Pass Labs X250 User Manual

Pass X250

Owner's Manual

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The X250 stereo amplifier embodies the design technology and refinements of the larger "X"
series amplifiers including extensions of the patented Supersymmetry circuit.

The Supersymmetry circuit topology was granted a U.S. patent in 1994, and is the result of 19
years of effort by Nelson Pass. The amplifier uses highly matched components in a classically
simple balanced Class A circuit. The amplifier contains only two simple stages: the first is a
balanced Class A voltage gain stage using low noise Jfet input devices followed by
complementary Mosfets. Its output drives a bank of high power Mosfets operated as voltage
followers.

These are inherently low distortion types of circuits, but their performance is improved when
operated in balanced mode through cancellation. Distortion and noise identical to both halves
of a balanced circuit will disappear at the output, and in a well-matched symmetric circuit, most
of the distortion and noise is identical.

Supersymmetry enhances this effect by providing a connection between the two halves of the
balanced circuit that further perfects the match. Any distortion and noise not already identical
to the two halves is made identical through feedback by a factor of about 10, and the result is
improved cancellation at the output.

Unlike feedback techniques where the goal is to correct for the distortion by feeding a gain
stage an inversely distorted signal, Supersymmetry seeks merely to create perfect matching.

Matched balanced power circuitry typically sees a distortion and noise reduction of about 90%
(20 dB) through a balanced connection without any additional effort. The Supersymmetric
circuit delivers another 90% reduction, so that the X series has about 1/100 of the distortion of
a conventionally simple amplifier. Actually this ordinary distortion and noise can still be seen at
the output of one half of the circuit, but since it is virtually identical on the other half, it goes
away at the speaker terminals. This gives good measured performance, which because it is
produced by a simple circuit, also sounds excellent.

Previously these kinds of simple Class A circuits have been popular for their sound quality in
low power amplifiers, but have not found application at high power levels due to excessive
distortion and low efficiency. Supersymmetry overcomes this barrier, delivering the
sweetness, staging, and detail of very simple circuitry up to kilowatt power levels and beyond.

The X Series amplifiers have the tremendous dynamic range (>150 dB) to do justice to the 24
bit recordings of the 21st Century. The simple but powerful circuitry moves easily from total
silence to explosive transient and back to silence without a trace.

So relax and enjoy your amplifier. Call us if you ever have a problem or question. Nelson
personally answers his email addressed to [e-mail], and you are welcome to
ask questions or offer comments. Thank you for buying our product.

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Setup

You can position the amplifier anywhere you want, but it requires ventilation. We do not
recommend placing it in enclosed cabinets or small closets without means for air to circulate
freely. The amplifier idles at about 270 watts.

Let’s talk about power requirements. The amplifier draws about 2.3 amps (continuous rms) out
of the wall during normal audio operation, and this reflects mostly the idle current that we run
through the output stage. If you are driving a low impedance load, you will draw more than
this, but this will not be typical.

The X250 is provided with the more conventional AC line cord, which is rated at 15 amps. The
circuit ground is attached to the chassis in the conventional manner. The power input to the
amplifier runs through a 3 pole RF filter which removes high frequency noise coming into and
going out of the power supply.

Under no circumstances should you defeat the ground connection of the power cord. For your
safety, the chassis of the amplifier should be earth grounded.

Looking at the rear panel you will see the AC power cord receptacle, a power switch, fuse
holder, two pairs of output connectors, a pair of 5 way connectors for remote turn-on, two RCA
input connectors and two XLR balanced input connectors.

Make sure that the power switch is off (down). Plug the AC cord into the back of the amplifier,
and then into the wall. Then turn the switch on (up). The lights in your house will blink when
the power supply charges the capacitors.

On the front panel, the "Standby" LED indicator should be glowing blue, indicating that the
power is on. The "Power" LED should not be on. If the "Power" LED is on, don’t get excited,
just use the front panel stand-by button to go to stand-by mode, with the "Standby" LED on
and the "Power " LED off.

OK, so the amplifier is sitting there in stand-by mode with just the single blue LED lit. No
speaker connected yet. You can go ahead and connect the source and speakers.

The amplifier can be driven single-end or balanced, if driving the amplifier single-ended leave
the supplied jumpers in place (between pins 1 & 3 on the XLR).

Now that the source component is connected, make sure there is no signal coming from it,
probably by turning the volume all the way down.

With the speakers connected, push the front panel button to activate the amplifier. The
"Power" LED will come on.

You are ready to play music.
Do everybody a favor and try not to have shorted output cables. It happens accidentally all the

time, and the amplifier is designed to survive, but I wouldn’t bet the farm on it.
Of course it’s always possible that something could go wrong. If so, don’t get excited, just

relax. It’s really aggravating when something like this doesn’t work, we understand, but it will

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get fixed. We go to a lot of trouble to make products reliable, and the failure rate of our
amplifiers is very low. This is small comfort to the few, but take it easy and give us a call if you
have problems.

