On 1/05/2016 11:18 PM, Dave Plowman (News) wrote:
In article ,
Phil Allison wrote:
They can also include a circuit to make them drop out if they detect a
large DC offset. Very good way of protecting speakers in event of a
fault.



** Few speaker relay systems provide reliable protection if that happens
- nor do they claim to.

I'm talking about those which do.

The simple fact being that the kind of relays used are only capable of
switching *off* DC voltages to a maximum of 30V at rated current. IOW,
their current breaking capacity is very limited for DC voltages.

Then use a relay fit for purpose. Fraction of the cost of a speaker repair.


**High Voltage (30 VDC) DC relays are quite expensive. Certainly more
expensive than mid-range bass drivers.

http://uk.farnell.com/te-connectivit...20a/dp/1386559

You STILL haven't told us the brand and model of the amplifier. Failing
that, you could point us to a schematic.

--
Trevor Wilson
www.rageaudio.com.au

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Dave Plowman (News) wrote:



** Few speaker relay systems provide reliable protection if that happens
- nor do they claim to.

I'm talking about those which do.


** Bet you have never seen one.



.... Phil

Trevor Wilson wrote:



Then use a relay fit for purpose. Fraction of the cost of a speaker repair.


**High Voltage (30 VDC) DC relays are quite expensive. Certainly more
expensive than mid-range bass drivers.


http://uk.farnell.com/te-connectivit...20a/dp/1386559


** Such a large and expensive relay is quite impractical for use in a domestic hi-fi amplifier - also note that it needs powerful magnets for "magnetic blow out" of the arc to meet the 125V spec with DC.

A simple triac "crow bar" gives low cost, very effective DC protection for speakers - but surprisingly few amps have them fitted, the Quad 405 being an exception.


..... Phil

In article , Phil
Allison wrote:

BTW; anyone familiar with the famous Crown DC300A ?

Both channels would emit a long squeal about 10 seconds after switch off
- it sounded much like a party balloon being let down while squeezing
the neck.


Yes.- I remember it well.

The Crown DC 300A was a ubiquitous studio workhorse in the UK.
I still have a copy of the manual:

Laboratory amplifier. 155W pc minimum RMS 1Hz to 20kHz.
FR +/- 0.1dB DC - 20kHz at 1W into 8 Ohms
FR +/- 1 dB DC - 100Khz
HD 0.001% 20Hz to 400 Hz increasing linbearly to 0.05% ar
20kHz at 150W RMS pc into 8 Ohms.
Output impedance 7 mOhms in series with 3uHenries

Unconditionally stable with any load including completely
reactive.

The IM and THD charts that come with the manual are blank
and marked "Disclosure prohibited 4.11.1974"

Our chief of maintenance described it as "a good amp,
incredibly reliable and clean as a whistle DC to daylight!" :-)

It was frequenty partnered with the Lockwood Major Monitor, a
studio speaker with a Tannoy 15" Gold. A very impressive
combination.

The Crown (Amcron in some territories) was also used as a vinyl
cutting amp, driven from a inverse RIAA preamp. It gave very
good results. Recorded Productions Ltd in London W1 had such
a set up.



Iain

In article ,
Iain Churches wrote:

[snip amp details]

The IM and THD charts that come with the manual are blank
and marked "Disclosure prohibited 4.11.1974"

I must confess I burst out laughing when I read that! Did they think the
detailed distortion performance of their amp was a 'trade secret' and that
no-one should be allowed either to know the spec or measure it for
themselves?!

8-]

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:



The IM and THD charts that come with the manual are blank
and marked "Disclosure prohibited 4.11.1974"


I must confess I burst out laughing when I read that! Did they think the
detailed distortion performance of their amp was a 'trade secret' and that
no-one should be allowed either to know the spec or measure it for
themselves?!


** The word "prohibited" ought to tell you there was an FCC law in the USA that prevented making such disclosures.

The same FFC law allowed only continuous RMS power specs and then after a 1/3 power pre-conditioning session for 30 minutes.

