Calculating a stepped attenuator resistors values...

As I plan to build a Gainclone, I'd like to put a stepped attenuator instead of a pot on it.

I already know a few things about how to cable it : it progressively shorts the output to ground after an input resistor which limits current and keeps input impedance high enough.

I will use two rotary switches (2poles * 6positions) in series, meaning there will always be 2 resistors in series, and allowing 36 different positions.

As I'm not so good at maths, I wondered how to calculate the resistors values, in order to get a linear acoustic attenuation.

I suppose it depends on the input impedance, and that it follows a log law, but I don't know how to use it.
:D

How easy it is to give a smoother attenuation to a part of the attenuation range ?


If anyone could help me, I'd be glad ! :smash:
 

moamps

diyAudio Member
2002-10-11 8:15 am
Croatia
www.moxtone.com
Hi,
The picture below shows one possibility.
On the audioXpress site http://www.audioxpress.com/magsdirx/ax/addenda/index.htm you can find a simple Excel spreadsheet (att.xls) for calculating attenuator values. There is also an article that goes with the spreadsheet, which was published in this month's issue of aX.

Regards,
Milan
 

Attachments

  • att.jpg
    att.jpg
    72.2 KB · Views: 587
Nice realization ! Your preamp seems really well-built and shielded !

This is the stepped attenuator solution I wanted to use, variable steps of resistors that shorts input to ground.

I made some calculations with formulas found on the Net but, as I cannot obtain a 24 or 36 positions switch, I won't be able to use a stepped attenuator in my project.

The resistance range goes from 20K to 6R so it's impossible to cover the whole range using 2 switches with resistors in series as I thought it was possible before calculating the values.

It means I need at least 3 switches which is really not practical !

I'll put an ALPS inside, I suppose it's a good pot too for my first amp project :)

Thanks for your replies !