VC-302
Impedance Matching Volume Control
14
The VC-302 is an impedance matching volume control with an IR receiver
knockout. The knockout allows Channel Vision’s IR-2400 to be
integrated with the volume control for applications that require IR
repeating. The VC-302 comes with white, ivory, and almond knobs and
face plates to easily match other wall plates in the room.
Features:
!
Protects your amplifier against low impedance loads
!
Knockout for mounting Channel Vision’s IR-2400
!
Supports up to 8 pairs of speakers
!
Fits in a single-gang low-voltage ring or open-back j-box
!
Supports 14 - 20 AWG speaker wire
Bottom View
Input... Speaker wire connections
from your amplifier
R+ R- L- L+
R+ R- L- L+
INPUT
OUTPUT
Output... Speaker wire
connections to your speakers
Side View
2.63”
2.73”
White (included)
2
IR knockout...
Not visible from the front
Note: This product must be installed in
either an open ring or an extra-deep box.
8X8X4X4X1/2X
Impedance Matching Jumpers
1/2X
Accessories
Ivory & Almond conversion
kits sold separately
Ivory
VC-3IV
Almond
VC-3AL
Changing the Knob & Face Plate Color
Remove the existing knob and face plate.
1. Pull the knob off.
R+ R- L- L+
R+ R- L- L+
INPUT
OUTPUT
Gently press tabs from
back side using a flat blade
screwdriver.
2. Remove the face plate by pressing
on the 4 tabs that protrude through
the metal plate.
To install the new knob and face plate, simply reverse the process
shown above.
How Impedance Matching Works
Amplifiers have minimum impedance specifications for the speakers that
will be connected to them. This is usually 8 or 4 ohms. Most speakers
are either 8 or 4 ohms. Thus, most amplifiers are designed to drive one
pair of speakers. When additional speakers are connected in parallel
(wired to the same amplifier terminals) the resulting impedance is
reduced. This is why impedance matching volume controls are used.
The VC-302 provides three impedance magnifier settings: 1/2x, 4x, and
8x. These settings are used to magnify or compensate for low impedance
loads that result from wiring multiple speakers in parallel. Although the
calculations needed to determine the correct magnifier setting are simple,
you can use the chart below as guide to quickly find the correct setting for
your application. (Note: when no impedance matching is required, use
the 1/2x setting).
See chart for more information.
With no impedance matching
Number of 8 Ohm
speakers in parallel
2
3
4
5
6
7
8
Impedance
(Ohms)
4
2.66
2
1.6
1.33
1.14
1
With impedance matching
volume control VC-302
Magnifier
Setting
1/2X
4X
4X
8X
8X
8X
8X
Resulting
Impedance (Ohms)
8
10.64
8
12.8
10.64
9.12
8
Example:
If you have 4
pairs of 8-ohm
speakers
connected in
parallel, then
move the jumpers
to the 4X position
as on the
following page.
3
Typical Installation Diagram
VC-302
TM
CHA NN EL V ISI ON
Central
VC-302 side view
8X8X4X4X1/2X
1/2X
8 Ohms
C-1001
8 Ohms
VC-302
16/4 speaker wire
Amplifier
8 Ohms
VC-302
Move both jumpers
to the 4X pins on
each VC-302.
8 Ohms
4
Understanding Impedance Calculations
Let’s examine a hypothetical system:
Joe Blow has one amplifier and he decides to hook 3 pairs of 8-ohm speakers in
parallel to it. (see Figure 1)
The amplifier is rated for a minimum load of 8 ohms.
Result: Joe Blow, blows up his amplifier (or at least puts it into thermal overload).
Figure 1
Left channel
connections
have been omitted
for simplicity.
Amplifier
Incorrect!
Minimum Impedance: 8 Ohms
8 Ohms 8 Ohms 8 Ohms
There are two approaches that Joe could have taken:
1. Buy one impedance matching volume control and hook 3 speakers up to it.
(see Figure 2)
2. Buy 3 impedance matching volume controls and connect one in line before
each pair of speakers. (see Figure 3)
Suppose Joe Blow chooses option one and buys one VC-302 volume control and
connects it in line between his amp and 3 pairs of 8-ohm speakers.
