Pass Labs X350, X1000, X600 User Manual

Pass Laboratories

Owner's Manual

X1000 / X600 / X350

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Now For Something Completely Different:

The X Series are the biggest and most powerful amplifiers we have created, but they are not
unique for that reason. We have chosen the biggest and most powerful as the proper temple
for a new concept in high performance audio amplification.

Called Supersymmetry™, the circuit topology was granted a U.S. patent in 1994, and is the
result of 19 years of effort by 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 single-ended Class A voltage gain stage. Its output drives a bank of high
power Mosfets operated as voltage followers without feedback.

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, 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
simple, 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. It’s a spooky experience.

Because these amplifiers do not rely on output feedback for high performance, their
performance is consistent across the audio band, from DC to beyond audibility. They are
unconditionally stable into any loudspeaker.

If you need even more power, X1000’s can be operated in series and parallel arrays. As a
practical matter, we offer peak output levels to 64,000 watts.

So relax and enjoy your amplifier. Call us if you ever have a problem or question.

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Setup

First off, let’s be clear about one thing. These amplifiers are very heavy. In the case of an
X1000, we strongly suggest that you get four, not two, people to help you lift it. Yeah, maybe
two people could do it, but if you get hurt, don’t say you weren’t warned. X600 and X350
models are not much lighter, either. Make the dealer lift it.

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 amplifiers all idle at about 600 watts per chassis, about the same as a hair dryer.

Let’s talk about power requirements. The amplifier draws about 5 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 hard, you will draw more
than this, but this will not be typical. On this basis, we estimate that you can put two channels
on a single 20 amp, 120 volt AC line without problems in most cases.

The X1000 is provided with a special standard 20 amp power cord that is not like most of the
ones you are used to. It is heavier, and mates to a standard receptacle at the rear of the
amplifier. It is probably not compatible with other “after-market” audiophile power cords, so be
sure to check before assuming that it will fit. The power cord we provide requires the more
modern high current wall sockets, so it is entirely possible that an electrician will have to come
out and install new ones. A diagram of the kind of AC wall outlet required is included here.

X600 and X350 models are being provided with the more conventional AC line cord which is
rated at 15 amps for your convenience.

The chassis of the X1000 is connected to the earth ground through the power cord. This is the
only thing it connects to. It is not attached to the circuit or to the amplifier signal ground. This
is essential to operating the amplifiers in series or parallel configurations. The X600 and X350
have their circuit grounds attached to the chassis in the conventional manner.

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. We aren’t kidding.

Series or parallel operation of X1000 amplifiers requires an interface adapter that is essential
for such operation. Do not attempt to operate the X1000 in series or parallel without this
interface. For more information, contact your dealer or the factory.

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

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

On the front panel, the single led indicator on the meter should be glowing blue, indicating that
the power is on. The meter lights should not be on, and the meter should be parked at the left.
If the meter lights are on, and the meter is up nearly to half-scale, don’t get excited, just use

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the front panel stand-by button to go to stand-by mode, with the meter light off and the meter
parked to the left.

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 now.

The amplifier requires a fully balanced source, that is to say a male XLR connector with pin 1
at ground, pin 2 positive signal, and pin 3 negative signal. Both pins 2 and 3 should be driven
by equal levels of signal with opposite phase.

In standby mode, the balanced inputs to the circuit are shorted, and the input impedance of the
amplifier will be 2000 ohms, just so you know.

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.

The next step is the connection of the speaker. If this is the first time out of the box with the
amplifier and you are particularly paranoid, you might connect up some cheap disposable
speaker first, but I have to say that we have not seen a failure yet, and are not expecting one.

On the X1000 and X600 the two sets of output connectors on the rear panel are in parallel, for
the convenience of those who wish to bi-wire their loudspeakers. The red terminal is
connected directly to the other red terminal, and so with both black terminals.

On the X350, there is one set per output, but the terminal should be large enough to
accommodate dual spade lugs if desired.

With the speakers connected, push the front panel button to activate the amplifier. The meter
lights will come on. The meter on the f ront should go to somewhere between one-third and
half way up, reflecting the bias on the output stage.

You are ready to play music.
The meters read current through the output stage in the X1000 and X600 and total supply

charging current in the X350. At low levels, they should stay pretty constant and if the meters
are bouncing around at a moderate or low listening level, it might indicate that you are driving a
very low impedance load or even a dead short. If your speakers are not known to be very low
impedance, say less than 4 ohms, then you should check this out.

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.

With the X350, you will see some meter bounce if the AC line is not constant. This is normal,
and simply shows the different charging rate of the power supply capacitors, not the actual
current going through the audio circuitry. You will also note on the X350 that the meter shows
the draw of both channels. We had only one meter.

