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Old 24th April 2002, 11:24 PM   #1
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Question Shunted volume: pot & resistor values

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Hi. I'm picking this up from Harry Haller's 'passive preamp' thread, from which I finally understood how a shunt volume works. I'm doing a passive preamp and I have a 10k linear pot. Will this work for a shunt setup? If so, what resistor value should I use? Is the 'Fake Law' arrangement mandatory for a linear pot? If this pot is really unsuitable, what are 'ideal' vlaues for the pot and resisitor? I'm talking pretty general nominal values, because I'm not at a super hi level of fi right now, and I have no idea of the impedance etc. properties of things like my my interconnects. What I do know: My source (CD) line out is 2v, (I don't know it's output impedance or even what this really means other than it may come into play...) My amps (2 monos) are 500mv input sensitive and 100k input impedance.

TIA
Peace and Music
Sanaka
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Old 24th April 2002, 11:32 PM   #2
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Default Law faking passive

http://sound.westhost.com/project01.htm

H.H.
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Old 25th April 2002, 12:55 AM   #3
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Default Calculating the values for stepped attenuators

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!)
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Old 25th April 2002, 01:11 AM   #4
fcel is offline fcel  United States
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Psarin: Do you know where I can buy a 4 gang 10K or 5K potentiometer KIT ..... like the ones that DACT makes ....... but sells for a lot less? Thanks.
I can built and I can solder.
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Old 25th April 2002, 01:25 AM   #5
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Default Thanks that helps on the Law Faking part....

I did see that before so thanks for making me read it more closely. So for law fake a linear pot is good In my case (10k linear pot): just run a 1.5k resistor between out and ground? Would the effective value of the pot change?

My main concern is the shunt, however. Do I go with shunt resistor ~10% of pot (like your 11k on the 100k pot)?

Also I still don't know if 10k is the 'right' pot. Seems like a lot of mention of more like 50 -100k going on.

Thanks!
peace and music
Sanaka
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Old 25th April 2002, 01:35 AM   #6
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Default thanks psarin

I thought about stepped but my currently manifesting system I think won't quite benefit from the extra quality or work. A pot/shunt is barely harder than just a pot, and that's good right now

What does one need to know to determine an appropriate pot value for a given system? I'm betting this is a can-of-worms question, but a very dumbed down newbie answer would be fine for me at the moment

Thanks!
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Old 25th April 2002, 03:16 AM   #7
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Default (to fcel)

try shallco (www.shallco.com) the makers of the rotary switches for most of the 'high-end' audio stepped attunuators. the tech department folks there were very helpful in discussing options for DIY'ers. however, be warned that most modern switches are built with SMD resistors in mind.
which is why i went the route of surplus 'oldie' switches. they show up often on Ebay, and most surplus places (like www.leedselect.com, www.apexjr.com) have them. i just used the above code to calculate the resistor values and then scrounged them from my parts bins.
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Old 25th April 2002, 03:51 AM   #8
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Try http://www.percyaudio.com/
He has good selection of switches. Shalco is a bit expensive but I had a god experience with Elma switches. Michael also sells remote control kit for Alps pots.
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Old 25th April 2002, 03:57 AM   #9
fcel is offline fcel  United States
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HPotter: Michael says he does not have any 4 gang pot. I did not check if he has those steps attunator kit the last time I check.
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Old 25th April 2002, 04:17 AM   #10
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That's true, 4-gang switches are not in his catalog. I know Sonic Frontiers used 4-gang switches from Electroswitch. They were wired as series attenuators. I got 3 of them, 50K ohm ea. If anyone is interested I might sell them. The Elma is the one in front, it's 2 deck. I used it recently in my Dac and it's not too bad. All of them are 24 positions.
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