People are interested in how long it takes for these amplifiers to break in. It takes about an
hour for them to warm up, and this is where we adjust them first. Then we adjust them again
and again over a couple of days, keeping the bias and offset in the sweet spot. Our
environment is about 23 degrees Cent igrade, room temperature, and the heat sinks will rise to
about 25 degrees C. above that, for a heat sink temperature of 48 degrees C.

In your setup the temperature may vary a bit due to line voltage and ventilation, but it is not a
big deal. You should be able to put your hands on the heat sinks without discomfort for 10
seconds or so.

The amplifier has a thermal cutout that will disconnect AC power if the temperature exceeds 75
degrees Centigrade. This should never occur in real life.

More things to know: You can remotely operate the stand-by mode by applying 12 volts DC to
the single pair of 5 way connectors on the rear of the amplifier. The positive of the 12 volts DC
goes to the red connector. This connection has an actual operating range of about 9 volts to
15 volts. This switching will override the front panel button, so if you want the button to
operate, leave the rear connection open.

So much for the most essential information.

Speaker Interface

The X250 is optimized for loads nominally rated at 4 ohms and above. You can run the
amplifiers into a lower nominal impedance without difficulty, and we are not aware of a speaker
on the market that presents unusual difficulty with these amplifiers.

The X amplifiers do not care particularly about the reactivity of the load. Reactive loads
typically will have slightly less distortion at a given voltage/current level than resistive loads,
but will make the amplifier run a little hotter. The X circuit was designed to be quite happy
driving electrostatic and other speakers.

When driving transformer-coupled loads directly, as in some electrostatic and ribbon designs,
some attention must be paid to the DC character of the situation. If the transformer primary is
being driven raw with no protection from DC and your source has DC voltage, or in cases
where the small offset of the power amplifier is still too much, you may create distortion in the
transformer and get less than optimal performance from it. Generally this is not the case with
transformer coupled loudspeakers, but it does occasionally surface. In these cases, take
special care that the source does not contain a differential DC component, and confirm the
differential DC offset of the amplifier is sufficiently low. This is easily adjusted by a qualified
technician armed with the service manual. Again, consult your dealer or call us.

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Interconnects and Speaker Cables

We have a general recommendation about interconnects, which is that they should cost less
than the amplifier. We have tried a lot of products and most of them work well, but as a
practical matter we cannot make blanket recommendations.

The amplifier is not sensitive to source interconnects. It is also not sensitive to radio frequency
pickup, which allows some flexibility in choosing source interconnects without shields.

We prefer speaker cables that are thick and short. Silver and copper are the preferred metals.
If you find any cable made of gold, please send me a couple hundred feet.

Fortunately the amplifier is not sensitive to the capacitive/inductive character of some of the
specialty speaker cables, so feel free to ex peri ment .

We have found that about 90 per cent of bad sounding cables are really bad connections, and
we recommend that special attention be paid to cleanliness of contact surfaces and tight fit.

Speaker cables should be firmly tightened down at the speaker output terminals, but do not
use a wrench. They will not withstand 100 foot-lbs of torque. Hand tightening without
excessive force is plenty.

Fun Hardware Facts

The X250 has a power transformer rated at 1200 watts, continuous duty. Under actual
conditions in the amplifier, it will do about 1800 watts for short duration.

The X250 has 20 computer grade capacitors at 10,000 uF and 50 volts each. These are used
to create the unregulated output stage rails at plus and minus 47 volts at 20 amps.

All the power transistors in the product are power Mosfets, actually Hexfets from International
Rectifier and Harris. These are hyper-matched parts, with gate voltages matched to 0.5% and
all devices taken from the same lot codes (made on the same wafer). The speed and noise
critical gain devices in the front end, (that is to say the actual balanced pair of transistors) are
ultra low noise and distortion matched JFETs having a low (.02 S) transconductance figure.
The JFETs are made on the same substrate for prefect matching.

The X250 has 32 output Mosfet power transistors in TO-3 metal packages. The output stages
can sustain transients of about 6,000 watts, but are not allowed to dissipate more than 1000
watts for any instant, even into a dead short.

So how long will this hardware last? It is my experience that, barring abuse or the odd failure
of a component, the first things to go will be the power supply capacitors, and from experience,
they will last 15 to 20 years. Fortunately they die gracefully and are easily replaced. After that,
the longevity will depend on the number of operating thermal cycles, but I can say that I have
had amplifiers operating in the field in excess of 20 years with no particular mortality except
capacitors. The answer is, I don’t have good information beyond that. More to the point, I
would suggest that you not worry about it. This is a conservatively built industrial design, not a
tweaky tube circuit run on the brink. If it breaks, we will simply get it fixed, so sleep well.

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