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In Reply to: Re: REALLY stupid attenuator questions. posted by Greg on October 19, 1999 at 08:44:45:

last night I plugged my CDP directly into the amp to get an idea how max volume would be. It was plenty loud. I just listened to the beginning of some familiar pieces which I knew started out fairly quitely. Very nice sound. I rigged up a breadbx and resisters I purchased from RS this morning and tried it out - not bad but certainly not the best connections. Regarding impedance matching, I can force the input impedance to match the output of the CDP at a particular volume setting but then the output varies. Is there a way to optimize this to a certain volume level? I have in place now a 10K in series and 4.7K shunt which is a quite low volume setting. I will be playing with the shunt K to find the right combination.
Scott W

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Re: Just tried it Greg 14:28:30 10/20/99 (2)
> > . Regarding impedance matching, I can force the input impedance to match the output of the CDP at a particular volume setting but then the output varies. Is there a way to optimize this to a certain volume level? < <
Scott, The amp imput impeadance and the output impedance of the CPU will be optimized at all volume levels provided you use the right series resistor. The 10K resistor you chose may be too small for your tube amp. If you are good with math, here is a fomula for precise calculations of resitors for ladder attenuators posted by a fellow tweaker. Good luck, Greg
Ladder Attenuator
For -3 dB attenuation,
R1/(R1+R2) = 10**(-3/20) = R1/(R1+R2) = .708 = R1 = .708(R1+R2)
For R1+R2 = 50K,
R1 = .708(50K) = 35.4K, and R2 = 50K - 35.4K = 14.6K
To double check: 20log(35.4K/50K) = 20log(.708) = -3 dB.
The example works for any attenuator value. From the example for -1 dB attenuation,
we know that: R1 = .89125(R1+R2)
Set R1+R2 to the attenuator value you want.
For R1+R2 = 50K, just plug in the numbers.
R1 = .89125(50K) = 44.6K. R2 must equal (R1+R2) - R1 which gives 5.4K.
To double check: 20log[R1/(R1+R2)] = 20log(44.6K/50K) = -1 dB
- I can handle the math but what do you set the values at? Scott W 17:51:50 10/20/99 (1)
  I developed the formula on an excell spreadsheet and can do the calculations but now how do I know what to set the Series resistor at. Is the goal to make the max output impedance match the max input impedance of the amp? If so, I can use a 10K series resistor.
  Scott W
  - Re: I can handle the math but what do you set the values at? Greg 08:42:00 10/21/99 (0)
    > > . Is the goal to make the max output impedance match the max input impedance of the amp? If so, I can use a 10K series resistor. < <
    The goal is to lose as little of your signal as possible, and have great dynamics and clean clear sound. This is a balancing act. We must try to keep several factors in balance. All at the same time. This is impossible. That is why you must have compromises. We must be skillful or we risk haveing an attenuator with poor dynamics, with lots of signal loss. I was in your shoes 3 months ago. I asked several very smart people what value my attenuator should be. I received several different answers!!! All I can tell you is what they told me: "I do not think the 15K values I received are a good match for my tube amp( 470K). Right? So what impedance value would you recommend?( My source is 100 ohms, my cables are short 1 meter. I realize that there is no perfect value for a pure passive attenuator. So, what would you recommend? 50k, 75K, 100K or should I just keep my 15K values. They do sound good".
    Answer:
    "Stick with the 15K attenuators. Best Regards, R. W."
    So there you have my final solution, which would be considered a bit of a strain on my source, which is the Linn Linto phono preamp(100ohms). I ask Linn if the Linto would be happy driving this load, and they said it would be more than happy with this. So, now we are all happy here and the sound is great! Don't you just love happy endings? Regards, Greg
Re: Just tried it mark 15:52:27 10/19/99 (0)
Hello All,
i was amazed as well when i built a passive source selector with remote control source selection and volume control.
I'm using relays for the source selection, and a motorized pot for the volume control.
It sounds absolutly amazing.

Mark

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