Help w/ Resistors To Pad Tweeter

[mike1234]

GIVEN: This is for my active 6.1 MTM PC system. I have some concerns about the impedance load which drops to 3 ohms. The bi-amp plates are rated for 8 ohm minimum. Philip has been using these with his 4 ohm HT MTM's with no problem so far and his drop as low as 2.9 ohms so I'm probably okay. But I'm sort of anal about pushing the limits so I'll probably design mine with 2 ohm non-inductive resisters which will raise minimum impedance to 5 ohms and average to 6 ohms. The plate amps are rated at 40/15 watts for woofer/tweeter at 8 ohms so I "assume" it's more like 50/20 at 6 ohms.

QUESTIONS: How much total power handling do I need for the resisters? Will a single 10 watt 2 ohm be okay for the tweeter amp? Will two 4 ohm 10 watt resistors in parallel be okay for the woofer amp? Or do I need to match/exceed the power output of the amps?

[philiparcario]

Provided Link: 2ohm 50watt 1% resistors


> GIVEN: This is for my active 6.1 MTM PC
> system. I have some concerns about the
> impedance load which drops to 3 ohms. The
> bi-amp plates are rated for 8 ohm minimum.
> Philip has been using these with his 4 ohm
> HT MTM's with no problem so far and his drop
> as low as 2.9 ohms so I'm probably okay. But
> I'm sort of anal about pushing the limits so
> I'll probably design mine with 2 ohm
> non-inductive resisters which will raise
> minimum impedance to 5 ohms and average to 6
> ohms. The plate amps are rated at 40/15
> watts for woofer/tweeter at 8 ohms so I
> "assume" it's more like 50/20 at 6
> ohms.

> QUESTIONS: How much total power handling do
> I need for the resisters? Will a single 10
> watt 2 ohm be okay for the tweeter amp? Will
> two 4 ohm 10 watt resistors in parallel be
> okay for the woofer amp? Or do I need to
> match/exceed the power output of the amps?
Remember I am crossing to the sats at 100hz that is why they handle the 4ohm mtm's or so I think. I use these resistors from newark. They will not overheat.

[billfitzmaurice]

By adding resistors you lower sensitivity, which results in using more power to reach the same SPL, which may overheat the amp as well. Resistors are not a panacea for all ills.

[mike1234]

And does it matter??

> Remember I am crossing to the sats at 100hz
> that is why they handle the 4ohm mtm's or so
> I think. I use these resistors from newark.
> They will not overheat.

[mike1234]

...but in this case it really doesn't matter. These are near-field speakers for my PC. I just want high quality reliable sound.

Are you saying that, in this particular case, it's better to run the amps into lower-than-advertised impedancs than to increase the impedance?

> By adding resistors you lower sensitivity,
> which results in using more power to reach
> the same SPL, which may overheat the amp as
> well. Resistors are not a panacea for all
> ills.

[philiparcario]

> And does it matter??
they are inductive. It would matter if the inductance is high enough.

[philiparcario]

> they are inductive. It would matter if the
> inductance is high enough.
the inductance can be measured I have the meter and a resister. wait till tomorrow I will look it up for you.

[aricaudio]

> ...but in this case it really doesn't
> matter. These are near-field speakers for my
> PC. I just want high quality reliable sound.

> Are you saying that, in this particular
> case, it's better to run the amps into
> lower-than-advertised impedancs than to
> increase the impedance?

I would say "no". I think Bill's saying that is the downfall of the resistor- but for all intents and purposes I say it's fine

[mike1234]

[mike1234]

...and I know this is a band-aid quick-fix type solution but it seems an okay/viable option in this case.

> I would say "no". I think Bill's
> saying that is the downfall of the resistor-
> but for all intents and purposes I say it's
> fine

[aricaudio]

> ...and I know this is a band-aid quick-fix
> type solution but it seems an okay/viable
> option in this case.
That the resistors WILL change value when they heat up, and again when they cool down- so it's better to parallel a bunch of 25 watters to achieve a target impedance rating (ex: (4) 25 watt 20 ohms in parallel= (1) 100 watt, 5 ohm resistor).

[mike1234]

...capability of the resistors (in parrallel of course) given the 50/20 watt amp spec? 200/80?

> That the resistors WILL change value when
> they heat up, and again when they cool down-
> so it's better to parallel a bunch of 25
> watters to achieve a target impedance rating
> (ex: (4) 25 watt 20 ohms in parallel= (1)
> 100 watt, 5 ohm resistor).

[aricaudio]

> ...capability of the resistors (in parrallel
> of course) given the 50/20 watt amp spec?
> 200/80?

I'm not sure what you're asking here. What does the 50/20, 200/80 refer to? I would basically just take what the amp puts out in R.M.S., and go higher on the resistor bank (ie: 100 watt r.m.s. amp, typically a 5-20 watt resistor is fine for a tweeter...but why not go for a 100 watt bank to allow no- heat related changes to happen to the impedance? Just take your target impedance for the resistors, and multiply by how many resistors you will use. Hope this helps, Aric

[mike1234]

...produce at 6 ohms average. The 200/80 refers to an interpolation of your recommendation of using 80 watts of resistor wattage capability to handle 20 watts of amplifier power. Is this overkill to prevent resistor overheating?

> I'm not sure what you're asking here. What
> does the 50/20, 200/80 refer to? I would
> basically just take what the amp puts out in
> R.M.S., and go higher on the resistor bank
> (ie: 100 watt r.m.s. amp, typically a 5-20
> watt resistor is fine for a tweeter...but
> why not go for a 100 watt bank to allow no-
> heat related changes to happen to the
> impedance? Just take your target impedance
> for the resistors, and multiply by how many
> resistors you will use. Hope this helps,
> Aric

[philiparcario]

Mike I have not forgotten I can't find my lcr/cap meter. I have the resister not the meter. It is in the attic or the gold mine as i like to think of it. LOL ; phil

[mike1234]