Parasound Stereo Amplifier A 51 User Manual

A 51 Amplifier  
Owners Guide  
 
TABLE OF CONTENTS  
A 51 Amplifier  
PLACEMENT GUIDELINES FOR YOUR A 51  
2
3
CONNECTING A SURROUND CONTROLLER  
TO THE BALANCED INPUTS ON YOUR A 51  
CONNECTING A SURROUND CONTROLLER  
TO THE UNBALANCED INPUTS ON YOUR A 51  
4
5
CONNECTING YOUR SPEAKERS TO THE A 51  
MANUAL AND AUTOMATIC TURN ON-OFF OPTIONS  
CONNECTING AN EXTERNAL DC SOURCE FOR AUTOMATIC ON-OFF  
UNDERSTANDING THE INDICATORS ON THE A 51  
CONNECTING THE AC POWER CORD  
MAINTAINING YOUR A 51  
6
7
8
9
10  
11  
12  
13  
15  
17  
TROUBLESHOOTING GUIDE  
SERVICING YOUR A 51  
TECHNICALLY SPEAKING  
PARASOUND A 51 DESIGN OVERVIEW  
PARASOUND A 51 SPECIFICATIONS  
 
2
PLACEMENT GUIDELINES FOR YOUR A 51  
Power amplifiers are usually heavier and generate more heat than other components. To avoid  
damage to the A 51 or other equipment and to reduce risk of fire, you must follow these guidelines:  
A cable with a 2.5 mm sub-mini plug on each end (provided).  
Place the A 51 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 airflow 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 clear-  
ance can be a little less.  
Do not block the front of the A 51 behind closed cabinet doors during use.  
Do not stack the A 51 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 51  
To mount the A 51 into a 19" wide equipment rack, you must first attach its rack mount brackets  
(provided). With its four feet removed, the A 51 chassis and front panel height occupies four  
rack spaces (7" or 176 mm). When mounting equipment below the A 51, you will also need to  
1
allow about 8" below the unit for the bottom chassis screws. A single standard rack space  
3
allows 1 4" vertical height in a 19" wide equipment rack. This measurement standard was devel-  
oped by the EIA (Electronic Industries Association) so manufacturers of electronic components  
and equipment racks could build products in standardized heights that would fit in a uniform  
space. Please call your Parasound dealer or Parasound Technical Services if you need additional  
advice about rack mounting the A 51.  
To attach the rack mount brackets:  
Remove the three screws from each side of the A 51. These are arranged vertically, behind its  
front panel and in front of its first heatsink fin.  
Line up the three holes on each bracket with the three holes on the A 51 and reinsert the screws.  
Make sure the screws are tight because they will support the entire weight of the A 51 in the  
equipment rack.  
Amplifier  
A
51  
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On-Off  
HiTemp  
Rack Bracket  
 
