Centre, speaker - twin drivers, use one enclosure or two separate enclosures side by side?
 

On Thu, 11 Dec 2014 08:11:05 -0000, "Woody"
wrote:

"Peter Chant" wrote in message
...
On 12/10/2014 07:46 AM, gregz wrote:

You could try a little series resistance on the woofers
to push up the Qts
and be more suitable for closed box?


Interesting. This might actually reduce the influence of
the amp and
cables. I have one book which I cannot find which was
obsessed with
measuring the amp impedance and matching the speaker to
the amp.


I can understand that approach with a valve amp where there
is an output transformer involved, but matching with a
transistor amp? Eh? The main factor with a transistor amp is
having a power supply that could deliver the (often short
term) currents needed. JLH proved it could be done easily by
having a regulated PSU. In the '80s I built a dual mono
power MOSFET amp with regulated supplies a la JLH and tests
showed it would do 110W into 8R and 220W into 4R - which is
what it is all about. [I would still be using it today save
it has developed a dc offset problem on one channel and I
have never had the time to sit down and find out why.] I had
a pair of the Bailey designed transmission line speakers at
the time (rather like the Imhoff TLS80] and that could
rattle windows at 10 paces with that amp!

I was designing and building PA amps back in the 70s which absolutley
relied on tight PSU voltage regulation to avoid blowing my chosen
output transistors (Motorola 2015s recovered from Gvt surplus acquired
analogue Computer PSU fan cooled heatsink assemblies - twenty to a fan
cooled heatsink module) which weren't particularly good performers
slew rate wise and only had a max Vce of 60 volts.

By using a 45v 10A analogue voltage regulated supply for each channel
(again, using more of those Motorola 2015s) along with a solid state
version of the polyfuse (yet another 2015) of my own invention and
using a bridge output amplifier design, I was able to make a 200W RMS
per 4 ohm load stereo power amplifier (800W PMPO in the language of
cheap hi fi except that it was sustained PMPO as in a total of 400W of
500Hz square wave into 4 ohm resistor loads on each channel).

I modified my 'electronic polyfuse' so that its overload point varied
according to the signal output voltage so that it only peaked at the
ten amp setting when full output voltage was present, effectively
turning it into an impedance overload protection system (at zero or
low voltage output, the trip current was set for a more modest 2 or 3
amps to avoid a halfway condition that would otherwise be guaranteed
to burn out the transistors at low to mid volumes if a short circuit
were to develop across the speaker output terminals).

You knew without any ambiguity, typical of the "output protection'
drive limiting nonsense circuits used on a lot of commercial home
hi-fi systems, when the amp had suffered such an overload by the fact
that the output would go silent and the peak indicator lamp went to a
permanent glow (normally extinguished other than for brief faint
flashes if you were driving it to the _actual_ voltage clipping limit
on bass peaks). Resetting was just a matter of removing the overload
(unplugging the appropriate speaker cable).

Now, in spite of the merits of regulated voltage rails for the
output stages of a Hi-Fi amplifier, there's also some merit to the use
of completely unregulated analogue PSUs, provided the output devices
have an ample margin of voltage rating.

PMPO ratings have some validity in that real music very rarely
contains sustained notes that need to be of higher amplitude than the
transient peaks in the rest of the mix. Since transient peaks can
easily exceed the maximum peaks of bass notes that can be auditioned
at realistically loud levels by some 10 to 20dB, the transient high
energy reserve in the analogue PSU's smoothing capacitors can be put
to very good use to allow such transient peaks in the music to be
reproduced with less clipping distortion, for a PSU with a given
sustained maximum power rating.

The difference in performance between two amplifiers with the same
PMPO rating, one using a regulated PSU where the PMPO can be sustained
indefinitely and the other using an unregulated PSU where the PMPO
can't be sustained for more than a few milliseconds may be
indistinguishable with real music sources, especially so with a lot of
orchestral classical works using traditional acoustic instruments
(organ recitals, otoh, are a different kettle of fish, more akin to
the modern electronic instrumentation of rock and pop music).

The amp using a regulated PSU will have a continous rating two or
three times that of the unregulated one. Sticking to analogue PSUs,
this makes the unregulated PSU option considerably cheaper to
manufacture yet still capable of reproducing the maximum sound level
peaks of the more expensive amp on most classical music recordings
(organ recitals excepted).

