Effect of adding 4 ohm resistor to tweeter

I know it's generally a bad idea to add resistors in series with a woofer voice coil in an attempt at bringing a 4 ohm impedance to 8 ohms... but is it just as bad to try to do this with a tweeter?

I'm wondering if one should be concerned about damping factor, qts, or anything else since my understanding of these parameters is kinda fuzzy.

What would be the effect of bringing a tweeter up to 8 ohms by putting a 4 ohm resistor inline with it? I'm using a BG Neo 3.
 

benb

Member
2010-04-24 1:52 am
The main effect would be to lower the output by half the voltage, which would be 6dB. Tweeters are usually substantially more sensitive than woofers, but that might be more output reduction than you need.

The good news is 1. tweeters run at a much lower power level than woofers, so you won't be "wasting" too much power in the series resistor, though it should still be a 5W or 10W or so, depending on the max power expected into the tweeter.

Also good is tweeters are always operated above their main resonant frequency, so all those things connected to woofers really don't matter much. Maybe someone can fill in more on this, but I suspect many tweeters can still have odd resonances, and driving them from a lower impedance is always better. You might could use an L pad to make the impedance the tweeter sees lower.

I think you should probably worry more about matching the sensitivity with the woofer than what the exact impedance turns out to be - if it's 3dB more sensitive than the woofer, add a resistor to reduce its output 3dB, and accept that the impedance in the tweeter's range is going to be a little less than 8 ohms.

And shouldn't this thread be in the multi-way forum?
 
I know it's generally a bad idea to add resistors in series with a woofer voice coil in an attempt at bringing a 4 ohm impedance to 8 ohms... but is it just as bad to try to do this with a tweeter?

I'm wondering if one should be concerned about damping factor, qts, or anything else since my understanding of these parameters is kinda fuzzy.

What would be the effect of bringing a tweeter up to 8 ohms by putting a 4 ohm resistor inline with it? I'm using a BG Neo 3.

Several old Magneplanar models had a 2ohm ribbon, instead of the current 3ohm and Magnepan introduced a 1ohm resistor to "compensate", so they could keep the HP XO values the same. Most true-ribbon Magnepans allow for an "attenuation resistor" to be inserted in series with the ribbon, to bring its level down a bit.

In my active Maggies, I have a series 1ohm resistor on my ribbons, so that the amp I use on the ribbons can cope (with the 3ohm total load) and I can't hear any negatives - in terms of the kind of things you mentioned (eg. damping factor).

But adding a 4ohm resistor to a 4ohm driver is a bit more "extreme" IMO - can't your amp handle a 4ohm load? :)

Regards,

Andy
 
Yes, it can handle a 4-ohm load. Problem is that my tweets are already 4 dB ahead of my lows, I don't need to compound the problem by making them even louder by having a lower impedance.

Actually, where I was headed eventually was to probably construct a fixed L-pad resistor network to bring the impedance to 8 ohms and then attenuate the tweeter by 4 or 5 dB. I just wonder if there will be a degradation in sound quality that is worse than the problems of mismatched impedance and efficiencies in my drivers.
 
Yes, it can handle a 4-ohm load. Problem is that my tweets are already 4 dB ahead of my lows, I don't need to compound the problem by making them even louder by having a lower impedance.

Actually, where I was headed eventually was to probably construct a fixed L-pad resistor network to bring the impedance to 8 ohms and then attenuate the tweeter by 4 or 5 dB. I just wonder if there will be a degradation in sound quality that is worse than the problems of mismatched impedance and efficiencies in my drivers.

Surely the optimal solution is to go all-active ... where volume levels of individual drivers are controlled by pots in the active XO? That's the way I do it, anyway. :)

Regards,

Andy
 

tinitus

Ex-Moderator R.I.P.
2005-11-24 1:47 am
Actually, where I was headed eventually was to probably construct a fixed L-pad resistor network to bring the impedance to 8 ohms and then attenuate the tweeter by 4 or 5 dB.

I just wonder if there will be a degradation in sound quality that is worse than the problems of mismatched impedance and efficiencies in my drivers.

L-pad is to maintain impedance, and not alter it
series impedance is ok too
but it needs to be incorporated with the crossover
impedance changes influence xo function, thus L-pad

individual driver impedance differences matters only if you use tube amps

SPL level adjustment is very important
a serious problem with speakers
it can change everything
and its not always done correctly
I have found a series resistor to sound better than L-pad
but my impression may have been influenced by small changes in xo function


when you have found the right resistor value, maybe change it to a DCA graphite
or you can make one yourself
http://www.troelsgravesen.dk/graphite.htm
 

benb

Member
2010-04-24 1:52 am
The Bohlender Graebener Neo 3 PDR is a planar tweeter.
Oh, now I see. I'm not familiar with the driver, didn't look it up, haven't read much of the planar forum, and I tend to think of planars as those big full-range electrostatic things...
It's also getting a dedicated amp in a bi-amp system, so I wasn't sure whether to put it in the planar forum or the multi way.
This makes it easy, as andyr responded - connect the amp directly to the tweeter and lower the gain of the tweeter's amp in relation to the other amp.

