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Crossover resistor in series with bass unit?

 
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audiolabtower
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PostPosted: Sat Aug 01, 2015 6:44 pm    Post subject: Crossover resistor in series with bass unit? Reply with quote

Some years ago I got a pair of Cresta 10s for background music in the study for around 30 quid.



They are not bad for the original price, which was around £100 I think? The silk tweeter is particularly good, the lower mid only suffering from almost universal boxy colourations, bass is surprising for the size, and they can go loud. Good for popular music.

The crossover is quite simple





Question is why is there a resistor in series with the bass unit? I can understand the tweeter resistor to balance the sensitivities, but the bass resistor must affect damping? Does it not matter because of the limited bass response?



Some kind of partial zobel network with the 10uF compensating for impedance?

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SaSi
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PostPosted: Mon Aug 03, 2015 8:01 pm    Post subject: Reply with quote

My ignorance begs the following question:

If there's a 1.5Ohm resistor in line with the woofer, and a 2.7Ohm resistor in line with the tweeter, would removing the 1.5Ohm and replacing the 2.7Ohm with a smaller one (1.2Ohm ?) make more sense?

I've read and accepted the rationale behind "never put a resistor in series with the woofer". That xover makes very little sense to me.
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audiolabtower
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PostPosted: Mon Aug 03, 2015 8:14 pm    Post subject: Reply with quote

Exactly my thoughts too Smile ... but this is a Kef speaker so I assume it is designed with a knowledge of what they are doing and it is there for a reason? They would not spend money on a component in a budget speaker in the most competitive area of the market (the £100 budget speaker?) without it giving some performance justification to make it worthwhile? And they do have the sophisticated computer design as a starting point too?
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SaSi
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PostPosted: Mon Aug 03, 2015 9:39 pm    Post subject: Reply with quote

A crazy thought...

I have a power amp (Technics SE-9200) that sports an output impedance selector. That affects damping factor. I can tell you that with some kinds of speakers, the lower damping factor with the fluffy bass sounded pretty appealing on smaller boxes.

But again, a resistor taking all the punishment of the power routed to the woofer makes no click with what I understand.
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speakerguru
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PostPosted: Mon Aug 03, 2015 10:27 pm    Post subject: Reply with quote

audiolabtower wrote:
... but this is a Kef speaker so I assume it is designed with a knowledge of what they are doing and it is there for a reason? They would not spend money on a component in a budget speaker in the most competitive area of the market (the £100 budget speaker?) without it giving some performance justification to make it worthwhile?

...after 1991, some very strange things went on as the change of ownership and the exodus of the Fincham era engineers progressed...

audiolabtower wrote:
...And they do have the sophisticated computer design as a starting point too?

I'm not sure the computer optimised target function approach to speaker design survived for budget designs. I know outside consultant designers who favoured trial and error with a bucket of components were used at times.
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audiolabtower
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PostPosted: Tue Aug 04, 2015 3:35 pm    Post subject: Reply with quote

Thanks Guru. So it could well be the old bucketful of components scenario? How sad.

Have you any idea why you would put a series resistor in the bass? I seem to remember some very strange crossover topologies in old Monitor Audio and maybe Tangent speakers in the past, and I always thought they looked as if were designed with a piece of wood, some nails banged in, and random components stringed about presumably by listening alone...
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audiolabtower
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PostPosted: Tue Aug 04, 2015 3:37 pm    Post subject: Reply with quote

SaSi wrote:
, the lower damping factor with the fluffy bass sounded pretty appealing on smaller boxes


That's a good theory Wink
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proffski
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PostPosted: Wed Aug 05, 2015 8:56 am    Post subject: Reply with quote

Tricky, but within my limited knowledge I'd assume that increasing the series resistance should increase Qes. This then influences the Qts which is an important parameter in a reflex tuned box.
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audiolabtower
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PostPosted: Wed Aug 05, 2015 11:29 am    Post subject: Reply with quote

I had to look that up Prof... but again a good point! Smile

I do believe they were originally supplied with some foam bungs to tune the bass (there is a big port on the back). When I had them direct on the desk top they were too boomy, so I cut some cylindrical hard foam for the reflex tubes and that was enough to cure it. Then I made small stands only 5cm high with free air underneath and the bass was too light and bungs were not needed anymore giving an overall better sound.
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proffski
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PostPosted: Wed Aug 05, 2015 12:37 pm    Post subject: Reply with quote

I'm sure that Speaker Guru will put me right if my assumptions are not correct. Wink

The most impressive effect of raising loudspeakers off a surface I once demonstrated to a friend who had a couple of small Tannoy Mercury enclosures on the floor, the effect was DRAMATIC!

Whilst I am no fan of "Hi-Fi Tweaks" I did once tune a pair of IMF Super Compacts by gradually adding BAF wadding inside the enclosure, again the changes were not only for the better but instantly audible, we could not believe our ears on the improvement... The tonal quality was infinitely for the better and not wishful thinking just because it was “different” as with many other tweaks.

IMF: http://www.imf-electronics.com/Home/imf/speaker-range/compacts/super-compact

Tannoy: http://www.hifidatabase.com/static/gallery/8/6748-tan_Mercury-M20.jpg
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iso
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PostPosted: Wed Aug 05, 2015 3:43 pm    Post subject: Reply with quote

If you try to use some popular vented box alignments with low Qt speakers like Altec 604E or Tannoy HPD 385, you will get suggestions to use 35-50 L enclosures with quite high f3 figures. Those 15" drive units will hardly fit in such small cabs.

However, in these cases it might be useful to use high DCR series coil to increase Qt slightly. Coil DCR of 2-2,5 ohms is not too dramatic for 16 ohm Altec, but it will increase Qt enough to make sensibly sized cab behave very well. f3 will drop to 40-50 Hz with approx 100 L cab volumes.

I do not know if this is true for Audiolabtower´s cabs, but it came to my mind anyway.

Best Regards

Kimmo
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audiolabtower
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PostPosted: Wed Aug 05, 2015 6:45 pm    Post subject: Reply with quote

That looks a pretty good theory to me! After all it is a very small box for a reflex design with very large and long port (box is stuffed full of wadding, presumably to stop mid frequencies resonating in the tube and getting out). You would imagine the normal route for a 130 mm drive unit would be a closed box?

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