Stepped Attenuator
Well folks, below is the formula and the wiring diagram to make and use
your very own stepped attenuator.
Below is a formula for the resistors for constructing your stepped
attenuator. A most humble thanks go out to Marc for supplying them to us. THANKS MARC!!!
In case anyone is wondering, the formula for attenuation is:
FORMULA: | CIRCUIT:
R1 |
Vout = Vin x --------- | Vin ----(R2)--------- Vout
R2 + R1 | |
| (R1)
| |
| GROUND
where:
Vin = voltage input
Vout = voltage output
R1 = shunt resistor value
R2 = series resistor value
The total resistance that the attenuator presents is:
R1 + R2 This does not include any impedance after
the attenuator. To calculate the total impedance
including the load on the output, calculate the parallel
resistance of the load and R1, and add this to the
value of R2.
The formula for converting a VOLTAGE ratio to decibels is:
20*log(n)
where (n) is the voltage ratio.
For instance, if R1 and R2 are both 5k ohms, the voltage
ratio will be 0.5, making this a -6dB 10k Ohm attenuator.
I'm pretty sure this is right, please correct me if I've made
a dumb mistake somewhere...
Regards,
Marc Yun
myun@sas.upenn.edu
Below is the wiring diagram to install you brannie newie stepped
attenuator. Many thanks go out to Stewart.
Source 1 sig ----------\
\ SW1 Pot
Source 2 sig ----------- <------------1
/ 1
Source 3 sig ----------/ 1
1<------------->To power amp sig
1
/1\------------->To power amp gnd
/ 1 \
Source 1 gnd ------------------------ 1 1
1 1
Source 2 gnd --------------------------1 1
1
Source 3 gnd -----------------------------1
In case that's not entirely clear (more than likely!), the signal
from each input connector centre socket is fed to the input
side of the source selector switch SW1. The common of SW1
goes to the input pin of the pot and the wiper of the pot is
connected to the output socket or hardwired to the power
amp interconnect. The ground returns of the sources and
of the power amp are connected to a single 'star point'
which is the ground pin of the pot. THIS IS IMPORTANT!
To maintain the star ground, it is necessary that the input
sockets have insulated grounds or that a plastic box is used
(easier and cheaper). Internal wiring should be solid core
twisted pair, I make mine from 26AWG wirewrap wire which
is silver plated oxygen-free copper with non-polar insulation
and cost a few cents per foot from any electronics store.
Obviously the above is one channel only :-) Wiring from
pot to power amp is recommended to be the same
solid-core twisted pair for best quality, but this won't take a
lot of flexing and a good quality interconnect cable may be
used for more durability if you'll be moving the cables a lot.
For the true believer, forget the input sockets and hardwire
directly from the switch terminals to RCA plugs at the
source components - that's what I do in my TV sound system.
It avoids one connector pair in the signal path and of course
it costs less too!
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