In article , Wally
wrote:
Jim Lesurf wrote:
[snip]


Two comments on that:

1) If the LF speaker was previously connected via that network, it may
now be producing a different frequency response than before.

Okay. Maybe that would explain why the bass seemed a bit muddy (and a
bit too loud) when the active crossover was set to 400Hz. Would it be
fair to say that the low-end roll off (70Hz downwards) would still be
more influenced by the cabinet/room?

Hard to be sure. However the above seems plausible. In general the LF
roll-away at frequencies below 50-100 Hz region does tend to be dominated
by how easily the speaker can produce pressure variations in the room.
Ideally, an infinite baffle speaker in an airtight room with no leaks and
firm walls would allow a response down to dc.! :-) In practice, though, the
LF response will depend on how much air pressure change the room will
experience for a given cone movement once you are below the lowest room
resonance.

FWIW my experience here is limited, but it is that there are two problems
which may combine if you are unlucky. One is the basic room pressure
resonance which will probably be somewhere below 100Hz. The other is the
'mechanical' resonance of the speaker cone mass/compliance which will also
be somewhere below 100Hz. If the two are at a similar frequency they may
produce a particularly severe 'boom', and the bass may fall away rapidly
below this. Which is 'more significant' will depend on the details of your
situation.

With the sub I use in my living room I can measure a change in the peak
frequency and level if I open/close the room door.

IIUC commercial subwoofers employ an amp with a correction network to try
and iron out some of the speaker effects and give a more extended response.
You'd need to experiment to determine what was required for a given system.


Not sure what might be meant by 'conflict'.

I mean a mismatch in the roll-off of the B110 in its sub-enclosure,
compared with that imposed on the B139 by the active filter (the B110 is
set up as a mid driver and the IB enclosure is quite small - I don't
know what the resonant frequency of the midrange system is, but I
suspect it's rather higher than that of the driver itself, which is
38Hz). Aren't the slopes supposed to be similar in terms of dB/octave?

Not sure. The amount of output (sensitivity) will vary in a different way
to the actual drive levels. With HF speakers you need to avoid LF as the
excursions might cause problems. With LF speakers you avoid HF as it can
excite cone resonances. So the degree of attenuation required depends on
different factors as well. However I'm not a speaker designer so don't know
much about the details of practical designs.


However the B110 and its enclosure presumable have a given frequency
response and set of resonances, etc. Just driving directly from a flat
amp with a LP roll-off may not deal with this. Hard to say as I'm
afraid I don't know the specifics of the actual arrangement. Rolling
away the HF will avoid things like cone breakup resonances. However in
the region where both the LF and HF speakers are active their outputs
will combine and this will also have effects that will be quite
specific to the individual arrangement.

Not sure we're talking about the same thing - the B110 is acting as a
midrange driver. Its LF response is rolled off both by the HP portion of
the passive crossover and the size of its sub-enclosure. The B139 is
being driven by a flat amp with a LP filter before it. The LF signal to
the B110 is also being restricted by the bass/mid crossover frequency in
the Behringer. However, switching off the bass amp and tweaking the
active xover freq reveals that it has a little effect on the B110 below
about 250-300Hz; from about 200Hz downward is only very slight. In other
words, the now-redundant bass/mid parts of the passive crossover could
be causing the roll-off on the B110, or its sub-enclosure, or a
combnation of both. What I'm wondering is, would it be a good idea to
remove the passive xover components and have only the sub-enclosure
acting to roll off the B110s bass response? I appreciate that, without
knowing the specifics, this might not be answerable. :-)

Afraid I have to go along with your final comments. :-) I don't know
enough to say for sure.

TBH though, personally I would not embark on speaker
development/design without having access to some kit to measure the
response, etc. There are simply too many variables and potential
problems which I'd find too hard to disentagle simply 'by ear'.

The way I see it, I can always undo the changes I make. If I don't like
a change, I can revert to the previous configuration.

Yes. The snag, though, is the phenomenon I tend to call "lost in space".
This is where there are many variables you can fiddle with, or which alter
out of your control. This means as you change some things you sometimes
don't know what is really happening, or if some other slight change would
be desirable, etc. This is worse when working entirely 'by ear' as your
reactions will vary according to how you feel and your 'recent'
experiences, and affected by the unreliability of memory.

The problem here is not just that the two power amps may have
different gains. They may also have different output impedances. In
particular if the valve amp has a 'high' output impedance this will
produce a different frequency response to an amp with a 'low' output
impedance.

The spec for the Cyrus 2 quotes 0.08 ohms, can't find a number for the
valve amp.

Afraid I don't know any figures for the valve amp you mentioned. However if
you look in reviews, values when quoted are often of the order of 0.5 Ohms
or more. i.e. somewhat larger than the value for your Cyrus,


... I'm assuming that the feeling of better clarity and improved bass
is down to having more power available, and perhaps splitting bass
and mid/top to separate amps.

Alas, I have my doubts that the reason you assume is the correct one.
Unless the amps were seriously current limiting I doubt that the
biamping will have had very much effect on the power levels available.
Could you hear previously clear signs of clipping?

When each was used to amplify the full range, the Cyrus was louder than
the valve amp, and stayed clean for longer. However, they both reached a
point where I felt the sound became unpleasant to listen to. Whether or
not that was clipping, I'm not sure - to me, that term denotes what you
get when a guitar amp is turned up to the point where it sounds like a
chainsaw. I don't let the hifi get anything like that bad, so I'm not
sure it's clipping that I'm hearing, or some other effect. I sometmes
wonder if the speakers themselves were being overloaded, but I don't
think this would be likely in the case of the valve amp - the Cyrus can
easily match it in terms of in-room volume, and stays cleaner as it does
so.

Hard to say, but if the sound becomes unpleasant at high levels then I'd
expect that to be due to some mix of:

1) Amp clipping (voltage) or current limiting

2) Speaker distortions

3) Amp distortion levels rising with output levels

With 'traditional' valve designs (1) and (3) may be factors, but can't
really say without more details.

[snip]

If the results sound better to you, that is fair enough as you have a
result you prefer. For that your lug-ole is king.

However it means that if that *is* the reason, then similar results
might be obtained by much cheaper and easier means. This may not
matter to you at this point, but if it were the case, knowing this
might save others time and money. :-)

Maybe that depends on what is causing the change in frequency response.

Yes. Indeed, there are some good reasons for feeling that an active
crossover and 'direct drive' of the speaker units can be optimum. But this
then may well require the crossover to also do some other alterations to
cater for any inherent variations in sensitivity with frequency of the
speaker units. However it is difficult to say without more specific
measured data.

My concern here is that people may buy relatively expensive active
crossovers and bi- or tru-amp, but then get result which could have been
obtained more easily and cheaply with a slightly better amp and a tweaked
speaker or room acoustic. Hence I am inclined to be wary of this without
suitable measurements, etc.

Slainte,

Jim

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