CONNECTING A SURROUND CONTROLLER  
TO THE BALANCED INPUTS ON YOUR A 51  
3
Left and Right Balanced Input Jacks  
Balanced connections will give you the best sound. If your 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:  
Five balanced interconnect cables with XLR connectors  
Male  
Female  
XLR Connectors  
A surround controller with balanced output connectors  
Before Connecting  
Leave the A 51s 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 Balanced - Unbalanced Input Select switch on the rear of the A 51 is in its  
Balanced (left) position and the Ground switch is in its Normal (down) position.  
To Connect  
1 Plug the male end of the first balanced interconnect cable into the Balanced Ch 1 input  
jack on the A 51.  
2 Plug the female end of the cable into the balanced output jack for the channel on your  
surround controller that you wish to correspond to Ch 1 on your A 51.  
3 0 Repeat steps 1 and 2 above to connect Ch 2, Ch 3, Ch 4, and Ch 5.  
OUTPUTS  
SURROUNDŁ  
CONTROLLER  
2
4
6
8
10  
Input Select Switch,  
set to left position  
Input Select  
Balanced  
A51 Amplifier  
Ch 2  
Ch 3  
Ch 4  
Ch 5  
Lift  
Parasound Products, Inc.  
San Francisco, CA USA  
1
3
5
7
9
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Manufactured under  
license from Lucasfilm  
Ltd. Lucasfilm andTHX  
are registeredTrademarks  
of Lucasfilm Ltd.  
WARNING  
Fuse  
T15A/250V1-115V Area  
T8A/250V-230V Area  
To Prevent Fire Or Shock  
Hazard, Do Not ExposeThis  
UnitTo Rain Or Moisture.  
AC 115V  
AC 230V  
60Hz  
50Hz  
CAUTION  
To Prevent Electric Shock,  
Do Not Remove Cover. No  
User-Serviceable Parts Inside,  
Refer Servicing To Qualified  
Service Personnel.  
Audio  
Man  
50 mV 250 Normal  
Sensitivity  
12  
V
Power Consumption: 1800W  
Input  
AutoTurn On  
Ground  
Ground Switch, set to down position  
YOU SHOULD KNOW  
Balanced XLR Jacks and Their Pin Configuration  
The balanced inputs of the A 51 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 51 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 SURROUND CONTROLLER  
TO THE UNBALANCED INPUTS ON YOUR A 51  
4
Ch 1-5 Unbalanced Inputs  
Use these inputs if your surround controller doesnt have balanced output connectors or if you  
simply prefer to use unbalanced connections.  
What Youll Need:  
Five shielded interconnect cables with RCA plugs  
A surround controller with five RCA output jacks  
RCA Plugs  
Before Connecting  
Leave the AC cord on the A 51 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 pulled or stretched once  
they are connected.  
Make sure the Balanced-Unbalanced Input Select switch on the A 51 is in its Unbalanced (right)  
position and the Ground switch is in its Normal (down) position.  
To Connect  
1 Plug one end of the cable into the Unbalanced Ch 1 Input jack on the A 51.  
2 Plug the other end of the cable into the unbalanced output jack for the channel on your  
surround controller that you wish to correspond to Ch 1 on your A 51.  
3 0 Repeat steps 1 and 2 for Ch 2, Ch 3, Ch 4, and Ch 5.  
OUTPUTS  
SURROUNDŁ  
CONTROLLER  
2
4
6
8
10  
Input Select Switch,  
set to right position  
Input Select  
Balanced  
Unbalan
A51 Amplifier  
Ch
Ch 2  
Ch 3  
Ch 4  
Ch 5  
Lift  
Parasound Products, Inc.  
San Francisco, CA USA  
1
3
5
7
9
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A
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Manufactured under  
license from Lucasfilm  
Ltd. Lucasfilm andTHX  
are registeredTrademarks  
of Lucasfilm Ltd.  
WARNING  
Fuse  
T15A/250V1-115V Area  
T8A/250V-230V Area  
To Prevent Fire Or Shock  
Hazard, Do Not ExposeThis  
UnitTo Rain Or Moisture.  
AC 115V  
AC 230V  
60Hz  
50Hz  
CAUTION  
To Prevent Electric Shock,  
Do Not Remove Cover. No  
User-Serviceable Parts Inside,  
Refer Servicing To Qualified  
Service Personnel.  
Audio  
Man  
50 mV 250 Normal  
Sensitivity  
12  
V
Power Consumption: 1800W  
Input  
AutoTurn On  
Grond  
Ground Switch, set to down position  
YOU SHOULD KNOW  
Write Down Your Connections  
Make a note of which channel on your surround controller now connects to each channel  
of the A 51 so you’ll always hear the channels in their intended position and to facilitate  
trouble-shooting in case you encounter a problem later on.  
 
