[James Romeyn]

Currently waiting for ducks to align before ordering an Atma-Sphere S-30 Mk3 OTL amp from Duke, who was nice enough to let me audition (um, translate, get addicted to) the amp in my system.

The only limitation of the OTL design powering my speakers is a highish output impedance.  Owners of speakers that match up well w/ the Atma-Sphere could arguably expect sound quality putting them permanently off the amplifier-search merry-go-round. 

The speakers employed here have a sum total of one impedance peak above the bass range.  Duke made an outboard parallel filter that flattened this peak, resulting in the most-favored sound ever in this room.  (IIRC Duke said he could duplicate this filter for speakers having a sum total one impedance peak above the bass range & possibly as many as two.)

IIRC Duke also mentioned even my current SS amp's performance might improve w/ this filter.  Sound quality was indeed improved: smoother, more ease, less tension, less grain, tighter focus + increased musicality, image/stage improved a lot...etc.  The difference can be heard even in the next room.  (Duke: the filter was connected next to the speaker inputs.  The French speaker's binding post spacing is too great to accomodate the dual bananas; the long filter input leads were replaced by two R.S. 18" alligator test leads twisted together.)   

The associated TRL CDP inverts phase, necessitating the speaker wire polarity to be inverted (at the speaker end).  Duke's outboard parallel impedance filter was auditioned w/ its polarity matching the speaker, then inverted w/ its polarity matching the amp (back & forth several times).  The sound quality was always preferred when the filter polarity matched the speaker & always disfavored matching the amp. 

Would appreciate Duke's comments on the filter polarity experience.  Could sound quality improve as the parallel-only filter burns-in?   


   

[Duke]

Hello ro7939,

Thanks for your post!

Okay, at this point you have more experience with that filter than I do!  I'm not using anything quite like it in my own speakers, and I'm very glad to hear that it's offering an improvement even with your solid state amplifier.   

For those wondering what we are talking about, high output impedance (low damping factor) amplifiers tend to deliver more power into a high impedance load than into a low impedance load, which is just the opposite of a solid state amp.  Ro7939's speaker was "voiced" on a solid state amp, and has a nominal 8-ohm impedance but the impedance peaks up around 20 ohms in the crossover region.   Assuming an output level equal to 1 watt into 8 ohms, a solid state amp will deliver less than 1/2 watt into that impedance peak but the Atma-Sphere OTL amp will deliver maybe 1.5 watts into that same impedance peak.  Remember that the speaker was voiced on a solid state amp - meaning that the frequency response curve is as intended with only 1/2 watt going into the impedance peak.  So with the OTL amp's increased power output into the impedance peak, we have maybe 4 or 5 dB too much energy in that region - giving the speaker a forward sound.  The filter I made for RLC smooths the impedance in that region so that the speaker presents something much closer to a smooth 8-ohm load across the spectrum - well except for the bass impedance peaks, but that's another story.

I don't think I could duplicate the filter for speakers having more than one impedance peak unless I could take measurements as I went along, because that's too many things interacting for me to predict the outcome.  And even with your filter, I think I included a couple of different resistors because I wasn't sure which would work best.

As for the polarity of the filter, again ro7939 you are in new territory for me.  In theory, I don't think the polarity should make a difference... but then either the theory or my understanding of it may well be incomplete.   

I would expect only a minor improvement from "burn in" of the elements in the filter, but some improvement is possible. 

Duke