Incidently, my clip montoring circuit wasn't reliant on using a
regulated PSU, it would work equally well with an unregulated supply.
This is an important consideration in that all you really need to know
about the volume setting is that it sounds loud enough and yet isn't
running into clipping (or severe clipping - at very high acoustic
power levels, your inner ear could be producing such clipping
artifacts indistinguishable from amplifier clipping, in a properly
designed amp that responds gracefully to such 'overload conditions').

IMHO, there's no real need to have a fancy VU meter monitoring the
amplifier's output voltage. A minimalistic clipping indicator is all
that is really required in practice. The only reason I have fitted
such an LED meter to the front panel of my miniature 50W per channel
stereo amp was to utilise an otherwise unused meter as a decorative
bauble to add a little bit of visual interest aside from the simple
on/off mains switch and an indicator lamp for which the meter now
serves.


A few decades ago there was an obsession with getting the
amp output impedance as low as possible to increase the
damping factor, until someone - could have been JLH or Doug
Self or someone like that - proved that it is easy to over
damp a circuit. Indeed the lower the amp output impedance
becomes the more effect the resistance of the
interconnecting cable has which was why I believe there was
a move to direct amping. I believe it was only with
increasingly powerful computer modelling that it was
discovered that much could be achieved by better driver,
cabinet, and particularly crossover design and at the same
time more could be made of signal level filtration so bi or
tri-amping became popular and to an extent survives today
(although very little in the UK from what I read.) As a
result of the work on crossovers, in my very limited
experience you will often find a small series resistor on
the output of the bass section of a crossover.

I take issue with the quoting of 'Damping Factor' figures expressed
as a ratio of speaker impedance to amplifier output impedance, eg DF
of 400 on 8 ohm speaker loads, implying an output impedance of just
20milli ohms. It would be better to simply quote this 20 mill ohms
figure (the lower the better) than to falsely claim that the amp can
dampen the speaker cone movement 50 times better than an amp with a Zo
figure of 1 ohm.

The whole thing is a nonsense. The marketing droid who came up with
this bit of pseudo technobabble rather conveniently forgets the 7.5
ohms or so resistance of the voice coil of an "Eight Ohm" speaker
drive unit which is effective in series with the ampfilier's output
impedance with respect to any electrical damping effect. Sadly, it
would seem that a lot of 'accoustic engineers' have also fallen for
this con.

When a typical bass driver is tested to determine its free air
resonance impedance you see typical values around 4 or 5 times its
nominal impedance (around 35 ohms for an 8 ohm voice coil drive unit).
What this means is that the actual damping factor is more like 4 to 1
regardless of whether the driving amp impedance is 20milli ohms or
half an ohm. Furthermore, it's important to measure the speaker's
resonance under damped conditions (ie, detect a dip in current when
driven from a low Z source rather than look for a voltage peak when
driven from a high Z source) since the damping will effect the
resonant frequency.

Now I know this will be contentious, but from what I heard
years ago the Motional Feedback speaker marketed by Philips
did far more to achieve purity of (bass) sound than anything
I've ever heard. I remember going to one of the hi-fi shows
in Harrogate probably in the '70s. I walked into the
ballroom at the Old Swan Hotel (of Agatha Christie fame) as
I could hear what I thought was a brass band playing -
possibly Grimethorpe - so you can imagine my surprise when
all I saw was two MFB loudspeakers. Staggered was not the
word. The only reason that I can think they never took off
was (a) the price which compared with the price of some
so-called hi-fi kit these days would now be seen as cheap
and (b) because it was done by Philips who were not
perceived to have hi-fi capability. I often wonder what
would have become of the technique if the design had been
done by someone like an early Linn?

The trick with such motion feedback is to eliminate any direct
electrical coupling between the two transducers and, more importantly,
bandwidth limit the response of the driver amp/feedback loop to avoid
negative feedback becoming postive feedback due to phase shift.

This technique can be quite effective, as you seem to have witnessed,
in cancelling the non-linear effects of air pressure loading on the
cone, as well as inherent non-linearities in the magnetic driving
forces over the 'throw' of the voice coil's working range.

