg C.): 53
Number of Chassis: 1
Unit Dimensions (W x D x H) (In.): 19 x 21.5 x 11
Unit Weight (LBS): 127