Halo A 21 User manual

A 21 Amplifier
Owner’s Guide

1
THANK YOU!
Congratulations and Thank You for Choosing Parasound
Your new Parasound Halo Series A 21 power amplifier presents the latest advancements
in amplifier technology. The A 21 is built to the strict quality and performance standards
set by Parasound. We’re proud to offer you this exceptional audio component that will
bring you many years of enjoyment and dependability.
Here at Parasound, we design our products to perform at a higher level of flexibility
and sonic performance than you may have expected. We encourage you to read
this entire manual to learn all the features and capabilities of your new Halo Series
A 21 Amplifier.
If you’re eager to get up and running right away, simply follow the basic step-by-step
instructions to connect and operate the A 21. If you want to learn about some of the
technical and design aspects of your A 21, refer to the Technically Speaking and
Parasound A 21 Design Overview sections in the back of the manual. If you run into
difficulties, the Troubleshooting Guide should help you quickly remedy the problem.
We appreciate you taking the time to read these instructions and thank you for
selecting Parasound for your listening pleasure.
The Parasound Staff
Unpacking Your A 21
Unpack your A 21 from the shipping carton, remove the enclosed rack mounts, AC
power cord, and the control wire with 2.5 mm sub-mini plugs on each end. (This is
the trigger connection wire). While you are unpacking your new amplifier, inspect
it thoroughly for possible shipping damage. If you see any, contact your Parasound
dealer right away. Be sure to save and store both the inner and outer cartons and the
packing inserts for possible future transport. To save room for storage, you can cut the
seams on the bottom of the cartons and flatten them.
Keeping Records for Future Reference
Record the serial number located on the bottom of your A 21 in the space below. Also
note your Parasound Dealer’s name and phone number. We recommend that you keep
your purchase receipt with this manual and store them both in a safe place. You may
need to refer to this information sometime in the future.
Parasound A 21 Amplifier Serial #: ____________________
Parasound Dealer: ____________________________________
Phone Number:_______________________________________
Date of Purchase: _____________________________________
YOU SHOULD KNOW
There is no Parasound warranty for this unit if it was not purchased from an Authorized
Parasound Dealer. Investigate any warranty claims made by unauthorized dealers very
carefully as you will need to depend entirely upon the dealer, and NOT upon Parasound.
Unauthorized dealers may lack the capability to arrange repairs of Parasound equipment.
Authorized Parasound Dealers are listed at www.parasound.com or you can call 415-397-7100
between 8:30 am and 4:30 pm Pacific time.
A missing or tampered serial number could indicate that this unit was stolen or sold by an
unauthorized dealer. If this unit is missing its serial number, you should return it to your
dealer immediately for a full refund.

PLACEMENT GUIDELINES FOR YOUR A 21
_________________________________________________________________________________
CONNECTING A STEREO PREAMPLIFIER OR A SURROUND
CONTROLLER TO THE BALANCED INPUTS ON YOUR A 21
_________________________________________________________________________________
CONNECTING A STEREO PREAMPLIFIER OR A SURROUND
CONTROLLER TO THE UNBALANCED INPUTS ON YOUR A 21
_________________________________________________________________________________
CONNECTING THE A 21 LOOP OUT JACKS
TO ANOTHER POWER AMPLIFIER’S INPUTS
_________________________________________________________________________________
CONNECTING SPEAKERS TO YOUR A 21
_________________________________________________________________________________
CONFIGURING THE A 21 FOR BRIDGED MONO OPERATION
_________________________________________________________________________________
MANUAL AND AUTOMATIC TURN ON-OFF OPTIONS
_________________________________________________________________________________
CONNECTING AN EXTERNAL DC SOURCE FOR A 21 AUTOMATIC TURN ON-OFF
_________________________________________________________________________________
CONNECTING THE A 21 TO TRIGGER ANOTHER COMPONENT
_________________________________________________________________________________
UNDERSTANDING THE INDICATORS ON THE A 21
_________________________________________________________________________________
CONNECTING THE A 21 AC POWER CORD
_________________________________________________________________________________
MAINTAINING YOUR A 21
_________________________________________________________________________________
TROUBLESHOOTING GUIDE
_________________________________________________________________________________
SERVICING YOUR A 21
_________________________________________________________________________________
TECHNICALLY SPEAKING
_________________________________________________________________________________
PARASOUND A 21 DESIGN OVERVIEW
_________________________________________________________________________________
PARASOUND A 21 SPECIFICATIONS
_________________________________________________________________________________
1
TABLE OF CONTENTS
A 21 Amplifier
19
17
13
12
15
14
11
10
9
8
7
5
4
2
1
3
6

