Last December Scott sent me this email with a attached filter modification for my Edingdale GT speakers:
Happy Christmas Colin!
Little something for you attached, just for fun. Optional addition for the Edingdale GT. No changes to the main filter, all exactly the same.
Basically, an impedance & phase correcting circuit strapped across the input terminals before the crossover. The GT is a fairly easy load as-designed. With this -see the attached before & after. Apart from the 1st impedance peak (largely irrelevant), the impedance and electrical phase are to all intents & purposes flat-line, unreactive 'ideal' resistive loads. Voltage source, SET, current source -it doesn't care. System distortion reduced, dynamic contrast enhanced. Two of the caps are large values, but are bipolar electrolytics so fairly cheap; one inductor is a big value, but an iron-core in a small winding gauge so not particularly expensive. Resistors we know about.
Partly inspired by Joe Rasmusen's work but where he's going into other slightly more esoteric fields, here I'm just focussed on a full-on strike at the load & electrical phase to enhance the main dynamic range / low distortion & clean FR objects we've always had with the Edingdale. I've drawn it separately, and as part of the full GT filter for grins. In the latter it looks at first glance like it's in the low pass, but the signal in fact passes this load linearising filter before it actually reaches either the low or high pass. Just a convenient way of drawing it.
I had been following this thread at the time but must admit to not trying to build the add-on filter with other things occupying me.
With the pandemic situation of what seems like continual lockdown and restrictions on movement since March it wasn't until recently that I cobbled together a pair the filters with components I had to hand and built them into a couple wooden boxes in similar fashion I did to the external main filters for these speakers. I did however later install the main XO'ers back inside the speakers so I had the original boxes that I modified for these impedance and phase correction filters.
Having completed the build and spoken to Scott regards what to expect he did advise that the filter would most likely be more suited my 300b SET amp rather than a SS amp. On listing this has proved the case.
At first when I added the filter fed from my 300b's after a initial 15 min listen I wasn't that convinced the change was for the better. There are some big value caps and inductors in the new filter so I decided to leave a CD on continuous play and see if things improved with running-in of the components. A couple hours later and wow what a transformation in sound quality and presence of the music. Big improvements in soundstage and listenability. The sound simply draws one into the music that I found really engaging.
I reckon Nick was and is on to something with the title of this thread. A big thumbs up from me. A pity no one can come round for a A-B comparison of with/without the Impedance/phase compensating filter. It really does work and is something to be given serious consideration for anyone with a multiway speaker run by valve amp, especially if it is a SET.
Scott also sent a before and after simulation plot:
As is plot
and with filter
I can post up the filter if anyone is interested?