5
CONNECTING YOUR SPEAKERS TO THE A 51  
Speaker Terminals  
The A 51 speaker terminals accept speaker wires with banana plugs or bare ends. Refer to Bare Wire  
SpeakerTermination in the Technically Speaking section for information about bare wire termination.  
Common Speaker Connectors  
What Youll Need:  
Five lengths of AWG 16 or thicker speaker wire  
with banana plugs or bare ends  
Five loudspeakers  
Dual  
Banana Plug  
Single  
Banana Plug  
Stripped  
AWG wire  
Before Connecting  
Remove power to all the components in your audio system.  
To Connect  
Because the speaker terminals are very close to each other, the bare wire ends insert diagonally  
into the terminal shafts. The - (negative) wires enter the black - terminals at 8 oclock. The +  
(wires) enter the red + terminals at 4 oclock. The resulting 45 degree angles of the speaker  
wires routes them outside the terminals of the channels below. Its easier to connect speaker  
wires if you start with the bottom terminals for Channel 5 and proceed upward.  
1 Insert one wire with the ridge or other marking into the red Ch 5 + (positive) speaker  
terminal on the A 51. Insert the wire without the ridge or other marking into the adjacent  
black Ch 5 - (negative) speaker terminal on the A 51.  
2 Insert the other end of the wire with the ridge or other marking to the red + (positive)  
terminal on the speaker. Insert the other end of the wire without the ridge or other marking  
to the adjacent black (minus) terminal on the speaker.  
3 0 Repeat steps 1 and 2 for Ch 4, then Ch 3, then Ch 2, and finally Ch 1.  
Input Select  
Balanced  
Unbalanced  
Ch 1  
9
7
5
3
1
A51 Amplifier  
Parasound Products, Inc.  
San Francisco, CA USA  
Ch 2  
Ch 3  
Ch 4  
Ch 5  
Lift  
L
U
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A
S
F
I
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Manufactured under  
license from Lucasfilm  
Ltd. Lucasfilm andTHX  
are registeredTrademarks  
of Lucasfilm Ltd.  
WARNING  
Fuse  
T15A/250V1-115V Area  
T8A/250V-230V Area  
To Prevent Fire Or Shock  
Hazard, Do Not ExposeThis  
UnitTo Rain Or Moisture.  
AC 115V  
AC 230V  
60Hz  
50Hz  
CAUTION  
To Prevent Electric Shock,  
Do Not Remove Cover. No  
User-Serviceable Parts Inside,  
Refer Servicing To Qualified  
Service Personnel.  
Audio  
Man  
50 mV 250 Normal  
Sensitivity  
12  
V
Power Consumption: 1800W  
Input  
AutoTurn On  
Ground  
SPEAKERS  
10  
8
6
4
2
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 all 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.  
Keep wire lengths for both channels as close to equal as possible.  
 
6
MANUAL AND AUTOMATIC TURN ON-OFF OPTIONS  
You can manually turn the A 51 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 51 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.  
Input Select  
Balanced  
Unbalanced  
Ch 1  
A51 Amplifier  
Parasound Products, Inc.  
San Francisco, CA USA  
Ch 2  
Ch 3  
Ch 4  
Ch 5  
Lift  
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Manufactured under  
license from Lucasfilm  
Ltd. Lucasfilm andTHX  
are registeredTrademarks  
of Lucasfilm Ltd.  
WARNING  
Fuse  
T15A/250V1-115V Area  
T8A/250V-230V Area  
To Prevent Fire Or Shock  
Hazard, Do Not ExposeThis  
UnitTo Rain Or Moisture.  
AC 115V  
AC 230V  
60Hz  
50Hz  
CAUTION  
To Prevent Electric Shock,  
Do Not Remove Cover. No  
User-Serviceable Parts Inside,  
Refer Servicing To Qualified  
Service Personnel.  
Audio  
Man  
50 mV 250 Normal  
Sensitivity  
12  
V
Power Consumption: 1800W  
Input  
AutoTurn On  
Ground  
AutoTurn On Switch, set to middle position  
Connecting an Active Audio Source for Automatic On-Off  
If you want to automatically turn on the A 51 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 Youll Need:  
An audio source thats playing  
Before Connecting  
Remove power to all the components in your audio system.  
Make sure the Auto Turn On switch on the A 51 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 oclock position.  
To Connect  
Connect an active audio source to either the Ch 1 Balanced or Unbalanced Input jacks.  
(Refer to to pages 2 and 3).  
Input Select  
Balanced  
Unbalanced  
Ch 1  
A51 Amplifier  
Parasound Products, Inc.  
San Francisco, CA USA  
Ch 2  
Ch 3  
Ch 4  
Ch 5  
Lift  
L
U
C
A
S
F
I
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M
Manufactured under  
license from Lucasfilm  
Ltd. Lucasfilm andTHX  
are registeredTrademarks  
of Lucasfilm Ltd.  
WARNING  
Fuse  
T15A/250V1-115V Area  
T8A/250V-230V Area  
To Prevent Fire Or Shock  
Hazard, Do Not ExposeThis  
UnitTo Rain Or Moisture.  
AC 115V  
AC 230V  
60Hz  
50Hz  
CAUTION  
To Prevent Electric Shock,  
Do Not Remove Cover. No  
User-Serviceable Parts Inside,  
Refer Servicing To Qualified  
Service Personnel.  
Audio  
Man  
50 mV 250 Normal  
Sensitivity  
12  
V
Power Consumption: 1800W  
Input  
AutoTurn On  
Ground  
AutoTurn On Switch, set to up position  
YOU SHOULD KNOW  
Turn Off Delay in the Audio Mode  
When the music stops, the A 51 will remain on for about five minutes.This delay prevents  
unwanted turn-off during silent passages in music or during the pauses between tracks.  
 
