using a resistor to attentuate a woofers response

[czag]

Would there be any harm in using a 1.5ohm resistor in series before the the low pass circuit to bring 2 8ohm woofers, wired in parallel, responses down 3 decibels? What would the negative effects be, if any?

I seem to remember reading that it would not be good to do it, but I don't remember why or where I read it.

Chris

[Pete Schumacher ®]

Would there be any harm in using a 1.5ohm resistor in series before the the low pass circuit to bring 2 8ohm woofers, wired in parallel, responses down 3 decibels? What would the negative effects be, if any?

I seem to remember reading that it would not be good to do it, but I don't remember why or where I read it.

Chris

It affects Qts when you do that, so any box alignment may need to be revisited, ESPECIALLY vented alignments.

[MSaturn]

The "it's not good" phrase comes from the fact that woofers are almost always the least sensitive part of a speaker already, and you're therefore losing precious sensitivity as compared to the rest of the system.

Now, it may adjust Qts for the better, and it may make the load easier on your amplifier; since you have two woofers, the loss is probably moot.

[billfitzmaurice]

What would the negative effects be, if any?
A fire, foremost. In any event before assuming the woofers are more sensitive than required be sure to calculate the baffle step, which can cost as much as 6dB.

[mzisserson]

A fire, foremost. In any event before assuming the woofers are more sensitive than required be sure to calculate the baffle step, which can cost as much as 6dB.

There are 25+ watt resistors available at Mouser, Digikey, Allied, andn Newark. Also 50+ watt, and flame proof.

I think the baffle step is a huge consideration, also the Qts change needs to be considered.

I think for the first time I agree with Saturn. It really is not a good idea since it is the least sensitive component that takes the bulk of the power. ASSUMING the resistor burns off the current 100% evenly, you will not destroy dynamic... That is assuming though.

[czag]

This is for an mtm that will go into my entertainment center. The speaker will be flush with the ent. center, so it will effectively make for a baffle about 52"x70". There aren't really any gaps around the speaker and the tv and ent. center make for a large relatively flat surface, so I don't think any baffle step is in order. That puts the sensitivity at about 95 dbs, I need to get it down to 92 dbs to go with a tweeter I already have. It was sitting out in front of the cabinet, but the wife says it must go in if I want to build bigger LR speakers.

Assuming I run a couple of 50 watt 3ohm resistors parallel to each other to get a power handling of 100 watts and run that in series before the low pass circuit, will there be any negative affects on the sound quality?

Chris

[philiparcario]

well I am running 6

6 ohm 50 watt resistors gives me 1 300watt resistor. it is a sub no fires yet. but it does change qts you need to calc new qts.

[dantheman]

Here's some simulations of what will happen to FR using a 9.2 Ohm series resistor. I know, much larger than you are using, but it will stress the deviations from measured response. These were done using MJK Mathcad.

No added resistance:
http://www.audiocircle.com/index.php?action=gallery;sa=view;picture=27273

With 9.2 Ohms in line:
http://www.audiocircle.com/index.php?action=gallery;sa=view;picture=27274

So you'll loose little efficiency in the bass d/t Q boost and more up in frequency.

Dan

[Zero the Hero]

wire them in series and attenuate the tweeter

[daveh]

I would be tempted to just run it as an MT instead of wasting power. Save some money.

My .02

DaveH

[czag]

wire them in series and attenuate the tweeter

Why would I do something that is very easy and makes complete sense......Oh yeah, because it's very easy and makes complete sense.

Thanks for pointing out the obvious solution that was not so obvious to me.

It took me all of about 1 minute to modify the circuit I had built in PCD to work with these in series.

Chris

[Æ]

I would be tempted to just run it as an MT instead of wasting power. Save some money.

My .02

DaveH

Yeah, cheer, applause!

[czag]

I would be tempted to just run it as an MT instead of wasting power. Save some money.

My .02

DaveH

That is one way to go, but running them in series lets me reuse the same enclosure and gets me by with half the excursion with same power to reach the same spl level.

Chris

[Wolf]

Uhm- whoa there!

You lose 6dB sensitivity of a parallel-pair when wiring in series, Czag.
You won't get the same output.
Wolf

[Jeff B.]

Uhm- whoa there!

You lose 6dB sensitivity of a parallel-pair when wiring in series, Czag.
You won't get the same output.
Wolf

Correct, and I don't think he's going for that 6dB on purpose. For flush mounting with no BSC, series would be a good way to go if he needs to use both drivers in the same box. I think he got some good wisdom and direction.

[Wolf]

It just sounded like he didn't know that, the way it was written.
Later,
Wolf

[czag]

I meant that I can get the same spl with half the excursion as compared to using one woofer in a MT.

I am definitely losing that 6 db on purpose so that they will be close to the same sensitivity as the tweeter I have. The xover I had for these drivers had about 4-5 db of BSC, but now I do not need any with the placement in the ent. center.

Chris

[Wolf]

Alright.
Wolf

[jmc]

It just sounded like he didn't know that, the way it was written.
Later,
Wolf

Buddy - he was referring to a reference SPL level as compared to a one woofer system.

LATER,
JMC

[auracle]

If you don't want to drop sensitivity that much, several parallel resistors may be the best ticket to reaching your goal. I'd make sure you have enough wattage for an insane drive level like 40 volts p-p continuous :eek: and you shouldn't have any trouble with fires :). Use the simple

Fc = R/(44/7)*L formula to make sure you're not rolling off the top end too much or as others suggested, model the extra resistance to ensure that you can live with the impact to overall response.

There seems to be a resurgence in high efficiency and sensitivity these days. Some pretty high profile companies are still dropping below the dreaded 3 ohm range (Wilson Audio) to keep efficiency around 90-92 db. So if efficiency/sensitivity are really important to you - I'd keep the resistor option open.

[MSaturn]

Put a pillow over the speaker, or stand very far away.

[Wolf]

Yeah- he already stated his case, so why restate? It's easy to see now.
Later,
Wolf

[curt_c]

Here's an example of why series resistance is a sub-optimal choice for woofers:

The enclosure volume would be MUCH larger.

I modeled a parallel pair of RS225-8, in LspCAD in a sealed enclosure. The additional 1.5 ohm resistor would require doubling the enclosure volume. (From 65 ltrs to 130 ltrs in the case of the RS225.)

The series resistor will potentially have high heat dissipation requirements.

Assuming 160 watts RMS as the maximum dissipation by the woofers, the resistor would be dissipating approximately 60 watts. Good design practice suggests the resistor power capability should be double the expected power dissipation, so a 100 watt resistor would be acceptable. Optimally it should be externally mounted and/or on a heat sink to be adequately cooled.

On the plus side, one could spec a low pass inductor with a high DCR and hopefully only add a small additional series resistance, which would reduce the resistors power dissipation requirements. A small gauge air core would be ideal for this option, and the increased enclosure requirements would still apply.

C

[auracle]

Put a pillow over the speaker, or stand very far away.

Exactly....but you should always make sure you use the pillows with spikes on the bottom - they couple better to the floor that way. I sometimes also put special resonance absorbing pebbles on my pillows - only the ones you get at the finest retailers, of course....the ones at Best Buy aren't tuned properly.

:D