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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!
     Stewart Pinkerton
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     
 

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