master volume question

[iknowjohnny]

i have tried a number of pots as master but i always find that regardless of the fact they have all been audio taper 1M, different pots...brands etc, have tapers that are never exactly the same. I just recapped my amp with sozos and also added a new CTS master since the old one was going bad. The last one was a bourns and it worked a lot better. As you know a pre PI master at least is very touchy at low volumes. with the bourns i could turn it slightly to get those bedroom volumes even tho it was very very sensitive. Theres always that point where it suddenly gets louder, but it was doable. This new CTS is 1M audio taper just like the bourns, but this thing is MORE than sensitive. It hits that point and just jumps like 5 times as loud, and the problem is the area in between can't be tuned in because it's just too small a range. you could blow on it and hear a volume change.

So the question is this....what value resistor and how would i strap it across the pot to smooth that out so it transitions gradually? I know i will be limiting the amp's volume, but it's a 2xel34 marshall circuit and a loud gig would be about 10-11 oclock. So i don't want to limit the amp's volume a lot, but i can afford a fair bit of volume loss. I tried several values and ways myself, but none worked so i'd rather ask you gurus so i don't have to resolder my new pot 50 times and have to get yet another. Or IS there even a way? Thanks.

[Firestorm]

Fun article on this by rgk here:http://www.geofex.com/Article_Folders/p ... tscret.htm

[iknowjohnny]

LOL! I just read that earlier. I've seen it before so i re-read it. But i didn't get anything from it that was obvious as a remedy to my exact problem.

[Firestorm]

Probably the easiest fix is to go to a 1M linear pot. It'll be a lot less jumpy at the bottom. The taper will be truer, too.

You could use a reverse audio taper pot, but they're pretty much only available in 50K or 3M.

I haven't tried them, but the Allesandro pots (looks like they're made for him by PEC) are supposed to be less jumpy. Expensive, tho.

[iknowjohnny]

I never thought to do that but it makes sense, at least assuming it's the range of the pot covering more ground in a small area and not the circuit itself responsible for it. Why does most all of what you read suggest audio for the master?

[Firestorm]

They really should be audio taper, because of the way we perceive loudness. The problem with log taper pots is they apparently make them by building up parallel linear tracks of different values to approximate a log taper. Sometimes these are jumpy. A linear taper pot will seem to get loud too fast, but you should have better control over it and the range will be awfully small.

Now that I actually think about it, the reverse taper pots would be worse, except that a 3M gives you more "room." You could wire it backwards and have a normal log taper 3M pot. Of course the pot will work backwards

[Structo]

I was looking inside a newer CTS pot the other day and was surprised to find instead of a single wiper contact there were four or five little fingers that rode on the multiple tracks of carbon.

[iknowjohnny]

I think they do that because with a single wiper if it gets contaminated you're out of luck. Multiple wipers insure there will always be at least one working with rare exception or unless the pot gets badly contaminated.

[VacuumVoodoo]

The sudden volume increase is caused by the non-zero resistance between pot wiper and beginning of resistive track, be it carbon or conductive plastic.
Carbon tracks are simply screen printed on the potentiometer base, so called "audio taper" is achieved by by printing two halves of the track with carbon paste having different resistivity.
The initial resistance is sometimes specified (by the better manufacturers) in the data sheets. Unfortunately we don't see this very often. There is one or two manufacturers in Eastern Europe who will specify on the pots manufactured on former Soviet mil spec.

Anyway, to solve the problem at hand, that is to give you a Master Volume for bedroom use, here is my solution ...

EDIT AUGUST 15. 2012 schematic & description removed due to it being used commercially without permission and as a selling argument by a third party in breach of "Fair Use Only". Some mild action taken.

[iknowjohnny]

i already have a relay activated volume boost, but i just wanted to have a master that works well enough to do vey low volume like the bourns did. so if i was going to go to that much trouble i'd just order another bourns. Just don't want to wait till my next order because otherwise the shipping will make it a $12 pot. So i'll try the suggestion for a linear and if that doesn't work i'll live with it till i order again and get a bourns. I image linear should work even better tho.

[VacuumVoodoo]

Then skip the toggle switch, take a dual pot, hardwire it in cascade and be done with it. It makes a MV usable over its full rotation. Or just sit and wait for that Bourns pot made by CTS based on their joystick pot.
A single linear pot will only make things worse.

[jjman]

My experience and understanding is that a linear is worse for your situation. And reverse log/audio the absolute worst. An audio taper is really a string of linear tapers.

[img:526:389]http://www.capacitor.com.hk/upload/0127.gif[/img]

Antilog=reverse taper
Log=mathematical only
Commercial log=actual audio/log pot results

We do not hear in a "linear" manner so the log is the goal for audio volume.

[VacuumVoodoo]

Thanks for posting this chart. Now I challenge you to do the maths and see how well two linear pots cascaded as per my schematic approximate true log curve. You'll be pretty surprised.

[Firestorm]

I was guessing that a linear pot might have a smoother taper than the hodge-podgey commercial log pots. But you'd have to be able to turn it in excruciatingly small increments even to tell.

You used to be able to find 2.2M log pots. Those might help.

Aleksander's idea of stacking the pots got me thinking about concentric pots instead of straight dual-gang (sort of a master for the master), but pretty expensive to implement.

How about a 1ML with a switch? Put a 1MR in front of the pot and wire the switch so it shorts it out. You'd have a stock MV and a half volume MV.

[Firestorm]

Thanks for posting this chart. Now I challenge you to do the maths and see how well two linear pots cascaded as per my schematic approximate true log curve. You'll be pretty surprised.

I was looking at that when you posted it. Absolutely true. Neat idea.