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Old 23rd March 2008, 09:09 PM   #1
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Default 1000 Watt Sub Amp: Design / Build

Another ambitious project! The goal: to build a sub amp that is capable of driving (up to) 1000 watts into a 2 ohm load.

Normal use for this amp would be a 4 ohm load, but for future consideration, I thought it would be better to over-build rather than just meet my immediate needs.

I am currently using a ~500 watt @4 ohm based on Rod Elliot's P68. This was my first ever amp build, and though it works very well, it looks pretty rough. I did it on an old circuit board recycled from a scrapped receiver. I sanded off all of the copper and wired all of the components point to point. Truly amazing it works at all! In another thread here, there's a picture of it (in the top right hand corner) and the sub it drives.

I have been planning this for a while now, and I've come up with a schematic that sims well and is relatively simple. Supply voltage is +/-70 volts from a 50-0-50, 850VA toroid.
During my last amp project (Patchwork ) I learned a lot, but I'm still nowheres near confident enough to try to pull this one off without some help and guidance.
Anyhow, here's the schematic. I ask for any advise and critical comment (good or bad).
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Old 23rd March 2008, 09:56 PM   #2
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Default Re: 1000 Watt Sub Amp: Design / Build

Quote:
Originally posted by MJL21193
Another ambitious project! The goal: to build a sub amp that is capable of driving (up to) 1000 watts into a 2 ohm load.
Hi John,

quick look, should be fine - I don't see any obvious problems. I trust that the majority of electrolytics is off-board and you won't use the 330uF cap on its own.

Since you will use it for a sub-woofer why not band-limit it to say 2kHz, just in case there are some sputious in the input and you fry the voice coils.
Kind regards

Nico
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Old 23rd March 2008, 09:59 PM   #3
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Sorry John a quick question, how will you compensate for temperature, are the diodes mounted on the heatsink?

Nico
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Old 24th March 2008, 01:28 AM   #4
Leolabs is offline Leolabs  Malaysia
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You may try to replace those bias diodes with 'diode connected transistors'.
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Old 24th March 2008, 01:43 AM   #5
kalmara is offline kalmara  Bulgaria
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Quote:
Originally posted by Leolabs
You may try to replace those bias diodes with 'diode connected transistors'.
a.k.a. Emitter follower, and mount the transistor on the main heatsink. He has it in his other project - the patchwork. On the other hand the main goal here is a subwoofer amplifier, wich means bias is not a big problem here with the amp working in almost ~class B, so the diodes will work fine. It might be a problem, and the emitter follower can be used...if the amp was another class above like "Hi-Fi" or so, for improving the performance in higher frequencies....i think.
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Old 24th March 2008, 03:22 AM   #6
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Default Re: Re: 1000 Watt Sub Amp: Design / Build

Quote:
Originally posted by Nico Ras


Hi John,

quick look, should be fine - I don't see any obvious problems. I trust that the majority of electrolytics is off-board
Hi Nico,
Good to hear from you here. For the power supply, I'm not quite sure how much capacitance I need. The transformer is being used now in the "rustic" version with 10,000uF on each rail. It doesn't seem to run out of gas, even on the big power demands from movie soundtrack LFE through the Linkwitz Transform. For a lower impedance, I expect I should have more per rail - maybe 20,000-40,000?


Quote:
Originally posted by Nico Ras
Sorry John a quick question, how will you compensate for temperature, are the diodes mounted on the heatsink?
Yes, thermal tracking with the diode string in contact with the heatsink. I will do the board layout to make this possible.

I did a fair bit of research on heatsink requirements for this amp. Using this formula: Pd = (Vpp^2 )/(R(Load)*2 * PI^2 to determine total dissipation at full power I get 490 watts at 2 ohm load. Lotsa heat! The heatsink I have for this has a thermal resistance of .37*/watt (BarrredBose on Ebay - 12" long, shown below). For continuous duty into low impedance I'd need some fans to lower the thermal resistance.
The fact that this amp will not be used for full audio spectrum, but limited to 80 Hz and lower, give a bit of breathing room. (hopefully)
Anyone with a better grip on the cooling requirements for this, feel free to chime in, especially if you think the heatsink is not enough. The one in the "rustic" version is a homemade job, with less mass than the new one. It doesn't get hot at all.

Here's the heatsink, with the 18 output devices laid out to see how they fit.
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Old 24th March 2008, 03:29 AM   #7
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Since you are driving such a low impedance load, you may want to try raising your VAS current.

