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Hum again...

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Re: Hum again...

Postby Hugh Robjohns » Tue Jun 11, 2019 11:30 am

Yes, quite likely -- that or a slight leak somewhere around the chassis gasket that was cured when you tightened the driver back into the cabinet.

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Re: Hum again...

Postby James Perrett » Tue Jun 11, 2019 2:21 pm

Martin Walker wrote:I recently solved the very occasional buzz on certain loud bass notes in one of my monitor speakers - I unbolted the woofer, lifted it out, couldn't see anything lose, then bolted it back in, and the noise hasn't happened since.

Did you put it back in the same orientation as before? Over the years the driver suspension can sag a little and cause the voice coil to rub on the magnet so simply turning the driver round can cure the problem.
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Re: Hum again...

Postby Arpangel » Tue Jun 11, 2019 2:55 pm

Hugh Robjohns wrote:I love it when the engineering bit of audio engineering proves its worth! :thumbup:

Yes, it's always amused men that I've never seen an engine in a studio....well, not one of them petrol or steam jobs....

:)
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Re: Hum again...

Postby Martin Walker » Tue Jun 11, 2019 4:17 pm

James Perrett wrote:
Martin Walker wrote:I recently solved the very occasional buzz on certain loud bass notes in one of my monitor speakers - I unbolted the woofer, lifted it out, couldn't see anything lose, then bolted it back in, and the noise hasn't happened since.

Did you put it back in the same orientation as before? Over the years the driver suspension can sag a little and cause the voice coil to rub on the magnet so simply turning the driver round can cure the problem.

I didn't, but if the buzz ever occurs again I'll certainly try that James - thank you! :thumbup:

I did after all buy these loudspeakers 22 years ago now.


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Re: Hum again...

Postby Hugh Robjohns » Tue Jun 11, 2019 4:25 pm

Ah... they'll just be nicely run in now then. Excellent! :lol:
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Re: Hum again...

Postby Martin Walker » Tue Jun 11, 2019 4:35 pm

But they are built by ATC, and were thus beautifully engineered in the first place ;)


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Re: Hum again...

Postby Hugh Robjohns » Tue Jun 11, 2019 4:43 pm

Naturally! :lol:

I have a couple of little ATC SCM7s turning up tomorrow for a new stereo listening system in a room upstairs and am really looking forward to firing them up.
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Re: Hum again...

Postby Hugh Robjohns » Tue Jun 11, 2019 10:10 pm

Hugh Robjohns wrote:... but it sounds like the 33 is grounded via the mains safety earth, but the 405 isn't... which is somewhat disturbing!

I happened to flick through the original manual for the Quad 405-1 today, and was reminded that it was intended to be powered directly from one of the 33's mains outlets which are two-pin and ground-free. The amp received its ground reference via the 4-pin audio cable screen from the 33 which was grounded via the mains safety earth.

So if that's the way your system is hooked up, my supposition above was correct; the 33 _is_ grounded via the mains (and hence the ground-loop hum when connected to a mains-grounded source) but the 405 -- if powered via the 33's two-pin mains outlets -- is only grounded via its audio inputs (and thus no ground-loop hum).

However, the 405 can be earthed directly by using a standard 3-core mains lead.
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Re: Hum again...

Postby Arpangel » Wed Jun 12, 2019 8:02 am

Hugh Robjohns wrote:
Hugh Robjohns wrote:... but it sounds like the 33 is grounded via the mains safety earth, but the 405 isn't... which is somewhat disturbing!

I happened to flick through the original manual for the Quad 405-1 today, and was reminded that it was intended to be powered directly from one of the 33's mains outlets which are two-pin and ground-free. The amp received its ground reference via the 4-pin audio cable screen from the 33 which was grounded via the mains safety earth.

So if that's the way your system is hooked up, my supposition above was correct; the 33 _is_ grounded via the mains (and hence the ground-loop hum when connected to a mains-grounded source) but the 405 -- if powered via the 33's two-pin mains outlets -- is only grounded via its audio inputs (and thus no ground-loop hum).

However, the 405 can be earthed directly by using a standard 3-core mains lead.

Both mains leads on the 405/33 are earthed, the 33 uses a Bulgin connector, I remember those from the past, again, another British hi-fi idiosyncrasy, along with DIN plugs, ugh.
The back of my 33 is festooned with those DIN to phono adaptors.
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Re: Hum again...

Postby Mike Stranks » Wed Jun 12, 2019 8:58 am

Arpangel wrote:.... another British hi-fi idiosyncrasy, along with DIN plugs, ugh.
The back of my 33 is festooned with those DIN to phono adaptors.

Nope!

Back in the 60s and and early 70s the DIN connector (known to some of our younger brethren as the MIDI connector) was the de facto connector on audio equipment across the board (not just hifi) - for both mic and line levels. I've owned kit by Philips (Dutch), Tandberg (Norwegian), AKG (Austrian) and Uher (German) that utilised 2, 3 and 5-pin DIN connectors.
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Re: Hum again...

Postby Hugh Robjohns » Wed Jun 12, 2019 10:15 am

Arpangel wrote:Both mains leads on the 405/33 are earthed...

Sure. They should be.

But the standard installation is with the power amp (303 or 405) powered from one of the two-pin mains outlets on the back of the 33 preamp (the other is normally for powering the FM3 tuner). That way, when you switch on the preamp, everything powers up too.

Image

Being two-pin mains outlets, the mains safety ground isn't extended to the power amp, and the ground reference is, instead, carried via the screen of the 4-pin audio cable linking to two.

