Somewhere on teh intarwebs Jim Lesurf wrote:
In article , ~misfit~
wrote:
Hi. I've been reading here a while but not posting so much. It's
mostly above me but I'm trying to learn....

I have a problem, forgive me if this is the wrong forum in which to
ask for help. However I feel that I'm quite a way away from being
able to sort it out myself (although I do have a budget LCR meter
that just arrived from Hong Kong).


I would very much like to put a simple low-pass filter on the woofer,
with the cut-off point the same (or as close as possible) as that of
the high-pass circuit on the tweeter. However, other than the woofer
being 3.6 Ohm and the tweeter being 8 Ohm (boxes rated at 4 Ohm) I
know nothing about them. They're small Philips units, quasi-ribbon
tweeters, well finished with a good rose-wood veneer and I bought
them at auction.

Ideally, you'd equip yourself and be able to do some basic
measurements.

Indeed. I'm trying to find downloads of any good freeware that I can use.
However, circumstances have left me an invalid, having lost my home and
business, with only welfare as an income, and a few remnants of my audio
gear. So, as I have little to no spending money I'm trying to get the best
listening experience that I can from what I have. Hence the question and the
attempt at learning as much as I can.

Sadly opioid painkillers rob me of a large chunk of my ability to learn, to
form memories. I used to learn as easy as pie. I graduated two years younger
than my class-mates... Now I feel like a dunce sometimes. :-/

But you could experiment 'by ear' if willing to spend some time doing
so. What I can't say is if this would be worth the effort instead of
replacing the speakers entirely.

Not really an option, see above.

Experiments may not yield a
satisfactory result if the speaker units aren't good enough to be
'improved' by such tweaks. Or if a simple crossover isn't up to the
job. Lacking measurements or experience, trying what you propose may
involve a lot of trial and error without ending up getting a good
return!

Frankly I was rather hoping there was maybe a software tool that I could
enter the values of what's already in the boxes, the drivers and high-pass
circuit and get some idea of where the high-pass point is and what
components I would need to build a low-pass for the woofer that works at the
same point.

Does you have a meter that gives a reliable AC voltage measurement
that doesn't vary much with frequency across the band from about 50Hz
up to a few kHz? If so, it could help you experiment. (I'm not sure
if your "LCR meter" also measures voltages.)

I have a fairly cheap DMM other than the LCR meter which measures AC but
have never discovered if it does what you ask, nor do I know how to test it
to find out. All I know about it is that it cost $30 and is rated for mains
electricity work. It seems quite good, with a 32-position rotary dial in the
middle, AC range has 5 positions from 200mV to 750V.

The problem with an LCR meter is that it may just do this at one
frequency, and not give a useful results for L/C/R for something that
has complex behaviour. Going that route you may really need to be
able to check the impedance at various frequencies.

I mainly bought the LCR meter so that I could measure inductors and
capacitors so that I might a) re-use components that I have from older
systems or b) as you mention below, maybe hand-wind my own inductors. My
existing DMM didn't measure inductance and has a lower range of capacitance.

Assuming you can measure AC voltages...

To test the meter, you could connect it as a voltmeter to measure the
output of your power amp and play some test tones from a CD, say, see
if the reading for the voltage stays steady as you change frequency.
However note that a cheap multimeter may only give sensible AC
voltage readings for 50-60Hz.

Ok, thanks. Seems I'll be searching for a download of either a test-tone CD
or, preferably, software that I can run on my second laptop that will output
variable tones.

By ear or by meter, the simple way to experiment is to try putting
something like an inductor in series with the woofer. Lacking
measurements you'd have to guess what may be the right value for
inductance. Then listen (and/or measure the effect on the ac voltage
at the woofer) to decide what effect it has had. Use that to decide
if a bigger or smaller inductance might be worth trying. Repeat until
happy or exhausted. :-)

Yes, that sounds to be roughly what I tought I might have to do. As they
are, the X-over is built onto a small PCB connected to the speaker terminal
block. I don't want to keep pulling the things to bits so might open them
and run the wires for the woofer out the reflex port. That way, if I do get
exhausted (a real possibility with my back pain) I can simply wire them back
to the terminals along with the amp output until next time. :-)

Lacking measurements you can 'guess' the speaker impedance is around
10 Ohms, then choose an inductor value on that basis.

As I mentioned, the woofer is in fact rated 3.6 Ohms. At least that's what's
written on it, with 8 Ohms written on the ribbons, and the sticker on the
back says nom. imp. 4 Ohms.

Chances are,
the guess will be wrong and the actual impedance is higher. However
you can then cheaply try connecting a resistor *across* the woofer
terminals to pull down the impedance at the roll-away point. Then
experiment with resistors - cheaper than trying different inductors.
But avoid choosing a value that is too low as the amp may not like
the loading.

Ok, thanks. I hope that the amp's protection circuitry's working fine as, if
it's not and it's needed I'm not in a position to replace it.

For real use you'd need an inductor that can handle high enough
currents and with a very low series resistance. But to save cost you
could experiment first with cheaper low-current inductors. Or even
'wind your own'.

I've been grabbing inductors out of scrapped computer / server
power-supplies lately (so many I didn't...). They're mostly toroidial and I
didn't think they'd be of use (other than a source of wire) but then I read
about a speaker company using toroidial inductors on their woofers, like
it's a great thing... The trouble is the highest indusctance I have is only
0.4mH but it's 1.3mm diameter wire.

Beyond that, I suspect others can give you more detailed advice, and
may have experience of speaking building, etc.

I was hoping so, hence holding off on replying. However it seems not.

Thanks Jim. I'd pour you a dram or two of of Uigeadial if only I still had
some...
--
Shaun.

"Humans will have advanced a long, long, way when religious belief has a
cozy little classification in the DSM."
David Melville (in r.a.s.f1)