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Old 20th Apr 2019, 1:56 pm   #1
RojDW48
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Default What might cause oscillation in a speaker?

I have a pair of AR cabs which had been modified to take different drivers. The x-overs were present but the inductors had been disconnected - presumably so that a different x-over could be used with the new drivers (the terminal posts were hard soldered into the x-over, making it difficult to take it out of circuit). I sawed out the odd holes and made up a false baffle to fit over with 3 drivers attached. I re-attached the inductors and connected the drivers to the appropriate sections. At low volume it worked OK but at any significant volume I got a nasty squawking oscillation - what might cause this? It was just an amusing exercise to see what I could do with the cabs (the Mem Sahib says they will make good beds for her runner beans, so all is not lost!), but I would love to know what was going on.
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Old 20th Apr 2019, 2:03 pm   #2
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Default Re: What might cause oscillation in a speaker?

Sounds like the amplifier you are driving it with doesn't like the impedance it is seeing. Try to work out which speaker is squawking by disconnecting bass, midrange and tweeter one at a time. Are both units squawking? if only one channel then you could have a failed capacitor in one of the crossover circuits. Cheers, Jerry
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Old 20th Apr 2019, 2:14 pm   #3
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Default Re: What might cause oscillation in a speaker?

I played about with making speaker cabinets many years ago when I was a lad, and I found it very interesting drilling different size holes etc, and observing how they affected the performance of the system. I soon realised there was more to cabinets than I first thought!
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Old 20th Apr 2019, 3:09 pm   #4
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Default Re: What might cause oscillation in a speaker?

Hello Jerry - you probably recall these cabs? Finally got around to playing with them - one only so far. Is that squawking likely to harm the amp's output? It's a Radford SPA50 solid state job. So far as I am aware the drivers are all 8ohm but I'd better check with my little meter - thanks for the advice.

John - I also fiddled around with cab design in my mis-spent youth. Back in the 70s a friend gave up on making a pair of Decca corner horns and gave them to me. I turned them upside down and made them into transmission lines....the earth moved!! So did the neighbours...with extreme prejudice in my opinion
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Old 20th Apr 2019, 4:00 pm   #5
Edward Huggins
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Default Re: What might cause oscillation in a speaker?

If I were you I would abandon the x-overs for the original AR drive untis and make up 2 very simple x-overs. You only need 3/4 components for each one, there are circuits aplenty. Maplin used to sell them and suppliers like Falcon can advise.
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Old 20th Apr 2019, 4:13 pm   #6
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Default Re: What might cause oscillation in a speaker?

I wonder if your activities in the crossover department have left a capacitor right across the amplifiers output. they REALLY don't like this.

Speakers are passive. Oscillation needs a source of energy. Either it's the amp hooting or you've just discovered a new source of energy!

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Old 20th Apr 2019, 8:16 pm   #7
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Default Re: What might cause oscillation in a speaker?

Thanks for the advice - I checked the drivers and gingerly connected up again - no problems in fact they sound quite nice. Pics of one in its raw, unprimped up state and the curious assortment of drivers.
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Old 21st Apr 2019, 6:25 am   #8
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Default Re: What might cause oscillation in a speaker?

The woofers a Kef I think, I have one upstairs, the others obviously nicked off Jap boomboxes of hifi. AFAIK the AR speakers were designed as closed baffle so the drivers would have been chosen for this design, no idea what the original drivers would have been, but a Seas 1" dome tweeter would be better than a Chinese Sony one would think.

I still have a few drivers Roj if you want to try something a bit more hifi in the speaker department.

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Old 21st Apr 2019, 8:41 am   #9
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Default Re: What might cause oscillation in a speaker?

Neat job with the replacement baffle boards - glad you were able to rejuvenate them! Cheers, Jerry
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Old 21st Apr 2019, 11:08 am   #10
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Default Re: What might cause oscillation in a speaker?

Thanks Andy - I shall think about how far I want to take this project. Yes I think both woofers are versions of the Kef B200. The unfitted one in the pics definitely - the fitted one I have had since the 60s - I think it's an early B200 but it has a cast basket so . I can't recall where the Asian ones came from!
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Old 21st Apr 2019, 11:38 am   #11
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Default Re: What might cause oscillation in a speaker?

Those little speakers need to be in a small enclosure within the main box or they will suffer from the air pressure created by the woofer. Most tweeters are closed back, even if the mid range is open to internal pressures. In fact the 'tweeter' looks more like a mid range/full range out of something of relatively low power. The white Pioneer clearly shows it's only rated at 3 watts max.
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Old 21st Apr 2019, 5:05 pm   #12
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Default Re: What might cause oscillation in a speaker?

As Andy suggests the bass unit is a KEF B200 which is a very capable driver used by a number of highly regarded manufacturers as well as KEF.

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Old 21st Apr 2019, 11:48 pm   #13
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Default Re: What might cause oscillation in a speaker?

Quote:
Originally Posted by vidjoman View Post
Those little speakers need to be in a small enclosure within the main box or they will suffer from the air pressure created by the woofer.
Spot on! I noticed this afternoon that they were flapping in an uncoordinated manner. The other curiosity is the difference between the two woofers (maybe not both B200s?). The definitely B200 does not have anything like the dispersion of the much older one (shown fitted in the first pic) so the relative volume of each speaker is very different.....unless I sit dead in line with both 'SP200s' - when they sound about equal.

I wasn't expecting much in the way of 'hi-fi' from this experiment. The x-overs seem to be good quality (at least the elements are - the copper trace is about a molecule thick and the hardboard (!) mounts were stapled to the cabs with a staple gun ). I have some better mid range drivers I could try but I obviously need to match the woofers more effectively before proceeding.
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Old 22nd Apr 2019, 8:34 am   #14
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Default Re: What might cause oscillation in a speaker?

A KEF B200 is a generic number - they made loads of variants and are distinguished by the SPxxx number that's usually on a sticker. The SP type is dependent on several factors and 2 different ones will probably give a different sound, being dependent on the cabinet design.
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Old 22nd Apr 2019, 8:27 pm   #15
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Default Re: What might cause oscillation in a speaker?

They certainly did make a range! I had a pair of Reference 103s which had been vandalised - lots of holes poked in the cones. No problem I thought - I have spares....but not the right ones. The right ones were on sale at eye-watering sums, so I repaired the cones and they sounded very nice!
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