Tri-amping, driver time alignment, and carbon fibre cones
 

In article , Stewart Pinkerton
wrote:
On Mon, 16 May 2005 19:58:04 GMT, "Tim Martin"
wrote:


"Stewart Pinkerton" wrote in message
.. .

That's true, but unless you have access to good SPL measuring gear,
you may have some difficulty knowing where 'flat' is unless
everything is matched.

You don't need to be able to measure the SPL, just the signal generated
by a microphone.

Only if you know that the mic response is flat!

The reason I was recommending measurements in this thread was to give
'before' and 'after' comparisions. Hence I was not concerned with an
accurate measurement of the actual in-room acoustic results, but in any
changes. For that the only real requirements is that the mic response
should be stable and give enough output for a clear measurement. Any
departures from flat mic response would be removable as 'common mode'
variations for before/after comparisons.

Slainte,

Jim

--
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Tri-amping, driver time alignment, and carbon fibre cones
 

In article , Tim Martin
wrote:

"Stewart Pinkerton" wrote in message
...

You seem to be missing a critical point here. Three drivers in a box
will not in themselves have flat responses across their working
frequency bands, so the fact that you are supplying a flat signal in
three parts will have very little to do with the FR of the final
speaker.

Sure, but that has nothing to do with whether identical or different
amplifiers are used for each frequency band.

But may have some relevance if there is a change in the slopes of the
crossovers, their turnover points, or the in-band network alterations of
frequency response.

In general passive 'crossovers' in speakers do not just divide the power by
frequency and distribute it to the drivers. They can also alter the
response to cater to some extent for the in-band variations of the drivers.
If you remove this then the reponse will change - even if you maintain the
same crossover points and slopes for the frequency bands.

Slainte,

Jim

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Tri-amping, driver time alignment, and carbon fibre cones
 

In article , Wally
wrote:
Stewart Pinkerton wrote:

You seem to be missing a critical point here. Three drivers in a box
will not in themselves have flat responses across their working
frequency bands,

Are there any that do? Flat to within what spec?

I think the point Stewart is referring to is that the passive networks
which the original speaker engineers employed may have included
arrangements to help flatten/alter the response. This is in addition to the
networks acting as 'crossovers'. If you remove these arrangements the
results will change accordingly.

... so the fact that you are supplying a flat signal in three parts
will have very little to do with the FR of the final speaker.

Check out the crossovers in real high-quality active speakers such as
Meridian or ATC, and you'll find that they are far from flat in their
electrical output.

If this becomes an issue, I have the option of moving the digital EQ to
before the crossover and using it to apply corrections to the full band.
(Or, indeed, adding additional EQ units.)

The snag here is that you don't have 'before' measurements and hence may
not know what changes you have made to the response. As has been
mentioned, trying to assess this by acousics measurements in a domestic
listening room is a nightmare. ;-

Slainte,

Jim

--
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Tri-amping, driver time alignment, and carbon fibre cones
 

"Stewart Pinkerton" wrote in message
...

True, but are you saying that you don't care what happens in the pass
band? You're not going to do any equalisation? You think that passive
crossovers *only* perform frequency division?

I said or suggested nothing like that.

The question we are discussing is whether, in a tri-amped speaker, all the
amplifiers should be the same.

You said they should be, and IIRC said it was otherwise difficult to get a
flat response. I've pointed out that whether or not the amplifiers are the
same makes no difference; with an uncalibrated microphone and test signals
within the frequency range of the LF and mid-range speakers, one can easily
adjust the gain so that the test signal is generating the same sound level
from the bass and mid-range; and one can use the same process to do the
same with mid-range and HF drivers.

Equalisation is a separate issue. It has nothing to do with the issue of
whether the amplifiers need to be the same ... that is, in a tri-amped
system, the process of equalisation will not be affected by whether the
amplifiers are the same. And in the context of equalisation, your comments
about the SPL-measuring equipment needed apply equally regardless of whether
all the amplifiers are the same.

Tim

Tri-amping, driver time alignment, and carbon fibre cones
 

"Bob Latham" wrote in message
...

As far as I understand it there is only one frequency at which two drive
units will radiate the same acoustic energy and this is at the exact cross
over frequency. Above or below this, one or the other driver should be
significantly louder.

That's with the crossover operating. When you're setting up the amplifiers
to match driver outputs, you don't have the crossover operating.

Tim

Tri-amping, driver time alignment, and carbon fibre cones
 

"Bob Latham" wrote in message
...

Oh, I see. Yes, you could do that. It would only be useful though if the
Xover was perfectly flat in each pass band and the same for the drive
units. In the reality drive units are not flat response objects and
consequently xovers have to compensate for this and so they don't have
flat outputs either.

