Just to let people know I've put up a new webpage at

http://www.audiomisc.co.uk/Armstrong...iliconEra.html

This documents the 1973-4 period when they released the 600 range and it
got a lot of attention. Can be interesting to compare reviews to see when
they agree and when they conflict. :-)

FWIW I'm planning two more pages.

One to deal with the 1975-1985 period which covers the dissapearance of
Armstrong as a manufacturer. This is actually quite a complex set of events
so needs a lot of details. Also covers more reviews of the 600 and those of
the 700 amps, what happened with the tuner, etc. In some ways its also a
history of what went wrong in UK manufacturing and how equipment was
reviewed and sold by retailers, etc. Some skeletons will be dug up, and
oddities in 'reviews' examined. 8-]

The other page is to document more extensively the 'radio chassis era' from
1932 to about 1960. During this period Armstrong made a lot of different
models, some without their name or logo on the front! I'm still working
though old references, magazines, etc, to sort this out!

However I'm also doing some work to try and build a decent website for the
Museum of Communication, so it will take a while to do the above pages.

Cheers,

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

On 2015-11-08 12:21:20 +0000, Jim Lesurf said:

Just to let people know I've put up a new webpage at

http://www.audiomisc.co.uk/Armstrong...iliconEra.html

This documents the 1973-4 period when they released the 600 range and it
got a lot of attention. Can be interesting to compare reviews to see when
they agree and when they conflict. :-)

FWIW I'm planning two more pages.

One to deal with the 1975-1985 period which covers the dissapearance of
Armstrong as a manufacturer. This is actually quite a complex set of events
so needs a lot of details. Also covers more reviews of the 600 and those of
the 700 amps, what happened with the tuner, etc. In some ways its also a
history of what went wrong in UK manufacturing and how equipment was
reviewed and sold by retailers, etc. Some skeletons will be dug up, and
oddities in 'reviews' examined. 8-]

The other page is to document more extensively the 'radio chassis era' from
1932 to about 1960. During this period Armstrong made a lot of different
models, some without their name or logo on the front! I'm still working
though old references, magazines, etc, to sort this out!

However I'm also doing some work to try and build a decent website for the
Museum of Communication, so it will take a while to do the above pages.

Cheers,

Jim

Search and replace "arid" by "and" - a typical OCR error?

I like like idea of the secondary feedback loop to make the
capacitor-coupled output impedance less reactive at low frequencies.

Arthur

--
real email arthur at bellacat dot com

In article , Arthur Quinn
wrote:

Search and replace "arid" by "and" - a typical OCR error?

Sorry, I thought I'd corrected that! Yes, the text was done via OCR, which
did give errors like that. Spend some time reading and correcting. But must
have missed one (or more)...

I like like idea of the secondary feedback loop to make the
capacitor-coupled output impedance less reactive at low frequencies.

It wasn't a new idea entirely as IIRC some valve amps used the same trick
to reduce the effects of output transformer imperfections. The challenge is
to apply 'just the right amount'. Particularly in the days when batches of
'the same' transistor varied wildly in their properties.

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

Jim Lesurf wrote:

Arthur Quinn
+

I like like idea of the secondary feedback loop to make the
capacitor-coupled output impedance less reactive at low frequencies.

It wasn't a new idea entirely as IIRC some valve amps used the same trick
to reduce the effects of output transformer imperfections. The challenge is
to apply 'just the right amount'.


** Taking some or even all NFB from the speaker side of the output capacitor was pretty common with amplifiers using one DC supply. Doing so however raises the issue of low frequency instability.

I recall seeing a Kenwood solid state receiver of late 60s vintage that when connected to a Philips 8 inch low resonance woofer ( AD8065) slowly went into oscillation at about 3Hz.

The trick was to use only a judicious amount OR have a capacitor or two internally rolling off the gain of the power stage at low frequencies - as is done with most valve amps.


.... Phil

On 2015-11-09 05:14:30 +0000, Phil Allison said:

Jim Lesurf wrote:

Arthur Quinn
+

I like like idea of the secondary feedback loop to make the
capacitor-coupled output impedance less reactive at low frequencies.

It wasn't a new idea entirely as IIRC some valve amps used the same trick
to reduce the effects of output transformer imperfections. The challenge is
to apply 'just the right amount'.


** Taking some or even all NFB from the speaker side of the output
capacitor was pretty common with amplifiers using one DC supply. Doing
so however raises the issue of low frequency instability.

I recall seeing a Kenwood solid state receiver of late 60s vintage that
when connected to a Philips 8 inch low resonance woofer ( AD8065)
slowly went into oscillation at about 3Hz.

The trick was to use only a judicious amount OR have a capacitor or two
internally rolling off the gain of the power stage at low frequencies
- as is done with most valve amps.


... Phil

Yes, the review says of the damping factor that "from about 2Hz to 50Hz
it tends towards slightly negative and infinity", implying that the
resistive component of the amplifier output impedance passes through
zero to negative within that frequency range. Oviously the negative
resistance will have been made much smaller than the expected
loudspeaker resistance to ensure stability.

Arthur

--
real email arthur at bellacat dot com

In article , Arthur Quinn
wrote:
Yes, the review says of the damping factor that "from about 2Hz to 50Hz
it tends towards slightly negative and infinity", implying that the
resistive component of the amplifier output impedance passes through
zero to negative within that frequency range. Oviously the negative
resistance will have been made much smaller than the expected
loudspeaker resistance to ensure stability.

