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Old 12th July 2001, 05:16 PM   #1
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I need some ideas on what to do for a sub amp. I've got 2 new sub drivers on their way to me. These drivers have dual 4-ohm voice coils, with a power rating of 1KW per VC (2KW per driver). I'd like to build a Class-A/B amp to drive them. Fortunately, the bandwidth will only be 15Hz-100Hz. Does anyone have any ideas that might help with this?

I'm currently considering building a bridged MOSFET amp to drive 2KW into a 2-ohm load (VCs in parallel), but I think the effective 1-ohm load might be too low for MOSFETs. Any opinions?

Thanks

Thoth
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Old 13th July 2001, 01:41 AM   #2
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Thoth,
You're saying bridged. That implies that you've got (or will have) two channels handy. Why not drive each driver separately with one channel? A 1 ohm load is going to be awfully low for most any circuit (whether bipolar or MOSFET--doesn't matter). A 2 ohm load is kinder, assuming that you're set on running the coils in parallel.
Try http://www.aussieamplifiers.com for some large amps. I believe they are MOSFET designs.
The dual voice coil feature opens up a lot of options, depending on your mood. I always liked the idea of driving the second coil with a signal that increases as the driver rolls off, giving another octave or so on the bottom. (Generally, drivers that say they're good to frequency X don't really get there *flat*.) But there are other options.
This sounds like it could be interesting...

Grey
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Old 13th July 2001, 04:24 AM   #3
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Quote:
Originally posted by GRollins
You're saying bridged. That implies that you've got (or will have) two channels handy. Why not drive each driver separately with one channel? A 1 ohm load is going to be awfully low for most any circuit (whether bipolar or MOSFET--doesn't matter). A 2 ohm load is kinder, assuming that you're set on running the coils in parallel.
Actually, there will be two of these subs. If I build amps (the alternative is to buy a pair of Crown K2 amps http://www.crownaudio.com/amp_htm/k.htm), I'll build as many channels as I need. I would prefer to build for a lower voltage (caps are easier to find, and cheaper), thus the preference for bridging. BTW, anybody got a high-power Class-D design handy?
Quote:
Try http://www.aussieamplifiers.com for some large amps. I believe they are MOSFET designs.
Those amps have MOSFET outputs and drivers in an EF(?) configuration, but have bipolar inputs, and VA (I'd use bipolar inputs, VA and drivers with MOSFET outputs in a CFP configuration). These are fairly good amps, from what I've heard, but the design looks a little dated. To get up to the power levels I'm looking for, I'd need four of his biggest amps (1KW into 4 ohms). For some reason, there's not much information on his page about those amps.
Quote:
The dual voice coil feature opens up a lot of options, depending on your mood. I always liked the idea of driving the second coil with a signal that increases as the driver rolls off, giving another octave or so on the bottom. (Generally, drivers that say they're good to frequency X don't really get there *flat*.) But there are other options.
This sounds like it could be interesting...

Grey
The drivers I'll be using are the Stryke HE15 (http://www.stryke.com/SATser.html). I've run the numbers through LspCAD, and the predicted in-room response (1W) will be flat (+/- 1dB) from 16.5Hz to 70Hz, with a 3dB notch centered at 85Hz. This is clean enough that I'll just pour the power in, without worrying about EQ. With 2KW in, each sub will put out better than 115dB in-room, from 16Hz, without the drivers leaving their linear range (bottoming is FAR past that), as per LspCAD.

In case you haven't guessed, I expect this project to take quite a while to complete, with the finished speakers weighing in at 200-250 lbs each. For space reasons (and SAF), I can't use the designs John has on his web page (http://www.stryke.com/kits.htm), so I'm rolling my own. The current plan uses 2 18" PRs facing forward (with the driver), but I'm still looking at my options (rejected sealed, ported and bandpass; checking out TL).

[Edited by thoth on 07-13-2001 at 10:56 AM]
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Old 15th July 2001, 12:57 AM   #4
blmn is offline blmn  Brazil
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Thoth,

I'm thinking about design a 1KW/2ohms class D sub amp with the HIP4080 IC from Intersil or even another more recently developed chip. I will start it from Intersil basic project from this web page.

In my case, I will use it in audio car systems, but I think it will be easy for you to adapt it for home use.

My first idea (no calculations yet) is to use 8 IRF540 or IRF640 to do the job.

I will drawn some drafts and I will be able to send some ideas for you, if you want.

