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Old 29th May 2005, 06:12 PM   #1
provio is offline provio  Finland
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Default Gainclone for subwoofer

Hello

I'm planning to build a subwoofer consisting of Peerless 12" XLS driver and Peerless 12" passive radiator. These will be placed in ~40litre box. Now I'm not sure about some things and I hope you guys could clear them out for me. Firstly are drivers meant for cars any good for home systems? That 4ohm nominal impedance would be great since I'm also building the amplifier myself. Since this is my first DIY audio project I'll probably start with easier amp (gainclone PA100 with LM3886 chips) but I need some facts will this be powerfull enough for driving this subwoofer at 8ohm load or should I buy the car version instead.. or go for bigger amp?

As for the amp, things I'm looking for are regulated psu, adjustable low-pass filter (I'll probably try Elliot's '
Eight Band Sub-Woofer Graphic Equaliser'), a subsonic filter, driver protection and volume control but more about this later. Have to decide the amp first


Thanks for your time and sorry about my messy english. Still learning
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Old 29th May 2005, 08:53 PM   #2
mateo88 is offline mateo88  United States
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I would say go with the 4 ohm subwoofer, because you can parallel either two lm3886's or two 3875's to get about 100 watts. With 8 ohms, you're only looking at about 50 watts, unless you bridge the chips. You may even want to see if you can find a 2 ohm subwoofer (or dual 4 ohm voice coils), that way you could parallel 4 lm3875's for about 200 watts.
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Old 30th May 2005, 02:11 AM   #3
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I would say the opposite... buy the 8-ohm and bridge two 3886s.
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Old 30th May 2005, 03:50 AM   #4
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I say get the 4 ohm and bridge parallel some 3886's
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Old 30th May 2005, 07:58 AM   #5
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I concur.
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Old 30th May 2005, 03:06 PM   #6
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Hi provio,

I don't know if you have already read this site, but if you don't have a look:

http://linkwitzlab.com/thor-intro.htm

It's describing a subwoofer with the driver you want and it also explains lot of things. I Have used it to calculate the power needed for my subs.
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Old 31st May 2005, 02:01 PM   #7
provio is offline provio  Finland
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Hi again

Thanks for your suggestions and for the link yesgrey3, it seems to be informative.

I will probably go with 8ohm version since I have never heard a subwoofer designed for car's cabin being used in bigger room and because
the T/S parameters betweed these 2 drivers depart from each other (I would probably have to use bigger box with the car driver) and also because
of the price of course. I also decided to use NIGC BPA200 with DC-servo for driving the sub. I know that size isn't straightly comparable with better
sound quality and that I should start with simple GC but what the heck, I like the challenge

I started designing the amp on the base of AN-1192. Is it ok as it is or is there something (critical) that I should take notice of? Should I use snubbers?
Is ±30V rail voltage ok or should I use ±35V? Will 500VA transformer do the job?

I haven't found LF411ACN/LF412ACN op amps from local electronic shops (yet), so I thought of using LF411CN/LF412CN instead. Are these suitable for
the job or should I use some other chips? The datasheets give the A-versions a supply voltage value of ±22V and ±18V for normal ones.
I presume these are minumum values and I could power the chips with my rail voltage? Or do I need another psu?

I'm also having troubles finding 1N456A diodes used in the DC-servo circuit. What whould be a good low-leakage diode for replacement?

Thanks for taking your time to answer these basic questions. I appreciate your help.
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Old 31st May 2005, 03:57 PM   #8
macboy is offline macboy  Canada
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Quote:
The datasheets give the A-versions a supply voltage value of ±22V and ±18V for normal ones.
I presume these are minumum values and I could power the chips with my rail voltage? Or do I need another psu?
No, these are maximum values. You will need to provide a separate power supply for the opamps. The most common voltage to use is +/- 15 V. You can use a couple of simple 3-terminal regulators to regulate your amp rails down to +/- 15V for the opamps (see "U8" and "U9" in AN-1192; did you overlook that?). The non-"A" version of the opamps will work just fine.

Quote:
Is ±30V rail voltage ok or should I use ±35V? Will 500VA transformer do the job?
For an 8 ohm load, go for +/- 35 V rails. That is just about the ideal voltage to maximize output power into 8 ohms. 500 VA will do just fine; even 300 VA or so would be adequate. Don't forget that when choosing a transformer, the AC voltage must be 71% of the desired DC voltage. So for +/- 35 VDC rails, you need a transformer with either dual 24 VAC to 25 VAC outputs, or a single 48 VAC to 50 VAC centre-tapped (often called simply "50 VCT") output. Dual secondaries give you more flexibility in how you build your power supply, since you can wire them up to be centre-tapped or leave them separate or even parallel them (if you only need one output not two).
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Old 31st May 2005, 09:38 PM   #9
provio is offline provio  Finland
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Quote:
Originally posted by macboy

No, these are maximum values. You will need to provide a separate power supply for the opamps. The most common voltage to use is +/- 15 V. You can use a couple of simple 3-terminal regulators to regulate your amp rails down to +/- 15V for the opamps (see "U8" and "U9" in AN-1192; did you overlook that?).
Yup I somehow missed that part. Thanks for clearing this out.

Quote:
Originally posted by macboy

Don't forget that when choosing a transformer, the AC voltage must be 71% of the desired DC voltage. So for +/- 35 VDC rails, you need a transformer with either dual 24 VAC to 25 VAC outputs, or a single 48 VAC to 50 VAC
I'm going to use linear regulators so do I need to worry about AC output, as LM338 is capable of "+40V, -0.3V input/output differential"? If my VDC secondaries are 38Vrms, I could theoretically drop the voltage to 15V with these chips right? Although that could get a little
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Old 31st May 2005, 11:15 PM   #10
Nuuk is offline Nuuk  United Kingdom
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Quote:
If my VDC secondaries are 38Vrms, I could theoretically drop the voltage to 15V with these chips right? Although that could get a little
That's right and you are closer to the ice cap than me so don't add to the global warming! I wouldn't want to drop more than 10 volts through the LM338 unless you use some serious heatsinks!
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