Luckily, these imperfections are far less noticable when dealing with
the mid to hi frequency ranges covered by mid range/tweeter drive
units. It's fortunate indeed that this technique is a practical
reality with bass drive units where it can offer the most benefit.
--
J B Good

Centre, speaker - twin drivers, use one enclosure or two separateenclosures side by side?
 

On 12/11/2014 09:39 AM, Jim Lesurf wrote:

I can understand that approach with a valve amp where there is an
output transformer involved, but matching with a transistor amp? Eh?

I'm not clear what "matching" means above. An engineer would probably
assume "arrange for the two impedances to be equal". However it may have
been used more vaguely, akin to "suit one another" in some other way.

I used 'matching' in the more general form not the specific electrical
engineering way. In the circumstances I was describing the book was
recommending measuring the output resistance of the amp (and presumably
cables) so that the exact figure for electrical damping could be used.

Now - say I was to build a transmission line...


Pete

Centre, speaker - twin drivers, use one enclosure or two separateenclosures side by side?
 

On 12/10/2014 10:22 PM, Peter Chant wrote:
On 12/10/2014 06:15 AM, Woody wrote:
Just as a thought, have you considered trying one or both
speakers (series or parallel) on an open baffle to see what
they sound like? If you are not wanting bass then a piece of
wood maybe a foot or so square might be a starting point?


No. If I do that I'll have to put a foot on the bottom. Perhaps
sides... Actually if I put a foot on the bottom then the speakers are
near the screen and effectively the TV is part of the baffle. Apart
from making sure it does not fall off the TV there are few reasons not
to try this.

Cheap adjustable hole saw has arrived. It it works it will pay for
itself in saved hassle in one use. However, if baffle is a min of a
foot wide one wavelength between front and back will be at over 400Hz,
so roll off will be quite high. I wonder if I can put the speakers in a
scrap piece and temporarily fix a larger sheet so I don't have to drill
my nice unsullied sheet of ply!

Centre, speaker - twin drivers, use one enclosure or two separate enclosures side by side?
 

In article , Johny B Good
wrote:


I was designing and building PA amps back in the 70s which absolutley
relied on tight PSU voltage regulation to avoid blowing my chosen output
transistors (Motorola 2015s recovered from Gvt surplus acquired analogue
Computer PSU fan cooled heatsink assemblies - twenty to a fan cooled
heatsink module) which weren't particularly good performers slew rate
wise and only had a max Vce of 60 volts.

By using a 45v 10A analogue voltage regulated supply for each channel
(again, using more of those Motorola 2015s) along with a solid state
version of the polyfuse (yet another 2015) of my own invention and using
a bridge output amplifier design, I was able to make a 200W RMS per 4
ohm load stereo power amplifier (800W PMPO in the language of cheap hi
fi except that it was sustained PMPO as in a total of 400W of 500Hz
square wave into 4 ohm resistor loads on each channel).

[snip]

Now, in spite of the merits of regulated voltage rails for the output
stages of a Hi-Fi amplifier, there's also some merit to the use of
completely unregulated analogue PSUs, provided the output devices have
an ample margin of voltage rating.

Indeed. At the end of the 70s/early 80s I deliberately chose to use
unregulated supplies specifically to let the amp deliver transient peaks
well about the amplifier's 'continouous' rated power.

The drawback is that this can remain hidden when people read adverts or
reviews. So doesn't help 'sell' the amp. But does let it deliver more when
it comes to music. Lets it play undistorted at much higher levels.



IMHO, there's no real need to have a fancy VU meter monitoring the
amplifier's output voltage. A minimalistic clipping indicator is all
that is really required in practice.

Indeed. I stuck a LED on the output of a monostable that triggerred when
the difference in voltage across a longtail pair rose when the amp clipped
or struggled. So if the LED stayed unlit you knew the output was
essentially just a scaled up version of the input with no clipping or
limiting. Even very short 'clips' would cause the LED to light for about
half a sec so you had time to see the event.

[snip]

I take issue with the quoting of 'Damping Factor' figures expressed as
a ratio of speaker impedance to amplifier output impedance, eg DF of 400
on 8 ohm speaker loads, implying an output impedance of just 20milli
ohms. It would be better to simply quote this 20 mill ohms figure (the
lower the better) than to falsely claim that the amp can dampen the
speaker cone movement 50 times better than an amp with a Zo figure of 1
ohm.

