Furman Sound Stereo Amplifier SP 20AB User Manual

 
SP-20AB MANUAL  
The SP-20AB’s compact size and 20 watts per  
channel make it ideal for modular use in large  
commercial installations with multiple small  
speakers. Its power capacity is also just right  
for driving the small near field monitors used  
in most recording studios as a comparison  
with the main control room monitors - it can  
power them without overpowering them.  
Options and Accessories  
Model HR-2 Headphone Remote Station:  
The HR-2 is compact, unobtrusive box that  
clamps to a mike stand or belt, providing two  
headphone jacks, each with its own volume  
control. Any number of HR-2 stations may  
be connected to a SP-20AB using standard  
microphone cables. The HR-2 comes with  
snap-on, snap-off mike stand clamp.  
The SP-20AB’s output is available through  
either of two connectors: standard banana  
jack binding posts for speakers, or through  
a male XLR connector. The Satellite/Remote  
XLR output allows the unit to become a  
headphone driver, linking any number of  
‘Furman HR-2 Headphone Remote Stations  
together via standard microphone cables.  
Each HR-2 station conveniently clips to a  
mic stand and provides individual volume  
controls for two sets of headphones. When  
used for overdubbing or rehearsal, this setup  
provides more than adequate loudness in  
all headphones (even older high-impedance  
models), and allows each musician to set his  
or her own comfortable volume level. When  
the overdubbing is complete and the musi-  
cians remove their headphones, the SP-20AB  
can also power the tape playback speakers  
through the banana jacks, as selected by  
pressing the SPEAKERS button on the front  
panel. An additional headset may be plugged  
directly into the SP-20AB’s front panel. A  
volume control is provided for it.  
HRKIT-1:  
The HRKIT-1 allows mounting of one SP-  
20AB in a single rack space. Unit is centered in  
the space, which equally distributes the weight  
and allows room for user customizations such  
as additional selector switches, indicator lights,  
extra banana jacks, etc.  
HRKIT-2:  
Allows side by side mounting of two SP-  
20AB’s in a single rack space or one SP-20AB  
in a standard half rack enclosure.  
The SP-20AB has a convenient input level  
control on the left side of the front panel. A  
pair of yellow LED’s - one for each channel  
- light up whenever signal is present (threshold  
-30 dBu). Another pair of red LED’s show an  
overload condition in either channel.  
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SP-20AB MANUAL  
5
4
0
6
5
4
0
6
3
7
OFF  
ON  
3
7
L
R
2
8
2
8
OVERLOAD  
SIGNAL  
1
9
PWR  
9
1
10  
20 WATTS / CHANNEL  
10  
HEAD  
PHONE  
STEREO AMPLIFIER  
INPUT LEVEL  
SPEAKERS  
VOLUME  
MODEL SP-20A  
volume may be adjusted without affecting  
the speaker level or the level of any other  
headphones connected through HR-2  
Remote Stations.  
Descriptions of Controls  
Front Panel  
SPEAKERS BUTTON: Depressing this button  
turns on the speakers connected to the  
SPEAKER banana jacks on the rear panel.  
This provides an easy way to mute the sound  
without changing the volume setting.  
POWER LIGHT: Glows red when unit is  
receiving power and the ON-OFF SWITCH  
is in the ON position.  
INPUT LEVEL: The Input Level control can be  
adjusted for maximum signal to noise and  
minimum clipping. The Input Level control is  
stereo - it affects both left and right channels  
simultaneously and affects all outputs.  
SIGNAL PRESENT LIGHTS: These yellow  
LED’s turn on whenever a signal exceeding  
125 millivolts RMS (40 dB below full power) is  
present at the SP-20AB’s input jack. They will  
increase in brightness as the signal strength  
increases. This feature is very useful as a  
clue to where the fault may lie if the expected  
program is not heard at the outputs.  
OVERLOAD LIGHTS: These LED’s glow red to  
indicate an overload in the SP-20AB. If the  
LED lights up, either the drive level to the  
unit should be reduced, or the setting of the  
Input Level control should be reduced until  
the LED remains off. The lights turn on when  
the signal level at either output is within 2 to  
3dB of clipping. Once triggered, the lights will  
stay lit for a minimum of 50 milliseconds so  
they can be easily seen, even if the actual  
duration of the overload is much shorter.  