As I said, that arrangement could have explained your issues... but if the power amp and preamp are being powered separately via their own earthed mains cables something else is going on... :?:
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Re: Hum again...

Postby Hugh Robjohns » Wed Jun 12, 2019 10:34 am

Mike Stranks wrote:Back in the 60s and and early 70s the DIN connector (known to some of our younger brethren as the MIDI connector) was the de facto connector on audio equipment across the board (not just hifi) - for both mic and line levels.

It was the only 'standard' of the time and came from the German Deutsches Institut für Normung (DIN). The specific standard for audio connections was DIN 41524, but that has been withdrawn and is now embodied in the current IEC 60130-9 standard.

Rather than being a matched-voltage interface with the wanted signal being generated as a voltage source, it uses a current source. That means the actual signal voltage received at the destination depends almost entirely on the input impedance of that device.

The DIN spec calls for a current source that provides a received signal voltage of 1mV per 1K Ohms input impedance. In practice most line level devices presented an input impedance of 100k Ohms, so received 100mV as the nominal input signal level. The Quad 33 follows this spec, as did almost all the European audio equipment of the 60s and 70s.

That nominal signal level is pretty low compared to more modern voltage-matched interfaces (which the Japanese hifi manufacturers introduced along with the horrid RCA-phono connectors). Most modern gear typically expects a nominal line input level of about 500mV (and up to 2V for the peak level from a CD player) and presents an input impedance of 10-50K Ohms.

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Re: Hum again...

Postby Zukan » Wed Jun 12, 2019 10:51 am

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Re: Hum again...

Postby Arpangel » Wed Jun 12, 2019 6:26 pm

DIN plugs had one advantage, no loud buzz on inserting the leads if powered up.
Also, they save space, compared to using Phono plugs/sockets.
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Re: Hum again...

Postby Hugh Robjohns » Wed Jun 12, 2019 7:06 pm

The physical plug has several advantages over RCA-phonos, but the DIN standard 1mV/kOhm thing is no longer appropriate.

In addition to the valid points you made, there's also the benefit of a single plug/cable for a stereo connection, and a single plug/cable for rec/rep connections with recording devices.

Naim like to claim a range of other benefits as well, to justify their continued use of DIN plugs... but most of them don't bear technical scrutiny... Like avoiding ground-loops in cable screens, or unmatched cable/plug impedances causing signal reflections. Both complete nonsense!
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Re: Hum again...

Postby Arpangel » Thu Jun 13, 2019 7:43 am

Hugh Robjohns wrote:Naturally! :lol:

I have a couple of little ATC SCM7s turning up tomorrow for a new stereo listening system in a room upstairs and am really looking forward to firing them up.

I am going to have to upgrade my monitoring someday, if my ageing KEF's stop working.
They were made from a project in Hi-FI Answers magazing in 1975.

Size 1000mm High x 405mm Wide x 470 mm Deep. Weight 48Kg each
Bass KEF B139 SP1044
Mid KEF B110 SP1003
Tweeter Audax TW025MI (replaces original KEF T27)
Supertweeter Coles ITT-STC 4001G

These sound really good, but if I do have to replace them it'll be with a apair of ATC s of some sort.
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Re: Hum again...

Postby Martin Walker » Thu Jun 13, 2019 5:49 pm

Arpangel wrote:Bass KEF B139
Mid KEF B110
Tweeter Audax TW025MI (replaces original KEF T27)
Supertweeter Coles ITT-STC 4001G

These sound really good, but if I do have to replace them it'll be with a apair of ATC s of some sort.

Ah, some real classic drivers there, and very well respected in their time :thumbup:


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Re: Hum again...

Postby Arpangel » Thu Jun 13, 2019 6:58 pm

Martin Walker wrote:
Arpangel wrote:Bass KEF B139
Mid KEF B110
Tweeter Audax TW025MI (replaces original KEF T27)
Supertweeter Coles ITT-STC 4001G

These sound really good, but if I do have to replace them it'll be with a apair of ATC s of some sort.

Ah, some real classic drivers there, and very well respected in their time :thumbup:


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Hi Martin, yes, and they still sound good. Surprisingly "neutral" the mid range is extremely good, bass is great for organ music!
If I get a pair of ATC' then I'll keep these and the Quads and set them up as a hi-fI in another room. I've got a Philips CD850 which is one of the best CD's I've heard, it sounds fanatasic through the KEF's. Strange how hi-fI seems to have reached a peak and not gone further at certain points in its history.
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Re: Hum again...

Postby James Perrett » Thu Jun 13, 2019 8:51 pm

Arpangel wrote:Strange how hi-fI seems to have reached a peak and not gone further at certain points in its history.

By the 70's most of the technology was well understood and there were plenty of manufacturers making good hifi equipment. There wasn't really anywhere for mainstream hifi to progress which is why the hifi market moved into all that snake oil and quackery. There has certainly been progress in making good sounding gear more compact and convenient since then but the basic building blocks have been around for over 40 years.
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Re: Hum again...

Postby Sam Spoons » Thu Jun 13, 2019 9:09 pm

I'd say that improvements in HiFi in the last 40 years or so have been in listening environments more than the gear. And while more people are into home cinema than HiFi I guess the room treatment the cinema buffs stick on their walls will have filtered down. But I don't really know, I listen in the studio and am not too much interested in cinema TBH, plus SWMBO would not countenance a raft of acoustic panels all over the living room :headbang:
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