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Aidan135711 22nd February 2008 09:58 PM

Log or Lin?
Finally got my LM4766T amp going.Now I need a pot.
Ready made stepped attenuators are too expensive and I can't seem to find enough info on how to build my own so I choose to use some sort of high accuracy stereo pot.
Do I use a 100k log or 100k lin pot for input?
Or can someone show me the way to some stepped att. diagrams?

richie00boy 22nd February 2008 10:12 PM

It depends on the input impedance of the chip amp. If you want to use a 100k pot which is quite high impedance, I would try a linear pot.

Aidan135711 22nd February 2008 10:21 PM

I just checked the datasheet and it shows 10k pot on input.My mistake.
So,if I use 10k pot ,should it be log or lin?

jaudio 22nd February 2008 11:09 PM

Use a log.

Profsparks 22nd February 2008 11:17 PM

Just found this on another post, enjoy!


richie00boy 23rd February 2008 08:55 AM

Yes if 10k use logarithmic.

AndrewT 23rd February 2008 11:00 AM

you can use a lin with log faking resistor added.

The slight advantage of the lin track is better matching of resistance. This can affect balance between channels and becomes a bigger problem at high attenuation rates.

When mimicing a log pot with a lin pot+faking resistor it is usual to select a lin~=10times log value and then add a wiper to signal ground resistor ~10%to 15% of lin pot value.
eg 10klog or 100klin+12k resistor.

danielwritesbac 23rd February 2008 05:53 PM

Hi guys!

I've noticed that some sources, usually (not always) headphone sources, may have a cap at the output that causes the bass to vanish if a 10k pot is used.

Unless tried, you wouldn't believe that Alpha's A20K ($1) plus ordinary 24 gauge solid conductor copper, could be a viable solution.

Whether that's true or not, it can give you a baseline from which to evaluate other options.

Aidan135711 23rd February 2008 06:44 PM

I just read profsparks link about building a ladder stepped att.
I might have a go at making one of those,however in his resistor calculator table,should position 1 read -80db not -8db,or am I mis-understanding something?

AndrewT 24th February 2008 09:49 AM

-80db or -infinitydb are the choices. The 8 is an attempt to write the symbol for infinity which is similar to an 8 on it's side with the right hand end open.
If Rg=0 then attenuation is theoretically infinity.
If Rg = 1r0 then attenuation is -80db.

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