Polarity & Phasing

Determining polarities/phasing thru the circuitry of an ESL is not so easy.  Some toroidal power transformers I've used to power ESL's aren't even marked with phasing dots, and even if they were, you couldn't know the phasing coming out of the transformers for sure without connecting a dual-trace oscilloscope and comparing the input and output sine waves.  

My friend and fellow ESL builder Richard, who also has an ESL web page, explains the polarity issue pretty well in this excerpt from his web site: 
ESL Polarity  

In a hybrid ESL, the panel and woofer polarities must match to play together, in phase.  When out-of-phase, the woofer and panel sound waves combine destructively; resulting in reduced volume and a dip in the frequency response.  Conversely, when the woofer and panel are in phase, their respective sound waves combine constructively, resulting in full volume and frequency response.  This being so, we can use the resulting volume effects to audibly check the phasing.  

To check the panel/woofer phasing in my speakers, I used a test tone matching the crossover frequency, and then determined correct phasing by swapping the (-/+) woofer connections and comparing the volumes, as described below. 

With woofer and ESL panel on left and right channels of same amp, and crossover frequency set at 200Hz, correct phasing was determined as follows:  

1)  Downloaded 200Hz tone and imported it into Itunes.  

2)  With test tone playing and left channel (woofer) defeated, used   
     preamp volume control to set panel volume to normal listening 
     level.  (preamp volume control to remain as set for remaining 

3)  With test tone playing and right channel (panel) defeated, and 
     using the gain control on the crossover, set woofer volume to 
     about the same level as the panel.  

4)  With test tone playing and both channels enabled, observed 
     their combined volume. 
5)  Swapped the [+/-] woofer connections to reverse phasing, then 
     compared the resulting volume to the previous volume. 

    After swapping the woofer connections the volume dropped     
    significantly, which indicated the panel and woofer were then 
    out-of-phase.  Swapping the woofer connections a second time 
    restored the higher volume; indicating the panel and woofer were 
    then back in-phase.  

6) Repeated steps 1-5 for the other speaker.