Technical Discussions of Audio and Loudspeaker Issues
the Audio Engineering
Topic No. 13
I first became fascinated with the possibility of "transforming" the response of an existing loudspeaker system into a "better" response when I read Siegfried Linkwitz' three part article in the very first issues of Speaker Builder magazine in 1980. The article appeared in Speaker Builder issues 2/80, 3/80 and 4/80. On page 16, Figure 25 of issue 4/80 Linkwitz outlines a method of designing a simple 1 op amp circuit that performs two critical changes to the frequency response of the loudspeaker with which it is used.
First it "neutralizes" the response of the existing speaker by precisely "undoing" the 2nd order high-pass response of a specified closed box speaker. Second, the circuit creates a new 2nd order high-pass response as specified by the designer. On the face of it it seems to be a method, "trick" if you will, for changing the response of a closed box to any other response desired. Say you're not happy with your too small speaker with F(sc) of 100 Hz and Q(tc) of 2. You just design up a simple one op amp circuit, connect it in series with your stereo preamp and BOOM your funky little speaker is magically transformed into a 35 Hz Q(tc) = .707 KILLER speaker system. Sound too good to be true? It's not. Read on.
The circuit is really quite simple. Here is what it looks like:
For details on calculating the component values you can see the original article or you can download my Linkwitz Spreadsheet. This is an M/S Excel file which calculates the component values based on your input of the F(sc) and Q(tc) of the EXISTING speaker and the F(sc) and Q(tc) for the NEW RESPONSE. It includes the schematic and design notes.
Here's the link for the spreadsheet download:
How accurate is the Linkwitz Transform?
I've verified my Linkwitz transform designs using various SPICE packages and usually find that the simulation matches the intended design quite nicely. Then as I move from paper to the circuit board I begin to see small errors creep in, mostly by way of small (5 to 10%) errors in component values.
The original design formulas Siegfried Linkwitz gave for calculating the component values (Speaker Builder issue 4/80, p. 16, fig. 25) seem to me to be very precise. The Excel spreadsheet I give away at my site accurately implements the Linkwitz design formulas.
Personally, I've found the largest source of error in the whole process of designing a Linkwitz Transform to be the accurate determination of the starting F(sc) and Q(tc) of the unassisted closed box system. When I actually measure the response achieved by my prototype circuits I see good agreement with the targeted response. The sanity check I use is to first model the closed box in software and print that response. Then I measure the response of the circuit as built.
I then manually plot the new combined response right on the closed box simulation page by taking the dB level for the EQ and adding it to the dB level for the closed box, point by point. If everything is correct you will plot out the intended new target response. Any error will be apparent in the new response. I usually get a good enough match that no further tweaking is necessary. But if you want to tighten up the match the place to start is with your component tolerances. The most common film capacitors are of the 5, 10 or 20% variety. If you can measure your capacitors and resistors you can select your components for 1% or better match to the target values. This usually means carefully creating composite capacitors by combining several smaller values in parallel. I can usually find 1% accurate resistors just by fishing around the 5% parts bin with my Ohm meter. But on occasion I have to gang up resistors to get a target value. You can readily purchase 1% R's if necessary.
All-in-all I think the Linkwitz Transform is a powerful way to customize the response of a closed box loudspeaker.
Here is a link to a related discussion of group delay and the improvement that can be obtained by using a Linkwitz Transform equalizer with your speaker system.
Visit Siegfried's web site at: www.linkwitzlab.com
Spreadsheet Revision History:
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