Log or Lin?

[Aidan135711]

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]

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]

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]

Use a log.

 

[Profsparks]

Just found this on another post, enjoy!

http://www.penguinlovers.net/audio/Attenuator.html


Cheers
Sparks

 

[richie00boy]

Yes if 10k use logarithmic.

 

[AndrewT]

Hi,
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]

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]

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?

http://www.penguinlovers.net/audio/Attenuator.html

 

[AndrewT]

-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.

 

[Aidan135711]

-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.

Yes,I understand now.
Thank you all for your responses.

 

[andySweden]

Clarification

AndrewT: Could you clarify your "fake log pot"? Between what should you put the resistor? Was it just between the pot and the signal ground? Probably it's just my English, that makes me not understand this...

I'm thinking of buying a simple Remote with preinstalled 100k (probably lin - not specified) pot.

Another thing: If you do this fake log pot, will that also resolve the problem with added noise with a high value pot (I guess it will, but ask the qst anyway...)?

Thank you!
Sincerely
Andy

 

[Nordic]

http://www.vt4c.com/shop/program/main.php?cat_id=60&group_id=2

cheap source for the switches 2x24 step switches starting at $14 for single items, discount with larger orders...

 

[andySweden]

What is the shipping for the above shop (roughly)? I tried to find some info about it, but couldn't...

Sincerely
Andy

 

[AndrewT]

Re: Clarification

Could you clarify your "fake log pot"? Between what should you put the resistor? ...................
Another thing: If you do this fake log pot, will that also resolve the problem with added noise with a high value pot .....

the faking resistor goes between the wiper and the audio/input ground.
Zin of the amplifier can also provide the faking resistor value if there is no DC blocking cap between pot and Zin. But 100k varies Rs too much for many power amps.

The amp sees a worst case source resistance of [100k+Rs]/4 // 12k ~=8k1. this is still much higher than the 2k5 seen when a 10klog is used. But lower than 25k without the faking resistor.

Have you tried the search facility on the Forum, or even Google?

 

[andySweden]

Thank you!

That was most informative! (I believe that the English word "wiper" refers to the arrow pointing to the pot!?)

Now, the Ebay seller answered my question about the pot. It is a 100k LOG pot. Is that a problem?

I will also search the forum for more info!

Sincerely
Andy

 

[Nordic]

first item on the menu.. How to buy, has shipping info...

I think to make a fake law pot, you need to start with a linear one...

 

[andySweden]

Thank you for helping me, Nordic!

It is a 100k LOG pot. Is that a problem?

What I meant was: I will not be faking a log pot if I already have one... But is having a log pot with such a high value a problem when making a 3886 GC?

Sincerely
Andy

 

[AndrewT]

It depends on the topology you adopt.

Variable Rs without a DC blocking cap results in variable output offset.

The higher the range of variable Rs the worse the change in output offset becomes.

100klog > Rs=0r0 to 25k
100klin//12k > Rs=0r0 to 8k1
10klog > Rs=0r0 to 2k5

 

[andySweden]

I will be using a DC blocking cap...