This is yet another advantage of biamping/multiamping, being able to compensate for different driver sensitivities and impedances at line level, where there's no significant power loss nor any worry of reduced damping factor.
 
Manufacturer says that driver is purely resistive. Sounds like a bad concept, at least theoretically, and can't be true, at least theoretically, but certainly makes it easier to design for.

With cheap amps available second-hand, passive crossovers make less sense for DIYer and only manufacturers have to bend themselves into pretzels making them today.

L-pads are a sorry mess when viewed from any angle. Instead, cobble your own little resistive pad to satisfy requirements, once you know the loudness you want (SPL is quite important, as someone said above).
 
Hi,

For the Neo 3 a series resistor would be fine, no advantage to an L-pad.

For ferrofluid domes the same is usually true, but rising inductance can
skew the balance to more top end brightness. For non ferrofluid tweeters
an L-pad is usually necessary due to the Fs impedance peak and here
you still will be significantly altering the tweeters roll-off response,
if its all accounted for properly in the design this is not an issue.

rgds, sreten.

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Manufacturer says that driver is purely resistive. Sounds like a bad concept, at least theoretically, and can't be true, at least theoretically, but certainly makes it easier to design for.

Ben, I'm interested to know why:
1. a purely resistive driver is a bad concept, and
2. why it can't be true.

Magnepan drivers are almost purely resistive, whether they are:
* the original round-wire on mylar drivers
* the newer flat-wire ("quasi-ribbon") on mylar drivers, or
* true-ribbons.

They sound magnificent - so they certainly can't be a "bad concept". I measured the impedance of my 3 drivers and, from memory, it was about 1/30th of the published figures for a Peerless cone driver. Hence, with this low inductance, they remain a constant load - which is good:
a) for a passive XO, and
b) for the amplifier, in an active setup.

Maybe your only experience has been with conventional cone drivers? :)

Regards,

Andy
 
andr -

The answer is I don't believe in perpetual motion, and I was talking at least theoretically earlier, if you recall.

Terms like "purely" should not be used in reputable descriptions. And if such terms are allowed to be introduced by the folks in the Marketing Department, you ought to be skeptical about the rest of the description too.

It is correct to say that being able to anticipate impedance is to (an unknown degree) helpful in designing passive crossover (where you deserve what you get anyway if you still use them) and to some probably insignificant degree for poorly designed amps (if you still use them... ). Moreover, constant-like resistance is more so, as you suggest.

Footnote: I'm a big believer in motional feedback. So any time I hear that the driver has nothing to say back to the amp, I am puzzled.
 
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tinitus

Ex-Moderator R.I.P.
2005-11-24 1:47 am
Its a ribbon, or more correctly a planar

due to the princible of having the lead as part of the diaphragm instead of an attached voice coil, they are commonly known to be similar or close to a resistive load
the point is, impedance is very linear, hence the called a resitive load

but its still placed in a magnet gap, and moving, and thus may not be that simple
but is known as an easier load
 
Footnote: I'm a big believer in motional feedback. So any time I hear that the driver has nothing to say back to the amp, I am puzzled.

The property of being a resistive driver that doesn't have any inductive properties (and so displays a contact resistance, rather than an impedance which increases with frequency) has got nothing to do with feedback from the driver back to the amp. Maggie drivers do exhibit back-EMF.

Regards,

Andy
 
The property of being a resistive driver that doesn't have any inductive properties (and so displays a contact resistance, rather than an impedance which increases with frequency) has got nothing to do with feedback from the driver back to the amp. Maggie drivers do exhibit back-EMF.

Regards,

Andy

... in which case, unless I don't have my grasp right, means they are not purely/theoretically resistive.
 
... in which case, unless I don't have my grasp right, means they are not purely/theoretically resistive.

I believe your grasp most certainly isn't right! :p

AIUI, in the case of Maggies, back EMF is generated through Newton's Law (due to the coupling between the large panel area and the air).

And as to being "purely resistive", if the inductance is at so low a level as to make no real-world difference in terms of the operation of an XO, the driver can be called non-inductive. After all, we never say "<i>Speaker X goes down to 10Hz</i>" - we always say "<i>Speaker X goes from 25Hz to xxKHz</i>". The "25Hz" generally refers to its -3dB point (some mfrs probably cheat and refer to the -6dB point) and the speaker ,i><b>does </b></i>produce sound below this frequency ... it's just that you won't be able to hear its output at 10Hz.

Regards,

Andy
 
AIUI, in the case of Maggies, back EMF is generated through Newton's Law (due to the coupling between the large panel area and the air).

And as to being "purely resistive"...

Back-EMF is a result of magnetic coupling - the same coupling that produces a force on the ribbon when you run a current through it. Force constant (N/A) and back-EMF constant (V*sec/m) are directly related.

The in-circuit result of back-EMF is an increase in the apparent resistive impedance seen by the amp.