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Old 13th January 2004, 01:20 AM   #1
boone is offline boone  United States
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Default Mbow1 / Ml Tl

Greetings,

I would like to build a ML TL enclosure for Dennis Murphy's MBOW1 (www.murphyblaster.com) or Danny Richie's A/V1 (www.gr-research.com). Both use the GR Research M130 woofer.

While looking at Martin King's site I noticed that he dealt with the FE-208 Sigma's low Qt by adding a resistor in series with the driver to simulate a higher Qt system. A 3.5 ohm resistor raised Qt from .22 to .335.

My questions are;

Does the 12 ohm resistor in the baffle step correction circuit also change Qt?

How do you calculate the change in Qt (Qe) for a given resistor?

Related to these questions;

In the section relating to his baffle step correction for the Focal transmission line and Fostex Ml TQWT speakers Mr. King used an iron core inductor to minimize DC resistance so it wouldn't change the effective Qe of the driver. I'm confused as to why the resistance of the inductor which might change 1 ohm from iron core to air core is significant but the resistor which parallels it can change by 3 ohms with no recalculation?

All of these questions relate to my confusion regarding how resistance in the crossover changes Qe. It seems I need to figure this out before I can use Mr. Kings computer models with any accuracy.

Thank you for your help.
boone
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Old 13th January 2004, 02:41 PM   #2
MJK is offline MJK  United States
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Hi boone,

The original circuit for the FE-208 Sigma used a series resistor, a Zobel circuit, and a baffle step correction circuit. This was the first circuit of this type, using a series resistor, that I had tried and since then I have improved on the circuit and the resulting speaker's performance. Please review my site for the latest circuit layout.

I look at these circuits from a little different perspective when trying to understand exactly how they work and why. If you follow my logic below then I think it will help you understand and design a circuit for your application.

1. The Zobel circuit makes the driver appear to be a pure resistor above approximately 200 Hz. This is very important and leaving out a Zobel is a mistake in my opinion. A Zobel should have no impact on a speakers sound, the voltage at the driver's terminals is unchanged by a Zobel. A Zobel cancels the rising impedance of the voice coil inductance. I have also found that a Zobel circuit is very forgiving of the exact values of the resistor and capacitor, I design these Zobel circuits based on the impedance measurements and not using any textbook formula.

2. With a Zobel in place, the impedance curve can be broken into two different behaviors. Above 200 Hz, the impedance is purely resistive. Lets assume the impedance is is equal to a DC Re of 8 ohms. Below 200 Hz, there are typically one or two very tall impedance peaks that may even approach 100 ohms in magnitude. So if a resistance is placed in series with a driver a voltage division is set-up. Below 200 Hz, almost all of the applies voltage is seen across the speaker terminals due to the ratio of the high impedance of the driver to the smaller series resistor. Minimal attenuation of the driver response is realized. Above 200 Hz, a simple voltage division also exists and the split between the voltage drop across the series resistor and the driver can be significant. If the series resistor is equal to the drivers DC Re, then the voltage applied to the driver will be one half of the total voltage and a 6 dB drop in efficiency will occur.

3. So based on the discussion in 2, the series resistance is only really significant at the higher frequencies above our assumed value of 200 Hz. The series resistor forms a voltage division that attenuates the efficiency of the driver in the pass band. The series resistor has almost no impact in the frequency range that the T/S parameters are used to set-up a bass alignment, below 200 Hz. So in reality none of the driver's T/S patrameters are effected, Qes and Qts do not change. People like to simulate a series resistor by adjusting Qes and Re, but all you are really doing is adjusting the shape of the response curve of the driver to pad down the efficiency. If you think about what is really going on, a simple voltage division, then I think it is a little less confusing and understandable. It is for me and I hope it is for you.

So back to the correction circuit I am using for the Fostex FE-208 Sigma ML TL. The Zobel takes care of the rising driver impedance due to the voice coil inductance. Then I have a correction circuit composed of an iron core inductor in parallel with a resistor in parallel with a capacitor. At low frequencies all of the current passes through the inductor, it is the path of least resistance due to the low DC resistance. The correction circuit is essentially invisible to the driver and all of the voltage is applied at the driver's terminals. As frequency increases the impedance of the inductor rises and the current starts to flow into the resistor because this is now the path of least resistance. The simple voltage division between this resistor and the driver produces the dB reduction required to correct for both the baffle step loss and the attenuation required to make a low Qts driver produce a midrange that is balanced with the bass output. At high frequencies, where an eight inch full range driver starts to roll-off, the capacitor shorts the correction circuit so that all of the applied voltage is once againn seen at the driver's terminals. This shorting by the capacitor causes the response to roll-up, due to the higher applied voltage at the driver terminals, and extends the high frequency a little higher before it starts to drop off.

