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Romy the Cat
Boston, MA
Posts 10,159
Joined on 05-28-2004
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Post #:
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218
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Post ID:
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23426
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Reply to:
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23422
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This weekend wife told me that I should not do nothing else
but to fix my playback and this was exactly what I wanted to do. Yesterday I took
good 14 hours to play with it, moving boxes, trying to find a good
configuration, making measurements, thinking about results. I think I have fond
a configuration that give to me more or less acceptable sound, actually to my
surprise. The surprise did not come from the fact the sound is good, in fact I
did not listen it critically but from the message that I got from all of it
that my “small blood” solution can work. If you do not build playback yourself and did
not play with all of it then you might not be interested for the rest. I post
it for the guys who experiment with it and for myself, to organize some of my thoughts
and to keep sanity.
So, I entered the fight with very well defined objectives. I
do not want to use any powerful SS amp to drive my playback, I want just one
Milq, one set of wires and one power switch. I want it to be more or less good enough
to let me to hang (a few years?) until I go to my new midbass horn project. I
would like to keep it as time-aligned as possible. I would like to have more or less flat frequency
response with individual channels decay very gradually and waterflow more or
less consistent, with no large timing-frequency gaps. I would like to have
first order filtration if possible. I would like it to sound if not good but acceptable.
I would like do not spend more money, use whatever I have and to finish within 2
days.
Well, I spend all yesterday with my right channel, the one
that has no corner and the more complex one.
Here is what I end up with. I use a single (not pair) Vitavox 15 driver
in sealed box and midbass channel and line array of 6 pimpled Scanspeak as LF.
I drive everything from Milq LF and … it kind of works. Here is the crossover I
end up with, it most likely will be changing more.
The response is not as smooth as I use to but all together
it is not as bad as I the room response to the topology of the acoustic system
I have. I have more of less consistent response down to 30Hz and then it decays
very nicely with a good -4dB bump and 24Hz. The LF tower of 5.3R impedance is
sitting at 45Hz lowpass. So, it doe the upper region being on low pass transition
slope. The midbass channel with 15R impedance is sitting at 130Hz low pass to
meet the upper bass horns. It is in a small 18x18x18 seals box and it drops at
50Hz like a rock. Generally it does not need to be high passed as the acoustic suspension
of the small sealed box will hold the driver tightly but my concern was about
the amp loading. It has not a typical for me over 10:1 output transformer but
5.3:1. It works very fine with my Scanspeak but at LF the Scanspeak begin to see
the Vitavox and overall load to the amp drops and Milq might run in current hanger.
It would be nice to have one more Milq channels but I do not have one. A consideration
would be to have a nice SS follower after the Milq LF transformer, specifically
to drive the Scanspeak. This would decouple the Scanspeaks from the Milq and
would let midbass to run very free. I do not see any ready to go SS followers available
for sale. I do not want to build it and I do not want to vandalize my SS amps
to covert thermo followers. So, I end up with filtering out LF from midbass in
order to help Milq a bit. The surprise I got that as I was changing the filtering capacitor, moving the Vitavox ‘s
high pass I got a very nice boost at the lower knee of my woofer tower. The amassing
thins that this LF boos as I have now is +2.5dB at 20Hz compare to midbass is
not connected or compare the Vitavox is not high passed! I have no idea why it
does it and it can’t be explained by Milq loading more and yield more power as
with no filtering cap in Vitavox the load should be even more. I asked Dima and
he thinks that the coil and filter former some kind of response chain that acts
like Zobel and rise impedance more than bypass and overcorrect the amp gain. Nevertheless,
the extra LF boost at 20Hz is very welcoming thing and it look like the boost
goes away at 30Hz. As I start listening it I will see how it sound but it is in
sub 30Hz, so it might be work out very nicely.
Now, is the elephant in the room: the midbass channels and what
I did with it? I named it phase joker. To position it at time-aligned position
was not possible as any of my experiment with crossovers did not allow me to
have horrible room modes. I tried zillion positions in the room, doe one, two
and three Vitavox cubes, with all possible polarity, all possible crossovers.
Nothing worked and if I serve the dams of upper bass I always get fucked by
lower bass. I want the slopes to be in phase and to be summing but I was not
able to find this configuration. Then I decided to screw it and stop worry
about timing and I flipped the midbass Vitavox at 90 degree. I was able to find a good position, in fact
near to time-alignment when I have summing boost on both LF and Upper bass with
midbass filing the amplitude gap very nicely.
I am setting today the second channel the same configuration
and will do some listening. I am very pleased that I was able to find this setting
but a few things need to be said. The idea of using my narrow bandwidth midbass
as “distributed, strategically injected phase anomaly”
might be considered as elegant in my situation, however from my perspective it
is a huge white flag. What I did was polluting my listening space with phase
junk and this is not something that I would advise as sane playback building techniques.
The way how the problem with my room shall be dealt if I have time should be
very different. I would need to come up with a topology of midbass that would
be able to work along with the rest of my system with coherent phase and time-alignment.
I presume that it will be a midbass horn where the location and size of the mouth
will overcome the room mode but it will be a project for another day.
Romy The Cat
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
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