CONNECTING AN EXTERNAL DC SOURCE FOR  
AUTOMATIC TURN ON-OFF  
7
To automatically turn on and off the A 51 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 Youll Need:  
A cable with a 2.5 mm sub-mini plug on each end (provided)  
A controlcomponent with +9Vdc to +12Vdc trigger voltage  
2.5 mm sub-mini plug  
Before Connecting  
Remove Power to all the components in your audio system.  
Make sure the Auto Turn On switch on the A 51 rear panel is in its 12V (down) position.  
To Connect  
1 Plug one end of the trigger wire into the 12V Input jack on the A 51.  
2 Plug the other end of this wire into the external DC source.  
Input Select  
Balanced  
Unbalanced  
Ch 1  
A51 Amplifier  
Parasound Products, Inc.  
San Francisco, CA USA  
Ch 2  
Ch 3  
Ch 4  
Ch 5  
Lift  
L
U
C
A
S
F
I
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Manufactured under  
license from Lucasfilm  
Ltd. Lucasfilm andTHX  
are registeredTrademarks  
of Lucasfilm Ltd.  
WARNING  
Fuse  
T15A/250V1-115V Area  
T8A/250V-230V Area  
To Prevent Fire Or Shock  
Hazard, Do Not ExposeThis  
UnitTo Rain Or Moisture.  
AC 115V  
AC 230V  
60Hz  
50Hz  
CAUTION  
To Prevent Electric Shock,  
Do Not Remove Cover. No  
User-Serviceable Parts Inside,  
Refer Servicing To Qualified  
Service Personnel.  
Audio  
Man  
50 mV 250 Normal  
Sensitivity  
12  
V
Power Consumption: 1800W  
Input  
AutoTurn On  
Ground  
1
AutoTurn On Switch, set to down position  
2
CONTROL  
CONTROL  
COMPONENT  
COMPONENT  
Output  
YOU SHOULD KNOW  
If the device you want to use to control your A 51 doesnt 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.  
 
8
UNDERSTANDING THE INDICATORS ON THE A 51  
a AC Present Indication  
When the A 51 is plugged into a live AC outlet, a soft blue halo glows behind its On-Off  
button and its red PBadge glows faintly. These indicate that the unit is plugged into a live  
AC outlet, even when it is turned off.  
b On, Off and Fault/Protection Status Indicators  
Whenever the A 51 is turned on, the soft blue glow behind its On-Off button will change  
to red for about five 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 51s protection circuits and no  
sound will be heard from the speaker.  
The A 51 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 51 will resume operation. If the A 51 remains in protectionafter it has cooled down  
and youve confirmed there are no external faults, it could indicate an internal problem.  
Please contact Parasounds Technical Service Department.  
c g Channel Status Indicators  
These are the five small round indicators in the center of the recess in the front panel.  
The indicator on the left displays the status of channel one and the subsequent indicators  
display the status of channels two through five.  
c g All Indicators Illuminated  
When all five channels are operating normally, these will all glow blue.  
c g No Illumination:  
When the A 51 is turned off, these indicators are off. An indicator that does not glow when  
the A 51 is turned on represents a fault in that channel.  
h High-Temp Indicator  
This indicator is near the right side of the panel recess. It will glow red if any channel over-  
heats. The On-Off button will also glow red if the A 51 overheats. Refer to the  
Troubleshooting Guide on page 11.  
Amplifier  
A 51  
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On-Off  
HiTemp  
 