PLACEMENT GUIDELINES FOR YOUR A 21
Power ampliers are usually heavier and generate more heat than other components. To avoid
damage to the A 21 or other equipment and to reduce risk of re, you must follow these
guidelines:
• Place the A 21 on a separate shelf that will adequately support its weight.
• Keep it away from heat sources such as air ducts or radiators.
• Avoid placing it on carpeting or another material that might obstruct airow into the openings
in the chassis bottom.
• Leave at least 3” of space around both sides and 6” of space above the top. The bottom
clearance can be a little less.
• Do not block the front of the A 21 behind closed cabinet doors during use.
• Do not stack the A 21 with other components inside a cabinet unless you use a fan to circulate
and exhaust the warm air that builds up between them.
Rack Mounting Your Parasound A 21
To mount the A 21 into a 19” wide equipment rack, you must rst attach its rack mount brackets
(provided). With its four feet removed, the A 21 chassis and front panel height occupies four rack
spaces (7” or 176 mm). When mounting equipment below the A 21, you will also need to allow
about 1/8” below the unit for the bottom chassis screws. A single standard rack space allows
1-3/4” vertical height in a 19" wide equipment rack. This measurement standard was developed
by the EIA (Electronic Industries Association) so manufacturers of electronic components and
equipment racks could build products in standardized heights that would t in a uniform space.
Please call your Parasound dealer or Parasound Technical Services if you need additional advice
about rack mounting the A 21.
To attach the rack mount brackets:
• Remove the three screws from each side of the A 21. These are arranged vertically, behind its
front panel and in front of its rst heatsink n.
• Line up the three holes on each bracket with the three holes on the A 21 and reinsert
the screws.
• Make sure the screws are tight because they will support the entire weight of the A 21 in the
equipment rack
1
HRA-4 Rack Adapter Mounted on JC 1
Rack Mount Brackets Attached to A 21

XLR Connectors
Male
Female
YOU SHOULD KNOW
Balanced XLR Jacks and Their Pin Configuration
The balanced inputs of the A 21 use XLR jacks that conform to the industry standard of:
Pin 1: Ground, Pin 2: Positive (+), Pin 3: Negative (--). The balanced outputs on some
components use terminals with 3 screws instead of XLR jacks. These are compatible with
the A 21 as long as you match the bare wires to the corresponding pins on the XLR plug: +
to pin 2, - to pin 3, and Ground to pin 1.
CONNECTING A STEREO PREAMPLIFIER OR A SURROUND
CONTROLLER TO THE BALANCED INPUTS ON YOUR A 21
1
2
3&4
2
3
Left
Right
COMPONENTS
SURROUND
SOUND
CONTROLLER
PREAMP
OUTPUTS
or
1 2
3
1 2
3
42
1
Left and Right Balanced Input Jacks
Balanced connections will give you the best sound. If your preamplifier or surround controller
has balanced XLR output jacks, we recommend that you connect them to these inputs. Refer
to the Balanced and Unbalanced Lines in the Technically Speaking section for additional
information about why we recommend using balanced lines.
What You’ll Need:
• One pair of balanced interconnect cables with XLR connectors
• A preamplifier or surround controller with balanced outputs
Before Connecting
Leave the A 21’s AC cord disconnected until you have made all other connections to
prevent any surprise burst of sound.
Make sure that all your cables are long enough so they are not strained or stretched
once they are connected.
Make sure the Inputs switch on the rear of the A 21 is in its Balanced (up) position and
the Ground switch is in its Normal (down) position.
To Connect
Plug the male end of the balanced interconnect cable into the Balanced Right Channel
input jack on the A 21.
Plug the female end of this cable into the balanced right channel output jack on
your preamplifier (or a channel on your surround controller).
Repeat steps 1 and 2 above to connect the left channel.

Left and Right Unbalanced Inputs
Use these inputs if your preamplifier or surround controller doesn’t have balanced
output connectors or if you simply prefer to use unbalanced connections.
What You’ll Need:
• A pair of shielded interconnect cables with RCA plugs
• A source component with RCA output jacks
Before Connecting
Leave the AC cord on the A 21 disconnected until you have made all other connections
to prevent any surprise burst of sound.
Make sure that all your cables are long enough so they are not strained or stretched
once they are connected.
Make sure the Inputs switch on the A 21 is in its Unbalanced (down) position and the
Ground switch is in its Normal (down) position.
To Connect
Plug one end of the cable into the Unbalanced Right Channel Input jack on the A 21.
Plug the other end of this cable into the unbalanced right channel output jack on your
preamplifier (or a channel on your surround controller).
Repeat steps 1 and 2 for the left channel.
CONNECTING A STEREO PREAMPLIFIER OR A SURROUND
CONTROLLER TO THE UNBALANCED INPUTS ON YOUR A 21
3
3
COMPONENTS
SURROUND
SOUND
CONTROLLER
PREAMP
OUTPUTS
or
4
2
1
Right Left
2
RCA Plugs
Right
Left
3&4
1
YOU SHOULD KNOW
Interconnect Cables and Their Color Codes
Common color codes for input and output jacks are red for right and white for left. Match
the colors at the outputs from your preamplifier or surround controller to the inputs on your
A 21 so you’ll always hear the channels in their intended position.