The original law was Draconian and eventually became relaxed.



..... Phil

In article ,
Jim Lesurf wrote:
In article ,
Iain Churches wrote:

[snip amp details]

The IM and THD charts that come with the manual are blank
and marked "Disclosure prohibited 4.11.1974"

I must confess I burst out laughing when I read that! Did they think the
detailed distortion performance of their amp was a 'trade secret' and
that no-one should be allowed either to know the spec or measure it for
themselves?!

Possibly because the results were so bad? The Crown amp was loved by the
pop boys because it was loud and brash. But when used at normal levels for
GP stuff, bettered by many others.

--
*7up is good for you, signed snow white*

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" wrote in message
...
In article ,
Jim Lesurf wrote:
In article ,
Iain Churches wrote:

[snip amp details]

The IM and THD charts that come with the manual are blank
and marked "Disclosure prohibited 4.11.1974"

I must confess I burst out laughing when I read that! Did they think the
detailed distortion performance of their amp was a 'trade secret' and
that no-one should be allowed either to know the spec or measure it for
themselves?!

Possibly because the results were so bad?

You snipped the figures I gave. They are excellent.


z The Crown amp was loved by the
pop boys because it was loud and brash. But when used at normal levels for
GP stuff, bettered by many others.


The Crown amp was popular everywhere, it had the power to be loud
but not brash. Driving large Tannoys or Lockwoods it made a very
impressive monitor system. A good vinyl cutting amp also.

Iain

In article , Phil
Allison wrote:
Jim Lesurf wrote:



The IM and THD charts that come with the manual are blank and
marked "Disclosure prohibited 4.11.1974"


I must confess I burst out laughing when I read that! Did they think
the detailed distortion performance of their amp was a 'trade secret'
and that no-one should be allowed either to know the spec or measure
it for themselves?!


** The word "prohibited" ought to tell you there was an FCC law in the
USA that prevented making such disclosures.

Why would the Federal Communications Commission prohibit a manufacturer of
audio amplifiers from publishing measurement charts of audio IM/THD?

The same FFC law allowed only continuous RMS power specs and then after
a 1/3 power pre-conditioning session for 30 minutes.

I can recall that the IHFA laid down IHFA-707 specs that required this. Had
to ensure the 700 power amps passed it to keep magazine reviewers happy.
But I don't recall it being mandated by the *FCC*. And the results weren't
then 'prohibited' from being published.

Do you mean FTC? I think the 1/3rd power malarky was their idea to stop all
the American claims for wildly overstated 'output powers'. But I can't
recall any FTC documentation I ever saw that made IM curves 'secret'.

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

In article ,
Iain Churches wrote:

"Dave Plowman (News)" wrote in message
...
In article ,
Jim Lesurf wrote:
In article ,
Iain Churches wrote:

[snip amp details]

The IM and THD charts that come with the manual are blank
and marked "Disclosure prohibited 4.11.1974"

I must confess I burst out laughing when I read that! Did they think
the detailed distortion performance of their amp was a 'trade secret'
and that no-one should be allowed either to know the spec or measure
it for themselves?!

Possibly because the results were so bad?

You snipped the figures I gave.

No I didn't.

They are excellent.


z The Crown amp was loved by the
pop boys because it was loud and brash. But when used at normal levels
for GP stuff, bettered by many others.


The Crown amp was popular everywhere,

No it wasn't. ;-)

it had the power to be loud
but not brash.

Not on the tests I heard.


Driving large Tannoys or Lockwoods it made a very
impressive monitor system.

Tannoys never were the most critical of speaker chassis. Merely loud.

A good vinyl cutting amp also.

No idea about that. But then some types of distortion isn't going to be
critical for that application anyway.

--
*If at first you don't succeed, try management *

Dave Plowman London SW
To e-mail, change noise into sound.