9
CONNECTING THE A 51 AC POWER CORD  
AC Power Connections  
We recommend that you plug your A 51 into the same AC wall outlet or power strip that pow-  
ers your other audio components, especially the preamplifier or system controller. Having all  
the audio components on the same power circuit helps prevent hum caused by possible ground  
loops. The A 51 requires AC power that is continuous rather than switched on and off.  
110 V - 120 V or 220 V - 240 V Operation  
The A 51 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 51s operating voltage involves  
rewiring internal connections for both its main power transformer and standby power trans-  
former, plus changing fuse values. Since there is a risk of electric shock, voltage changes must  
be referred to a qualified technician.  
What Youll 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  
1 Plug the female end of the AC cord to the AC receptacle on the rear of the A 51.  
2 Plug the male end of the AC cord to an AC outlet or power strip.  
Input Select  
Balanced  
Unbalanced  
Ch 1  
A51 Amplifier  
Parasound Products, Inc.  
San Francisco, CA USA  
Ch 2  
Ch 3  
Ch 4  
Ch 5  
Lift  
L
U
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A
S
F
I
L
M
Manufactured under  
license from Lucasfilm  
Ltd. Lucasfilm andTHX  
are registeredTrademarks  
of Lucasfilm Ltd.  
WARNING  
Fuse  
T15A/250V1-115V Area  
T8A/250V-230V Area  
To Prevent Fire Or Shock  
Hazard, Do Not ExposeThis  
UnitTo Rain Or Moisture.  
AC 115V  
AC 230V  
60Hz  
50Hz  
CAUTION  
To Prevent Electric Shock,  
Do Not Remove Cover. No  
User-Serviceable Parts Inside,  
Refer Servicing To Qualified  
Service Personnel.  
Audio  
Man  
50 mV 250 Normal  
Sensitivity  
12  
V
Power Consumption: 1800W  
Input  
AutoTurn On  
Ground  
1
2
 
10  
MAINTAINING YOUR A 51  
Your Parasound A 51 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 amplifiers 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.  
Notes:  
 
11  
TROUBLESHOOTING GUIDE  
TROUBLE  
PROBABLE CAUSE  
REMEDY  
Power cord is disconnected  
Connect power cord  
No power  
Auto Turn On selector switch  
is in wrong position  
Check for correct position of the Auto  
Turn On switch  
AC fuse blown  
Replace with same value fuse  
DC trigger source wired in  
reverse or voltage too low  
Check polarity of DC source with a voltmeter.  
Try connecting a 9V battery to the 12V DC input  
Auto turn on doesn’t  
function with a trigger  
voltage  
Auto Turn On selector switch  
Make sure Auto Turn On selector switch is  
is in wrong position  
in its 12V position  
Auto Turn On selector switch  
is in wrong position  
Make sure Auto Turn On selector switch is  
in its Audio position  
Auto turn on doesn’t  
function with an audio  
input signal  
Sensitivity incorrectly adjusted  
Turn the Audio trigger Sensitivity knob  
counterclockwise  
Sensitivity level too high  
Turn Sensitivity knob clockwise toward 250 mV  
A 51 will not turn off when  
audio signal is removed  
Bad connection from surround  
controller  
Check input connections or try a different  
set of cables  
Power on, but no sound  
Over-current protection circuitry  
has been activated- one or both  
blue channel LEDs not illuminated  
Check speaker load impedance. Check speaker  
wires and connections  
On-Off button glows red  
and channel indicator(s)  
off  
A 51 has overheated - red Hi  
Temp indicator is on  
Allow A 51 to cool. Check that there is adequate  
ventilation around the A 51. Be sure the load  
impedance of each channel is no less than 4 ohms  
Damaged speaker  
Connect each speaker to another channel on the  
A 51. If the same speaker remains distorted, the  
speaker or its wire is damaged. If the other speak-  
er distorts, the problem may be with the A 51, the  
surround controller, or a source component  
Distorted sound  
Problem with preamplifier  
or source component  
Switch the input cables. If distortion moves  
to another channel, the problem is with a  
component other than the A 51  
Balanced-Unbalanced Input  
switch is in the incorrect position  
Move to the correct position for the jacks  
in use  
Ground loop between surround  
controller and A 51 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  
Hum and / or buzz  
through speakers  
Ground loop from cable TV  
Move the Ground switch to its Lift position.  
Install a cable isolation device such as the  
Xantech 634 Ground Breaker.  
Other ground loop  
Contact Parasound Technical Service  
Load impedance at speaker  
terminals is too low  
Make sure the speaker or speaker selector  
load is 4 ohms or higher  
A 51 is overheating  
Not enough ventilation  
Make sure the A 51 has adequate ventilation  
 
12  
SERVICING YOUR A 51  
If All Else Fails Call Us for Help  
Call your Parasound dealer or Parasounds Technical Service Department toll free at  
1-866-770-TECH (8324). We can often solve the problem with simple diagnostic tests you can per-  
form yourself. If we determine that your A 51 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 51 to Parasound for Service  
If Parasound determines that you should send your A 51 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 sellersmisrepresentations 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 51  
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 peanutsor otherwise  
improperly packed.  
We cannot stress enough the importance of properly packing your A 51. 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.  
 
