Another Goodmans Magnum project

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Dr Bunsen Honeydew
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Re: Another Goodmans Magnum project

Unread post by Dr Bunsen Honeydew »

Who says they have gone out of spec, have you measured them, and what with. It is not as easy as you think to measure capacitors, you can't just use a multimeter. And what spec matters and how far out matters. It is the classic saying of a little knowledge is a dangerous thing!

Visual usually tells you if a cap is duff. Does it have scab or crust developing on or around it, is part of the case swollen. Is there any liquid leaking. Is the capacitor getting too hot to touch in use.
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Lindsayt (Mon Jun 01, 2020 11:08 am)

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jayman67
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Re: Another Goodmans Magnum project

Unread post by jayman67 »

fair point there.
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Re: Another Goodmans Magnum project

Unread post by Geoff.R.G »

jayman67 wrote:
Dr Bunsen Honeydew wrote:Mostly! if something doesn't sound broke then it ain't broke..
yep i understand that bit but assuming the fact that the old caps go out of spec with age, are you not by replacing them for new ones bringing the speaker back to how they left the factory so to speak?
As with most of the nonsense, there is a grain of truth in there. Some capacitors manufactured and used in the 50s and early 60s can deteriorate with age, especially if they aren't used. I can't think what the construction of said items is but they do have a nasty habit of exploding when they fail. It is not common for this to happen as the people who really know their stuff will replace them when refurbishing equipment of this vintage. Now, to complicate matters, the said capacitors are electrolytic and equipment from that era uses rectifiers that also, occasionally fail. If you apply AC to an electrolytic it can explode. So there is the background, is the problem the capacitors or the rectifiers of is it all just a con to justify installing expensive modern capacitors?

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Re: Another Goodmans Magnum project

Unread post by jayman67 »

im intrigued to know what your thoughts on elcap electrolytic capacitors are please? i would think they are cheap as they seem to be in every speaker ive looked inside of.
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Re: Another Goodmans Magnum project

Unread post by Dr Bunsen Honeydew »

They are used for cheapness, especially for high mfd values, nothing more. Electrolytic capacitors are not really suited to AF (audio frequency) circuits, they are used mainly for removing noise from DC circuits.

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Re: Another Goodmans Magnum project

Unread post by Dr Bunsen Honeydew »

BTW I am talking about things that are used. Capacitors (especially electrolytic) thrive on being used, they deteriorate from not being used and can deform (a tech term, it doesn't mean they go ugly). So if I had an old Jap amp from the 70's that had been in the loft for 20 years then I would change them.
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Lindsayt (Mon Jun 01, 2020 11:09 am)

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Re: Another Goodmans Magnum project

Unread post by jayman67 »

Dr Bunsen Honeydew wrote:Its nonsense. Just think about it. If the driver moves in then the air is being pushed out through a hole somewhere. You then let go of the driver and the air will slowly return under the pump of the driver re-centering. If the cabinet is air tight there will be strong resistance to the driver being pushed in, and when released the increased air pressure will bounce the driver out to its balance point immediately.

Typical of the eeediots who call themselves experts. There must be an awful lot of IB speaker out there with air leaks.
just so you dont think im mad he's another thread over the pond about the whether the drivers should return fast or slow when pushed in :)

http://www.audiokarma.org/forums/index. ... t-10199501
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Re: Another Goodmans Magnum project

Unread post by Dr Bunsen Honeydew »

Think about it, they are wrong.

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Re: Another Goodmans Magnum project

Unread post by _D_S_J_R_ »

The small Doc-rod cabs I have - It takes a huge amount of effort to push in the bass cones and they spring back immediately when let go of. The air in the box really is a powerful 'spring.'
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Re: Another Goodmans Magnum project

Unread post by Geoff.R.G »

The pressure increase in a speaker cabinet resulting from a 10cm cone moving 12mm is about 80hPa (mB) depending on the volume of the cabinet and assuming a 100% seal, so hardly enough to cause a problem to a suitable amplifier. (based on a cabinet 12x10x18cm, less pressure increase for a bigger cabinet)

Barometric pressure is usually around 1013 hPa so a 20 hPa change might move the neutral position of the voice coil by a couple of mm but, if the drive unit is intended for use in an IB cabinet, the design of the coil and former should have taken that into account.

These are just samples to show the effect moving the cone has on the internal pressure, I make no comment on sound quality.

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