Passive Sub and Resistors

[tubesaregood]

I have a passive subwoofer with satellite outputs. I've seen people selling other passive subs with the input/output deal and the outputs have resistors on them (looks like modest wirewound resistors) to act as a dummy load in place of satellites. If I were to use a 50 wpc amplifier to drive said subwoofer and I were to use resistors in place of satellites, what power rating should I get? Would say, 5 or 10 watts work or should I aim higher? I'm guessing it depends on how loud I'll be going?

And what's the advantage of attaching the resistors anyway? Are they only there to bleed off some of the excess power or does a crossover like that only work correctly with a load on both the low pass and high pass sections? I can't figure that out for some reason.

 

[j_garcia]

Never heard of anyone doing this. The only thing I can think of is it allows the receiver to think there is a speaker connected to the outputs, but I can't actually see that being necessary, especially if you filter the signal before the sub rather than feeding it a full range signal, which would be the best way to do it. Meaning, feed the sub's amp with the sub pre out from a receiver that is already crossed over and you shouldn't need to worry about it.

 

[tubesaregood]

I could also see it used as impedance adjustment for the system and to bleed off some of the power fed to the sub, but yeah, I'm trying to see if there is any other reason for doing it.

 

[mtrycrafts]

I have a passive subwoofer with satellite outputs. I've seen people selling other passive subs with the input/output deal and the outputs have resistors on them (looks like modest wirewound resistors) to act as a dummy load in place of satellites. If I were to use a 50 wpc amplifier to drive said subwoofer and I were to use resistors in place of satellites, what power rating should I get? Would say, 5 or 10 watts work or should I aim higher? I'm guessing it depends on how loud I'll be going?

And what's the advantage of attaching the resistors anyway? Are they only there to bleed off some of the excess power or does a crossover like that only work correctly with a load on both the low pass and high pass sections? I can't figure that out for some reason.

First I'd like to fully understand this passive sub. It has speaker level inputs. Does it have a crossover in it? I suppose it also has speaker outputs? And that is where you have seen the resistor?

I don't see why anyone would do that other than not understanding what is going on there. Then, the urban legend begins.

That resistor doesn't bleed anything off the sub, if it has an internal crossover. Nor does it impedance match anything, from what I am understanding from your initial post.

If you want to bleed power off, just turn down the volume.

 

[dave1490]

I have a passive subwoofer with satellite outputs. I've seen people selling other passive subs with the input/output deal and the outputs have resistors on them (looks like modest wirewound resistors) to act as a dummy load in place of satellites. If I were to use a 50 wpc amplifier to drive said subwoofer and I were to use resistors in place of satellites, what power rating should I get? Would say, 5 or 10 watts work or should I aim higher? I'm guessing it depends on how loud I'll be going?

And what's the advantage of attaching the resistors anyway? Are they only there to bleed off some of the excess power or does a crossover like that only work correctly with a load on both the low pass and high pass sections? I can't figure that out for some reason.

i did somthing like this with a technic,s reciever that only had high level outputs and needed low level to drive a exsteral power amp.i just put 1 75 ohm resistor in each + and- signal path from the high output to the low level input of the power amp worked great.it just lowers the voltage output to drive the power amp.carver has 1 of these for thier 400 series.why pay $45 for 2 resitors in a fancy housing.the loudness will very with the resisance of the resistor being used.

 

[tubesaregood]

First I'd like to fully understand this passive sub. It has speaker level inputs. Does it have a crossover in it? I suppose it also has speaker outputs? And that is where you have seen the resistor?

I don't see why anyone would do that other than not understanding what is going on there. Then, the urban legend begins.

That resistor doesn't bleed anything off the sub, if it has an internal crossover. Nor does it impedance match anything, from what I am understanding from your initial post.

If you want to bleed power off, just turn down the volume.

Yes, yes, and yes.

I only know the basics about electricity. This sub has a low pass on the inputs and a high pass on the outputs. What exactly is going on in there with all that? I understand the basic concept of crossovers etc. (I think!), but I don't fully understand what you said in relation to this particular situation.

 

[dave1490]

Yes, yes, and yes.

I only know the basics about electricity. This sub has a low pass on the inputs and a high pass on the outputs. What exactly is going on in there with all that? I understand the basic concept of crossovers etc. (I think!), but I don't fully understand what you said in relation to this particular situation.

the low pass on the sub is the low level input{for the sub out on the reciever}.the high pass is the high level input{for the speaker outputs on the reciever}.
the highpass output is if you want to use the amp and crossover in the sub to power tweaters,if you want to bypass the reciever.only use the high level input if you want the full frequency range whitch is what you need to power tweaters from the high level outputs on the sub.

understand that if you use the high level inputs on the sub it will be steped down to run the sub amp,then amped using the sub amp for the high level outputs to run tweaters.

 

[tubesaregood]

the low pass on the sub is the low level input{for the sub out on the reciever}.the high pass is the high level input{for the speaker outputs on the reciever}.
the highpass output is if you want to use the amp and crossover in the sub to power tweaters,if you want to bypass the reciever.only use the high level input if you want the full frequency range whitch is what you need to power tweaters from the high level outputs on the sub.

understand that if you use the high level inputs on the sub it will be steped down to run the sub amp,then amped using the sub amp for the high level outputs to run tweaters.

