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Class D Switching Power Amplifiers and Power D/A conversion

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Old 16th July 2009, 11:34 PM   #401
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I'm guessing the 23.6 litres is at Qb 0.707. As I explained in post 370, you divide the Vb by Pi (3.14), and multiply Fb with Squareroot of 2 (1.414) to get optimum Qb for outdoors purposes 1.212.

Try both configurations. In 8 Ohms it will have the volume as with a single pair in 4 Ohms but will use half the power. In 2 it'll have 4 times the volume (+6dB) of a single pair in 4 Ohms but will use twice the power, assuming the amp can even handle 2 Ohms loads.
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Old 17th July 2009, 08:06 AM   #402
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Hi Saturnus

Quote:
Originally posted by Saturnus
I'm guessing the 23.6 litres is at Qb 0.707. As I explained in post 370, you divide the Vb by Pi (3.14), and multiply Fb with Squareroot of 2 (1.414) to get optimum Qb for outdoors purposes 1.212.
Thanks for repeating, I obviously overlooked that on my first 2 trawls through the thread, probably because I didn't even know what Vb etc was at that point. Apologies if I am spelling this out slowly.

I was thinking that sealed would be easier to start with, however there is no Fb then. Does that mean I just divide the volume by pi (from 23.5 to 7.5 liters) resulting in an Qtc changing from 0.707 to 0.97? Or do I decrease the Vb until the Qtc = 1.212 (4.2 liters)? What is the relationship between Qtc and Qb?

If I was to go with a ported box (which now makes more sense given the smaller volumes required), the initial recommended Vb is 44.0 and Fb = 60.85. Decreasing the Vb by pi and increasing Fb by 2^(1/2) yields Vb = 14 and Fb = 86.05.

A comparison of frequency response between sealed vs vented shows much better performance at lower freq with vented and seems preferable. Actually, in the interests of my own knowledge , I might just make one of each.

In terms of port placement, am I correct in thinking that as long as the port is close to the driver (within 1/4 of the wavelength of the Fb) the actual location is not that important.

Quote:
Originally posted by Saturnus
Try both configurations. In 8 Ohms it will have the volume as with a single pair in 4 Ohms but will use half the power. In 2 it'll have 4 times the volume (+6dB) of a single pair in 4 Ohms but will use twice the power, assuming the amp can even handle 2 Ohms loads.
I will try both, though leaning towards in series (8ohm) as I think the amp will be more comfortable there.

If I get super-motivated (and it rains this weekend), I may just attempt to summarise as much of the information contained within this thread as I understand into one document.

Cheers,
-Vinnie
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Old 17th July 2009, 10:39 AM   #403
coit is offline coit  Australia
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Saturnis it's great to see you back!
I am running the 4 ohm 10" PAudio speakers with the Jaycar piezo tweeters with 5A wirewound 100ohm resistors.It sounds GREAT.
Do you think the inductance of the wirewound resistors is adversely affecting the sound?
Also,reading about piezo tweeters,most suggest 20ohm or so to protect the amp at high frequencies.You say the resistor cuts out the bottom frequencies of the piezo tweeter.I hope this isn't too esoteric;I just want to know.
This unit is a mind blast.
thanks so much
Paul (raging) Curtis
PS I originally ran it with 2 Sure TA2024 amps at 16V.
I now have 2 of the Sure 2050 amps running at 26.4V while I'm waiting for a 36V power supply.
thanks again for putting us onto these speakers
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Old 17th July 2009, 11:00 AM   #404
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motorbikevinnie,

the trick only works with ported boxes. With closed boxes you tune to Q=1, and add electronic compensation.

Your numbers for the box seems fine. I'd go with that. And actually the port placement doesn't matter at all as long as the airflow is unobstructed.



coit,

the series resistor indeed has many effects when combined with a piezo. One of them is that it prevents the amp from going into self-oscillization because the amp doesn't see a reactive load at high frequencies but for that effect it doesn't matter whether it's 20 Ohms, 200 Ohms or even more.

I sincerely suggest you try exchanging the wirewound resistor to a carbon resistor, or alternatively if you can't get that, a metalfilm one. I was quite surprised by the improved performance myself, I have no doubt you will hear it too.



to both and everyone else,

I'm just very happy that all my hard work is helping so many people around the world. Boomboxes is one of the first audio things people get interested in. And to inspire others to strive for good sound even under difficult conditions has been very awarding in itself.
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Old 17th July 2009, 12:50 PM   #405
coit is offline coit  Australia
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Thankyou for your quick reply.
I have 1watt resistors but can't find 3watt.
I will make them.
thankyou
Paul
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Old 17th July 2009, 09:56 PM   #406
coit is offline coit  Australia
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If I put 3 33ohm 1watt carbon film resistors in series,is that 99ohms at 1watt or 99ohms at 3watts?
thanks
Paul
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Old 17th July 2009, 10:18 PM   #407
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99 Ohm 1 Watt
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Old 17th July 2009, 11:29 PM   #408
coit is offline coit  Australia
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Sorry about the basc questions.
What about 2 by 330ohm and 1 by 270ohm 1watt resistors in parallel giving 102 ohms? What would be their power dispersion?
thanks
Paul
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Old 17th July 2009, 11:52 PM   #409
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2.612 Watt, close enough.
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Old 18th July 2009, 12:09 AM   #410
coit is offline coit  Australia
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Thankyou for your help.
4 390ohm resistors give 97.5 ohms.
This should be better
Paul
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