Pass Laboratories XA100.5 User manual

pass

XA100.5 Owner’s Manual

April 25, 2007

XA100.5 Owner’s Manual

Nelson Pass has been designing audio electronics since about 1971,

first with ESS (remember Heil transformers?), and then forming a

new company, Threshold in 1975. Threshold pioneered the design

of high power Class A power amplifiers and later, high power

amplifiers using only local feedback (the Stasis series).

Pass sold Threshold and created Pass Laboratories in 1991, where he

concentrated first on elevating single-ended Class A power amplifiers

to new power levels and performance, the Aleph series.

Along the way he found the time to design successful lines of

amplifiers for such companies as Adcom and Nakamichi, and has

contributed approximately 60 designs (so far) to the public “Do-It-

Yourself ” hobbyist community.

Over the years, Nelson Pass has made power, simplicity, and

performance his design signature. The hardware tends to run heavy

and hot, but elicits high performance and reliability from simple

circuits with little or no negative feedback..

In 1998 Pass Labs released the X series of audio power amplifiers,

based on the “SuperSymmetric” topology (U.S. Patent #5,376,899)

which elicits high power and performance from simple circuits with

minimal feedback.

The first X amplifier, the X1000 was intended as the premier

example of the power of this principle, delivering 1000 watts rms

into 8 ohms at low distortion. By itself of course, this is no miracle,

but you have to consider that products with comparable performance

have complicated circuits with as many as nine consecutive gain

stages and lots and lots of negative feedback. The X1000 had only

two stages and used only local feedback.

The difference was the unique balanced circuit topology in which

circuit errors are replicated at both output terminals so as to cancel

and disappear across the loudspeaker terminals. The high quality of

the sound reflects both the low distortion and simplicity of the gain

path.

The SuperSymmetric circuit consists of two identical matched

circuits arranged like the wings of a butterfly, showing symmetry

from left to right, and operating balanced to the loudspeaker. The

amplified signal appears with opposing phase across the loudspeaker.

Most of the distortion and noise appears in phase across the

loudspeaker, and is not seen.

XA100.5 Owner’s Manual

We start with simple FET circuits already having low distortion and

noise, and arrange them in two symmetrical halves. The two halves

of the amplifier channel are closely matched, eliminating a large

portion of distortion and noise without feedback. A small amount

of feedback is also applied, not so much for the purpose of reducing

distortion but to make the distortion as identical as possible on both

polarities of the balanced output.

It is easier to make distortions more identical than to remove them

with feedback, and this is the operating principle of X amplifiers.

Since the X1000, Pass Labs products have continuously evolved

toward higher performance. The Class AB “X” series was joined

by the Class A “XA” amplifiers. Subsequent improvements to the

Class AB “X” amplifiers resulted in the “X.5” products, and their

phenomenal success has led to the Class A “XA.5” series.

As always the goal has been the best musical and objective

performance possible with minimal parts in the signal path and

minimal feedback. This process is not completely understood, and

many of the improvements are the result of trial and error and

extensive listening.

Measurement of performance is important to us, and we feel that

well-designed product sounds good and measures good. Apparently

it is possible to make an amplifier which measures well but which

does not please the ears, and so we let our ears be the final judge.

X.5 and XA.5 amplifiers utilize the same basic circuit topology

and gain devices. They differ in the sizes of the power supplies,

the chassis and heat sinks, and the number of output devices. The

output stages of the X.5 series are operated in heavily biased Class

AB, with an idling dissipation similar to the rated output. The XA

amplifiers operate pure Class A with idling dissipation more than

twice the rated output.

Within any given chassis size, the X and XA amplifiers are designed

to dissipate about the same power, and it is not surprising that this

leads to two channel amplifiers with half the output power per

channel of comparable mono amplifiers, and Class A amplifiers with

half the output of Class AB amplifiers.

The X amplifiers deliver more output power per dollar, and the XA

amplifiers have a better subjective performance. All of the amplifiers

drive known loudspeaker loads without misbehaving, and all are

unconditionally stable into low impedance and reactive loads.

XA100.5 Owner’s Manual

All the usual performance metrics - power, distortion, noise, input

impedance, damping factor and bandwidth have all been improved

in this latest generation. The power supplies are larger, with fast

rectifiers gating to larger capacitor banks. The increases in AC line

noise worldwide have been addressed with heavier EMI filtering

and dramatically quieter power transformers. In addition, through

improved biasing regulation, the circuits give much more consistent

performance under varying AC line voltages and over a variety of

ambient temperatures.

All this is very nice, but only if it improves your listening experience.

We listen very carefully and critically to our amplifiers with a wide

variety of associated equipment and we take our time with it.

Musical performance is most important, and is the basis of our

success over the years.

It’s true that we also value the other things. The design and

manufacturing decisions have to balance subjective performance

against reliability, cost, specs, and ego (in approximately that order).

That is why we make more than one amplifier, but all are built with

the same commitment to quality.