Loop Out Connector
The Right and Left Loop Out jacks provide connections to route the same input signals from
your preamplifier or surround controller to an additional power amplifier. In most instances,
you won't use the loop out feature so you can skip this section.
What You’ll Need:
• One pair of shielded interconnect cables with RCA plugs
• An additional amplifier
Before Connecting
Leave the AC cord on the A 21 disconnected from the wall until you have made all of
the connections in the system to prevent any surprise burst of sound.
Make sure that your interconnect cables are long enough so they are not strained or
stretched once they are connected.
To Connect
Plug one end of the cable to the right channel Loop Out jack on the A 21.
Plug the other end of this cable to the right channel input jack on the other power amplifier.
Repeat steps 1 and 2 for the left channel.
RCA Plugs
Right
Left
CONNECTING THE A 21 LOOP OUT JACKS
TO ANOTHER POWER AMPLIFIER’S INPUTS 4
YOU SHOULD KNOW
These jacks eliminate the need for two “Y” connector cables to split the outputs from the
two source channels between your A 21 and another amplifier. The A 21 Loop Out Jacks
jacks provide the same signals that are received at either the balanced or the unbalanced
inputs. Refer to Loop Out Jacks in the Technically Speaking section for more information
about when to use the Loop Out Jacks.
3
COMPONENTS
1
24
POWER AMP
Right Left
INPUTS
2
1
3&4

Stripped
AWG wire
Dual
Banana Plug
Common Speaker Connectors
Single
Banana Plug
Spade Lug
Red usually
designates right
The GND notch
signifies ground
or speaker left
Speaker Terminals
The five-way binding post speaker terminals on the A 21 accept most speaker wire
terminations including single or dual banana plugs, 1/4” spade lugs, or bare wire.
Refer to Bare Wire Speaker Termination in the Technically Speaking section for
information about bare wire termination.
What You’ll Need:
• Two lengths of speaker wire
AWG 16 or thicker with banana
plugs, spade lugs, or bare wire
• Loudspeakers or a speaker
selector box
Before Connecting
Remove power to all the components in your audio system.
To Connect
Insert one wire with the ridge or other marking into the red Right + (positive) speaker
terminal on the A 21. Insert the wire without the ridge or other marking into the black
Right - (negative) speaker terminal on the A 21.
Insert the other end of the wire with the ridge or other marking into the red right +
(positive) terminal on the speaker. Insert the wire without the ridge or other marking
into the black right - (negative) terminal on the speaker.
Repeat steps 1 and 2 for the left channel.
YOU SHOULD KNOW
Correct Speaker Polarity is Important
Polarity refers to + and – connections. Speaker wires are coded with printing or a ridge on
the insulation on one of the leads so you know which lead was connected to the + and –
terminals at the other end. This coding will help you keep the + and - polarity consistent
for both channels.
Speaker Wire Length and Gauge (thickness)
When selecting speaker wire, follow these guidelines:
• Keep the length of your speaker wire as short as possible
• Use the thickest wire practical. For lengths greater than 50 feet, use speaker wire with an AWG
(gauge) of 14 or lower. The smaller the AWG, the thicker the wire.
• Do not use speaker wire that is thinner than 16 AWG.
CONNECTING SPEAKERS TO YOUR A 21
5
1
2
3&4
2 4
1 3
Speakers

CONFIGURING THE A 21 FOR BRIDGED MONO OPERATION 6
YOU SHOULD KNOW
IMPORTANT! - Before Trying Bridged Mono
You should not connect a speaker with an impedance of less than 8 Ω to the A 21 when it is
configured for bridged mono operation. Refer to Bridged Mono Operation in the Technically
Speaking section for additional information about. Use the right channel Gain control to
adjust level in the bridged mode.
1
2
3
6
4
5
7
8
Speaker
7
6
4 5
88
2
3
8
1
RightLeft
Components
Surround
Sound
Controller
Preamp
OUTPUTS
In certain applications, you may require your A 21 to power a single 8 Ω sp eaker by
using its bridged mono feature. In most instances, you won’t use the A 21 this way and
you can skip this section.
What You’ll Need:
• One length of speaker wire with banana plugs, spade lugs, or bare wires
• One shielded balanced or unbalanced interconnect cable for each A 21 used
in bridged mono
• One loudspeaker with an impedance of 8
Ω or higher
Before Connecting
Remove power to all the components in your audio system.
Set the A 21 Stereo - Mono Bridge Switch to its Mono (up) position.
To Connect: From the Source
Plug one end of the interconnect cable into the Balanced or Unbalanced Right
Channel & Mono Input jack on the A 21
Plug the other end of this cable into the desired output channel on the preamplifier
or surround controller.
Do not make any connection to either Left Channel Input jack on the A 21.
To Connect: To the Speaker
Insert the wire with the ridge or other marking into the red Right + (positive) speaker
terminal on the A 21. (This is the positive speaker terminal in bridged mono.)
Insert the wire without the ridge or other marking into the Left + (positive) speaker
terminal on the A 21. (This is the negative speaker terminal in bridged mono.)
Insert the wire with the ridge or other marking into the red + (positive) terminal on the speaker.
Insert the wire without the ridge or other marking into the black -- (negative) terminal
on the speaker.
Do not make connections to either of the
A 21's black speaker terminals.