13  
TECHNICALLY SPEAKING  
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 conductors  
with respect to ground. Therefore, it is canceled or  
rejected by the differential input circuit.  
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 250 mV (fully clockwise).  
If you set this control to 50 mV, the A 51 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 250 mV, the A 51 might not turn on during  
quiet musical passages. The detented position (click  
stop) at 12 oclock corresponds to 100 mV. Its a good  
starting point and will be suitable for most systems.  
This phenomenon of rejecting noise signals common  
to both positive and negative conductors is called  
common mode rejection. Differential inputs are speci-  
fied 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 12" (13 mm) length of bare wire. You can insert  
the stripped wire into the hole that goes sideways  
through the terminals 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.  
Balanced and Unbalanced Lines  
Recording and broadcast studios use balanced con-  
nections 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 bal-  
anced inputs with XLR jacks so you can take full  
advantage of their inherent noise reduction capability  
and superior sound quality.  
Speaker Wires - Banana Plugs Instead of Bare Ends  
For convenience, weve supplied ten gold-plated  
banana plugs with your A 51. To attach these plugs, first  
pull the flexible red and black insulators off the plugs  
and slide them on the + and - speaker wires. Next, use  
a wire stripper to remove insulation to expose about  
3/4" (19mm) of bare wire. Insert the bare wire into the  
barrel of the banana plug and crimp the barrel down  
with a pliers or crimper to hold it in place. Finally, slide  
the colored insulator down the wire so it is over the  
barrel of the plug. The only exposed metal should be  
the tip of the plug.  
Unbalanced connections with RCA jacks are found on  
all home audio equipment. RCA jacks and two-conduc-  
tor 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 lines  
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.  
Choosing Interconnect Cables and Speaker Wire  
We are often asked to recommend specific brands of  
interconnect cables and speaker wire. Its 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.  
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 bal-  
anced signals requires special differential circuitry.  
Ground Loops - Eliminating Hum and Buzz  
Audible hum and buzzing noises in a system are usu-  
ally 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 isnt at zero, there can be an audible 60 Hz  
A differential input circuit amplifies only the difference  
between the positive and negative signals. For exam-  
ple, when a 1 Volt signal arrives at a balanced input  
stage, the differential input seesa positive 1 Volt  
 
14  
TECHNICALLY SPEAKING continued  
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 addi-  
tion to the hum.  
1. Use balanced input lines with your Parasound A 51.  
(See Balanced and Unbalanced Lines in this section).  
2. When rack mounting, always use the insulated  
shoulderwashers. These break the ground loops  
caused by metal-to-metal contact between the rack,  
the components, and their rack-mount bolts. Extra  
washers are available from rack manufacturer Middle  
The ideal of zero voltage ground for all the compo-  
nents in a system is practically impossible, because  
some resistance between the ground points of differ-  
ent components is inevitable. By keeping components  
close together with their power cords plugged into a  
common AC outlet or power strip, youll avoid the  
problems created by resistance in the houses wiring.  
3. Use the Ground switch on the A 51 to eliminate  
most ground loops. This separates the A 51s 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 finished before you try moving the  
Ground switch from its Normal to its Lift position.  
Hum and buzz is also caused when unwanted voltage  
flows through multiple component ground points  
called ground loops. Here are three tips to avoid  
ground loops:  
Input Select  
Balanced  
Unbalanced  
A51 Amplifier  
Parasound Products, Inc.  
San Francisco, CA USA  
Ch 1  
Ch 2  
Ch 3  
Ch 4  
Ch 5  
Lift  
L
U
C
A
S
F
I
L
M
Manufactured under  
license from Lucasfilm  
Ltd. Lucasfilm andTHX  
are registeredTrademarks  
of Lucasfilm Ltd.  
WARNING  
Fuse  
T15A/250V1-115V Area  
T8A/250V-230V Area  
To Prevent Fire Or Shock  
Hazard, Do Not ExposeThis  
UnitTo Rain Or Moisture.  
AC 115V  
AC 230V  
60Hz  
50Hz  
CAUTION  
To Prevent Electric Shock,  
Do Not Remove Cover. No  
User-Serviceable Parts Inside,  
Refer Servicing To Qualified  
Service Personnel.  
Audio  
Man  
50 mV 250  
Sensitivity  
AutoTurn On  
Normal  
Ground  
12 V  
Power Consumption: 1800W  
Input  
Ground Switch, set to up position  
 