Regards
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Old 15th July 2001, 05:30 AM   #5
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$345 http://www.apexmicrotech.com/pdf/pwm...fiers/sa04.pdf A full bridge PWM amplifier rated at 4KW maximum.Use an isolation transformer.Run the coils in series for an 8 ohm load.An evaluation module with circuit board,heatsink,and filter is available.This model cannot be easily used above 2Khz.Other models are available for high frequency use.Also available are the 10KW SA08 and SA18 for higher impedance high voltage use.The SA03 will drive 1.8KW at 2 ohms if you want to parallel the coils.A surplus isolation transformer will be easier to find for the SA04 though.
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Old 15th July 2001, 07:43 AM   #6
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djk,

I checked out the Apex Microtechnology website, and found that the SA04 doesn't have the current ability to drive 2KW into 8 ohms. In fact, none of their devices can drive 2KW into 8 ohms. Furthermore, none of them can drive 2KW into 2 ohms (current limits). On the bright side, the SA03 can drive 1KW into 4 ohms (the SA13 can't).

This would require 4 SA03 devices at a cost of $375 each. This is equivilent to the cost of one Crown K2. The transformers and other power supply components will add another $600-$800 to this. The case, heatsink and other, miscellaneous components will add still more. I think that, in this case, the minimal savings I would get by building from the SA03, will be more than offset by the probable higher reliability of the Crown amps.

Too bad they don't have just the PWM driver for an H-bridge circuit. That would allow me to hang 4/8 N-channel MOSFETs on the outputs, and use a single supply rail.


blmn,

Check out the Cirrus Logic CS44210. It appears to be a smarter driver chip. Unfortunately, it seems to need a digital input, just like the HIP4080. This is one of the places I run into problems when designing a Class-D amp.

As for MOSFETs, I'd check out the IRFP3710 (100V). With a 57A rating, there would be no need to run them in parallel. Also, the TO-247 package is easier for a DIYer to mount to a heatsink. The input capacitance is a little high, but not much above two IRF540s.

Finally, many industriasl applications that used to use MOSFETs are now moving towards IGBTs. I think that it would be interesting to explore the implications of using an IGBT in a Class-D amplifier. The IRG4P254S looks like a good candidate for this test.
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Old 15th July 2001, 09:56 AM   #7
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I understand how easy it is to misread a spec sheet.Most FETs are rated at 25*C and must be de-rated for real temperatures.The output stage of the SA04 is rated at 20A continuous, 30A peak, guaranteed, at 85*C case temperature.2000W at 8 ohms is 15.81A RMS or 22.36A peak.30A peak at 8 ohms would be 3.6KW [(30 X .707)X(30 X .707)] X 8 = 3.6KW .If you really wanted to know you could call Apex and ask them what die they are using.My guess would be the IRF 260 series rated at 200V 50A (200A peak) at 25*C.The total maximum Ron is less than 0.22 ohms for a package dissipaion of 55W at 2KW out.Looks OK to me.The 10KW series from Apex use IGBT for their 500V ratings.

[Edited by djk on 07-15-2001 at 05:21 AM]
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Old 15th July 2001, 10:14 PM   #8
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djk,

Thanks. I didn't even check the 'maximum' current rating. This allows me to do the job with only 2 devices. Also, I checked Plitron, and found a 7.5KVA isolation transformer (80 lbs!) that should work quite well(shielded with electrostatic screen and overtemp switch), for $364. With dual 100/120 primaries, it will even allow me to run 240V in (I run ALL my amps at 240V; they're plugged into a dryer outlet), and boost the voltage slightly (using 1 or 2 100V taps, as required).

This cuts the cost down to about what one Crown amp would be. There's still the space and reliability issues, but I expect that those can be adequately solved.

Now, I need to figure out how to clean up the 22.5KHz switching signal (I wonder if I can bump this to 30KHz; my girlfriend can hear 22.5KHz). This shouldn't be too hard, as Apex has several application notes that look like they might apply.

I'd still like to go to some high switching rates (100KHz-1MHz; the Crown uses 250KHz), but I may have to accept this.

Thanks again.
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Old 18th July 2001, 07:19 PM   #9
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I've been doing some thinking about Class-D amplifiers, and I came up with an idea. Please see 'http://www.games.accs.com/ER/wizards/thoth/class-D.gif', and tell me what you think.

This is a high-level diagram for a dual PWM amplifier. The diagram doesn't contain any protection circuits, or the output filter.

There is some overlap between the two PWM circiuts. This is an attempt to provide more accurate sound at lower output levels.

Please remember that this is for a 2KW subwoofer amplifier. It is NOT for an audiophile-grade full-range amplifier.

Thanks,

Thoth.
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Old 18th July 2001, 11:36 PM   #10
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Thoth,

Actually, I have some of HIP4080 and IRF540/640 in my lab, so, I will trying use them first.

Regards



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