Resistors valus for shunt stepped attenuator?

I want do do a shunt stepped attenuator 10K with 23 steps, the question: is this table showing the right values?

Step 1, Attenuation = Infinity dB, Rx = 10000 ohms, Ry = 0 ohms.
Step 2, Attenuation = 75 dB, Rx = 10000 ohms, Ry = 2 ohms.
Step 3, Attenuation = 72 dB, Rx = 10000 ohms, Ry = 3 ohms.
Step 4, Attenuation = 69 dB, Rx = 10000 ohms, Ry = 4 ohms.
Step 5, Attenuation = 66 dB, Rx = 10000 ohms, Ry = 5 ohms.
Step 6, Attenuation = 63 dB, Rx = 10000 ohms, Ry = 7 ohms.
Step 7, Attenuation = 60 dB, Rx = 10000 ohms, Ry = 10 ohms.
Step 8, Attenuation = 58 dB, Rx = 10000 ohms, Ry = 13 ohms.
Step 9, Attenuation = 56 dB, Rx = 10000 ohms, Ry = 16 ohms.
Step 10, Attenuation = 54 dB, Rx = 10000 ohms, Ry = 20 ohms.
Step 11, Attenuation = 52 dB, Rx = 10000 ohms, Ry = 25 ohms.
Step 12, Attenuation = 50 dB, Rx = 10000 ohms, Ry = 32 ohms.
Step 13, Attenuation = 48 dB, Rx = 10000 ohms, Ry = 40 ohms.
Step 14, Attenuation = 46 dB, Rx = 10000 ohms, Ry = 50 ohms.
Step 15, Attenuation = 44 dB, Rx = 10000 ohms, Ry = 63 ohms.
Step 16, Attenuation = 42 dB, Rx = 10000 ohms, Ry = 80 ohms.
Step 17, Attenuation = 40 dB, Rx = 10000 ohms, Ry = 101 ohms.
Step 18, Attenuation = 38 dB, Rx = 10000 ohms, Ry = 127 ohms.
Step 19, Attenuation = 36 dB, Rx = 10000 ohms, Ry = 161 ohms.
Step 20, Attenuation = 34 dB, Rx = 10000 ohms, Ry = 204 ohms.
Step 21, Attenuation = 32 dB, Rx = 10000 ohms, Ry = 258 ohms.
Step 22, Attenuation = 30 dB, Rx = 10000 ohms, Ry = 327 ohms.
Step 23, Attenuation = 28 dB, Rx = 10000 ohms, Ry = 415 ohms.

Thanks for help.
 

wakibaki

Banned
2008-01-08 11:51 pm
Here's the attenuation with R1 as series element followed by R2 to ground, signal taken at junction of R1, R2.

shunt.jpg

W
 
Now the Shallco 23 stepped attenuator is with these resistors & sounds very good:

Step 1, Rx = 10000 ohms, Ry = 0 ohms.
Step 2, Rx = 10000 ohms, Ry = 2 ohms.
Step 3, Rx = 10000 ohms, Ry = 3.6 ohms.
Step 4, Rx = 10000 ohms, Ry = 6.8 ohms.
Step 5, Rx = 10000 ohms, Ry = 13 ohms.
Step 6, Rx = 10000 ohms, Ry = 22ohms.
Step 7, Rx = 10000 ohms, Ry = 39 ohms.
Step 8, Rx = 10000 ohms, Ry = 68 ohms.
Step 9, Rx = 10000 ohms, Ry = 120 ohms.
Step 10, Rx = 10000 ohms, Ry = 180 ohms.
Step 11, Rx = 10000 ohms, Ry = 270 ohms.
Step 12, Rx = 10000 ohms, Ry = 360 ohms.
Step 13, Rx = 10000 ohms, Ry = 510 ohms.
Step 14, Rx = 10000 ohms, Ry = 820 ohms.
Step 15, Rx = 10000 ohms, Ry = 1K1 ohms.
Step 16, Rx = 10000 ohms, Ry = 1K6 ohms.
Step 17, Rx = 10000 ohms, Ry = 2K4 ohms.
Step 18, Rx = 10000 ohms, Ry = 3K ohms.
Step 19, Rx = 10000 ohms, Ry = 4K7 ohms.
Step 20, Rx = 10000 ohms, Ry = 6K8 ohms.
Step 21, Rx = 10000 ohms, Ry = 9K ohms.
Step 22, Rx = 10000 ohms, Ry = 15K ohms.
Step 23, Rx = 10000 ohms, Ry = without resistor / not connected

There is a mistake or something wrong?
 
I'm hearing Shallco between positions setp 13 to step 18 so attenuation is:

Step 13, Rx = 10000 ohms, Ry = 510 ohms. 26.28065 dB
Step 14, Rx = 10000 ohms, Ry = 820 ohms. 22.40827 dB
Step 15, Rx = 10000 ohms, Ry = 1K1 ohms. 20.07861 dB
Step 16, Rx = 10000 ohms, Ry = 1K6 ohms. 17.20676 dB
Step 17, Rx = 10000 ohms, Ry = 2K4 ohms. 14.26421 dB
Step 18, Rx = 10000 ohms, Ry = 3K ohms. 12.73644 dB

If I use my first table minimum attenuation step 22 = 30dBs!!!!! because step 23 not connected = attenuation infinity.

Possible solution to lower the line amp gain, or do a voltage divider? but I'm very confort playing vinyl & there is not a lot of difference with DAC gain...
 
Last edited:
Hi,
the dB of attenuation will change with different loads on the output.
A 10k pot on a 100k amp will be different from a 20k pot on a 50k amp.

Do any of the calculators take account of load value?

eg.
a 10k pot set to -20dB, feeding a receiver with Rin equal to:
100M sends -20.00001dB to the receiver.
100k sends -20.08dB
22k sends -20.35dB
10k sends -20.75dB
 

wakibaki

Banned
2008-01-08 11:51 pm
Ah, yes, AndrewT, I am aware of that, and no, the calculations reflect only the values of the resistances in the attenuator. I felt, however, that entering into a discussion about this was one complication too many, given that the difference between a load of 100M & 10k is ~0.75dB at -20dB. Obviously the impact varies throughout the range of attenuation but stepped attenuators are commonly employed without consideration of such niceties as far as I can see and I thought on balance it was better to give the OP some input, albeit a possible oversimplification.

If anyone feels inclined to provide a more comprehensive explanation. feel free to step in...

@merlin

Here is a spreadsheet with the formula built in, you can change the values as you please. As AndrewT points out, these values ignore the effects of load impedance, and will be inaccurate to a greater or lesser degree.

View attachment attn.zip