Figure 2
Jumper set to X4
Correct!
VC-302
Amplifier
Minimum Impedance: 8 Ohms
Joe needs to do the math: He remembers that he can calculate the total parallel
impedance (Z ) by finding the reciprocal of the sum of reciprocals.
Thus: Z =
Total
Total
1
1
1
Z
1
1
Z
Z
3
2
= 8 Ohms
Z
1
1
18181
Z
= 8 Ohms
2
1
8
3
8
1
0.375
Z
= 8 Ohms
3
2.66 Ohms
2.66 Ohms is much lower than the 8-Ohm minimum that he was supposed to
connect to his amp Joe sets the jumpers on his VC-302 to the X4 position.
This magnifies the impedance times 4.
Thus: 2.66 ohms x 4 = 10.64 ohms. This is more than the 8-ohm minimum so
the amplifier should operate safely.
5
Suppose Joe Blow buys three VC-302 volume controls and connects each
one in line before each pair of speakers. In this case the impedance of each
pair of speakers is magnified so the impedance presented to the amplifier is
more than the speaker’s nominal value. Suppose that Joe sets the jumper to
the X4 position. This will magnify the 8 ohm speakers to look like a 32 ohm
load for the amplifier.
Thus: Z =
Total
1
1
1
Z
1
1
Z
Z
3
2
1
1321321
32
1
3
32
1
0.9375
10.66 Ohms
10.66 Ohms is more than the 8-ohm minimum so the amplifier should operate
safely.
Z
= 32 Ohms
1
Z
= 32 Ohms
2
Z
= 32 Ohms
3
Figure 3
Correct!
Jumper set
to X4
Amplifier
Minimum Impedance: 8 Ohms
VC-302
8 Ohms
Jumper set
to X4
8 Ohms
VC-302
VC-302
Jumper set
to X4
8 Ohms
What if you don’t like Math?
If you don’t want to be a mathematician, then you can always set the
magnifier to the X8 setting, but the speakers won’t be as loud.
To avoid doing the math shown on the previous pages, simply follow the
chart on page 3.
6
Integrating an IR Receiver
IR-2400 IR receiver
Remove the IR knockout
Use a drill for best results
The IR-2400 can be easily mounted inside the
VC-302 to provide inconspicuous IR repeating.
When using this option remember that the IR
receiver will require its own wiring to connect
back to an IR hub. IR signals will NOT run on
the speaker wire.
IR receiver head
Snap IR receiver
head into place as shown
Attach IR-2400 with
double sided tape
R+ R- L- L+
INPUT
R+ R- L- L+
OUTPUT
IR knockout hole
Secure excess wire with wire tie
For details regarding IR system connections please refer to the
IR-2400 manual.
7
10
Channel Vision Technology will repair or replace any defect in
material or workmanship which occurs during normal use of this
product with new or rebuilt parts, free of charge in the USA, for ten
years from the date of original purchase. This is a no hassle warranty
with no mail in warranty card needed. This warranty does not cover
damages in shipment, failures caused by other products not supplied
by Channel Vision Technology, or failures due to accident, misuse,
abuse, acts of God, or alteration of the equipment. This warranty is
extended only to the original purchaser, when purchased from an
authorized distributor.
A purchase receipt, invoice, or other proof of original purchase date
from an authorized distributor will be required before warranty repairs
are provided.
Mail in service can be obtained during the warranty period by calling
(800) 840-0288 toll free. A Return Authorization number must be
obtained in advance and can be marked on the outside of the shipping
carton.
This warranty gives you specific legal rights and you may have other
rights (which vary from state to state). If a problem with this product
develops during or after the warranty period, please contact Channel
Vision Technology, your dealer or any factory-authorized service
center.
Channel Vision products are not intended for use in medical,
lifesaving, life sustaining or critical environment applications.
Channel Vision customers using or selling Channel Vision products
for use in such applications do so at their own risk and agree to fully
indemnify Channel Vision for any damages resulting from such
improper use or sale.
w ww .c ha nn el vi si on .c om
234 Fischer Avenue, Costa Mesa, California 92626 USA
(714)424-6500 (800)840-0288 (714)424-6510 fax
email: techsupport@channelvision.com
500-151 Rev D
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