Jan 11, 2017

Pots and attenuators

In every audio equipment there is a gain or volume control to set music reproduction to an enjoyable level . This control is usually devoted to a simple potentiometer mostly for cost reasons. On pricey materials you can find more elaborated and expensive devices like L pad attenuators but if there are just 2 or 3 resistors in signal path to minimize sonic alteration the reflected impedance constantly vary like in basic potentiometers.

In professional gears, like mixing consoles, pots are very seldom used and signal attenuation is provided by more elaborated units. T pad and bridged T pad are the most widely used.
If they essentially remain voltage dividers these particulars setups are intended to keep a constant impedance whatever the attenuation. Very important when used in 600 ohm lines they can be of real benefits to prevent bandwidth alteration in amateur constructions.

Such attenuators are by far the best way to control signal. Unfortunately good NOS units by Daven, Tech Lab or Langevin are scarce and expensive but it is possible to the average DIY'er to build its own one with the great advantage of choosing the impedance to meet particular loads (I have on hand some excellent transformers with uncommon 900 ohm loading impedance).

They are quite easy to build, you just need a good soldering iron, some quality pliers, a bunch of resistors and plenty of time. I personally use Dale RN60D, Dale RLR7C or Philips/Vishay MRS25 (excellent for the task, easy to find and affordable). I also prefer the bridged T attenuator over the more classic T pad because it is fully symmetric.

pic taken from allaboutcircuits.com

Bridged T calculator greatly eases resistors calculation.

A good rotary switch is essential. Must be of the shorting type (make before brake) and have at least 20 contacts. Some old Siemens are excellent but hard to source.
Fortunately very high quality equivalents have been made in the former East Germany by RFT and are easy to source. They have 24 contacts, silver or palladium on copper. With such a switch you can build a 0 to -40 dB & off / 2 dB by step attenuator.

Step by step construction

First edit a file with impedance load, desired attenuation and resistors values.
For a 600 ohm one with a 2dB step the following values are:

RFT rotary switches are the best available choice for the price. Cold war time material, they where intended to work under adverse conditions. Very well constructed they are easy to disassemble and reorganize to suit our purpose.

1- Fully disassemble switch (except rotary mechanism) to access the phenolic wafers.
The silvered ones need some gentle cleaning with a piece of fabric (Never ever use chemicals !)

2- From a 3 wafers unit, set 2 wafers back to back for the R1 series resistors using the small spacers.
These will be populated later. I choose to make the common loop first because it is the trickiest part to do.

3- Make a common (Ground) loop from a silvered copper wire and insert a short fiberglass (or any other insulation material) sleeve to prevent any unwanted contact

4- Common loop hold in place and soldered on contact 1, this is the off position

5- Shunt resistors R2 bent and cut to proper length prior to solder. It helps a lot to solder first the resistor on the opposite contact to the common loop. It makes this one stiffer.

6- Common loop fully populated

7- Now it is very easy to feed the R1 series resistors wafers

All R1 resistors soldered...

8- Reassemble the shunt resistors wafer, add the two Z0 impedance matching resistors and you are done. Takes 8/10 hours for a complete unit, it's worth the effort for a good attenuator.

Any question, feel free to write