Jim Lesurf wrote:

Unfortunately, I didn't have the meter before the changes started - I
got it after I split the bass from the rest and added the EQ.

In that case the following experiment would be of interest.

1) Make a note of the settings at present and the measured (acoustic)
frequency response.

2) Alter the arrangement by using a set of series resisors to sum
together the outputs from the xover, then run the combined result
through just one power amp. Then run the output of the power amp to
the speakers in parallel.

If this involves running the o/p through the passive xovers, then that would
be a pain - I did a bunch of soldering to sort out the connections, and I'm
not keen on pulling it all apart again. (See closing comments below.)


This would help establish if the differences you hear are due to the
system simply acting as a 'tone control' and have little or nothing
to do with using more than one power amp. I'd be interested to know
what you discovered...

The most significant changes are better bass, cleaner volume, and better
dynamics. The EQ is only on the bass channel, so, aside from removing the
passive crossovers, I don't see how there can be much 'tone control' effect
on the mid or top (unless the active crossover isn't as flat as it's
purported to be). I have to assume that the amps are as flat as one can
reasonably expect.

Adding the bass EQ made a huge difference over the interim bi-amped set up
(where the bass was on one amp, and the mid/top was on the other with the
passive xovers still in place). The EQ on the bass hasn't brought a subtle
change - it's order of magnitude stuff.

Less gross is the change in dynamics and volume, but I'm satisfied that this
has definitely improved - when I first replaced the 20W valve amp with the
Cyrus 2, there was a clear increase in volume and, to a lesser extent,
dynamics. However, I was a little disappointed that the quality still
dropped markedly when I played it up loud. I wasn't sure if this was the amp
running out of puff, or the speakers starting to distort (50Wpc was the most
power I'd used with these speakers). Adding the second Cyrus and bringing
the valve amp back into the set up has given me available power of something
like 120Wpc, with no indication of the straining that I was previously
hearing at higher volumes - aside from that previously-mentioned tendency
for the mid to be a bit shouty at times.

So, with regard to an overall 'tone control' effect, the only real changes I
can think of are adding the active crossover and removing the passive ones.
My feeling is that the nett change due to this is likely to be rather more
subtle than obvious. Certainly, for now, my awareness/attention is focussed
on the much improved bass response, the cleaner sound at volume, and the
better dynamics. I feel that I'll have to live with these changes for quite
a while before I can get into the more subtle stuff.

To be honest, I'm not particularly interested in trying to establish how
much difference there is between the system as it stands now, and how it was
with all-passive xover and the single Cyrus amp - it was better than the
valve amp on its own, but was still ultimately less satisfying than it
could/should have been.

Looking back into the mists of time, this all started with the idea of
building isobaric subs out of the collection of KEF B139 drivers that I have
kicking around - since this would change the nominal impedance of the bass
end to 4 instead of 8 ohms, I felt that bi-amping was the way to go. (Some
people suggested tri-amping at the time, but active crossovers were too
expensive for me then - I had planned to bi-amp and build some sort of
active crossover since I had a scope and other kit available.) I'm now less
skint, the Behringer crossover is much cheaper than other kit was back then,
and I'm now interested in seeing what can be done with tri-amping for no
real reason other than the hell of it. First indications are positive.

To my mind, the system as it stands is the baseline - for the foreseeable
future, I'm essentially committed to a tri-amped set up, and the idea is to
try and make it better without spending a fortune. While I was initially
less than conducive to using measurements, I would say that I'm more willing
to sing from that particular hymn sheet after seeing driver free air
resonance graphically displayed on the scope, and having heard the
improvement to the bass that came about from using the SPL meter (compared
to my MkI lug-'ole attempts). With this in mind, I'll be taking a set of
measurements of the overall frequency response, such that the effect of
subsequent changes can be analysed objectively.


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