If you have purchased this amplifier, we thank you for your

confidence, and hope that you enjoy it for many years. If you have

any questions or comments, or if we can help you with your audio

system, pelase don’t hesitate to contact us.

service@passlabs.com

www.passlabs.com

Tel (530) 367 3690

Fax (530) 367 2193

Pass Labs,

PO Box 219

24449 Foresthill Rd,

Foresthill CA 95631

XA100.5 Owner’s Manual

SET UP

X amplifiers run at about the same power as their output rating, and

XA amplifier run more than twice that. The heat sinks on the side

of the amplifier run hot to the touch – about 25 to 30 degrees C.

above ambient, so you can expect 50 to 55 degrees C. temperatures

on the heat sinks. It is very important to the long life of the

amplifier that they get good ventilation.

You can put this amplifier anywhere you want, but it must have good

ventilation. Do not place it in enclosed cabinets or small closets

without means for air to circulate freely. Stacking these power

amplifiers directly upon each other is not recommended for the same

reason. You should have a minimum of about 6 inches clearance to

the top and sides for adequate air circulation.

When not actually intending to play music the amplifier should be

left in standby mode, where it draws only a few watts of bias on

the output stage. If you wish to reduce the power draw to 0, you

can shut the amplifier off via the rear panel switch, however we

recommend that for minimum noise this switch should be toggled in

stand-by mode.

We have provided a standard IEC power cord that fits into the

standard 15 amp IEC receptacle at the rear of the amplifier chassis.

With the exception of the largest of the X and XA series, there is no

difficulty running two amplifiers on a single 15-amp circuit. Given

the high current draw, we generally do not recommend active line

conditioners, although they are unlikely to damage the amplifier.

This amplifier is equipped for operation with an earth ground

provided by the users AC outlet. Never defeat this ground

connection. The signal ground of this amplifier is connected to

earth through a power thermistor, which provides a safety ground

but provides isolation from system ground loops.

For maximum safety we suggest that the power cord should be the

last cable installed on your power-amplifier. You may, if you wish,

substitute a 15-amp aftermarket power cord for the one we have

supplied. Only use power cords, which meet all local safety standards

and carry acceptance marks from the local regulatory authority. The

power cord should be attached to the amplifier prior to plugging

into house power. Place the rear mounted switch in the off (down)

position before plugging in this amplifier.

XA100.5 Owner’s Manual

The amplifier’s voltage and current rating are indicated on the

rear panel. It will be either 230 volts, 115 volts or 100 volts. The

larger amplifiers have a circuit breaker of the appropriate capacity.

The smaller power amplifiers use a slow-blow 3AG type fuse. The

frequency rating of the AC line source is 50 to 60 Hz. Please verify

that the amplifiers indicated voltage requirements are consistent with

the supplied voltage and current at your location.

On the rear panel you will find both RCA and XLR type input

connections for single-ended or balanced inputs. If you use the RCA

input, be certain that the XLR connector has a gold shorting plug

between pins 1 and 3, which connects the “minus” input to ground,

otherwise the amplifier will not operate properly. If you lose this

plug, just contact the factory – we give them away all the time.

The question comes up all the time – are balanced inputs (XLR)

better than single-ended (RCA)? The answer is that balanced lines

generally give lower noise and slightly less distortion. They are

recommended unless your favorite source component has only RCA

outputs.

Also on the rear panel are the output connections for the

loudspeakers. On stereo amplifiers there is one pair per channel, and

on mono amplifiers we provide two pairs wired in parallel to facilitate

“bi-wiring” a loudspeaker. As you are probably familiar, the red one

is positive polarity, and the black one is negative.

Please note that, being a balanced output, both of these terminals

are electrically live – neither is grounded and should not be treated

as ground. This is particularly important if you are using the

output of the amp to also drive a subwoofer amplifier or other

active component. If there is any question, please contact the

manufacturers of the products involved – both they and we will be

happy to advise you.

If you happen to need a clean signal ground connection for any

reason, we provide a dedicated 5 way binding post just for that on

the rear of the amplifier.

On the rear panel you will also find a pair of 5 way binding posts

for external turn-on control. Applying 5 to 12 volts DC at this

connection will override the front panel stand-by switch and turn the

amplifier on.

XA100.5 Owner’s Manual

So the amplifier is sitting there unconnected.

Make sure that the rear panel power switch/breaker is off (down).

Plug the AC cord into the back of the amplifier, and then into the

wall. Then turn the power switch/breaker on (up). The lights in

your house will dim for a moment as the power supply charges the

capacitors.

If your amplifier uses a circuit breaker and not a fuse, please note

that this circuit breaker is not a “safety device” or “emergency

disconnect” for this product. It will provide current limiting which

will protect the amplifier, but it does not provide personal shock

protection in the manner of a ground-fault interrupter. One more

reason why it is important not to defeat the Earth connection of

your power cord.

On the front panel, the “Standby” LED indicator should be glowing

blue, indicating that AC power is available and the capacitor banks

are charged. If the “Power” LEDs are lighting the meter, use the

front panel stand-by button to go to stand-by mode.

