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In Reply to: RE: Questions posted by Jian on May 11, 2014 at 12:14:03
You are bi-amplifying the speaker. You are using a dbx223 as a hi-pass filter in front of a Wolcott amplifier that drives the treble transformer. The treble transformer seems to go right into the mixer box. That's fine for the "ESL panel".But what the heck are the speakers you have labeled "back ribbons" and "front ribbons"? Are those ESLs or ribbon speakers? If ribbon speakers, I can indeed imagine that the impedance can go down to one ohm, all of which is in parallel with the Wolcott driving the ESLs. So you are using that 8-ohm L pad in SERIES with the "front ribbon" speaker. That will raise the impedance of the front ribbons as seen by the amplifier, but it has no effect on the impedance seen by the amplifier due to the "back ribbons", and it dissipates lots of amplifier power as heat in the Lpad. (Parenthetically, if that is the Sound Lab Lpad, it is very bad, sonically, in my opinion.) Thus the impedance of those back ribbons is dominating the load seen by the Wolcott. (The net impedance seen by the amp will be the sum of the inverted values of the impedances. Therefore, the impedance can never be higher than the lowest value of all those different speakers.) If indeed they are ribbons, I would guess that the impedance can go quite low, down to one ohm or below, even. That is not good for the Wolcott, if you want the best sound quality. In fact, it's not good in any way.
With respect, this set-up could be a lot better. Best of all would be to ditch those extra speakers or to run them completely independently using another amplifier. At the very least, you have no need for that "12db" passive crossover coming after the Wolcott, if the dbx is itself an active crossover.
I am editing this post to say that I did not take into account the fact that the passive crossover should, if it really does afford a very high impedance at frequencies below 12kHz, ameliorate some of the potential problems I cited here. (I think I must have been thinking "12db", as in slope, instead of "12kHz", as in the -3db point of the passive filter.) In my next post, I did take the effect of the passive crossover into account. Here the active dbx crossover and the passive one were not in fact redundant, because they affect very different parts of the spectrum.
Edits: 05/12/14Follow Ups:
I think there is a misunderstanding caused by my diagram. The drawing is not the latest one. I don’t use dbx 223 crossover now. I updated the diagram.
Why I use ribbon tweeters? I am a long time Magneplan speaker owner and feel that SL speakers sound somewhat dark and lack high frequency air, sparkle that I heard from Magneplanar 3.6, or 20.1, or Apogee Ribbons. The ribbon tweeters I use are Fontek ribbon. See link below.
http://www.parts-express.com/fountek-neo-x-30-ribbon-tweeter-black--296-713
I use 4 ribbon tweeters on front and two on the back to simulate the sonic characteristics of a electrostatic panel. I am not sure how the impedance of these ribbon tweeter changes. Of course, it could be better to use third amplifier to drive ribbon tweeters only.
Some one may think it is better off not using additional ribbon tweeters because it may destroy the coherence of full range electrostatic speaker. In my opinion, it is not true as long as choosing the right crossover point and optimal output level of ribbon tweeters.
and no one should tell you how to set it up, least of all me. But apart from the fact that without the dbx you do need a passive crossover, I still see problems here, assuming that THIS diagram does represent your set-up.
First of all, as far as I can see, you have no hi-pass filter at all in front of the SL toroidal treble transformer. Without the dbx, you need one. If the treble sounds closed in and dark, perhaps this is part of the cause. The toroid will be severely compromised by unfiltered low frequencies coming through the Wolcott. This, I am sure, Dr. West would not feel good about.
Second, what I said before still goes; the impedance of the "back ribbons" will dominate the total impedance seen by the Wolcott. So, whatever is the impedance v frequency of the back ribbons will be seen in parallel with everything else, and the net impedance seen by the Wolcott will be lower than that maximum value. I am pretty sure this would compromise the performance of the Wolcott. I guess you are getting away with that ok, if the crossover does function properly as a hi-pass filter starting at 12kHz, so the impedance problems I have outlined only apply at very hf where there is not much energy demand, but the problem with no hi-pass filter in front of the treble transformer of the SL still stands.
I don't mean to sound dictatorial; I am just trying to help. Will would have told you the same thing, maybe in stronger terms.
My advice: Take the ribbons out of this system for a moment and add a 22uF capacitor in series with the input to the SL treble transformer. That's all. Nothing else but Wolcott to 22uF cap to toroid. Remove for a moment the "12db"crossover and the L pad, as well as the ribbons, front and rear. I guess you can continue to drive the bass transformer full range with the Spectron. The SL bass transformer poops out above 2kHz, anyway. Have a listen.
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