HEADPHONE JACK / HEADPHONE VOLUME:  
Any type of headphones wired with a  
standard 1/4” stereo phone plug may be  
used successfully. Note that the headphone  
Rear Panel  
ON-OFF SWITCH: This switch turns the SP-  
20AB on and off. For convenience, it is  
recommended that the SP-20AB and any  
other rack-mount equipment be powered  
through a switchable outlet box, such as  
the Furman Sound PL-8 or PL-PLUS Power  
Conditioner and Light Module. The PL-8  
provides an easy way to power up the  
whole rack with one front-panel switch, and  
provides discreet illumination on dark stages  
and studios as well. If you do use a switcha-  
ble outlet box, leave the SP-20AB’s on-off  
switch in the ON position at all times.  
GROUND LIFT SWITCH: In many installations,  
hum-causing ground loops are formed by  
the common connection of various pieces of  
equipment to the power line ground, and by  
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SP-20AB MANUAL  
GND  
LIFT  
MONO  
STEREO  
BRIDGED  
ON  
OFF  
120 VAC  
60 Hz  
SPEAKERS  
LEFT RIGHT  
130 Watts  
only one input (plugged into the “Left/Mono  
Input” jack) and all the SP-20AB’s power is  
to be concentrated into one output. Note  
that under some circumstances, this can  
amount to more than double the rated 20  
watts pet channel for stereo .- to as much  
as 50 watts. Therefore, in bridged mode, for  
adequate protection, the speaker connected  
should be able to handle approximately 100  
watts without damage. See “SPEAKER  
OUTPUTS” below for details of how to  
connect the speaker.  
contact between chassis, as in a rack with  
metal rails. Sliding the Ground Lift switch up  
completely isolates all signal grounds from  
the chassis, breaking any ground loops. The  
chassis always remains connected to the  
ground pin on the AC cord for safety and to  
provide shielding against RF interference.  
Try both positions of the Ground Lift switch,  
and leave the switch in the position that  
results in the least hum in your system. Note:  
The ground lift can be rendered ineffective by  
3 pin audio connectors that tie pin I (signal  
ground) to the metal shell of the connector  
(chassis ground). The connection from pin 1  
to the shell is optional on all 3 pin connec-  
tors, and can be removed if present, by  
opening up the connector on the cable and  
disconnecting it.  
SPEAKER OUTPUTS: These outputs should be  
used for driving speakers. The connections  
are via multiple-use terminals, which can  
accommodate either dual banana plugs,  
bare wires, or wires terminated in spade lugs.  
For stereo or dual mono operation, connect  
one speaker to the left pair of terminals and  
the other speaker to the right pair. Observe  
proper polarity - make sure that the wire  
connected to the top (red) terminal on the  
left channel connects to the red terminal of  
the left speaker, and that the wire connected  
to the top (red) terminal on the right channel  
connects to the red terminal of the right  
speaker. If the wires to one speaker are  
reversed, the speakers will be of opposite  
polarity (or “out of phase”), and a severe loss  
of low frequencies may, result.  
MODE SWITCH: This three position slide  
switch determines the SP-20AB’s mode of  
operation The three choices are:  
MONO: This position is also referred to as  
“Dual Mono” because while both channels  
are operating separately (each driving its  
own speaker), there is only one input, which  
appears at both outputs. The input must be  
connected to the “Left/Mono Input” jack.  
STEREO: This position is for normal stereo  
operation, where there are two inputs (left  
and right) and two outputs.  
If Mono Bridged operation is desired, the  
single speaker must be connected to the  
BRIDGED: This position should be used  
for straight mono operation, when there is  
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SP-20AB MANUAL  
two top (red) terminals. In this case, nothing other applications. The HR-2 units are compact  
should be connected to the bottom (black) boxes which easily attach with a thumbscrew  
terminals. Positive polarity will be on the left to any microphone stand Each HR-2 has two  
channel (red) terminal.  
stereo volume controls and two headphone  
SATELLITE/REMOTE OUTPUT: This output jacks. There are also two XLR connectors, one  
is a single male XLR connector containing male and one female. These may be used to  
both the left and right channel outputs. It is “daisy chain” HR-2 boxes together with standard  
wired as follows: Pin 3 -left (high); Pin 2 - microphone cables.  
right (high); and Pin 1 - low, common to both  
Begin the chain at the SATELLITE/REMOTE  
channels. This output should be used as the connector on the rear of the SP-20AB, as shown  
starting point for a chain of HR-2 Remote in the above diagram. The SP-20AB connects to  
Stations (satellites). It necessary, it may also the first HR 2 box; another mic cable connects  
be used to connect a second set of speakers. the first HR-2 box to the second HR-2 box-, a  
This output is also turned on and off by the third mic cable connects the second HR-2 to  
SPEAKERS button. This output should not the third HR-2; and so on. If low impedance  
be used with HR-2 Stations when in Mono headphones are used, up to twelve HR-2’s may  
Bridged mode.  
be in the chain. If medium or high impedance  
headphones are used, 25 or more Remote  
Stations may be chained.  
