Increasing Qts by series R for OB

[grantnsw]

Hi,

I have 2 old 12 inch woofers with Qts = about 0.3, that I want to use for OB, if possible. I have read that Qts will be increased by a series resistor.

Sreten kindly gave me a 'calculator' link some years ago, but I couldn't get it to work then. So, I'm wondering if there is a simple formula to increase a drivers Qts by series R and what other effects this would have on performance/sound of the driver, and, if then, would it be high enough for OB?

It seems obvious that Re and Qts will be increased, but anything else?
I'm just trying to get something for nothing!

For example, raising Eminence Beta15A's from roughly 0.6 to approach the Alpha15A's 1.1? Qts. Is this possible? cheers, grant

[CLS]

http://en.wikipedia.org/wiki/Thiele_small


Qes = 2 * Pi * fs * Mms * Re / (BL) ^2

Now you just add the series R into Re and get a new Qes.

And then you'll have a new Qts.

For most cases, Qes dominates anyway.

In the end, you may probably tune it by ears.

[MJK]

You can raise the Qts by adding series resistance, but the efficiency will go down. For OB this may not be a good trade-off, sorry no free lunch.

[forr]

Efficiency of the speaker itself won't go down.
There is a power loss in the resistor, quite different matter.
To avoid this, an amplifier can be modified to get the required output resistance.
Note that a series resistor has sometimes been used by JBL, I think, because of too low Qts.
As the technique is of so low cost, it is worth to try it.

[MJK]

Originally posted by forr
Efficiency of the speaker itself won't go down.
There is a power loss in the resistor, quite different matter.
To avoid this, an amplifier can be modified to get the required output resistance.
Note that a series resistor has sometimes been used by JBL, I think, because of too low Qts.
As the technique is of so low cost, it is worth to try it.

Technically the Qts and the efficiency of the driver will not change. But considering his application where he is inserting a resistor between his amp and a particular driver, the net simple way of looking at the system level effect is higher Qts and lower efficiency. While you are technically correct, you are making it more confusing then it really needs to be.

[multisync]

also the effect of the amplifiers' damping factor will go down

[Rudolf]

I don't get it how people can see a series resistor as a solution for anything in this application. Look at my simulation for a Visaton TIW 300 / 8Ohm with Qts= 0,28 on an open baffle. I added a 8 Ohm resistor and a 16 Ohm resistor to raise Qts. What you get is loss all the way (upper diagram).

A second order filter (in this case designed for a upper limit of 150 Hz) would be much more effective and already include a working low pass (lower diagram).

[forr]

MJK,
The Qt..(well how to call it with an OB ?) is affected by the series resistor, that's prescisely its purpose.
Note that the higher power needed for the same level is dissipated in the resistor not in the voice coil.


Mutlisync
---also the effect of the amplifiers' damping factor will go down---

The resistance of the generator seen by the loudspeaker is higher, once again, it is its purpose to decrease the excessive damping.
The damping factor which characterises the output impedance of the amplifier dos not change.

[john k...]

If you really want to look at this the correct way then consider the resisted in series with the driver a voltage divider. The effect of the resistor is greatest where the impedance of the driver is the lowest. This means the resistor has little effect around fs where the impedance of the driver is high and the maximum effect through the midrange where the driver impedance is low. If Le is significant then the resistor has decreasing effect as the driver impedance again rises due to Le. This tends to yield a sway back response in a driver which would otherwise be flat though the midrange.

The arguments over changing Qt can be discussed one way or the other depending on what is considered the "system". In any event they are misleading because they consider only the T/S definition of efficiency which applies to the midband assuming the VC impedance is equal to Re alone.

Furthermore, the effective Qt can not be changed much w/o significant wasted power. With a 16 ohm resistor between the amp and a nominal 8 ohm load there could be as much as 9 dB of power wasted midband.

[theAnonymous1]

Here are the measured specs from a driver with and without a 20R series resistor.....

Without resistor:

Re 23.69
Qes 3.930
Qts 1.587

With 20R series resistor:

Re 3.67
Qes 0.603
Qts 0.492