Now turn off the rear power switch and unplug the AC power cord

from the wall while you connect the speakers and inputs.

Check to make certain that the loudspeaker cables aren’t shorted at

the loudspeaker end, connect speaker cables to the output binding

posts on the amplifiers, observing correct polarity. The output

binding posts will accept either bare wire or spades. (Regulatory

agencies in many countries have banned binding posts that will

accept a banana plug, forcing us to abandon this option.) Make sure

that your speaker wires only attach to the outputs of the amplifier,

not to each other and never to the amplifier case.

Remember the warning about treating the output connections as

ground, particularly if you are using a powered subwoofer with this

amplifier. If you need a ground, use the one provided specifically on

the rear panel of the amplifier.

The amplifier may be driven single-end or balanced, your choice.

Single-ended input will always occur through the RCA connector and

balanced input will always occur through the XLR connector.

Again, if you drive the amplifier single-ended then leave the supplied

jumpers in place between pins 1 & 3 on the input XLR.

On the XLR connector pin 1 is ground, pin two is positive input and

pin 3 is inverted (negative) input. Pin numbers are marked on the

XA100.5 Owner’s Manual

XLR. If you re-install the jumper incorrectly or leave it lying about,

the amplifier will not work properly, and you will be able to tell.

Choose either XLR or RCA inputs. If you run both at once from

different components, the sound is not likely to be optimal, but it

shouldn’t cause damage anywhere.

Now that the source component is connected, turn the volume all

the way down. Plug the power cord back into the wall, and turn on

the rear power switch.

Push the front panel button to activate the amplifier. The lights will

come on.

You are ready to play music

With any luck, you can now enter that blissful state, and we won’t

expect you back for a while.

It’s always possible that something may go wrong. If so, don’t get

excited. We know It’s really aggravating when a product doesn’t

work, but it will get fixed, and often it’s something really simple.

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. Depending on the resolution of the rest of your audio

chain, you may or may not notice any break in of the amplifier

beyond the first few hours of operation. You will very likely notice

the difference in sonic character between cold and normal operating

temperature over the first hour, so we recommend letting these

amplifiers warm up before any serious listening.

This generation of amplifiers warms up faster and is more bias-stable

during warm up than previous versions, but we still recommend an

hour for critical listening. At the factory we adjust the bias and offset

values initially and then after warm-up and then again after 48 hours,

If necessary. The “sweet spot” is a sink temperature between 50 and

55 degrees C., but this is not critical, and will vary with your room

temperature. You should be able to put your hands on the heat sinks

without undue discomfort for 5 seconds or so.

The amplifier has a thermal cutout that will disconnect AC power if

the temperature exceeds 75 degrees Centigrade. This thermal cutout

XA100.5 Owner’s Manual

should never occur in real life. This is a very rare occurrence, and

would indicate a serious problem.

Interconnects and Speaker Cables

We have a general recommendation about interconnects, and

speaker cables: They should cost less than the amplifier, and contain

at least some conductive material. We have tried a lot of products

and most 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, though shields are

usually a very good idea. For long runs balanced cables are highly

recommended for their inherent rejection of noise.

We prefer speaker cables that are short and stout. Oxygen-free

copper and silver are the suggested materials. If you find any really

exceptional cable made of gold, please gift us a couple hundred feet.

Fortunately this amplifier is not sensitive to the capacitive/inductive

character of some of the specialty speaker cables, so feel free to

experiment.

We have found that about 90 per cent of bad sounding cables are

really bad sounding connections, and we recommend that attention

be paid to cleanliness of electrical contact surfaces and proper

connector fit.

Speaker cables should be firmly tightened down at the speaker

output terminals, but not with a wrench. Output terminals will not

withstand the levels of torque that may be easily applied by wrench.

Hand tightening without excessive force is plenty. Cleaning contact

surfaces with one of the commercially available electronic contact

cleaners should be part of your annual system maintenance.

Product Life

So how long will this hardware last? It is our 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

30 years. Fortunately they die gracefully and are easily replaced by a

good technician.

After that, the longevity will depend upon the number of operating

XA100.5 Owner’s Manual

thermal cycles, but we can say that we have had amplifiers operating

in the field in excess of 20 years with no particular mortality, and we

don’t have good information beyond that.

More to the point, you should not worry about it. This is a

conservatively built industrial design, not a frail tube circuit run on

the brink of catastrophic failure. If it breaks, we will simply get it

fixed, so sleep well.

Pass Laboratories

PO Box 219

24449 Foresthill Rd

Foresthill, CA 95631 USA

www.passlabs.com

sales@passlabs.com

service@passlabas.com

tel:

(530) 367 3690

fax:

(530) 367 2193

“

pass

”, “Pass”, “Pass Labs”, “Pass Laboratories”,

“Supersymmetery”, “Aleph”, and “Zen” and are all registered

trademarks of Pass Laboratories, Inc., and all rights thereto are

protected by law.

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