The whole thing is a nonsense.

Damping Factor made more sense back in the days when power amps were
valve and almost all of them had high output impedances. But even then it
was a weird term given the details. e.g. Amps having an output impedance
that varies with frequency, level, etc, and generally *not* being
resistive. And speakers also being nothing much like a resistor. Alas we
seem stuck with the term.

The amps output impedance can matter, purely due to any effect by
interacting with the speaker's impedance variations with frequency or
signal level. Not really an issue of 'damping' but of fiddling about the
frequency response in a way that mirrors the speaker's impedance-frequency
curve.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Centre, speaker - twin drivers, use one enclosure or two separateenclosures side by side?
 

Jim Lesurf wrote:


The whole thing is a nonsense.

Damping Factor made more sense back in the days when power amps were
valve and almost all of them had high output impedances. But even then it
was a weird term given the details. e.g. Amps having an output impedance
that varies with frequency, level, etc, and generally *not* being
resistive. And speakers also being nothing much like a resistor. Alas we
seem stuck with the term.


** Damping factor has been relegated to a non issue with hi-fi amplifiers for many decades - despite which it still looms large in the minds of most audiophools. Bull**** baffles brains and good marketing gimmicks never die.

In another area of amplifier design, damping factors vary enormously from one model to another and yet rates no mention in advertising at all.

I am speaking of guitar amps, where the effective DF may be anything from 100 to 0.1 or lower - making for very audible differences.

Famous valve amps like Marshall and Fender have DFs of about 1 due to use of modest amounts of NFB. Early VOX amplifiers were class A and used no NFB at all resulting if very low DF numbers like 0.1. When VOX released their first SS models, the DF was even lower than the valve ones - due to using a combination of voltage and current feedback.

The same idea is still used in lot of modern SS guitar amps to get DFs of between 0.3 and 2, so mimicking the tonal character of popular valve models.

But makers keep it all a big secret.

How very odd.


..... Phil

Centre, speaker - twin drivers, use one enclosure or two separate enclosures side by side?
 

On Sat, 13 Dec 2014 01:58:55 -0800 (PST), Phil Allison
wrote:


Jim Lesurf wrote:


The whole thing is a nonsense.

Damping Factor made more sense back in the days when power amps were
valve and almost all of them had high output impedances. But even then it
was a weird term given the details. e.g. Amps having an output impedance
that varies with frequency, level, etc, and generally *not* being
resistive. And speakers also being nothing much like a resistor. Alas we
seem stuck with the term.


** Damping factor has been relegated to a non issue with hi-fi amplifiers for many decades - despite which it still looms large in the minds of most audiophools. Bull**** baffles brains and good marketing gimmicks never die.

In another area of amplifier design, damping factors vary enormously from one model to another and yet rates no mention in advertising at all.

I am speaking of guitar amps, where the effective DF may be anything from 100 to 0.1 or lower - making for very audible differences.

Famous valve amps like Marshall and Fender have DFs of about 1 due to use of modest amounts of NFB. Early VOX amplifiers were class A and used no NFB at all resulting if very low DF numbers like 0.1. When VOX released their first SS models, the DF was even lower than the valve ones - due to using a combination of voltage and current feedback.

The same idea is still used in lot of modern SS guitar amps to get DFs of between 0.3 and 2, so mimicking the tonal character of popular valve models.

But makers keep it all a big secret.

How very odd.


.... Phil

Guitar amplifiers are a case apart. The maker does what is necessary
to get the sound he wants - fidelity doesn't come into it. He will use
low damping factors to encourage speaker resonances, that being part
of the overall instrument sound. That is why Celestion - pretty much
undisputed rulers of the guitar driver unit world - have so many
models with varying, highly resonant responses.

There was a lot of early resistance to SS guitar amps for this very
reason. Makers didn't really understand what was needed and tried to
adapt standard op-amp type circuits with a dominant pole and heavy
negative feedback. This was a failure on so many counts, from the
over-damped control of the speaker to the disastrously harsh limiting
characteristic that stopped people playing at high volume. This
resulted in the feeling that solid state watts were smaller than valve
watts.

