This is the fully installed LT correction circuit board in our A350 amps. The circuit board has a toggle switch that can bypass the LT circuit. It is only available when you order with A350 amp. This option is only available with our drivers. We no longer provide fully installed LT for 3rd party drivers. For 3rd party drivers, please use the half installed boards.

	Description	Price
	Full assembled LT board for A370 (only available for our drivers)	$40
	Full assembled LT board and a A370SE nonservo Amp	$259

The best way to save money on LT boards with 3rd party drivers is to do-it-yourself. There are two steps required. The first step is to solder all the components using the schematic provided at the bottom of this page. The second step is to install the connector to the pre-amp board of a A350 amp. This requires some soldering skills. The complete procedure of second step is in here.

	Description	Price
	LT board + connector + switch + 1 Op amp	$20
	A370SE 12db nonservo amp plus the above LT half-assembled board	$249
	A370PEQ 12/24db nonservo amp plus the above LT half-assembled board	$289
	A370XLR 12/24db nonservo amp plus the above LT half-assembled board	$289

What is Linkwitz transform (LT) circuit?

The origin of Linkwitz Transform circuit dates back as early as 1980 when Siegfried Linkwitz published the article "A three-Enclosure Loudspeaker System" in Speaker Builder magazine (now AudioXpress). One of the objective was to extend the frequency response of a sealed box subwoofer module by altering its corner frequency and the Q value to a new set of values. It not only extends the frequency response, but also changes the Q value. There are several sites which cover this topic.

The Case for Linkwitz Transform Correction Circuit

If you have already decided to make a sealed box sub and have done some research, you may have discovered the Thiele/Small parameters impose restrictions. The first restriction is enclosure size. Most likely you will be told a larger box gives better bass extension. Normally, a small enclosure limits bass extension. Second, the interdependency of Q value on the bass extension vs enclosure size. To get lower Q for better musical sound, you need to build a literally huge box. In some cases, a 4 cubic foot box is required to get a low Q of 0.5. For many this is unacceptable. Not only it is difficult to build and move, it is even more difficult to make it inert. Third, the inflexibility of amplifier selection. Often the result is that either the amplifier doesn’t have sufficient power, or the excursion limits of the driver is exceeded.

Linkwitz circuit can address these problems, however, not by the circuit itself. First, to effectively use an LT circuit, the amp should be large enough to provide the required output headroom. Second, the amp should have a flexible extension control to allow one to tradeoff bass extension vs maximum SPL level. In another words, our A350 amps that have the adjustable extension filter settings are perfect for this task (as shown below).

The design process of LT-based subs is also different. After you pick an A350 to start with, one can calculate the enclosure size. The enclosure size should be such that the driver will not exceed the excursion limit. For most common drivers with a A350 amp, this will be around 2 cu ft internal volume. One can go smaller than that, but keep in mind that a smaller enclosure delivers less SPL output. After determining the driver and enclosure size, then we can go about setting the LT circuit board. When everything is finished, you can play with the extension setting to see which of the frequency and damping setting suit you best. In some cases, you may find best settings are different for music and movie. You will be the one to judge.

In short, we think that building a sealed box sub based on T/S parameters (for sealed box in particular) should be a thing of the past. Many manufacturers of sealed box subs are using LT correction circuit in their products without explicitly letting you know. We really recommend you consider this route to make your project more rewarding. The experience that one acquires in building our LT combo kit is equal to that one can acquire in building several conventional subs combined. We would certainly not recommend you go to the latter route.

To give you a real-world example using our drivers, the maximum SPL output from a 2 cu ft internal volume mated with our A350 amps (350WRMS) is equal or higher than the same driver in a 2.6 cu ft internal volume box driven by a 250WRMS amp. SPL outputs are equal at frequency below 20 Hz, and above 30 Hz, the A350 in 2 cu ft box has 3db higher max SPL output. Which one would you rather to choose? A more detailed explanation why an LT-based sub is superior can be found in our discussion of excursion utilization.

The above picture is the close-up shot of component side of LT circuit board. The circuit below is the for our LT circuit board. R1-R3 and C1-3 are components that determine what the original fs and Q values and what the new fs and Q values are. The second op amp is to keep the overall phase non-inverting. The component CSH and RSH can be used to implement a shelf-up EQ. This is very useful for subwoofer with large voice coil inductance. However, for most subwoofer drivers, there is no need to install CSH and RSH.

There are complex formulae to calculate the required component values. One can come up with these values from other web sites, or for our drivers, we do it for you. The procedure that we recommend is as follows. Start with C2 being a commonly used value, such as 0.1uF or 0.056uF. Calculate all R1-R3 values and pick the closest values that you can find. C1 and C3 values may not be commonly used values. In this case, we recommend parallel 2 caps to get the desired value. That is exactly why on the LT board we reserved 2 parallel caps for C1 and C3, respectively. The board itself is only 1-3/8"(H) x 2-1/4"(D). So we recommend small footprint components. Cp are power supply filter caps. I often use stacked 0.1u caps for them. Rf1 and Rf2 can be any value from 10k to 200k as long as they are equal. I normally pick one of the values from R1-R3. However, if one wants to implement the shelving function, then Rf1 and Rf2 need to be carefully calculated. After soldering, the board should look like as follows:

These components were purchased from Digikey and I will strongly recommend them (less than $25 order has a $5 handling charge though). Although, in the above picture I use 1/4w metal film resistors, 1/8w carbon/metal film resistors will also work. The solder pads for caps provides lead spacing for either 0.2" (5.0mm) or 0.3" (7.5mm) for C1, C2, C3. For Cp it is 5mm. One of C1 has spacing of 0.3" (7.5mm) and 0.4" (10mm) reserved for large cap value. After all the soldering has been done, put the board back with soldering side facing up. In the following are some examples:

Example component value table

The response has a Q value of 1.1 at 40 Hz based on curve fitting (one can also check the 90 degree cross point). In addition, the f3 is slightly above 30 Hz. With LT circuit, one can either equalized it to 19 Hz with Q value of 0.7 or 14 Hz with Q value of 0.5.

How about extension filter in A350?

When you order LT module from our bundle kit, we will also change the Q value of extension filter in A350. The new Q values are: 0.5 for high damping, 0.7 for mid damping, and 0.95 for low damping. The main reason is with LT module, there is no need for Q value larger than 0.95. The high damping should give a detailed bass sound. The low damping setting is recommended for high playback volume and is only recommended for 28 Hz extension filter setting.