MANUAL AND AUTOMATIC TURN ON-OFF OPTIONS
7
YOU SHOULD KNOW
Turn Off Delay in the Audio Mode
When the music (audio signal) stops, the A 21 will remain turned on for about five minutes.
This delay prevents unwanted turn-off during silent passages in music or pauses between
1Switch in up position
Start Sensitivity at 12 o'clock
You can manually turn the A 21 on and off with its front panel On-Off button. It can also be
triggered to turn on and off automatically when your preamplifier, surround controller, or
system controller is turned on and off.
Manual On-Off from the Front Panel
If you want to manually turn on and off the A 21 at any time, you need to first select
the Manual On-Off option by placing the Auto Turn On switch (on the rear panel) to
its middle Man (Manual) position.
Connecting an Active Audio Source for Automatic On-Off
If you want to automatically turn on the A 21 whenever music is playing (an audio
signal is present), choose the audio triggering option by moving the Auto Turn On
switch to its upper Audio position.
What You’ll Need:
• An audio source that's playing
Before Connecting
Remove power to all the components in your audio system.
Make sure the Auto Turn On switch on the A 21 is in its Audio (up) position.
Set the Audio triggering Sensitivity adjustment knob on the rear panel to the desired
level. Start with the 12 o’clock position.
To Connect
Connect an active audio source to either the Left or Right Balanced or Unbalanced
Input jacks. (Refer to to pages 2 and 3).

CONNECTING AN EXTERNAL DC SOURCE
FOR A 21 AUTOMATIC TURN ON-OFF 8
1
2
YOU SHOULD KNOW
If the device you want to use to control your A 21 doesn’t have a 2.5 mm trigger output
connector, you can cut one plug off the cable and terminate the end as required. The lead
with the red stripe is positive and the lead without the stripe is negative.
CONTROL
COMPONENT
Input
1
2
2.5 mm sub-mini plug
If you want to automatically turn on and off the A 21 with an external DC voltage, choose
the DC Triggering option by moving the Auto Turn On switch on the rear panel to its lower
12V position.
What You’ll Need:
• A cable with a 2.5 mm sub-mini plug on each end (provided)
• A “control” component with +9Vdc to +12Vdc trigger voltage
Before Connecting
Remove Power to all the components in your audio system.
Make sure the Auto Turn On switch on the A 21 rear panel is in its 12V (down) position.
To Connect
Plug one end of the trigger wire into the 12V Input jack on the A 21.
Plug the other end of this wire into the external DC source.

B
CONNECTING THE A 21 TO TRIGGER ANOTHER COMPONENT
9
1
2
YOU SHOULD KNOW
If the device you want your A 21 to control doesn’t have a 2.5 mm trigger output connector,
you can cut one plug off the cable and terminate the end as required. The lead with the
stripe is positive and the lead without the stripe is negative.
COMPONENT
TO CONTROL
Input
1
2
2.5 mm sub-mini plug
Whenever the A 21 is turned on either manually or automatically, there is 12 Vdc present
at its own DC Trigger Out jack so it can trigger additional amplifiers or other components
on and off.
What You’ll Need:
• A cable with a 2.5 mm sub-mini plug on each end
(in addition to the one provided with the A 21)
• A component that can be triggered with an external
+9 Vdc to +12 Vdc source. The A 21 DC Trigger output
provides up to 150 mA of current
Before Connecting
Remove power to all the components in your audio system.
To Connect
Plug one end of the trigger wire into the 12V Output jack on the A 21.
Plug the other end of this wire into the component that you want the A 21 to trigger.

UNDERSTANDING THE INDICATORS ON THE A 21
AC Present Indication
When the A 21 is plugged into a live AC outlet, a soft blue halo glows behind its On-Off button
and its red “P” Badge glows faintly. These indicate that the unit is plugged into a live AC
outlet, even when it is turned off.
On, Off and Fault/Protection Status Indicators
Whenever the A 21 is turned on, the soft blue glow behind its On-Off button will change to
red for about ve seconds as its internal circuits stabilize. Then the red glow is replaced
by a brighter blue glow to indicate normal operation. If the glow remains red after
turn on or while the amp is playing, it indicates activation of the A 21’s protection
circuits and no sound will be heard from the speaker. The A 21 protects itself from external
conditions such as excessive heat, load impedance that is too low, or a short-circuited
speaker connection or wire. After you correct the fault, the A 21 will resume operation. If
the A 21 remains “in protection” after it has cooled down and you’ve conrmed there
are no external faults, it could indicate an internal problem. Please contact Parasound’s
Technical Service Department.
Left and Right Channel Status Indicators
These are two small round indicators in the center of the recess in the front panel. The indicator
on the left displays the status of the left channel and the indicator on the right displays the
status of the right channel.
Both Illuminated
When both channels are operating normally, these both glow blue.
No Illumination:
When the A 21 is turned off, these indicators are off.
High-Temp Indicator
This indicator is near the right side of the panel recess. It will glow red if either channel
overheats. The On-Off button will also glow red if the A 21 overheats.
Gain Control Knobs
These adjust the gain (level) of the A 21. We recommend that you rotate the
Left and Right Gain controls to their maximum clockwise positions, marked THX Reference.
Refer to Gain Controls in Technically Speaking for more information about these adjustments.
Ground Switch
The Ground switch can assist in reducing audible hum caused by ground loops in your
system. This switch should be in its Normal (down) position. If you hear hum you can move
it to its Lift (up) position to eliminate it. Refer to the Technically Speaking section for more
information about ground loops.
10
A
BE
C D
A 2
F
G
F
G
A&B
C&D
B
E
F
C&D
C&D