If desired, the clip on an HR-2 box may be  
hung on a belt it no microphone stand is  
convenient.  
INPUTS: The SP-20AB is equipped with  
left and right balanced female XLR  
connectors. Balanced lines offer the  
benefits of cancellation of hum, noise, and  
RF interference which may be picked up in  
the interconnecting cables. For balanced  
connections, shielded twisted pair cable  
must be used. The wiring is as follows: Pin  
1-shield; Pin 2-signal positive; and Pin 3-  
signal negative.  
Use of the HR-2 Headphone  
Remote Stations  
The HR-2 Remote Stations may be used  
to increase the number of headphones which  
can be accommodated by an SP-20AB, and to  
place a volume control within easy reach of each  
individual needing it. This allows the SP-20AB  
to be used to power a headphone system for  
a recording studio, rehearsal studio, language  
lab, demonstration of audio equipment, or for  
Daisy-chaining HR-2 Remote  
Stations to the SP-20AB  
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SP-20AB MANUAL  
electronics troubleshooting experience and  
training undertake repairs on their own. There are  
no user serviceable parts inside the SP-20AB. All  
problems should be referred to the Factory or to  
other qualified service personnel.  
Installation  
For installation as a free-standing unit,  
attach the four adhesive rubber feet supplied  
with the SP-20AB to the bottom of its case.  
With either the HRKIT-1 or the HRKIT-2  
adaptor kits the SP-20AB may be mounted in  
standard 19” equipment racks. The HRKIT-1 is  
for mounting one SP-20AB in the center of a  
19” rack. The HRKIT-2 is for mounting two units  
side by side in a 19” rack (or for mounting a  
single unit in a “half rack”). Complete installation  
instructions are supplied with the kits.  
The input connectors are XLR balanced. The  
inputs to the SP-20AB proceed from the input  
connectors to op-amp buffers wired as differential  
(balanced) amplifiers. If a mono signal is inserted  
this will convert the differential op-amp to a non-  
inverting amp. After the buffers, the left and right  
signals proceed to the two decks of the stereo  
Input Level control.  
When installing any equipment in racks, keep  
in mind that standard racks come equipped  
with mounting rails with holes tapped for 10-32  
machine screws. Be sure to use only 10-32  
screws for this purpose (in particular, avoid  
10-24 screws, which will fit if forced but will strip  
the threads). To avoid marring the panel when  
tightening the screws, use nylon washers under  
the screw heads.  
The SP-20AB’s power amplification is derived  
from a pair of LM1875 integrated circuits, which  
have internal short-circuit and thermal overload  
protection. They are set for a voltage gain of 26 dB  
and drive the speaker outputs through a standard  
RLC network. The network establishes the high  
frequency rolloff, as well as preventing voltages  
generated by the speakers from reaching back  
into the IC and causing damage. The headphone  
output passes through a special high-power pot  
wired as a volume control.  
For best results, the input cables plugged  
into the SP-20AB should be shielded. Balanced  
connections should use shielded, twisted-pair  
cable. Headphone cables and speaker cables  
do not need to be shielded, though no harm is  
done it shielded cables (such as microphone  
cables) are used.  
The SP-20AB’s power supply provides ±30  
volts unregulated DC (with 120VAC line input) to  
the LM1875 stages, which may drop to ±23 volts  
under full load. A network of dropping resistors  
and zener diodes provides ±15 volts to the rest of  
the circuit.  
Circuit Description  
Both the Overload LED’s and the Signal  
Present LED’s monitor signal levels at the output  
of the LM1875, but before the headphone volume  
control and the Speaker On/Off Switch.  
CAUTION! This section is intended to assist the  
professional user with considerable experience  
in electronics to achieve a better understanding  
of the operation of the SP-20AB. Under  
no circumstances should persons without  
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SP-20AB MANUAL  
Limited Warranty  
not warrant against damages or defects arising  
out of improper or abnormal use of handling of  
the Product; against defects or damages arising  
from improper installation, against defects in  
products or components not manufactured by  
Manufacturer, or against damages resulting  
from such non-Manufacturer made products or  
components. This warranty shall be cancelable  
by Manufacturer at its sole discretion if the  
product is modified in any way without written  
authorization from Furman Sound. This  
warranty also does not apply to Products upon  
which repairs have been affected or attempted  
by persons other than pursuant to written  
authorization by Manufacturer.  
THIS WARRANTY IS EXCLUSIVE. The sole  
and exclusive obligation of Manufacturer shall  
be to repair or replace the defective Product in  
the manner and for the period provided above.  