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
get fixed. At Pass Labs, we go to a lot of trouble to make products reliable, and the failure rate

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of our amplifiers is almost non-existent. This is small comfort to the few, but take it easy and
give us a call if you have problems.

Now that the channels are up and running, we can take a moment to note a few things. The
meter lights are blue, subtle lighting in daylight, a little more dramatic at night. The meters
themselves read the amount of current going through the amplifier, and that is why they sit
near the half-way point, reflecting the bias current we run through them to get low distortion.

The bias current seen by the meters will vary slightly, going down a bit as the amplifiers warm
up. Two channels will not always be at exactly the same position. Don’t worry about it. If they
drift upwards or vary dramatically from each other, give us a call.

Later, as you start listening to music at higher levels, you will start to see the meters move up
above the bias point. Basically, the amplifier will be operating in pure Class A at current levels
that do not cause the meters to move. As the current to the speaker exceeds the bias level,
you will see the meter bounce upward from the idle reading.

It’s pretty simple: If the meter isn’t moving, you are still in pure Class A mode. You will
probably be surprised how loud you have to play it before the meter moves.

The meter has been calibrated to reflect the 600 watts idle draw of the amplifier, which is about
one-third full scale for the X1000 and X600, and one-half for the X350. The meter is intended
as a general indicator of the status of the amplifier and has not been calibrated to reflect any
particular values.

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 Centigrade, room temperature, and the heat sinks will rise to
about 22 degrees C. above that, for a heat sink temperature of 45 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.

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.

The front end of the amplifier draws about 25 watts even in stand-by mode, so the top cover
will always be a little warm. We recommend that you shut the amplifier down using the rear
power switch if the amplifier is going to be unused for any extended period. It will not hurt the
amplifier to be left either in operating or stand-by mode constantly, but it is potentially a waste
of energy.

Again, it is our experience that a 1 hour warm up is adequate for even the most critical
listening experience. Stand-by mode allows a faster approach to optimal conditions as it
keeps the front end circuit fully operation al.

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 pair of terminals is fully floating and isolated, and drives the
high impedance coil of a 12 volt relay. This connection has an actual operating range of about

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9 volts to 15 volts. This switching is independent of the front panel button, so if the amplifier is
placed in operating mode by the button, the relay will not turn it off, only on.

So much for essential information.

Speaker Interface

The X Series 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.

If you wish to maximize power or minimize distortion for loads rated below 4 ohms, we suggest
that you parallel 2 or more X1000 channels, which will allow multiples of the 20 amp rating.
For example, if you desire to achieve 16,000 watts peak into 1 ohm, you will want to use 4
amplifiers in parallel. If you want lower distortion at that figure, you will use more. Remember
that running arrays of X1000 channels requires an interface unit from Pass Labs. Feel free to
consult the factory for the answers to any questions.

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.
The X circuit was designed to be quite happy driving electrostatic and other speakers, since it
is unconditionally stable and operates without feedback.

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 DC component, and confirm the 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.

The damping factor of this amplifier is not extremely high as it is with some other high power
products, but it is the same at all audio frequencies and occurs naturally without feedback.
Some people are concerned that they need high numbers for good bass performance, but it
just doesn’t seem to be so. We suggest that you listen with your ears and not your meter.

The Power Line

Please keep in mind that the data we present on output power for this amplifier is based on a
solid AC power line. In order to get these high continuous figures, the AC line cannot be
allowed to collapse. Typically in maximum power testing, a household line will fall from 120
volts to around 105 volts. This impacts the power that can be delivered on a continuous basis,
and it is probable that you will not get full continuous power at specification.

This should not be the subject of great concern, since in audio usage, these kinds of power will
only occur as brief transients, and under these circumstances the AC line will not load badly,

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and the energy reserve held in the capacitors will do its job. As a practical matter, it will not
make a lot of difference. Just don’t call us up and complain that you only measured 1900
watts in your living room. If you do, we’ll want to know what speakers can take it.

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.

Source Interaction

The amplifier does not care what the source impedance is, although its gain will drop a bit with
high source impedances. Since we have 30 dB of gain (26 dB in the X350), we can afford to
throw a little bit away, since the quality is unaffected. Using a balanced source attenuated
through a balanced passive attenuator is perfectly OK, and we often use this approach
ourselves to minimize components in the signal path and also to reduce bit loss in CD players
with digital volume controls.

The differential input impedance of the amplifier is 22000 ohms, although this drops to 2000
ohms when the amplifier is in stand-by mode. 22000 ohms is not a problem for any balanced
source circuit we ever heard of, and will not be a problem for you.

As pointed out elsewhere, this power amplifier is the first ever to truly take advantage of the
concept of balanced operation for more than just noise reduction. As such it prefers balanced
input symmetry for optimal operation.

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