HELP with DIY ladder attenuator !!!!!

[mcgreg66]

Hi,

I want to built a 24step 100k stereo ladder attenuator with this switch -
4 pole 24-step Attenuator / Potentiometer *GOLD Plated* bei eBay.de: Video Cables Connectors (endet 09.07.11 03:15:16 MESZ)
for a KT88 SE tube amp

For calculating the resistor values I used this website
Stepped Attenuator Resistor String Calculator - Neville Roberts

Question 1.......
I wonder about the strange resistor value steps between step 6 and 8..are they correct...?!
Step 6 and 8 are nearly the same and I need to use the same resistors as I surely don't find the exact values!!!!

Step 1, Attenuation = Infinity dB, Rx = 100000 ohms, Ry = 0 ohms, Resistor = 0 ohms.
Step 2, Attenuation = 66 dB, Rx = 99950 ohms, Ry = 50 ohms, Resistor = 50 ohms.
Step 3, Attenuation = 58 dB, Rx = 99874 ohms, Ry = 126 ohms, Resistor = 76 ohms.
Step 4, Attenuation = 51 dB, Rx = 99718 ohms, Ry = 282 ohms, Resistor = 156 ohms.
Step 5, Attenuation = 45 dB, Rx = 99438 ohms, Ry = 562 ohms, Resistor = 280 ohms.
Step 6, Attenuation = 41 dB, Rx = 99109 ohms, Ry = 891 ohms, Resistor = 329 ohms.
Step 7, Attenuation = 38 dB, Rx = 98741 ohms, Ry = 1259 ohms, Resistor = 368 ohms.
Step 8, Attenuation = 36 dB, Rx = 98415 ohms, Ry = 1585 ohms, Resistor = 326 ohms.
Step 9, Attenuation = 34 dB, Rx = 98005 ohms, Ry = 1995 ohms, Resistor = 410 ohms.
Step 10, Attenuation = 32 dB, Rx = 97488 ohms, Ry = 2512 ohms, Resistor = 517 ohms.
Step 11, Attenuation = 30 dB, Rx = 96838 ohms, Ry = 3162 ohms, Resistor = 650 ohms.
Step 12, Attenuation = 28 dB, Rx = 96019 ohms, Ry = 3981 ohms, Resistor = 819 ohms.
Step 13, Attenuation = 26 dB, Rx = 94988 ohms, Ry = 5012 ohms, Resistor = 1031 ohms.
Step 14, Attenuation = 24 dB, Rx = 93690 ohms, Ry = 6310 ohms, Resistor = 1298 ohms.
Step 15, Attenuation = 22 dB, Rx = 92057 ohms, Ry = 7943 ohms, Resistor = 1633 ohms.
Step 16, Attenuation = 20 dB, Rx = 90000 ohms, Ry = 10000 ohms, Resistor = 2057 ohms.
Step 17, Attenuation = 18 dB, Rx = 87411 ohms, Ry = 12589 ohms, Resistor = 2589 ohms.
Step 18, Attenuation = 16 dB, Rx = 84151 ohms, Ry = 15849 ohms, Resistor = 3260 ohms.
Step 19, Attenuation = 14 dB, Rx = 80047 ohms, Ry = 19953 ohms, Resistor = 4104 ohms.
Step 20, Attenuation = 12 dB, Rx = 74881 ohms, Ry = 25119 ohms, Resistor = 5166 ohms.
Step 21, Attenuation = 9 dB, Rx = 64519 ohms, Ry = 35481 ohms, Resistor = 10362 ohms.
Step 22, Attenuation = 6 dB, Rx = 49881 ohms, Ry = 50119 ohms, Resistor = 14638 ohms.
Step 23, Attenuation = 3 dB, Rx = 29205 ohms, Ry = 70795 ohms, Resistor = 20676 ohms.
Step 24, Attenuation = 0 dB, Rx = 0 ohms, Ry = 100000 ohms, Resistor = 29205 ohms.

Question 2
Some DIY tell that they order many more pieces of one resistor value to measure and select them for exact channel balance...
Is this really necessary if I'm using high-quality resistors like Kiwame or Shinkoh?!

And...
Question 3
How to calculate the max attenuation needed?!
Sometimes it starts with -65dB sometimes with -75dB?!?

Depends on power of amp?!
Triode mode: 5+ watt output per channel
Ultralinear mode: 10+ watt output per channel
And efficiency of the speakers (89dB)?!

Thanks for HELP!!

Gregor

 

[sregor]

Q1 - the steps are not all the same - starts out at 8 db, then gets smaller until step 8, where it is 2 db.

Q3 - the simplest way is trail and error. Other factors would be listening volumes and habits, back ground noise, personal choice. You are looking at the volume at the lowest step - my own experience is that I never listen to music on the lowest position, so it is kind of not a factor. YMMV.