AC Power Connections
We recommend that you plug your A 21 into the same AC wall outlet or power strip that
powers your other audio components, especially the preamplier or system controller.
Having all the audio components on the same power circuit helps prevent hum caused
by possible ground loops. The A 21 requires AC power that is continuous rather than
switched on and off.
110 V - 120 V or 220 V - 240 V Operation
The A 21 can operate on either 110 V - 120 V or 220 V - 240 V. The correct line (or “AC
mains”) voltage for your unit is marked on its carton. Changing the A 21’s operating
voltage involves rewiring internal connections for both its main power transformer and
standby power transformer, plus changing fuse values. Since there is a risk of electric
shock, voltage changes must be referred to a qualied technician.
What You’ll Need:
• An IEC 65 AC Cord (provided)
• An AC outlet or high quality AC power strip within reach of the AC cord
Before Connecting
Remove power to all the components in your audio system.
To Connect
Plug the female end of the AC cord to the AC receptacle on the rear of the A 21.
Plug the male end of the AC cord to an AC outlet or power strip.
CONNECTING THE A 21 AC POWER CORD
11
2
1
1
2

MAINTAINING YOUR A 21
Your Parasound A 21 power amplifier requires no periodic maintenance and has no
user-serviceable parts inside. To avoid the risk of electric shock, do not remove its top
cover. The amplifier’s exterior can easily be cleaned with a soft cloth pre-moistened only
with a few drops of water or glass cleaner.
Main Power Fuse
If this fuse blows, please contact Parasound Technical Service for further advice.
12
Notes:

TROUBLESHOOTING GUIDE
13
A 21 will not turn off when
audio signal is removed
PROBABLE CAUSE
Power cord is disconnected
REMEDY
Hum and / or buzz
through speakers Ground loop between preamplifier
and A 21 or at the equipment rack
Move the Ground switch to its Lift position
Install nylon shoulder washers on both sides of
the panels of all equipment to insulate metal
from touching the rack
Check polarity of DC source with a voltmeter
Try connecting a 9 V battery to the 12V DC input
Make sure Auto Turn On selector switch
is in its Audio position
Auto Turn On selector switch in
wrong position
Auto Turn On selector switch in
wrong position
Connect each speaker to the opposite channel
on the A 21. If the same speaker remains distorted,
the speaker or its wire is damaged. If the other
speaker distorts, the problem may be with the
A 21, the preamp, or a source component
Move the Ground switch to its Lift position.
Install cable isolation device such as the
Xantech 634 Ground Breaker
www.xantech.com
Ground loop from cable TV
Contact Parasound Technical Service
Other ground loop
Problem with preamplifier or source
component
Switch the left and right input cables. If
distortion moves to the other channel, the
problem is with a component other than the A 21
Load impedance at speaker terminals
is too low
Make sure the A 21 has adequate ventilation
Move switch to its Stereo position
Mono Bridge switch is in the Mono
bridged position
Damaged speaker
Move to the correct position for the
jack in use
Balanced-Unbalanced Input switch
is in the incorrect position
Not enough ventilation
Make sure Auto Turn On selector switch
is in its 12V position
A 21 is overheating
No power
TROUBLE
Distorted sound
A 21 has overheated - red Hi Temp
indicator is on
Allow A 21 to cool. Check that there is adequate
ventilation around the A 21. Be sure the load
impedance of each channel is no less than 4 Ω
Check speaker load impedance. Check speaker
wires and connections
Check input connections or try a
different set of cables
Over-current protection circuitry has
been activated- one or both blue
channel LEDs not illuminated
Bad connection from preamplifier or
controller
Sensitivity level too high
Turn level controls clockwise
Turn Sensitivity knob clockwise toward 200 mV
AC Voltage selector switch is in the
wrong position
Auto Turn On selector switch is in
wrong position
Make sure the switch is in the
correct position for your region
Level controls turned too low
DC trigger source wired in reverse
or voltage too low
AC fuse blown Replace with same value fuse
Turn the Audio trigger Sensitivity
knob counterclockwise
Sensitivity incorrectly adjusted
Check for correct position of the Auto Turn On
switch
Connect power cord
Make sure the speaker or speaker
selector load is 4 ohms or higher
Power on, but no sound
On-Off button glows red
and channel indicator(s) off
Auto turn on doesn’t
function with a
trigger voltage
Auto turn on doesn’t
function with an audio
input signal

