Have we all stopped arguing? May I ask a question?
What's the best way to check if a power amp is likely to be stable
into a capacitive load such as an ESL?
I could put a 2uF cap on the output and look at the output on a scope
while feeding it with a square wave.
But is it likely to damage anything if the amp fails the test?

One old Rotel amp has a warning in the manual against capacitive loads,
which is a pity as it is a nice, low-power amp and not likely to make
sparks if the kids turn it up to 11.

--
Eiron.

"Eiron"

Have we all stopped arguing? May I ask a question?
What's the best way to check if a power amp is likely to be stable
into a capacitive load such as an ESL?

** ESLs are not capacitive loads - that is an absurd myth.


I could put a 2uF cap on the output and look at the output on a scope
while feeding it with a square wave.

** Creates a load condition that does not exist with any hi-fi speaker.

Totally stupid.


One old Rotel amp has a warning in the manual against capacitive loads,

** Can you quote the actual wording ?

Some amplifiers are not suitable for direct connection to * transformer
input * speakers like the Quad ESL57 - mostly because they lack sub-sonic
filters and current limiting OR because they are simply too powerful.

If you have a combination of speaker and amplifier in mind, then post the
info.



..... Phil

Phil Allison wrote:
"Eiron"
Have we all stopped arguing? May I ask a question?
What's the best way to check if a power amp is likely to be stable
into a capacitive load such as an ESL?

** ESLs are not capacitive loads - that is an absurd myth.


I could put a 2uF cap on the output and look at the output on a scope
while feeding it with a square wave.

** Creates a load condition that does not exist with any hi-fi speaker.

Totally stupid.


One old Rotel amp has a warning in the manual against capacitive loads,

** Can you quote the actual wording ?

http://www.rotel.com/content/manuals/ra930ax_eng.pdf page 2
"Do not use speakers that have very high capacitive load,
say over 0.1 microfarads, as it may cause a damage to the
power amplifier section. Normally, most speakers are low
capacitance type, but there are a few which do have
excessively high capacitance."

No other Rotel manuals seem to mention that, so the RA930AX must be a lemon.

Probably best not to risk it. Plenty more amps in the junkroom.

--
Eiron.

"Eiron"

** Can you quote the actual wording ?

http://www.rotel.com/content/manuals/ra930ax_eng.pdf page 2
"Do not use speakers that have very high capacitive load,
say over 0.1 microfarads, as it may cause a damage to the
power amplifier section. Normally, most speakers are low
capacitance type, but there are a few which do have
excessively high capacitance."


** Nothing there suggests Rotel are referring to ESLs of any kind

No other Rotel manuals seem to mention that, so the RA930AX must be a
lemon.


** You make too many ****ing stupid assumptions.


Probably best not to risk it.


** Risk what ??

All assumptions- no information.

Wot a stupid pommy puke.



...... Phil

In article , Eiron
wrote:
Have we all stopped arguing? May I ask a question? What's the best way
to check if a power amp is likely to be stable into a capacitive load
such as an ESL? I could put a 2uF cap on the output and look at the
output on a scope while feeding it with a square wave. But is it likely
to damage anything if the amp fails the test?

The old test used to be something like a 2.2uF cap. But TBH you would be
better experimenting by trying various capacitances, and some inductances.
I've know amps that were stable with 2.2uF but oscillated with 10,000pF.

What happens will depend entirely on the amp design. So unless you have
other info the only way to know is to experiment.

If in doubt you can try using a small fuse in the output. Or, better, to
put smaller dc fuses inside on the rails. They may blow needlessly, but
might also save the amp if it would otherwise damage itself. However if it
is OK, you may then need to re-test without the output fuse as that can
alter the loading it saw!

One old Rotel amp has a warning in the manual against capacitive loads,
which is a pity as it is a nice, low-power amp and not likely to make
sparks if the kids turn it up to 11.

If in doubt, put a low-value resistor in series with each output. Say 1.5
Ohms. Alternatively, if you know the amp lacks an output network, add one.
These changes may alter the frequency response, but will help avoid
instability!

Slainte,

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"
Eiron

Have we all stopped arguing? May I ask a question? What's the best way
to check if a power amp is likely to be stable into a capacitive load
such as an ESL? I could put a 2uF cap on the output and look at the
output on a scope while feeding it with a square wave. But is it likely
to damage anything if the amp fails the test?

The old test used to be something like a 2.2uF cap.


** Never was and still is NOT a relevant test of an amp's ability to cope
safety with an ESL speaker like the Quad '57.


But TBH you would be
better experimenting by trying various capacitances,

** Absolute CRAP !!


What happens will depend entirely on the amp design.