The type of inductor used in the correction circuit does impact the very low frequency response a little. I use an iron core inductor to get the lowest DC resistance, the smallest and lightest inductor, and the least expensive. An air core inductor could also be used but the DC ressitance may become large enough to have some impact in the bass response. I prefer iron core inductors and have not seen any real problems using them. I know the purists will object to this statement, but then how many of them would consider using a driver like the Fostex FE-208 Sigma in an ML TL. To each his own, use what works for you.

Hope that helps,
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Old 13th January 2004, 09:53 PM   #3
sreten is offline sreten  United Kingdom
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Default Re: Mbow1 / Ml Tl

Quote:
Originally posted by boone
My questions are;

Does the 12 ohm resistor in the baffle step correction circuit also change Qt?

How do you calculate the change in Qt (Qe) for a given resistor?

boone
A) no it doesn't.

Given A you cannot use either design with a different
Qt without a complete redesign of the crossover.

So you should design the ML TL to suit the drivers and crossover
as its stands, otherwise choose a different design /drivers.

A series resistor to change Qt is crude engineering at its worst.
A far better idea if necessary is an amplifier with positive resistance.

sreten.
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Old 13th January 2004, 11:06 PM   #4
MJK is offline MJK  United States
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Quote:
A series resistor to change Qt is crude engineering at its worst. A far better idea if necessary is an amplifier with positive resistance.
sreten,

I could not disagree more with your first statement. I think a well designed shelving circuit is really a simple and elegant solution to balancing the SPL response for a low Qts driver. You maintain all of the dynamic advantages of a large magnet motor and still obtain a balanced frequency response. I will agree that this circuit can be overdone and the sound degraded. You need to be careful and adjust the value of the resistor by ear, it is obvious when the right value has been found.

Putting the filter in the amp circuit, or between the amp and the preamp, is going to yield the same overall system efficiency. You will have to push the amp harder to get the same SPL at the listening position.

Using these simple passive circuits has worked well for me and when people finally work up the courage to try them on their own systems, even though all of the "experts" say they are going to ruin the sound, most prefer the results with the circuit in place and don't remove it. Simple, inexpensive, and easy.
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Old 14th January 2004, 05:52 AM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by MJK


sreten,

I could not disagree more with your first statement. I think a well designed shelving circuit is really a simple and elegant solution to balancing the SPL response for a low Qts driver. You maintain all of the dynamic advantages of a large magnet motor and still obtain a balanced frequency response. I will agree that this circuit can be overdone and the sound degraded. You need to be careful and adjust the value of the resistor by ear, it is obvious when the right value has been found.

I realise now I must have misread the question, I thought the
BSD reistor referred to was in the the MBOW1, in general BSD
circuits do not change Qt.

You must agree a amplifier with postive output resistance is a far more elegant solution than a power sapping passive resistor ?
Especially for low power amplifiers , class A and valve.

sreten.
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Old 14th January 2004, 11:18 AM   #6
MJK is offline MJK  United States
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Quote:
You must agree a amplifier with postive output resistance is a far more elegant solution than a power sapping passive resistor ?
I am afraid I cannot agree with this statement either. A low powered tube amp with a filter upstream of the amp will still require more power to the speakers to get the same SPL at the listening position. I am not sure that this arrangement is the magic answer to the problem. I have seen this tried and the limitations of the low power and the required SPL produced by the speakers were no better. This is one of the reasons I like high power solid state amps, never have to worry about power limitations.

The second advantage to a passive filter mounted in the speaker enclosure is that the speaker system is now self contained. You can pick them up and take them to somebody else's home, plug them in, and listen. You are not required to rewire their system to place a filter between the amp and preamp. What happens if they are using a receiver? Do you try and add the filter in a tape loop? I like simple.
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Old 14th January 2004, 06:46 PM   #7
sreten is offline sreten  United Kingdom
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The positive resistance is added by the amplifier
feedback loop using current sensing of the output.

There is no amplifier power loss associated with this approach.

sreten.
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Old 14th January 2004, 10:49 PM   #8
boone is offline boone  United States
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Greetings Martin & Sreten,

Thank you for responding to my questions. As my understanding of speaker building is very limited, I wanted to take an established design and put it in a TL to, hopefully, enhance bass/mid bass quality. As I looked at Martin's site I became worried that the crossover would alter Qe/Qt from what is listed for the driver and I would wind up designing a cabinet that was a poor match. Sounds like that is not the case so I'll proceed. Thanks again.

boone
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