SERVICING YOUR A 21 14
If All Else Fails –Call Us for Help
Call your Parasound dealer or Parasound’s Technical Service Department toll free at
1-866-770-TECH (8324). We can often solve the problem with simple diagnostic tests you can
perform yourself. If we determine that your A 21 will need further inspection or servicing,
we will:
a) Refer you to an authorized Parasound repair center near you, or
b) Authorize return of the unit to us and advise you of the correct procedure.
Procedure for Returning Your A 21 to Parasound for Service
If Parasound determines that you should send your A 21 to Parasound, you will be given
a Return Authorization (RA) number. This RA number must be clearly marked on the outer
carton only.
IMPORTANT: Enclose a copy of your original purchase receipt. A unit is eligible for warranty
repair ONLY when the purchase receipt shows that the unit was purchased from an Authorized
Parasound Dealer. A unit obtained through unauthorized channels is not eligible for warranty
repair. Parasound is not responsible for any sellers’ misrepresentations about our warranties
or other service policies.
We do not accept any of the following:
Units with collect shipping charges
Units without a valid RA number
Units without a suitable shipping carton
Units for which we see or hear evidence of improper packing
For a non-warranty repair, contact us for an estimate of the repair charges before you ship the
unit to us. The same packing and Return Authorization number procedures apply.
Important Notice - Shipping the A 21
Before shipping the unit to Parasound, you MUST re-pack the unit into its fitted molded foam
insert sandwich and its original carton. If you do not have the original packing cartons and foam
inserts, call us for new packing materials that we can provide to you for a nominal charge.
Use of any other carton and packing materials will probably result in shipping damage, and refusal
of the unit. Common carriers such as UPS seldom pay claims for damage incurred during
shipment when a product is surrounded only with Styrofoam "peanuts" or otherwise improperly
packed.
We cannot stress enough the importance of properly packing your A 21. Shipping damage
resulting from inadequate packing can cost you a lot of money and significantly increase the
time required for repair.
Ship the unit with adequate insurance. After repair under warranty, the unit will be returned to
you via prepaid UPS within the continental United States.

Audio Trigger Sensitivity Adjustment
The Audio Sensitivity Control sets the threshold
of the audio trigger signal. You can adjust this
level from a maximum sensitivity of 50 mV (fully
counterclockwise) to a minimum sensitivity of
200 mV (fully clockwise). If you set this control to
50 mV, the A 21 might be falsely triggered on by
non-musical or noisy signals that can appear in the
system, such as when you switch preamp inputs
at high volume levels. If you set this control to
200 mV, the A 21 might not turn on during quiet
musical passages. The detented position (click stop)
at 12 o’clock corresponds to 100 mV. It’s a good
starting point and will be suitable for most systems.
Balanced and Unbalanced Lines
Recording and broadcast studios use balanced
connections exclusively because of their inherent
ability to reject noise and hum, thus assuring the
best sound. Certain high quality preamplifiers and
surround controllers built for residential use utilize
balanced connections with XLR jacks for the same
reasons. All Parasound Halo series power amplifiers
have balanced inputs with XLR jacks so you can
take full advantage of their inherent noise reduction
capability and superior sound quality.
Unbalanced connections with RCA jacks are found
on all home audio equipment. RCA jacks and two-
conductor wires are less costly than the additional
circuitry, higher priced XLR connectors and three-
conductor wiring required for balanced connections.
In an unbalanced line, the positive audio signal
appears at the center pin of the RCA jack and the
negative signal on the outer shield wire, which also
functions as the ground connection. Unbalanced
interconnect cables are vulnerable to hum from an
AC line, or other noise, such as RF (Radio Frequency),
which can be reproduced through your loudspeakers.
Since the unbalanced line’s ground also carries the
audio signal, there is no way for the connected
amplifier or preamplifier to distinguish between the
audio signals you want and unwanted noise emanating
from external sources.
Balanced lines are superior because they utilize
separate conductors for audio and ground: two inner
conductors carry the positive and negative audio
signal, and a third outer wire connects the grounds
and also shields the two signal conductors. When the
positive and negative signals appear at the component
receiving the signal they are equal, but 180 degrees
out of phase with each other with respect to ground.
To send and receive balanced signals requires special
differential circuitry.
A differential input circuit amplifies only the difference
between the positive and negative signals. For
example, when a 1 Volt signal arrives at a balanced
input stage, the differential input “sees” a positive 1
Volt minus a negative 1 Volt, or 2 Volts total. External
hum and noise that somehow gets into a balanced
line is common to both its positive and negative
TECHNICALLY SPEAKING
15
conductors with respect to ground. Therefore, it is
canceled or rejected by the differential input circuit.
This phenomenon of rejecting noise signals common
to both positive and negative conductors is called
common mode rejection. Differential inputs are
specified according to how well they reject signals
common to both conductors. This is measured in
dB and is called the common mode rejection ratio or
CMRR.
Bare Speaker Wire Ends
If you plan to use connections with bare wire ends,
use a wire stripper to remove just enough insulation
to expose a 1/2” (13 mm) length of bare wire. You
can insert the stripped wire into the hole that goes
sideways through the terminal's metal post. Before
inserting the wire, twist its bare strands to prevent
any of the strands from making contact across the
two speaker terminals. If you have a soldering iron,
you can “tin” (apply a small amount of molten
solder) to each stripped bare wire to prevent it from
unraveling, fraying and oxidizing.
Bridged Mono Operation
In normal operation, the left and right amplifier
channels each amplify both the positive and negative
halves of the musical signal. In bridged mono
operation, the A 21’s entire left channel drives only
the positive half of the musical signal and its entire
right channel drives only the negative half of the
musical signal. This doubles its voltage swing.
This doubled voltage swing enables the A 21 to
deliver nearly double its 400 watts per channel 4 Ω
power, or 750 watts, into a single 8 Ω speaker.
The audible benefit of this higher power is increased
dynamic range, or headroom, so that musical peaks
can be reproduced with less distortion. Considering
that musical peaks and crescendos require 10-100
times as much power as average listening levels,
this added headroom is a substantial advantage
for unrestrained, undistorted listening, even if your
average listening level is moderate.
Bridging allows an amplifier to deliver more power
into a single speaker because in this mode, the load
appears to the amplifier as only half of its rated
impedance. Thus, the single 8 Ω speaker appears as
a 4 Ω load and a 4 Ω speaker appears as only a
2 Ω load.
The A 21 is not capable of driving 2 Ω for extended
periods because it will draw more current and
generate more heat than it can dissipate. We strongly
recommend against using speakers rated at less than
8 Ω when bridging.
Remember that mono bridging requires a separate
A 21 for each speaker. You can’t simultaneously drive
two speakers in stereo and one speaker mono
bridged.