** And has nothing to do with Eiron's dopey question - ****HEAD



..... Phil

Phil Allison wrote:

The old test used to be something like a 2.2uF cap.


** Never was and still is NOT a relevant test of an
amp's ability to cope safety with an ESL speaker like the
Quad '57.


But TBH you would be
better experimenting by trying various capacitances,

** Absolute CRAP !!


What happens will depend entirely on the amp design.


** And has nothing to do with Eiron's dopey question -
****HEAD


There's a suggested dummy load he

http://www.quadesl.com/graphics/quad...cs/ampload.jpg

which I found he

http://www.quadesl.com/quad_ref.html

But there's no accompanying text. Seems to lack primary and
secondary winding resistance, but I don't know how
significant they are.

No info on how true it might be outside the audio band,
either, but presumably it's intended with stability testing
in mind, although maybe what's OK for testing a typical
valve amp may not be adequate for testing an SS amp with a
wider bandwidth.

Hopefully not another red herring (which are smoked, BTW, so
fishing for one would be even more futile than a wild goose
chase).

Ian

In article , Ian Iveson
wrote:


There's a suggested dummy load he

http://www.quadesl.com/graphics/quad...cs/ampload.jpg


FWIW That is quite similar to the representation I used for

http://www.audiomisc.co.uk/57and303/interact.html

which also shows the resulting impedance versus frequency over the audio
band.


which I found he

http://www.quadesl.com/quad_ref.html

But there's no accompanying text. Seems to lack primary and secondary
winding resistance, but I don't know how significant they are.

No info on how true it might be outside the audio band, either,

I'm also unsure how much the impedance of the 'ESL57' design changed over
the years as the design developed. So I wonder if the variations you see in
the 'equivalent circuits' is due to people meauring different examples and
getting different results!

but presumably it's intended with stability testing in mind, although
maybe what's OK for testing a typical valve amp may not be adequate for
testing an SS amp with a wider bandwidth.

Alas, when the original Quad ESL was released no-one really took much
interest in the impedance response or stability issues. I suspect PJW's
attitude was "Just buy *our* amps to drive it as they are made to work
together". :-)

Indeed, even now, no-one seems to ever bother to check LS impedances above
the audio band.

Later on, the early SS designs could easily struggle with the capacitive
load in the treble region. So the 2,2uF test seemed to evolve and was
mainly used with squarewaves to see how much rounding or ringing (or slew
limtiing) was produced. Again, I suspect there was some confusion here as
more than one old review interpreted any ringing as a "sign of low
stability margin".

It may have been than with some amps, but in other cases it was just a
series output inductor in the amp interacting with the 2.2uF load to
produce a damped resonance. However the 2.2uF test is useful for seeing if
the amp can cope happily with high current bursts into a nasty load. But
with SS amps my personal experience was that much lower capacitance was
more useful as a probe for stability problems. As was the load that Gordon
King used to use which was a big inductor and resistor to get about 50
degrees and 5-6 Ohms in the few hundred Hertz region. The main claim to
fame of that was exposing the over-protection of the original 405! :-)

Slainte,

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

FWIW That is quite similar to the representation I used for

http://www.audiomisc.co.uk/57and303/interact.html

which also shows the resulting impedance versus frequency over the audio
band.

** Which is an UTTER LOAD OF CRAP from beginning to end.

Cos it is all based on false data.


which I found he

http://www.quadesl.com/quad_ref.html

But there's no accompanying text. Seems to lack primary and secondary
winding resistance, but I don't know how significant they are.


** There is a ****ING LOT you do not know and recklessly don't give a
**** about.

Just like any other pig arrogant pommy ****.


I'm also unsure how much the impedance of the 'ESL57' design changed over
the years as the design developed.

** There is a ****ING LOT you do not know and recklessly don't give a
**** about.

Just like any other pig arrogant pommy ****.


Alas, when the original Quad ESL was released no-one really took much
interest in the impedance response or stability issues.

** More wild and FALSE suppositions.


I suspect PJW's attitude was ...

** Another wild supposition.

The Quad ESL speaker handbook contains PW's comments on the topic, circa
1957.


Indeed, even now, no-one seems to ever bother to check LS impedances above
the audio band.

** Modern day reviews never actually check anything.

Cos that are all written by complete morons.



..... Phil

"Ian Iveson" wrote in
message

There's a suggested dummy load he

http://www.quadesl.com/graphics/quad...cs/ampload.jpg

which I found he

http://www.quadesl.com/quad_ref.html

But there's no accompanying text. Seems to lack primary
and secondary winding resistance, but I don't know how
significant they are.

Also, no info about the ESR of the components. This is a serious problem
because the inductors in particular almost certainly have signficiant ESR
unless they are huge.