Volume pot - suitable?

[guddler]

I was asked about suitable pots for Atari volume controls by PM today but after researching it a little, I want to know myself now

To take my Asteroids Cabaret as an example, but my Tempest is just the same, IIRC the manual doesn't give a value but I think it's a 50R wirewound pot (manuals not to hand right this moment)

Would a 1W wirewound pot be sufficient or does it need to be beefier?

RS Part Number 173-164 looks to me like it may fit the bill perfectly but I wanted to be sure.

The other thing is that's a Linear pot as opposed to a logarithmic pot but TBH, I've not got a bloody clue what that means anyway

 

[virtvic]

I'd like to know this, as my Asty caberet sounds like a pre-pubescent teenager, with it's voice all over the place!

 

[trm]

Linear vs Logarithmic.

A linear pot means that when you set the pot to half-way deflection, the resistance is half of the maximum. Move it to 3/4 of maximum and the resistance is 3/4 of the max resistance. If you plot the pot position against the resistance you'd see the points made a straight line at 45 degrees on the graph. You would (but for the caveat later) use these e.g. on motor controllers, so if you want to move a robot arm the deflection of the pot is directly proportional (linearly) to the movement of the arm. This is the green line on the graph below.

A logarithmic pot is entirely different. If you plotted a graph as I mentioned above, you will see something like the blue line. Log pots are normally used as volume controls as hearing works logarithmically - moving from volume pot from 10 to 11 on a log pot doesn't make the sound a 10th louder which it would on a linear pot.

And just to make this nicely inconsistent, most arcade boards use lin pots for volume controls! This is probably because you can approximate a log pot with a lin pot and external resistor, and proper log pots are more expensive.

This graph shows a plot for lin (the f(x) = x line in green) and a standard log pot (the f(x)=log(x) plot in blue). The linear plot has a linear directly proportional relationship between the input value x and the output value, also x. The logarithmic plot has a non-linear relationship plot.

Sorry can't help on the other point till later tonight when I can look at the schems; I'm meant to be on day 2 of moving house with my bro and sis-in-law but I seemed to break my toe half an hour ago so I'm just strapping it up

 

[trm]

Is the volume pot before or after the power amp? If before then I'd say 1 Watt was overkill and plenty, but if it's after the power-amp then it'll need to be 5 or so Watts at least.

 

[guddler]

Linear vs Logarithmic.

[Snippity Snip] Cheers dude, that's perfectly clear

Sorry can't help on the other point till later tonight when I can look at the schems; I'm meant to be on day 2 of moving house with my bro and sis-in-law but I seemed to break my toe half an hour ago so I'm just strapping it up

Ouch! That sounds like a perfectly reasonable reason to bail and head for the pub to me

I'll dig the schems out in a minute but I would think the answer is that it's pre amplification but honestly can't remember at the moment.

 

[guddler]

I'll dig the schems out in a minute but I would think the answer is that it's pre amplification but honestly can't remember at the moment.

Actually, it looks like it's between the amp and the speaker

 

[guddler]

Ok, found it in the Asteroids Deluxe manual.

Original is a 50R, 12.5W Wirewound Rheostat

Closest RS have would cost £40 + VAT

Farnell have what looks like the exact item - £30.87 + VAT (and US Stock)

Will go on a hunt...

 

[guddler]

A bit better: http://uk.digikey.com/1/1/540164-rheostat-50-ohm-12-5w-res50re.html

But not much!

Does appear to be the exact original part though.