Manufacturer shall not have any other obligation  
with respect to the Products or any part  
thereof, whether based on contract, tort, strict  
liability or otherwise. Under no circumstances,  
whether based on this Limited Warranty or  
otherwise, shall Manufacturer be liable for  
incidental, special, or consequential damages.  
Manufacturer’s employees or representatives’  
ORAL OR OTHER WRITTEN STATEMENTS  
DO NOT CONSTITUTE WARRANTIES, shall  
not be relied upon by Purchaser, and are not  
a part of the contract for sale or this limited  
warranty. This Limited Warranty states the  
entire obligation of Manufacturer with respect to  
the Product. If any part of this Limited Warranty  
is determined to be void or illegal, the remainder  
shall remain in full force and effect.  
Furman Sound, Inc., having its principal  
place of business at 1997 South McDowell Blvd.,  
Petaluma, CA 94954 (“Manufacturer”) warrants  
its SP-20AB (the “Product”) as follows:  
Manufacturer warrants to the original  
Purchaser of the Product that the Product sold  
hereunder will be free from defects in material  
and workmanship for a period of three years  
from the date of purchase. The Purchaser  
of the product is allowed fifteen days from  
the date of purchase to complete warranty  
registration by mail or on-line at the Furman  
website. If the Product does not conform to this  
Limited Warranty during the warranty period (as  
herein above specified), Purchaser shall notify  
Manufacturer in writing of the claimed defects.  
If the defects are of such type and nature as to  
be covered by this warranty, Manufacturer shall  
authorize Purchaser to return the Product to  
the Furman factory or to an authorized Furman  
repair location. Warranty claims should be  
accompanied by a copy of the original purchase  
invoice showing the purchase date; this is not  
necessary if the Warranty Registration was  
completed either via the mailed in warranty card  
or on-line website registration. Shipping charges  
to the Furman factory or to an authorized repair  
location must be prepaid by the Purchaser of  
the product. Manufacturer shall, at its own  
expense, furnish a replacement Product or, at  
Manufacturer’s option, repair the defective  
Product. Return shipping charges back to  
Purchaser will be paid by Manufacturer.  
THE FOREGOING IS IN LIEU OF ALL  
OTHER WARRANTIES, EXPRESS OR  
IMPLIED, INCLUDING BUT NOT LIMITED  
TO THE IMPLIED WARRANTIES OF  
MERCHANTABILITY AND FITNESS FOR A  
PARTICULAR PURPOSE. Manufacturer does  
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SP-20AB MANUAL  
Service  
Before returning any equipment for repair,  
please be sure that it is adequately packed and  
cushioned against damage in shipment, and  
that it is insured. We suggest that you save  
the original packaging and use it to ship the  
product for servicing. Also, please enclose a  
note giving your name, address, phone number  
and a description of the problem.  
NOTE: All equipment being returned for  
repair must have a Return Authorizatilon (RA)  
Number. To get an RA Number, please call the  
Furman Service Department: (707) 763-1010.  
Please display your RA Number prominently  
on the front of all packages.  
8
 
SP-20AB MANUAL  
SPECIFICATIONS  
OUTPUTS:  
Power:  
20 Watts per channel, stereo, into either 4 Ohms or  
8 Ohms, or 40 Watts mono into 8 Ohms, 20 Hz to  
20 KHz  
Connectors:  
Speakers: 5-way binding posts (accommodates  
dual banana plugs, spade lugs, bare wires, etc.)  
Remote Headphone: XLR.  
INPUTS:  
Input Impedance:  
Sensitivity:  
Connectors:  
20K Ohms.  
-6.5 dBU required for full output.  
XLR, balanced.  
GENERAL:  
Distortion:  
.01 % THD at full rated power at 1 KHz; .05% THD  
20 Hz to 20 KHz.  
Dynamic Range:  
Greater than 99 dB, A weighted  
Frequency Response: +0, -1 dB from 20 Hz to 20 KHz, 1 Watt output  
Dynamic Headroom: 2 dB, stereo, measured with 10 ms toneburst at 1  
KHz, 1 % duty cycle.  
Power Requirement: SP-20AB: 120 VAC, 60 Hz, 70 watts  
SP-20ABE: 230 VAC, 50/60 Hz, 70 watts  
Mechanical:  
Dimensions: 1.75” H x 8.45”W x 8.25” D.  
Weight: 9 lbs (4.2 kg).  
NOTE:  
0 dBu equals .775 Vrms.  
 
SP-20AB MANUAL  
 
Furman Sound, Inc.  
1997 South McDowell Blvd.  
Petaluma, California 94954-6919 USA  
Phone: 707-763-1010  
Fax: 707-763-1310  
 

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