In article , Wally
wrote:
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.)

OK. Fair enough. It may be worth noting here that you have bypassed the
original passive networks for reasons I mention 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.

The point to bear in mind that the overall frequency response may be
different now for a combination of reasons:

1) Removal/bypass of the original networks. These may not just have acted
as crossovers. They may have alttered the in-band levels and shapes of
response.

2) The power amps may have different gains.

3) Your 'crossover' points and slopes may be different to before.

4) The effective gains for the active crossover may now be different in the
various bands.

Hence it remains plausible that you might have obtained much the same
effect by using the active crossover simply as a 'tone control' whilst
using the speakers with fewer amps and leaving the passive networks in
place. Indeed, of the passive networks included arrangements to deal with
some detailed aspects of the speakers, the results *might* have been better
in some way. Problem is that we can't tell without a suitable comparison
test.

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.

How are you defining "available power" here? It may not be as simple as
just adding together the ratings for the individual amps.

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.

See above. :-)


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.

That is fair enough from your point of view. You have spent your money, put
in the effort, and obtained a result which you prefer. That seems an
excellent result to me, and clearly justifies the effort. However I am
simply trying to point out for consideration by yourself and others that
there may well be a cheaper and easier way to get similar 'improvements'.
This may save others some cost and effort.

[snip]

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.

One of the things I would recommend people do is to make some 'baseline'
measurements of the in-room response before they start any changes. [1]
This helps to assess any perceived effects and decide what may be the real
reasons for improvements. This in turn aids making decisions, allows the
user to focus on what is worthwhile, and to avoid waste of time or cash!
:-)

[1] In this context it does not matter much if the sound meter is a
cheap/poor one, or if the room has an odd acoustic. The aim is to do a
'before/after' comparison and have the info as a reference.

Slainte,

Jim

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