Why a 50 watt midrange resistor on the RS 2.5?

I am not understanding the logic behind the use of a 50 watt resistor in the midrange circuit of the RS 2.5. In most power supplies of the tube amps I've rebuilt only 5 or 10 watt resistors are used. What is the purpose of such a huge high wattage resistor in the mid circuit?

Larry D.

quantums are filled with 25 watt resistors....

The best guess that I could come up with is that the larger winding in the larger watt resistors would pass more information to the EMIMs on the 2.5s. I may sub in a 10 watter to see if I hear any difference. I'd also test by touch for a temperature difference.

Larry D.

also would add inductance, right...?

I'm not sure what would be added or subtracted. That's why I'm asking the question.

I would like to parallel a pair of Duelund 10W 1.5 ohm resistors in there, but I'm searching whether this would be a worthwhile endeavor since it would be an $80 cost to do it.

Larry D.

since I'm anal and not a big fan of sandcast resistors I used Mills 1% 12 watt in my Q2's...I just divided what ever resistance I needed and put them in series to get 24 watts...supposed to be 25 watt but I think i'll be all right...

they are high quality and its another option

Another thought....

I never use the midrange pot. I always play everything flat. What is the use of it being in the circuit if it is basically unused?

To properly bypass the mid pot would you have to put in a 0.2 ohm resistor or just wire around it? I guess I could measure the pot while it is set flat to see the resistance.

If I wanted to go for the simplest circuit I might remove the 50 watt resistor and bypass the mid pot and parallel a pair of 1.8 ohm resistors, if that is what I end up measuring.

I'm targeting the midrange because that is where the meat of the music is. I have some poly caps coming to try out in the woofer section of the crossover. I'll have to see if they measure up to their specs.

I look at these speakers as a big experimental toy that I can learn from. I can find out what does and does not work. Yeah, I grew up a long, long time ago, but we still need our toys.

Larry D.

is it the typical 5 ohm pot...?

if it is you can bypass it with a 2.5 ohm resistor...which will give you the middle (flat)

idadude said:

The best guess that I could come up with is that the larger winding in the larger watt resistors would pass more information to the EMIMs on the 2.5s. I may sub in a 10 watter to see if I hear any difference. I'd also test by touch for a temperature difference.

Larry D.

Click to expand...

I'd guess they use 50W resistors because a fair bit of power needs to be dissipated. Why does it need to be dissipated I could only guess. I opine it's not because the larger winding is there simply to pass more information.

You should replace it (or just leave it alone) with resistance having similar dissipation. Some pretty smart people put that specific part specification there for a reason, despite the reason not being immediately apparent.

Yeah, I know the 'smart' guys put it in there for a reason, but I just have to know 'why'. I'm not one that just accepts things from the most popular or educated people if I cannot clearly see the logic behind it. That's why I'm here asking questions. For all I know the resistor salesman made them an offer they could not refuse on some big honkin 50 waters for the price of 1/4 watt resistors.

If there was a lot of power being dissipated on the 0.7 ohm resistor, then there should be some heat generated, but it never feels to be anything except room temperature even when playing the speakers loudly for a long period of time. That was why I wanted to experiment with a smaller, say 10 watt resistor to check for any rise in temperature when music was being played loud. I do not want to make any changes to the speakers that would be detrimental to them in any way, but I am into careful experimentation.

Larry D.

Genuinely curious why you feel inclined to change it? Just because?

It was used perhaps because the inductance of the 50watt resistor was considerably less than a lower wattage resistor of the same value. Maybe the internal coil of resistance wire was further away from the the previous spiral winding.

Maybe. I have to believe there is a sound reason to use it, no pun intended, not simply because they got a good deal on 50W resistors.

Would be interesting to see if a schematic (assuming there is such a thing for the speaker) calls out an inductance spec/value for the 50W resistor.

whoaru99 said:

Maybe. I have to believe there is a sound reason to use it, no pun intended, not simply because they got a good deal on 50W resistors.

Would be interesting to see if a schematic (assuming there is such a thing for the speaker) calls out an inductance spec/value for the 50W resistor.

Click to expand...

Have at it
Perhaps something to do with the nature of the magnetic planar Emim?

http://www.infinity-classics.de/technik/manuals/RS_2.5_technical_sheet.pdf

whoaru99 said:

Genuinely curious why you feel inclined to change it? Just because?

Click to expand...

As I said previously, the meat of the music comes from the midrange, so I'm trying to get the most out of the midrange drivers. Here's some info about the Duelund resistors that got me interested in considering them for this application:

"Designed by DUELUND Coherent Audio of Denmark for Loudspeaker Cross-Overs these 5 to 10 watt non-inductive (they are purely resistive, since they are a solid element and have no electrical characteristics of a coil) resistors utilizing graphite as the resistive element, and pure silver wire for the lead out material. A phenolic cylinder is used to protect the resistive element itself.

The special characteristic about this resistor is that it displays a negative temperature coefficient. For example, when the voice coil of a speaker heats up, it's impedance rises. The DUELUND Graphite Resistor counteracts this by means of a decreasing impedance. The results are greatly improved dynamics from the driver."

That's what caught my attention. These look like a good fit for these drivers, but I want to do some research right here and pick the forum's brains and maybe do a little experimenting before I go throwing money at an idea.

Larry D.

tubed said:

Have at it
Perhaps something to do with the nature of the magnetic planar Emim?

http://www.infinity-classics.de/technik/manuals/RS_2.5_technical_sheet.pdf

Click to expand...

The schematic indicates 0.05mH inductance in series with the 50W resistor. Is there actually/physically a separate 0.05mH inductor?

Yes, there is one on my crossover.

If the inductance of the 50W resistor was important/critical to the design I'd think it would be noted. That is, however, only my opinion. Without the designer to answer it may remain speculation at best.

I was thinking somebody here might know why they used 50W, but if not I will have to do some experimenting. I don't have a 0.7 ohm resistor, but I do have a 0.6 ohm ten watter. I'll try it as soon as I get time and see if it warms up at all.

Larry D.