In a decent valve guitar amp feedback is pretty much confined to the
taming of the wildly varying bias condition of the output valves. The
entire rest of the chain will be run open loop to achieve the desired
smoothly curving transfer characteristic.

d

Centre, speaker - twin drivers, use one enclosure or two separateenclosures side by side?
 

Don Pearce wrote:



In another area of amplifier design, damping factors vary enormously from one model to another and yet rates no mention in advertising at all.

I am speaking of guitar amps, where the effective DF may be anything from 100 to 0.1 or lower - making for very audible differences.

Famous valve amps like Marshall and Fender have DFs of about 1 due to use of modest amounts of NFB. Early VOX amplifiers were class A and used no NFB at all resulting if very low DF numbers like 0.1. When VOX released their first SS models, the DF was even lower than the valve ones - due to using a combination of voltage and current feedback.

The same idea is still used in lot of modern SS guitar amps to get DFs of between 0.3 and 2, so mimicking the tonal character of popular valve models.

But makers keep it all a big secret.

How very odd.



Guitar amplifiers are a case apart.


** Sure - where wildly varying damping factors actually have a big effect on sound and makers studiously avoid any mention of it.

While the complete opposite occurs with hi-fi amps.



.... Phil

Centre, speaker - twin drivers, use one enclosure or two separate enclosures side by side?
 

On Sat, 13 Dec 2014 15:27:20 -0800 (PST), Phil Allison
wrote:

Don Pearce wrote:



In another area of amplifier design, damping factors vary enormously from one model to another and yet rates no mention in advertising at all.

I am speaking of guitar amps, where the effective DF may be anything from 100 to 0.1 or lower - making for very audible differences.

Famous valve amps like Marshall and Fender have DFs of about 1 due to use of modest amounts of NFB. Early VOX amplifiers were class A and used no NFB at all resulting if very low DF numbers like 0.1. When VOX released their first SS models, the DF was even lower than the valve ones - due to using a combination of voltage and current feedback.

The same idea is still used in lot of modern SS guitar amps to get DFs of between 0.3 and 2, so mimicking the tonal character of popular valve models.

But makers keep it all a big secret.

How very odd.



Guitar amplifiers are a case apart.


** Sure - where wildly varying damping factors actually have a big effect on sound and makers studiously avoid any mention of it.

They don't studiously avoid mention of it. The high output impedance
is an integral part of the design philosophy. They design to ensure
that there is a low enough damping factor to allow the speaker cone to
sing properly. And getting the figure right is part of their
intellectual property, not a bragging item.

d

Centre, speaker - twin drivers, use one enclosure or two separateenclosures side by side?
 

Don Pearce wrote:


Guitar amplifiers are a case apart.


** Sure - where wildly varying damping factors actually have a big effect on sound and makers studiously avoid any mention of it.

They don't studiously avoid mention of it.


** Yes they do - you bull****ting old fool.


The high output impedance
is an integral part of the design philosophy.

** Except for all the ones that have low and medium output impedance.


They design to ensure
that there is a low enough damping factor to allow the speaker cone to
sing properly.

** That is utter ********.

And getting the figure right is part of their
intellectual property, not a bragging item.


** Completely insane crap.

Most would say that Don has lost it - but I know he never had a single clue in the first place.




..... Phil

Centre, speaker - twin drivers, use one enclosure or two separate enclosures side by side?
 

On Sun, 14 Dec 2014 00:31:17 -0800 (PST), Phil Allison
wrote:

Don Pearce wrote:


Guitar amplifiers are a case apart.


** Sure - where wildly varying damping factors actually have a big effect on sound and makers studiously avoid any mention of it.

They don't studiously avoid mention of it.


** Yes they do - you bull****ting old fool.


The high output impedance
is an integral part of the design philosophy.

** Except for all the ones that have low and medium output impedance.


They design to ensure
that there is a low enough damping factor to allow the speaker cone to
sing properly.

** That is utter ********.

And getting the figure right is part of their
intellectual property, not a bragging item.


** Completely insane crap.

Most would say that Don has lost it - but I know he never had a single clue in the first place.




.... Phil



And normal service has resumed.

d