May be worth adding that for the 600 amp the relevant capacitor values,
etc, were changed more than once during the period of manufacture. But in
all the cases I know of the results always gave negative output impedance
magnitudes well below an Ohm. So weren't a problem in general use.

There was sometimes a more devious problem, though. This was due to the use
(in early versions of the 600) of a resistor put into the dc rail feed to
limit the current inrush at switch-on.

After a few seconds this resistor was bypassed by a thermally operated
switch. However before that - or if that switch failed - the low output
impedance and flat LF response could cause the set's output to wag up and
down at LF. Wasn't good for the amp or speakers, and also could cause the
lamps to keep fading up and down! The effect was similar to the
'motorboating' others may know from amps driven by unsuitable power
supplies when the amp has an awkward load.

The problem wasn't common in use, but showed up particularly on speakers
with a very low near-d.c. impedance. It did cause some people to say the
600 wasn't 'stable' with electrostatics, but the problem was actually this
LF interaction with the power supply when the inrush resistor was in the
way. Not the classic HF bursting into song of an amp unstable into a
capacitive load.

FWIW in later sets I reduced the feedback at LF and also increased the
reservoir and output capacitors. So the results relied less on the feedback
from the speaker side of the output caps. I also removed the thermal switch
and just used a mains diode bridge that wasn't bothered by the switch on
inrush. Simpler all around. But to be fair, when the 600 amp was originally
designs, the components I used weren't available. So Ted did the best he
could with what was available at the time. I could simplify things because
I could find the components to do it!

Components improved a *lot* over th 1970s and into the 1980s. You only have
to compare a 1970 'ab' volume pot with a 1980 Alps 40mm one to see what
amazing changes were made.

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

Arthur Quinn wrote:


** Taking some or even all NFB from the speaker side of the output
capacitor was pretty common with amplifiers using one DC supply. Doing
so however raises the issue of low frequency instability.

I recall seeing a Kenwood solid state receiver of late 60s vintage that
when connected to a Philips 8 inch low resonance woofer ( AD8065)
slowly went into oscillation at about 3Hz.

The trick was to use only a judicious amount OR have a capacitor or two
internally rolling off the gain of the power stage at low frequencies
- as is done with most valve amps.



Yes, the review says of the damping factor that "from about 2Hz to 50Hz
it tends towards slightly negative and infinity", implying that the
resistive component of the amplifier output impedance passes through
zero to negative within that frequency range.


** The get such test results implies the amplifier was being driven at its output with a varying frequency of known current while watching the voltage and phase at the terminals. It's a powerful technique that not many reviewers ever used.

BTW

The output stage of the 626 has very similar topology to the 40watt design published in the Philips "Audio Amplifier Systems" application book of 1970 - minus the regulated PSU.


..... Phil

Once upon a time on usenet Phil Allison wrote:
[snipped]
I recall seeing a Kenwood solid state receiver of late 60s vintage
that when connected to a Philips 8 inch low resonance woofer (
AD8065) slowly went into oscillation at about 3Hz.

I thought I recognised that number - or nearly. I've just checked my 'store
room' and found two AD8066 W4s. I could have sworn I had four of them, maybe
the other two are in boxes somewhere....

I seem to remember that they are good drivers - I must have kept them for a
reason. The surrounds are still nice and supple and they seem to be in good
condition.
--
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)
(*Diagnostic and Statistical Manual of Mental Disorders)

They were surely called Trio then.
Brian

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)
"~misfit~" wrote in message
...
Once upon a time on usenet Phil Allison wrote:
[snipped]
I recall seeing a Kenwood solid state receiver of late 60s vintage
that when connected to a Philips 8 inch low resonance woofer (
AD8065) slowly went into oscillation at about 3Hz.

I thought I recognised that number - or nearly. I've just checked my
'store room' and found two AD8066 W4s. I could have sworn I had four of
them, maybe the other two are in boxes somewhere....

I seem to remember that they are good drivers - I must have kept them for
a reason. The surrounds are still nice and supple and they seem to be in
good condition.
--
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)
(*Diagnostic and Statistical Manual of Mental Disorders)

On 08/11/2015 12:21, Jim Lesurf wrote:
Just to let people know I've put up a new webpage at

http://www.audiomisc.co.uk/Armstrong...iliconEra.html

This documents the 1973-4 period when they released the 600 range and it
got a lot of attention. Can be interesting to compare reviews to see when
they agree and when they conflict. :-)

FWIW I'm planning two more pages.

One to deal with the 1975-1985 period which covers the dissapearance of
Armstrong as a manufacturer. This is actually quite a complex set of events
so needs a lot of details. Also covers more reviews of the 600 and those of
the 700 amps, what happened with the tuner, etc. In some ways its also a
history of what went wrong in UK manufacturing and how equipment was
reviewed and sold by retailers, etc. Some skeletons will be dug up, and
oddities in 'reviews' examined. 8-]

The other page is to document more extensively the 'radio chassis era' from
1932 to about 1960. During this period Armstrong made a lot of different
models, some without their name or logo on the front! I'm still working
though old references, magazines, etc, to sort this out!

However I'm also doing some work to try and build a decent website for the
Museum of Communication, so it will take a while to do the above pages.



Below is a quote from your page at
http://www.audiomisc.co.uk/Armstrong...airandmod.html
'And listening to Radio 3 or 4 using the FM tuner demonstrates how much
nicer these can sound via FM than via DAB.'
I quite agree but would you say this is a result of the low bit rate the
bbc uses or is an indication of the inherent superiority of analogue vs
digital sound reproduction?