hi
i would highly recommend a DIY stepped attenuator (for example using a 20-position 2 or 4 pole switch). i have attached below simple qbasic code to calculate the resistance values for the three canonical types of attenuators:
TYPE 1: POTENTIOMETER REPLACEMENT
(worst of the 3 types, because there are the most number of solder joints and resistors in the signal path)
A=0
B=0
N=0
PRINT "Number of switch positions"
INPUT S
PRINT "Step Size (dB)"
INPUT D
PRINT "Output (Load) resistance"
INPUT L
PRINT "Input resistance"
INPUT R
DO UNTIL N=S-1
Y=((R-L/10^(-A/20)) + SQR((L/10^(-A/20)-R)^2 + 4*R*L))/2
C=R-Y-B
PRINT A;"dB ";C;"ohms"
B=B+C
A=A+D
N=N+1
LOOP
PRINT A;"dB "; R-B;"ohms"
TYPE 2: SERIES-SHUNT type, a good approximation to type 3, since it only has 2 resistors in the signal path. However, the input resistance is not constant, so the source must be current-capable.
A=0
B=0
N=0
PRINT "Number of switch positions"
INPUT S
PRINT "Step Size (dB)"
INPUT D
PRINT "Output (Load) resistance"
INPUT L
PRINT "Input (SERIES) resistance"
INPUT R
DO UNTIL N=S-1
A=A+D
C=(R*10^(-A/20))/(1-10^(-A/20))
Y=1/(1/C-1/L)
PRINT A;"dB SHUNT=";Y;"ohms"
N=N+1
LOOP
PRINT "SERIES RESISTANCE IS:"; R; "ohms"
TYPE 3: TRUE LADDER. BEST of the 3, each position involves two separate resistors. requires a 6 pole switch for stereo, and a lot of well matched resistors and soldering!
A=0
B=0
N=0
PRINT "Number of switch positions"
INPUT S
PRINT "Step Size (dB)"
INPUT D
PRINT "Output (Load) resistance"
INPUT L
PRINT "Input resistance"
INPUT R
DO UNTIL N=S
Y=((R-L/10^(-A/20)) + SQR((L/10^(-A/20)-R)^2 + 4*R*L))/2
X=R-Y
PRINT A;"dB ";Y; "ohms(to GND) ";X;"ohms(to Signal)"
A=A+D
N=N+1
LOOP
i recently built a stereo attenuator of type 2 using an excellent JBT 19 position 2 pole switch from leeds electronics (
www.leedselect.com, surplus switch for $5), with very satisfactory results. I really would not recommend type 1, since you still end up needing a 2 pole switch, and the number of resistors/solder joints in the signal path adds up fast!
hope this helps!
-p
(ps: note on the "Load resistance" above: it is important to use the actual input resistance of your following stage here: for SS gear, this would be the input impedance of the amplifier, for tube gear, i use the grid leak resistor attached to the input(driver) tube. Many potentiometer-type attenuators rely on the shunt-to-ground resistance of the potentiometer itself, which doesn't make sense to me!)
Shunted volume: pot & resistor values
Law faking passive
In my case (10k linear pot): just run a 1.5k resistor between out and ground? Would the effective value of the pot change?