15  
PARASOUND A 51 DESIGN OVERVIEW  
Circuit Designed by John Curl  
Each channels +/- 80 Vdc B+ and B- supply rails use  
high-speed rectifier diodes and four enormous 8,200 uF  
electrolytic filter capacitors, chosen for their low  
Equivalent Series Resistance (ESR) and dielectric absorp-  
Parasound design consultant John Curl has been a  
legend among audiophiles and electronic engineers  
for decades. He pioneered measurements to corre-  
late musical accuracy with the materials used in parts, tion. In addition, these filter capacitors are bypassed  
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 pre-  
amplifiers; 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.  
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 51 is first turned on, there is a significant  
amount of in-rush current required to charge the enor-  
mous power supply capacitors. In order to suppress  
this in-rush current and to prevent nuisance tripping of  
circuit breakers, we employ NTC (negative temperature  
coefficient) resistors. These resistors cut the in-rush cur-  
rent by approximately 50%. Once they heat up, they  
essentially become a jumper with zero ohms resist-  
ance. However, the A 51 goes one step further for this  
circuit. After the NTC resistors have done their job of  
suppressing in-rush current a gold contact relay auto-  
matically 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 51 is in full operation.  
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 sophisti-  
cated and are typical of products that are far more  
expensive. I cant think of any other audio products  
that offer nearly as much bang for the buck.”  
Parts Selection  
Every part within the A 51 is carefully chosen for its  
accuracy and reliability. Metal film resistors with 1%  
tolerance are selected for their precision and because  
their values dont drift as they heat up during opera-  
tion. Polypropylene and mica capacitors are used  
extensively for their superior linearity and low dielec-  
Audio Circuit Path Topology  
Parasounds circuit topology is a hybrid of carefully  
chosen discrete transistors that result in superior per-  
tric absorption. Semiconductors are selected for supe- formance at each stage. We use JFETs (Junction Field  
rior 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 select-  
ing parts makes the difference between a very good  
amplifier and an outstanding amplifier.  
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 transis-  
tors 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 Power Supply  
The heart of the power supply is a 2.4 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 Input Stage  
The A 51 power transformer employs multiple inde-  
pendent 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 instru-  
ments, dialogue and effect are positioned.  
The A 51s 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.  
 