TECHNICALLY SPEAKING continued 16
Choosing Interconnect Cables and Speaker Wire
We are often asked to recommend specific
interconnect cables and speaker wire. It’s true that
with some amplifiers, sound quality will vary greatly
according to interconnect cables and speaker wires.
However, Parasound amplifiers use robust circuitry
that sounds superb regardless of interconnects and
speaker wires. Therefore, we feel that choosing a
brand of cable for Parasound amplifiers is purely a
matter of personal taste.
Ground Loops - Eliminating Hum and Buzz
Audible hum and buzzing noises in a system are
usually related to issues with the component grounds.
Ground (sometimes called common) is a point of
reference for voltages in virtually all audio and video
components. Ground is supposed to remain at zero
volts while the audio signal swings positive (voltage
above ground) and negative (voltage below ground).
If ground isn’t at zero, there can be an audible 60 Hz
hum (or 50 Hz hum in regions with 50 Hz AC). The
harmonics of these frequencies (120 Hz, 240 Hz, 480
Hz or 100 Hz, 200 Hz, 400 Hz) may add buzz in addition
to the hum.
The ideal of zero voltage ground for all the components
in a system is practically impossible, because some
resistance between the ground points of different
components is inevitable. By keeping components
close together with their power cords plugged into
a common AC outlet or power strip, you’ll avoid the
problems created by resistance in the house’s wiring.
Hum and buzz is also caused when unwanted voltage
ows through multiple component ground points
called ground loops. Here are three tips to avoid
ground loops:
1. Use a balanced input line with your Parasound A 21.
(See Balanced and Unbalanced Lines in this section).
2. When rack mounting, always use the included
insulated “shoulder” washers. These break the ground
loops caused by metal-to-metal contact between the
rack, the units, and their rack-mount bolts. Extras
are available from rack manufacturer Middle Atlantic
Products, www.middleatlantic.com.
3. Use the Ground switch on the A 21 to eliminate most
ground loops. This separates the A 21’s signal input
ground from its chassis ground to isolate unwanted
voltage in the ground shield of the unbalanced (RCA)
input cable. Be sure your system installation is nished
before you try moving the Ground switch from its
Normal to its Lift position.
Loop Out Jacks
The Right and Left Loop Out jacks are convenient
connections to route the same input signals from your
preamplifier or surround controller to an additional
power amplifier. These jacks eliminate the need for
“Y” connectors to split the source signal between
your A 21 and another amplifier. The Loop Out jacks
work with either the Balanced or the Unbalanced
Input jacks. Here are some examples of where you
could use the Loop Out jacks:
1. When multiple speakers and amplifiers are to be
played simultaneously, you can route the output of
your preamplifier or controller to the first A 21 then
connect its Loop Out jacks to the Unbalanced Input
jacks a second A 21 (or other amplifier) to power
additional speakers for the same channel.
2. Some high-end speaker manufacturers recommend
bi-amplifying or “bi-amping” without using an
electronic crossover. When bi-amping this way, one
A 21 drives only the woofers and another A 21 drives
only the tweeters. For example, you could connect
the left and right channel outputs of the preamplifier
to the inputs of the first A 21 to power the woofers,
and then connect its Loop Out jacks to the second
A 21 to power the tweeters.
Gain Controls
When the gain knobs are turned fully clockwise, the
actual gain controls are essentially out of the circuit.
The left and right channels are perfectly matched,
and your A 21 can reach its fullest sonic potential.
In some cases, you’ll need to reduce the gain settings.
For example, if you find it difficult to make precise
volume adjustments at low listening levels, your
preamplifier or surround controller probably has a
very high output (or gain) that you can match by
reducing the A 21’s gain. Conversely, if you need to
advance your preamplifier or surround controller’s
volume to nearly the top of its range for average
listening levels, you should increase the gain settings
on the A 21. The A 21 gain adjustment that optimizes
the range of motion on the preamplifier’s volume
control is sometimes called “gain staging”.

