Jim Lesurf wrote:

IIUC the above means that the HF signals now go through the same
network as before, but with the LF speaker disconnected from that
network.

Yup. Well, mid and high still go through the network, and bass bypasses it.


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?


2) Removing the impedance of the LF speaker from the network may also
change the response of the network/HF speakers.

Hence in addition to other factors, the changes indicated above may
change the frequency response of the system.

Righto. Very interesting...




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?


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. :-)



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.


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. FWIW, I've always felt that both amps have a similar character to the
sound - ie, pretty clean and flat when used within their available headroom.
By contrast, both amps are substantially better than the Arcam Alpha -
they're much closer to each other than either is to the Arcam.


... 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.


The problem is that the changes you perceive may simply be due to you
altering the overall frequency response. This seems particularly
likely (and your assumption unlikely) if you hear the benefits at
moderate to low sound levels as well as at peak levels.

I haven't listened much at low volumes since I added the active crossover
(rather, only had it on as background music and not been paying attention).
Generally, though, I think it still sounds cleaner.


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. The
overall aim of this is to add headroom to be sure that the amplification
isn't a factor at high volumes, and to possibly improve the fidelity by
removing the passive crossovers. It started with the plan to double up the
bass drivers by converting them to a pair of isobaric enclosures - it seemed
that the easiest way to account for the change in impedance at the bass end,
but not at the mid or top, was to split the amplification up. I then thought
that, if I'm going to bin the passive crossovers at the bass end, then it
might not hurt to bin them altogether.

It would be fair to say that my curiosity has been piqued, and I'm quite
happy to the follow the tri-amping route just to find out what it sounds
like.


--
Wally
www.artbywally.com/FiatPandaRally/index.htm
www.wally.myby.co.uk