This time, a BBC AM8/16. Basically, a BBC modified Quad 405 to drive the
LS5/8. Has a crossover added to the input to the amps.

One of the two produces a pretty loud squawk on switching off. Not a fault
as such, as plenty early BBC ones in service did the same. But the BBC did
seem to find a cure. The pair I had were ok.

Sadly, the three circuits I've got for the crossover are all the same.

I've only got the one amp here, so can't just look for any differences.

I can post a link to the crossover circuit if it helps. It gets its power
from the + side of the Quad PS and regulates that down to 30 volts.

--
*A cartoonist was found dead in his home. Details are sketchy.*

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

In article , Dave Plowman (News)
scribeth thus
This time, a BBC AM8/16. Basically, a BBC modified Quad 405 to drive the
LS5/8. Has a crossover added to the input to the amps.

One of the two produces a pretty loud squawk on switching off. Not a fault
as such, as plenty early BBC ones in service did the same. But the BBC did
seem to find a cure. The pair I had were ok.

Sadly, the three circuits I've got for the crossover are all the same.

I've only got the one amp here, so can't just look for any differences.

I can post a link to the crossover circuit if it helps. It gets its power
from the + side of the Quad PS and regulates that down to 30 volts.


Are you saying that one channel squawks whist the other doesn't?.

If this was a problem overall with the 405 I bet QUAD would have done a
mod for it even in the balanced input splitter/driver board

Very much expect its a duff cap or caps somewhere well worth changing at
the age thats likely to be..
--
Tony Sayer

In article ,
tony sayer wrote:
In article , Dave Plowman (News)
scribeth thus
This time, a BBC AM8/16. Basically, a BBC modified Quad 405 to drive
the LS5/8. Has a crossover added to the input to the amps.

One of the two produces a pretty loud squawk on switching off. Not a
fault as such, as plenty early BBC ones in service did the same. But
the BBC did seem to find a cure. The pair I had were ok.

Sadly, the three circuits I've got for the crossover are all the same.

I've only got the one amp here, so can't just look for any differences.

I can post a link to the crossover circuit if it helps. It gets its
power from the + side of the Quad PS and regulates that down to 30
volts.


Are you saying that one channel squawks whist the other doesn't?.

It seems to be both channels (LF & HF) on the one amp.

If this was a problem overall with the 405 I bet QUAD would have done a
mod for it even in the balanced input splitter/driver board

I don't think it's anything to do with the Quad part.

Very much expect its a duff cap or caps somewhere well worth changing at
the age thats likely to be..

Thing is I well remember new ones doing it at work, when they first came
out. My pair here didn't, but then now no longer have AM8s.

This amp I'm looking at seems to have a later crossover, by my BBC
schematics. But I dunno how many versions there were. And measures just
fine. There are very few electrolytics on this PCB and to change all the
caps on spec would be a very difficult job since many are very odd values.

--
*Why is the word abbreviation so long? *

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

Dave Plowman (News) wrote:



This time, a BBC AM8/16. Basically, a BBC modified Quad 405 to drive
the LS5/8. Has a crossover added to the input to the amps.

One of the two produces a pretty loud squawk on switching off. Not a
fault as such, as plenty early BBC ones in service did the same. But
the BBC did seem to find a cure. The pair I had were ok.

Sadly, the three circuits I've got for the crossover are all the same.

I've only got the one amp here, so can't just look for any differences.

I can post a link to the crossover circuit if it helps. It gets its
power from the + side of the Quad PS and regulates that down to 30
volts.


Are you saying that one channel squawks whist the other doesn't?.

It seems to be both channels (LF & HF) on the one amp.

If this was a problem overall with the 405 I bet QUAD would have done a
mod for it even in the balanced input splitter/driver board

I don't think it's anything to do with the Quad part.

Very much expect its a duff cap or caps somewhere well worth changing at
the age thats likely to be..

Thing is I well remember new ones doing it at work, when they first came
out. My pair here didn't, but then now no longer have AM8s.

This amp I'm looking at seems to have a later crossover, by my BBC
schematics. But I dunno how many versions there were. And measures just
fine. There are very few electrolytics on this PCB and to change all the
caps on spec would be a very difficult job since many are very odd values.



** The simple solution is to add a DPDT relay to mute both inputs of the 405 whenever there is no AC power.

Ideally, a miniature sealed type with gold contacts powered by rectified AC from the secondary of the transformer - plus a suitable series resistor.