Your schematic shows 2x 3.3K resistors on 70 volt rail. I built a 350W 4 ohm amp, that uses 2x 2.2K 2W resistors to get 16ma for VAS. They get hot, and the VAS is warm on small heatsink, but I get good sound and drive, even at tested 2 ohm loads, even though the amp is rated four 4 ohm.

Since you want an amp RATED at 2 ohm...............
You may want to try 2x 1.5K ohms for VAS since 2 ohm load requires twice the regular current of 4 ohm. Just get some big 2W (or 5W) resistors so they can handle the power. Use a good VAS transistor and don't let it run too hot.

Also, since it's 1000W, if you have enough transistors, try 10 pairs of outputs. I used 5 pairs of MJL4281/4302 for 350W, and the output stage survived a shorted speaker coil and just blew the fuse.

Overkill is good, especially if you blow a speaker, or have any other fault, hopefully the fuse blows before the transistors because there's so many to handle the load.
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Old 24th March 2008, 03:53 AM   #8
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Quote:
Originally posted by Leolabs
You may try to replace those bias diodes with 'diode connected transistors'.
Hi Leolabs,
I wanted to keep it as simple as possible, given the intended use for this I used the diodes. I boroughed heavily from Rod Elliot's P68 here, which is a class B. I added the third diode to give some quiescent current (~85mA/device as things are now). I may change my mind on this though...

Quote:
Originally posted by kalmara

a.k.a. Emitter follower, and mount the transistor on the main heatsink. He has it in his other project - the patchwork. On the other hand the main goal here is a subwoofer amplifier, wich means bias is not a big problem here with the amp working in almost ~class B, so the diodes will work fine. It might be a problem, and the emitter follower can be used...if the amp was another class above like "Hi-Fi" or so, for improving the performance in higher frequencies....i think.
My thoughts exactly, concerning the "hifi" quality. The diodes do the trick, but there are some drawbacks: the inability to easily change idle current and putting a limit on the VAS current. The VAS may be the biggest problem.
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Old 24th March 2008, 04:18 AM   #9
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Quote:
Originally posted by EWorkshop1708
Since you are driving such a low impedance load, you may want to try raising your VAS current.


Also, since it's 1000W, if you have enough transistors, try 10 pairs of outputs. I used 5 pairs of MJL4281/4302 for 350W, and the output stage survived a shorted speaker coil and just blew the fuse.

Overkill is good, especially if you blow a speaker, or have any other fault, hopefully the fuse blows before the transistors because there's so many to handle the load.

Hi EWorkshop1708,

The problem with raising the VAS current in the present configuration, is that idle current will also increase. The VAS is running with ~12mA right now. I'd like to increase it - up to 20mA or more (the device can handle it), but that means either removing one diode and let the amp run B or ditch the diodes and use a Vbe multiplier.
I'm wondering if it's really necessary to raise VAS current though. The simulation runs fine and the nature of the output stage, with the CFP providing tons of current for the outputs.
Am I seeing it right?

I worried about how many outputs to use. The one I use now has 4 outputs plus the CFP. It seems fine for 4 ohm loads. I searched on google and found
this site where the guy has a spreadsheet to figure out how many outputs are needed to maintain a safety margin. I found the 8/side was good with these devices at the 2 ohm load, given the intended use for the amp.
Also, the available space on the heatsink is limited. I could probably squeeze one more per side.
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Old 24th March 2008, 05:00 AM   #10
GK is offline GK  Australia
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Quote:
Originally posted by MJL21193


Hi Leolabs,
I wanted to keep it as simple as possible, given the intended use for this I used the diodes. I boroughed heavily from Rod Elliot's P68 here, which is a class B. I added the third diode to give some quiescent current (~85mA/device as things are now). I may change my mind on this though..

Using diode connected transistors instead (collector tied to the base) is a good idea. You can use a low power, small die (for quick thermal response) TO-126 packaged device such as a BD139. They are much easier to mount on the heatsink than a string of 1N400X's. You can even screw them down onto the top of three of the MJL21193/4 trannies with the same three mounting screws. This makes the PCB layout easier as you then hard wire the trannie-diode string, and the thermal response will be better than a heatsink mount.

An opamp based low pass filter preamp with selectable turnover frequencies and roll off slope for this amplifier would also be something to look into.

Cheers,
Glen

PS, with a 1kW subwoofer amplifier Iím glad youíre not my neighbour.
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