T-pad in shunt mode

TonyM · « **on:** 12 Mar 2006, 12:28 am »

Greetings

I'm trying to work out how to turn my Alps blue 50k pot into a T-pad shunt mode attenuator. I'm not very good with schematics, so I need help with which resistors go to which terminal. I know how to work out the values of the resistors but am unsure of what goes where. I realise this stuff is basic to you guys but my understanding of electronics is so poor that I dare not attempt anything without asking those who know first.
help would be greatfully appreciated.

Greg Erskine · « **Reply #1 on:** 12 Mar 2006, 08:08 am »

Hi TonyM,

I can't offer you the answer, but I have found this site very useful. T-pads are mentioned towards the end.

http://www.siteswithstyle.com/voltsecond/12_posistion_shunt/12_Position_Pure_Shunt.html

regards

TonyM · « **Reply #2 on:** 12 Mar 2006, 09:58 am »

Thanks greg

I have seen this site and while useful in some aspects, still very technical. I'll have another look though.

cheers

AKSA · « **Reply #3 on:** 12 Mar 2006, 10:46 am »

Hi Tony,

A T-pad is an attenuator used to control input and output impedance AND attenuation. It comprises three resistors, all of which must change to alter attenuation. Here's the website here:

http://www.icycolors.com/nu9n/tpad-calculator.html

This enables you to put in the attenuation required, the input and the output impedance. You can see all three resistors must change, which means that for stereo control there must be six variable resistors. This is a tall order, as the relationship between them is not conducive to a single pot.

It can be done, but it's mighty complex......

In audio, a simple potentiometer will usually suffice.

Cheers,

Hugh

TonyM · « **Reply #4 on:** 12 Mar 2006, 10:49 am »

Thanks Hugh,

I've always had this problem with excessive loudness with my 50k Alps pot, is there an easier way to tame it?

cheers
Tony

AKSA · « **Reply #5 on:** 12 Mar 2006, 09:50 pm »

Tony,

Ah, that's the problem? Perhaps you have too much gain!

Try this:

Signal source to 33K resistor, then to top of pot.

Wiper of pot now goes to ground.

Signal is taken from the join of the 33K resistor and the top of the pot.

Wire both gangs of the pot up for both channels; one gang per channel.

You may have to establish which is the 'top' of the pot empirically, so that turning up the wick in the usual way increases rather than decreases the volume.

This is a shunt volume control. At max setting the attenuation will be down to (50/33+50) or around 60% of input signal, and input impedance will vary from around 75K down to around 25K (which is no bad thing if you preamp/source has an output impedance below about 3K) but it will reduce the loudness handily and pay dividends in clarity.

Cheers,

Hugh

TonyM · « **Reply #6 on:** 13 Mar 2006, 07:26 pm »

Hugh,

Just to clarify:

Remove the signal wire from the wiper lug, solder on a 33K resistor and solder this to the ground terminal. Do same for both channels.
Done! or am i missing something?

BTW i also have a 20k blue alps, if wanted to use this in shunt mode what would be the optimum value resistor to use?

cheers

Tony