High Quality Potentiometer vs. Mills resistor

inspiredsports

As I understand it, on some of their crossover designs, Polk uses a single resistor to "voice" the high-pass section. In other words, the resistor determines how much signal reaches the tweeters, determining their volume relative to the mid woofers.

As an example, on an SDA 1B, the first component behind the Red binding post (after the safeguard/polyswitch) is a 2.0 ohm resistor.

Removing this 2.0 ohm resistor would decrease resistance and allow the tweeters to sound louder in relation to the midwoofers. Replacing it with a 2.7 ohm would increase resistance and lower the volume of the tweeters, causing the speaker to sound more laid back.

I know Polk spent a lot of time (meaning money) voicing each speaker, but what if you wanted to determine an exact resistor value for your specific speaker in your specific listening environment.

Is there an ultra-high-grade potentiometer out there, with perhaps a range of from 0 to 5 ohms, that would perform as quietly as the 12 watt resistors we all seem to use when rebuilding our crossovers ??

mmadden28

I was just thinking along those lines--I'm in the process of upgrading my 2B's and SRS 2s, and have been following yours and other's progress and noted the variation in the resistor used sometimes-even from stock. Such as in your SRS 2 TL mod where you forgot to account for the .5 ohm resistance of the removed polyswitch, but then felt it was better for you with the lower resistance anyway. And another thread where one value might be better for SS vs Tube.
I think a HQ Pot or at least a switched resistor bank would be an interesting tweak so to speak, of course you'd be adding some additional connection points, but for the flexibility....Hmmm.
Afterall, you see that kind of thing on alot of older speakers (70's-ish)--then again there might be a reason its not as common anymore. Again hmmm.

mmadden28

I'd be reluctant to use a Pot that goes all the way to zero unless you had another resistor in series to set the min. Perhaps a HQ Alps Stepped Pot, using HQ Vishay-Dale resistors--Don't they make blanks to add you own resistors too?

inspiredsports

mmadden28 wrote: »

I'd be reluctant to use a Pot that goes all the way to zero unless you had another resistor in series to set the min. Perhaps a HQ Alps Stepped Pot, using HQ Vishay-Dale resistors--Don't they make blanks to add you own resistors too?

That's the thing, I have no idea how many db's of volume are associated with 1 ohm of resistance. I also don't know if it is linear, so it would be easy to use a pot to dial it in. Then my thought process is why take it out was of the same (or a higher) quality than the 12 watt Mills resistor.

I have an old set of 1B's that were water damaged that I picked up for $20 because they have all of the same parts as my '86 SDA SRS 2's with the exception of a 2.0 ohm resistor (versus 3.5 in the SRS 2's) and a 12" passive versus 15" in the 2's. If zero ohms causes an overload, it would cook the SL2000's in use, but they are pretty easy to find so I'd almost like to try it. I don't actually think zero ohms would hurt anything.

zingo

Focal uses adjustable jumpers on their crossovers to set the different values so you can adjust tweeter attenuation, etc. On some of their earlier stuff, the tweeter resistor was located on the outside of the crossover panel, and you could snap it out and one of a different value in. Both these methods avoid pots or longer circuits than necessary, and the pop-up tweeter design would be fairly easy to recreate DIY.

inspiredsports

I think the tolerance of Mills resistors is very tight, 1% or less, and many Pots seem to be in the 20% range so that might be the single best reason to avoid.

mmadden28

What's the max current through the resistor? I realize the resistor wattage is 12 watts, but the original was what, 5w or 10w?

I was just looking around for a rotary selector switch, (like a 1 pole 3 or 4 way like this) to use to switch between a couple different value Mills resistors (that way you control the compnents and tolerances uses)--But so far in my limited searching the max current handling of these 'audio quality' switches are 2 amps. So not sure if thats good enough or not.

The idea of an external jumper like in that Focal thing is not that bad an option

inspiredsports

mmadden28 wrote: »

What's the max current through the resistor? I realize the resistor wattage is 12 watts, but the original was what, 5w or 10w?

I was just looking around for a rotary selector switch, (like a 1 pole 3 or 4 way like this) to use to switch between a couple different value Mills resistors (that way you control the compnents and tolerances uses)--But so far in my limited searching the max current handling of these 'audio quality' switches are 2 amps. So not sure if thats good enough or not.

The idea of an external jumper like in that Focal thing is not that bad an option

I've already been doing napkin sketchs of what this might look like

Face

Just buy a few cheapo ceramic or MOX resistors, find the value you like best, go out and buy the appropriate Mills or Duelund resistor.

inspiredsports

Face wrote: »

Just buy a few cheapo ceramic or MOX resistors, find the value you like best, go out and buy the appropriate Mills or Duelund resistor.

The cost isn't what I'm concerned about as much as crafting a very quick, convenient way to change the value and be back in my listening chair in about 1 second.

Face

Not 1 second, but quick: http://i03.i.aliimg.com/photo/217759519_6/Multi_Colour_Crocodile_Alligator_Clips_Test_Leads_v0.jpg

Radio Shack usually stocks a few sizes.

inspiredsports

Face wrote: »

Not 1 second, but quick: http://i03.i.aliimg.com/photo/217759519_6/Multi_Colour_Crocodile_Alligator_Clips_Test_Leads_v0.jpg

Radio Shack usually stocks a few sizes.

I don't smoke that stuff anymore

Face

inspiredsports wrote: »

I don't smoke that stuff anymore

How about any less?

mmadden28

Hey guys, real quick slightly off topic question-(I'm placing my Sonicraft order)
With respect to the Mills resistors in the 2B TL, and SRS 2 upgrades--Should I go 5w or 12w?
Is the inductive difference between the two relevant at all?
From the Sonicraft site:

The MRA-5 (5 watt power rating) exhibits even lower inductance than the MRA-12 due to its very compact size

Face

For most of us(except Sal:D), a 5w should work fine.

inspiredsports

Face wrote: »

For most of us(except Sal:D), a 5w should work fine.

I know when I updated mine (exact same 2 models as mmadden28) the consensus seemed to be to go for the 12w, but I don't know enough about it to know why.