Yes. Hi. This is a passive subwoofer. There are no low level inputs. And no, low pass is not synonymous with low level inputs - it is a low pass filter. You know, lets the bass end of the audio spectrum pass to the speaker and blocks out high frequencies and such. There's a low pass filter on the speaker level inputs of a powered sub too. And you wouldn't use the high level outputs to power tweeters alone because even the lowest frequency tweeter I can think of (Epicure inverted dome, and barring 6 foot tall Bohlender Graebener planar transducers) starts its response at about 1800 Hz. The speaker level outputs on a passive sub would start at probably 80~150 Hz, and even in this passive sub the crossover frequency is about 200 Hz.

 

[dave1490]

the low pass on the sub is the low level input{for the sub out on the reciever}.the high pass is the high level input{for the speaker outputs on the reciever}.
the highpass output is if you want to use the amp and crossover in the sub to power tweaters,if you want to bypass the reciever.only use the high level input if you want the full frequency range whitch is what you need to power tweaters from the high level outputs on the sub.

understand that if you use the high level inputs on the sub it will be steped down to run the sub amp,then amped using the sub amp for the high level outputs to run tweaters.

forget what i said.this is for a active sub{with amp}.

 

[Ampdog]

Yes, I was also wondering all the time.

A passive sub as I know it has no electronics inside, e.g nothing fed by a mains cord. I am not familiar with commercial products, but would guess that all that could be inside is a low-pass filter for the sub itself, and a high-pass filter for convenience of connecting - exactly what?

A sub is supposed to be there for (say) below 150 Hz. Then I suppose there could be more than one filter inside (IOW cross-over) for convenience; something for mid-range and for tweeter. In that case I would expect to "slave outputs". My guess is that these are terminated externally in the absence of those drivers.

Tubesaregood, how do you drive the rest of your system of loudspeakers? Directly from the amplifier's (full frequency response) output? In that case the sub with terminations would waste power, as it functions as anonther full-frequency load on the amp. Ideally you should then leave those "outputs" (if indeed that is what they are) open. But it is very possible that the low-pass part of the sub cross-over is dependent on its characteristics, at least in the pass band between low and middle range, on the middle part being terminated. No cross-over is two separate independant sections.

In that you may have a problem, and someone might need to get inside and measure. Or you can drive the rest of your loudspeaker set-up from the sub's filters..... But note that in all this I used "should be" and "might". Guesswork by me, and one needs to know what exactly is inside your sub-enclosure. Or what is standard: Others could reply there.

Hope this helps.

 

[mtrycrafts]

Yes, yes, and yes.

I only know the basics about electricity. This sub has a low pass on the inputs and a high pass on the outputs. What exactly is going on in there with all that? I understand the basic concept of crossovers etc. (I think!), but I don't fully understand what you said in relation to this particular situation.

OK. I must have missed your post.
If the sub has speaker type connectors on its back, both input and output, then the crossover in it is so you can daisy chain the speaker with another set that will handle the frequency above the crossover of that sub. The amp would drive both the sub and the other speaker in chain with it..

If you only need to sub for its performance, no resistor is needed, period.
If you are also connecting other speakers to the amp terminal with the sub, you need to reconsider this approach as the sub will be in parallel with the low driver of the other speaker. Best if you daisy chain the speakers then.

 

[tubesaregood]

Sweet, so there's no good reason to put a dummy load on the satellite outputs.

Here's an example of what I'm talking about, on eBay right now... <p><img src="http://i9.ebayimg.com/04/i/000/98/6f/1d40_1.JPG">

note the resistors on the outputs.

I assume that's what they are. I think I'll ask the seller his logic on that and report back!

 

[mtrycrafts]

Sweet, so there's no good reason to put a dummy load on the satellite outputs.

Here's an example of what I'm talking about, on eBay right now... <p><img src="http://i9.ebayimg.com/04/i/000/98/6f/1d40_1.JPG">

note the resistors on the outputs.

I assume that's what they are. I think I'll ask the seller his logic on that and report back!

Sorry, the picture is not good enough to be sure what that is. Is there any writing on it? Color bands on it?
A stable closeup shot?

It surely has 4 terminal pairs. Left/right in and out so it would be what I said.

 

[tubesaregood]

Yeah, I have the exact same speaker system and another 15" passive sub by dbx. I asked the guy what they were there for and he basically didn't know (he thought the outputs were for connecting other subwoofers, which obviously would do no good because the high pass is set at about 150 Hz). But he did say they were resistors, I guess. At any rate, what else could they be that would actually serve a purpose?

 

[mtrycrafts]

Yeah, I have the exact same speaker system and another 15" passive sub by dbx. I asked the guy what they were there for and he basically didn't know (he thought the outputs were for connecting other subwoofers, which obviously would do no good because the high pass is set at about 150 Hz). But he did say they were resistors, I guess. At any rate, what else could they be that would actually serve a purpose?

Nothing else could, but neither does that resistor. Initially from that picture I was wondering if it was a cap instead

But that 150Hz tells the story. Below it stays in the sub, above it to the other speaker connected. Problem solved. I'd just take those resistors off so you don't have any power loss into it.