You can use the contacts to either ground the two signals or open circuit them.



..... Phil

In article ,
Phil Allison wrote:
** The simple solution is to add a DPDT relay to mute both inputs of the
405 whenever there is no AC power.

Ideally, a miniature sealed type with gold contacts powered by rectified
AC from the secondary of the transformer - plus a suitable series
resistor.

You can use the contacts to either ground the two signals or open
circuit them.

Snag is this squawk happens immediately at switch off. So the relay would
have to operate extremely quickly.
But I was curious why some do it and some don't.

The DC in the Quad drops off pretty slowly after switching off. The panel
LED takes ages to extinguish. Just why the crossover goes unstable so
quickly, I don't know. The DC from the Quad PS is regulated down to 30v
for the crossover.

--
*If vegetable oil comes from vegetables, where does baby oil come from? *

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

Dave Plowman (News) wrote:


** The simple solution is to add a DPDT relay to mute both inputs of the
405 whenever there is no AC power.

Ideally, a miniature sealed type with gold contacts powered by rectified
AC from the secondary of the transformer - plus a suitable series
resistor.

You can use the contacts to either ground the two signals or open
circuit them.

Snag is this squawk happens immediately at switch off. So the relay would
have to operate extremely quickly.



** With the relay powered with rectified AC only, the switch off time is less than 15mS.

That I why I suggested it.

Why not actually try it ?



..... Phil

In article ,
Phil Allison wrote:
Dave Plowman (News) wrote:


** The simple solution is to add a DPDT relay to mute both inputs of
the 405 whenever there is no AC power.

Ideally, a miniature sealed type with gold contacts powered by
rectified AC from the secondary of the transformer - plus a suitable
series resistor.

You can use the contacts to either ground the two signals or open
circuit them.

Snag is this squawk happens immediately at switch off. So the relay
would have to operate extremely quickly.



** With the relay powered with rectified AC only, the switch off time is
less than 15mS.

That I why I suggested it.

Why not actually try it ?


If you'd seen an AM8, you'd know there's not exactly much space inside it.
It could be done, but not easily. It would probably need to be sited on
the far side of the amp from the crossover.

I was more hoping someone could come up with why some of these crossovers
squawk, but some not.

--
*When it rains, why don't sheep shrink? *

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

In article , Dave Plowman (News)
wrote:

I was more hoping someone could come up with why some of these
crossovers squawk, but some not.

I guess that we can't say much more than the obvious. i.e. that something
is different between examples of amps and crossovers/speakers! Not exactly
news for you. :-/

I've never seen an AM8/16 so have no idea what changes the BBC people may
have made. I can't tell if the 'quawk' is due to a protection circuit
rattling on and off, bursts of RF instability, or what. Might be fixable by
a bigger output inductor or a cap in just the right place. But where?...

Ideally, you'd be best asking in some place were ancient BBC engineers are
listening. But again, I'm not sure where to suggest!

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
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Audio Misc http://www.audiomisc.co.uk/index.html

On 31/05/2016 9:13 PM, Dave Plowman (News) wrote:
In article ,
Phil Allison wrote:
** The simple solution is to add a DPDT relay to mute both inputs of the
405 whenever there is no AC power.

Ideally, a miniature sealed type with gold contacts powered by rectified
AC from the secondary of the transformer - plus a suitable series
resistor.

You can use the contacts to either ground the two signals or open
circuit them.

Snag is this squawk happens immediately at switch off. So the relay would
have to operate extremely quickly.

**Phil's idea is a good one. I used exactly that solution a short time
back with a Sony integrated amp. The preamp section was going unstable
at switch-off. The job was rapidly becoming unviable, so, after
consultation with the client, he agreed that a fast operating relay
would be acceptable. I used it to short the output of the preamp to
earth at switch-off. Perfect solution. Not ideal, but it does work well.

Try it.

--
Trevor Wilson
www.rageaudio.com.au

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Dave Plowman (Nutcase) wrote:


If you'd seen an AM8, you'd know there's not exactly much space inside it.
It could be done, but not easily. It would probably need to be sited on
the far side of the amp from the crossover.


** Miniature DPDT relays measure only 2x1x1cm.

DIL bridges are tiny.



I was more hoping someone could come up with why some of these crossovers
squawk, but some not.

** Same wrong thinking as with all your absurd questions.


.... Phil