No, the purpose of this particular exercise isn't to obtain flat frequency
response from the speaker - that's a different activity. The purpose of
this activity is simply to turn the volume knob on one of the two different
amplifiers, so that both amplifier/driver combinations will deliver the same
sound level from a given single-frequency test signal within the operating
range of both drivers.

An example is the KEF B110. Great driver but not flat over its working
range. Xovers should feed it around 6db more at 300Hz than at 2000Hz if
you don't do this, the acoustic output will be all over the place. Even
that depends on the box its mounted in.

Sure, but that's nothing to do with whether the same amplifier is used for a
different driver used with the KEF B110.. You have exactly the same problem
obtaining a flat response from the driver, regardless of whether you bi-amp
it, and whether you use the same or different amplifiers when bi-amping.

And of course you can use exactly the same passive EQ technique with the
driver, regardless of whether it's being bi-amped.

Tim

Tri-amping, driver time alignment, and carbon fibre cones
 

"Bob Latham" wrote in message
...

"Yes, that can be done but it gets you nowhere. Stick to Kef units, say you
set the B110 and B139 to radiate the same volume at 300Hz. Move the
frequency up to 400Hz with no crossover and they would be different again
so what is the point?"

The point is simply to adjust the gain when using non-identical amplifiers.
It's very simple to do. You still need to do whatever it was you were going
to do for EQ if you were using two identical amplifiers.

Tim

Tri-amping, driver time alignment, and carbon fibre cones
 

In article , Bob Latham
wrote:
In article , Tim Martin
wrote:

[Snip]

I've pointed out that whether or not the amplifiers are the same makes
no difference; with an uncalibrated microphone and test signals
within the frequency range of the LF and mid-range speakers, one can
easily adjust the gain so that the test signal is generating the same
sound level from the bass and mid-range; and one can use the same
process to do the same with mid-range and HF drivers.

As far as I understand it there is only one frequency at which two drive
units will radiate the same acoustic energy and this is at the exact
cross over frequency. Above or below this, one or the other driver
should be significantly louder.

Tim has already pointed out on of the potential flaws in the above, so I'll
comment on the other one. :-)

There is a problem with your "significantly louder" as it seems to imply
that any combined effect only occurs at one frequency, or in a very narrow
range. This often is not the case.

Consider a frequency were we have moved away (in frequency) from the
equal-output frequency by enough that one speaker unit is producing 10dB
less than the other. Depending on the relative phases of the two units in
question this can alter the combined result over a range of about +2 dB to
-3 dB. Thus the region where one unit is not "significantly" louder than
the other may be rather wider than you may assume. :-)

Slainte,

Jim

--
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Tri-amping, driver time alignment, and carbon fibre cones
 

In article , Bob Latham
wrote:
In article , Tim Martin
wrote:

"Bob Latham" wrote in message
...

As far as I understand it there is only one frequency at which two
drive units will radiate the same acoustic energy and this is at the
exact cross over frequency. Above or below this, one or the other
driver should be significantly louder.

That's with the crossover operating. When you're setting up the
amplifiers to match driver outputs, you don't have the crossover
operating.

Oh, I see. Yes, you could do that. It would only be useful though if the
Xover was perfectly flat in each pass band and the same for the drive
units. In the reality drive units are not flat response objects and
consequently xovers have to compensate for this and so they don't have
flat outputs either.

Also bear in mind that the passive 'crossovers' also connect to the speaker
units which tend to have frequency dependent impedances. This means the
interaction between the speaker impedance and that of the network is part
of the 'filtering'. By connecting the speaker unit directly to an amp you
change this interaction as well as the 'obvious' effects of the filter as
seen on a crossover circuit diagram.

Slainte,

Jim

--
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Tri-amping, driver time alignment, and carbon fibre cones
 

Jim Lesurf wrote:

I think the point Stewart is referring to is that the passive networks
which the original speaker engineers employed may have included
arrangements to help flatten/alter the response. This is in addition
to the networks acting as 'crossovers'. If you remove these
arrangements the results will change accordingly.

I realise this.


If this becomes an issue, I have the option of moving the digital EQ
to before the crossover and using it to apply corrections to the
full band. (Or, indeed, adding additional EQ units.)

The snag here is that you don't have 'before' measurements

Why are the 'before' measurements so important? If my previous speakers were
crappy little 2" computer jobbies, and I suddenly scored the pile of kit I
have now, unheard and with no history, would it suddenly become impossible
to 'sort' the sound?


and hence
may not know what changes you have made to the response.

I recently acquired an SPL meter, which I mentioned in here a few weeks ago.
The idea is to measure the response as it is now, and then compensate as
required.


As has been
mentioned, trying to assess this by acousics measurements in a
domestic listening room is a nightmare. ;-

But the domestic listening room is where they'll be listened to - isn't
accounting for that part of the whole deal anyway?


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