PARASOUND A 21 DESIGN OVERVIEW
17
chosen for their low Equivalent Series Resistance
(ESR) and dielectric absorption. In addition, these filter
capacitors are bypassed with smaller polypropylene
capacitors to reduce AC ripple in the DC supply and
to further eliminate noise and interference that is
generated in AC power lines from computers and
other appliances in the home.
Relay-Bypassed Soft Start Circuit
When the A 21 is rst turned on, there is a signicant
amount of in-rush current required to charge the
enormous power supply capacitors. In order to
suppress this in-rush current and to prevent nuisance
tripping of circuit breakers, we employ NTC (negative
temperature coefcient) resistors. These resistors cut
the in-rush current by approximately 50%. Once they
heat up, they essentially become a jumper with zero
ohms resistance. However, the A 21 goes one step
further for this circuit. After the NTC resistors have
done their job of suppressing in-rush current a gold
contact relay automatically is activated to jump across
the NTC resistors to completely bypass them. This
extra step insures that the resistors do not restrict any
current whatsoever to the power supply once the A 21
is in full operation.
Audio Circuit Path Topology
Parasound’s circuit topology is a hybrid of carefully
chosen discrete transistors that result in superior
performance at each stage. We use JFETs (Junction
Field Effect Transistors) for the input stage; MOSFETs
(Metal Oxide Field Effect Transistors) for the driver
stage and bipolar transistors for the output stage.
Discrete transistors are more sonically accurate than
integrated circuits commonly used by other brands.
Complementary Configuration
Each stage of amplification has transistors fed by
the positive DC power supply and complementary
transistors fed by the negative DC power supply.
Thus, half of the devices amplify the positive half
of the musical waveform while the other half of the
devices amplify the negative half. This complementary
topology is inherently linear, which reduces distortion
and improves sonic accuracy.
The Input Stage
The A 21’s input stage uses matched pairs of discrete
JFETs arranged in a differential configuration. JFETs
are ideal for the input stage because their inherently
high impedance is unaffected by the impedance
of source components. Differential configuration
provides superior noise reduction. These precision
input JFETs are also cascoded to produce the current
necessary to drive the MOSFET drivers in the following
stage.
Circuit designed by John Curl
Parasound design consultant John Curl has been a
legend among audiophiles and electronic engineers
for decades. He pioneered measurements to correlate
musical accuracy with the materials used in parts,
worked with world-class touring companies, has
designed highly coveted audio classics, including
the original Mark Levinson JC-2, Denneson JC-80,
Vendetta Phono Preamplifier, and CTC Blowtorch
preamplifiers; master recorders for Wilson Audio and
Mobile Fidelity; and the mixing consoles used in live
concerts by The Grateful Dead and the Montreux Jazz
Festival in Switzerland.
As our featured amplifier designer since 1990, he has
created many products that have earned Parasound
worldwide acclaim. John is particularly proud of
what he and Parasound have accomplished together:
“The circuits I design for Parasound are extremely
sophisticated and are typical of products that are far
more expensive. I can’t think of any other audio
products that offer nearly as much bang for the buck.”
Parts Selection
Every part within the A 21 is carefully chosen for
its accuracy and reliability. Metal film resistors with
1% tolerance are selected for their precision and
because their values don’t drift as they heat up during
operation. Polypropylene and mica capacitors are
used extensively for their superior linearity and low
dielectric absorption. Semiconductors are selected
for superior performance in their specific roles in
the circuit. Gold has the best conductivity of any
metal, so we use high quality gold-plated input
connectors and speaker terminals. The double-sided
circuit boards are FR4 glass epoxy for long-term
durability. The chassis is made of heavy gauge steel
to safely house the internal circuitry. This attention to
detail when selecting parts makes the difference between
a very good amplifier and an outstanding amplifier.
The Power Supply
The heart of the power supply is a 1 kVA toroidal
power transformer, chosen for its efficiency, low
hum field, and high power rating. Encapsulating this
massive power transformer in an epoxy-filled steel
canister assures ultra-quiet performance.
The A 21 power transformer employs multiple
independent secondary windings so that each
amplifier channel has its own power supply, assuring
more than ample DC voltage at all times and under
all conditions. It also reduces inter-channel crosstalk
that can blur the sound and impair the correct sense
of where instruments are positioned.
To create the +/- 80 Vdc B+ and B- supply rails for each
channel, we use high-speed rectifier diodes and four
enormous 25,000 uF Elna electrolytic filter capacitors,