16  
PARASOUND A 51 DESIGN OVERVIEW continued  
The Driver Stage  
Total Protection - DC Servos  
The driver stage provides critical amplification for  
which we employ a complementary matched pair of  
MOSFETs selected for their tube-like sonic qualities.  
MOSFETs tend to generate less odd-order harmonic  
distortion than bipolar transistors. This is important  
because odd-order distortion sounds unnatural and  
fatiguing to the human ear, whereas even-order dis-  
tortion is less offensive because it is consonant,  
rather than dissonant. Our MOSFET driver stage pre-  
vents the harshness and brittle sound so often found  
in other amplifiers.  
Direct Current (DC) burns out speakers. Every power  
amplifier must have some way to insure that DC from  
its power supply never reaches its + or - speaker ter-  
minals. Most amplifiers simply use trim controls to  
reduce their DC offset or capacitors to block DC.  
Unfortunately, trim controls can allow DC offset to  
increase over time, and even the most expensive  
capacitors in the audio signal path will veilsonic  
clarity and attenuate bass response.  
Parasound power amplifiers incorporate ingenious and  
fast-acting DC servo circuits, completely eliminating  
the need for coupling and blocking capacitors. The  
The Output Stage  
The amplifiers sonic characteristics are established by A 51 is direct-coupled from its input jacks to its  
its input and driver stages. Now, the sole job of its  
output stage is to deliver the enormous current and  
speaker terminals. This advanced circuitry never  
needs adjustment or maintenance. It operates out-  
voltage from its power supply to the speakers. Bipolar side the audio signal path to keep the DC offset at  
output transistors are better than MOSFETS in the  
output stage because of their higher safe operating  
area (SOA) and inherent ruggedness. Each channels  
output stage employs four pairs of high current (15-  
ampere) bipolar transistors to insure long-term reliabil-  
ity, even with continuous high power operation and  
challenging speaker loads. Lightning-fast (60 MHz)  
transistors respond instantly to complex demands in  
the musical signal, virtually eliminating distortions that  
occur with slower transistors. Slew rate limiting and  
Transient Intermodulation Distortion (TIM) are simply  
not an issue in the A 51.  
the output of the A 51 at a constant 0.00 Vdc. The  
results are startling clarity, freedom from listening  
fatigue, and formidable bass response.  
Total Protection - Relays  
Each channel of the A 51 has a high-quality protection  
relay with gold-plated contacts for long-term reliability.  
These relays function to protect either the amplifier,  
the speakers, or both. When the A 51 is first powered  
on, these relays remain open for three seconds as the  
positive and negative power supplies stabilize and  
reach equilibrium. This prevents annoying popping or  
other transient noises. Relay protection also prevents  
damage to your speakers in case of a catastrophic  
amplifier failure. Any amplifier that doesnt use relay  
protection for its speaker outputs compromises the  
safety of the amplifier and your speakers.  
Class A-A/B Operation  
Pure class A operation provides the purest sound.  
However, an amplifier operating entirely in class A  
operation would be enormous, highly inefficient, and  
generate too much heat. Class A/B combines some of  
the advantages of Class A with the efficiency of Class  
B operation. It is a compromise that reduces the heat  
generated in pure class A operation and the odd-order  
harmonic distortion created in class B. In class AB,  
the driver and output stages are always partially  
turned on, which provides a nominal amount of pure  
class A operation. At higher power levels, when the  
musical waveform swings from positive to negative  
and vice versa, each bank of transistors is allowed to  
rest momentarily. This resting, or quiescent time,  
makes it possible to deliver high amounts of power  
without overheating. It also makes it possible to use  
passive cooling and avoid fans, whose noise can be  
heard over the music. The A 51 input and driver  
stages employ pure Class A while its output stage  
operates with higher pure Class A power than many  
amplifiers selling for twice or three times its price.  
The result is less fatiguing, more natural sound.  
Total Protection - Current Overload  
Specialized current-sensing transistors are connected  
to the output stages of the A 51 to constantly moni-  
tor the current flow through the output transistors.  
If the current drawn by this stage exceeds a predeter-  
mined safe level due to a load impedance below  
1 ohm or a short circuit at the speaker terminals, the  
output relay will open immediately to prevent any of  
the output transistors or other parts from failing.  
Total Protection - Fuses  
Each channel of the amplifier has a separate fuse for  
its positive and negative DC voltage rails. These fuses  
provide backup protection in case the over-current  
protection does not work in time, or if an internal part  
fails. In the event of a part failure, these fuses halt  
operation to minimize damage to additional parts.  
 
17  
PARASOUND A 51 SPECIFICATIONS  
Power Output - All Channels Driven  
Input Sensitivity for 28.28 V Output into 8  
250 watts RMS x 5, continuous,  
Unbalanced: 1 V  
20 Hz - 20 kHz, 8  
Balanced: 1 V per leg  
400 watts RMS x 5, continuous,  
20 Hz - 20 kHz, 4Ω  
S/N Ratio  
> 112 dB, input shorted, IHF A-weighted  
> 102 dB, input shorted, unweigh  
Current Capacity  
60 amperes peak per channel  
Damping Factor  
> 1100 at 20 Hz  
Slew Rate  
> 130 volts per microsecond  
DC Trigger Requirements  
+9 Vdc to +12 Vdc, 2 mA  
Frequency Response  
5 Hz - 100 kHz, +0/-3 dB at 1 watt  
Audio Trigger Requirements  
50 mV AC - 250 mV AC  
Total Harmonic Distortion  
< 0.2 % at full power  
Dimensions  
1
< 0.03 % at typical listening levels  
Width: 17 2" (445 mm)  
Panel height: 7" (178 mm)  
IM Distortion  
5
Height with feet: 7 8" (194 mm)  
< 0.04 %  
Depth: 20" (508 mm)  
TIM  
Unmeasureable  
Power Requirement  
Standby: 25 Watts  
Dynamic Headroom  
Full Power into 4: 3000 Watts  
> 1.5 dB  
Net Weight  
Interchannel Crosstalk  
80 lb. (36.4 kg)  
> 78 dB at 1 kHz  
> 63 dB at 20 kHz  
Shipping Weight  
87 lb. (39.5 kg)  
Input Impedance  
Unbalanced: 47k Ω  
Balanced: 94k Ω  
V 1.0 Features and specifications subject to change without notice.  
© Parasound Products, Inc. 2002.  
 
Parasound Products, Inc. 950 Battery Street, San Francisco, CA 94111  
415-397-7100 / Fax 415-397-0144 www.parasound.com  
 

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