I asked Bobby about the impedance boxes awhile back and he said I didn't need them.
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Speltz Zeros are effective in some implementations and deleterious in others. Multiplying impedance is of no help with Merlins, as by design these speakers have a high impedance curve. I experimented with Zeros & VSM-MX to find out if a 3wpc SET amp would be complementary. It was not. Yet the Zeros had previously worked well to mate this low-powered SET to Wilson Watt/Puppies. Wilsons are 4R speakers with difficult dips to 2R. They are also several db more sensitive than the Merlins. The Zeros make Wilsons look like high-efficiency 16R speakers, whereas with Zeros the Merlins look like medium-efficiency speakers with anomolous 20-40R impedance. Bottom line is that Zeros work well when there is a mismatch to resolve.
with the Speltz you can increase the impedance to your heats desire and things will tighten up as you go however there is a sweet spot where you do not want to go higher as the sound becomes dry and too lean, you can definetly ruin the sound with too high an impedance. Also you are near the 'sweet spot' impedance of 12 - 14 ohms according to some of the posts on his website and in my own experience and you will retain the sound of his wire instead of your current wire - weather you will like that part of it or not remains to be seen.
If the ZERO is not loaded properly it will ring (adding harshness and obscuring detail) like any other inductive device. Ideally the speaker load should be 4,3 or 2 ohms, and if loaded that way the input will be 16 ohms. Other combinations are possible, but *in general* the ZERO is usually not be particularly helpful on a speaker that is already 6-8 ohms.
IMO you are far better off trying different speaker cables on the Merlins rather than ZEROs. If you had Magnaplanars it would be an entirely different story!