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Old 23rd August 2004, 04:35 AM   #1
Mr Teal is offline Mr Teal  Canada
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Default High powered amp choice

I'm planning on building a very high power amp some time in the near future, and amp trying to settle on a design.

Right now, I have two of these
1.9kVA trafos. I also have 4 x 22mF 100V caps, and 4 x 10mF 100V caps.

I also have a bunch (30 of each) of OnSemi MJE4281/4302 BJTs, so the design needs to use those.

Given the size of the PS, and the amount of output transistors I can thow at it, I'm looking for something that puts out around 2kW or more at the lowest stable impedance. Since it'll be (obivously ) a sub amp, super high fidelity isn't the most important criteria, efficiency is moreso.

Right now I'm leaning to a slightly modded ESP P68. Is there anything else out that that would be good to look at before I put an order in with Rod?
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Old 23rd August 2004, 07:42 AM   #2
djk is offline djk
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How many leads out of that transformer, three, or four?

If four leads, a pair of transformers will give about 100V DC no load, about 1.2KW~1.5KW at 2 ohms.

You will need to run about 15 pairs of outputs.

Watch the power in the MJE15034/35, at 100V the forward bias SOA stinks.
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Old 23rd August 2004, 08:43 AM   #3
OliverD is offline OliverD  Germany
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Default Re: High powered amp choice

Quote:
Right now, I have two of these [...] 1.9kVA trafos
Are they 208V primary and 120V secondary as stated in the auction? Then it would be difficult to hook them up to standard mains voltages... And the secondary voltage is somewhat high... Adding/removing windings is nearly impossible because you cannot remove the resin without damaging the transformer.

Quote:
I also have 4 x 22mF 100V caps, and 4 x 10mF 100V caps.
These are good for 80..85V rail voltage... Quite a common number for high power audio amplifiers. But won't get the power you want out of these rails unless you go bridge mode.

Quote:
I also have a bunch (30 of each) of OnSemi MJE4281/4302 BJTs, so the design needs to use those.
Very good devices.

I think if you get new transformers and build two pretty standard +/- 85V amplifiers, using them in bridge-mode, your goals are achievable.
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Old 23rd August 2004, 11:52 AM   #4
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Arrow Commentable Thoughts

Acc. to me plz consider Mosfets instead of Bipolars if u plan to go beyond 500Wrms or more for something real.

Regards
Ampman
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Old 24th August 2004, 03:51 PM   #5
Mr Teal is offline Mr Teal  Canada
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I'm loath to use mosfets, as I already have so many good BJTs.

The trafos are single primary/single secondary, and hooked up the the mains, the no load voltage is 66V, so about 93VDC
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Old 24th August 2004, 05:45 PM   #6
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Quote:
I'm loath to use mosfets
I don't blame you.
So you are looking at +/-93V rails and you want 2kW into what speaker load...8-ohms?
I can't find MJE4281 or MJE4302 on OnSemi's website. Have you a datasheet? Without this it is impossible to determine how may transistors you'll need.
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Old 24th August 2004, 06:22 PM   #7
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Oh, I see. You meant MJL4281. 230W 15A devices in plastic cases. Power must be derated at 1.84W/C.

As has been stated already, you won't et 2kW output into 8-ohms without bridging. In single-ended (not bridged) the most you'll get is approx. 500W into 8-ohms, 1000W into 4 and 2000W into 2.

Bridging is probably the most effective way to get the max reading on the Richter scale. You'd need to make sure the P68 is capable of this or not and whether it has adequate fault protection (I am not familiar with the design). Bridging with an 8-ohm subwoofer will give you 2000W.

If you bridge using an 8-ohm nominal sub-woofer and you want good reliability it is prudent to design the amp to withstand a 4-ohm resistive load. In this case, and assuming a class AB with low bias current, the average dissipation of all the transistors can be as high as 1700W. That is the total for both amps in bridge mode. Assuming your heatsinks/fans can keep the transistor cases at no more than 70C, each transistor will withstand 147W. Ergo, you need at least 1700/147=12 transistors. But the peak dissipation is the limiting factor: with 12 devices they will each suffer 360W momentarily - not ideal. Best to get this below 230W in which case you'll need 20 devices in total. That's 5 per rail per amp.

In summary, you can reliably drive an 8-ohm nominal subwoofer in bridge mode up to 2000W if you use 20 of your transistors and keep them at 70C or lower. This is a conservative calculation. You could use less but I would strongly recommend power limiting circuitry to protect the transistors from exceeding their SOAs. In any case, you should have some protection against accidentally shorting the outputs and a thermal switch on the heatsinks to cut-out at 70C.

Whether or not you can adapt the P68 to this end is unknown to me.
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Old 24th August 2004, 06:59 PM   #8
Mr Teal is offline Mr Teal  Canada
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Sorry, it's the MJLs.

Given the power of this amp and it's use for subs, I'd have no problem with it putting out max power at 2 ohms, with 500W or so at 8 ohms. Unless I'm going with a small box Tumult in a Linkwitz tranform or something, I'd never use it on just one sub anyway.

Also, as it is a sub amp, some measure of distortion is not a big deal. Even a class b output stage is fine. The biggest thing is efficiency. Given the large amount of power, even a small increase in efficiency would translate into a large savings in heatsinking needs.
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Old 24th August 2004, 07:19 PM   #9
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Treat single-ended into 2-ohms nominal the same as 8-ohms bridged in terms of number of devices.

Note that you'll need some pretty sturdy wiring and very thick pcb traces for a 2000W into 2-ohm amp. Your average speaker current could be over 30 amps (>45A peak).
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Old 24th August 2004, 07:25 PM   #10
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In terms of efficiency your best class AB amp will do 71% with worst-case output amplitude. Note that the worst efficiency does not occur at maximum output power.

Rule of thumb: to work out the approx. power dissipation of your electronics, take the max average speaker power and multiply by 3/7.
Eg: 2000W max average speaker power => at least 870W of heat to get rid of in your electronics (depending upon efficiency).
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