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