[BobM]

I'm looking to build a passive attenuator to turn down the level on my McCormack amp feeing into the mid-bass drivers in a biamping setup. The amp signal is a wee bit too loud compared to the tube amp driving the tweeter.

I've seen a few designs like the Axiom, and figure I can easily build this myself:
http://www.luminousaudio.com/axiomrca.html

I'm looking for a schematic to duplicate this type of design. Here's a description of the insides, from what I can gather:

- single Caddock resistor from the + on the input to the + on the output jacks (but what value?)

- somehow a Noble pot is also connected to the +kacks and regulates how much of the resistor is in the circuit (how is this connected?)

- the - ground is just bridged between the input and output jacks

So - what value Caddock resistor, and how to conenct it to the Noble pot so only the resistor is in the circuit and not the pot itself?

Thanks,
Bob

[JoshK]

I think what they are doing is a shunt type pot, or law-faked linear pot.  This is where the a single resistor is put in series with the + side of the signal and the pot is used to vary the resistance to ground.  Aspen Audio has written a bunch about it and there is a lot written about it on diyaudio.com (try 'shunt linear pot').  I am not sure what value you'd need. 

[mgalusha]

In the image below, imagine a pot instead of the switch and resistors. This is a shunt attentuator.



For the value of the caddock (or whatever series resistor you choose] it will depend on the input impedance of your amp and the output impedance of your source.

Oops, Josh beat me to it. He's right, it sounds like this is what they're doing but it's possible they are doing something else, difficult to say for sure. Shunt attenuators work quite well as a rule but I've only used them in line stages, not between components but it should work fine if you get the correct values.

[BobM]

Thanks guys,

I think I've got it. Basically it's just a voltage divider type of circuit with the pot varying what gets "shunted" off to ground. Easy enough.

The Axiom, I believe is a 50K impediance, so I guess that's the value of the resistor they use.

My McCormack has a 100K input impediance. Isn't the rule of thumb 1-10? If I'm mostly looking for it to be used at the most open end of its range (just reducing the input voltage between .1 - 1V at most I would guess) what would you suggest?

Thanks,
Bob

[BobM]

Still confused about the value of the primary, in-line resistor needed, and the Noble pot also. Can anyone shed some light?

The input impediance of the amp is 100K, so I'm guessing a resistor value of between 1K-10K would probably work fine?

The Noble pots, whcih would act as a variable shunt to ground is 50K, I believe.

Thanks for your assistance,
Bob

[JoshK]

No you want 100K resistor.  Basically this is the impedance that your preamp will "see" which should be >10x the output impedance of your preamp.  The pot should roughly be equal to the input impedance of the amp. 

[Davey]

Bob,

This sounds like it's going to be a permanent setting eventually?  If it were me I'd just connect your Noble pot (which you already have?) conventionally (forget about the shunt hookup) and experiment with the setting until you find a final result.  You could then remove the pot and replace it with two resistors for a fixed pad of the attenuation you need.

Cheers,

Davey.

[BobM]

OK, let me see if I've got this straight, before I go and order some expensive Caddock resistors:

use a 100K resistor for the + leg (matching the input impediance of the amp)
use a 100K Noble pot for the variable shunt resistor to ground

Then get a metal case & some decent RCA's and I'm all set.

Sounds about right? Please let me know if I'm mistaken.

Thanks all.
Bob