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Old 23rd September 2012, 08:02 PM   #1
JPM14 is offline JPM14  United Kingdom
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Default Maplin 150w mosfet amp. bridging?

I am trying to build up two pcb's for bridging and what I want to do is have each board driving quite a few fets, so that each pcb is 2 ohm stable. That way, I can achieve a substantial power bridged into 4 ohms. I have built a mosfet kit in the past with a long L bracket enough for 6 TO3 fets, with component upgrades to allow +/- 72v operation, where you get something like 300w into 4R. The wiring is done so that all the cases (source) are electrically joined, with a separate gate resistor for all 6 (I tried 470R here). my pcb used MJE340/350 as the pre-fet drivers. I was wondering how many fets can be driven by a pcb using MJE340/350's? I want to end up with a monobloc using two bridged pcb's with possibly 8 fets on each board, running on a lower supply of about +/-60v where the 8 fets should manage 2 ohms with ease. (therefore 4 ohms as bridged load)
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Old 24th September 2012, 09:30 AM   #2
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As long as you don't expect HF out of it you could probably drive 6 pair of FETs.
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Old 24th September 2012, 10:34 AM   #3
AndrewT is online now AndrewT  Scotland
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Do you know the difference between driving a 2r0 load and driving a 2ohm speaker?
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Old 24th September 2012, 11:35 AM   #4
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The Hafler DH600 with 3 pair on 93V could drive 600W/3R, or a 4Ω reactive load quite well. The XL600 added another pair and was able to drive 2Ω loads.

I think his idea of 4 pair per half of the bridge running on 60V will have no problems in 4Ω bridge mode with an adequate heatsink.
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Old 24th September 2012, 07:12 PM   #5
JPM14 is offline JPM14  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Do you know the difference between driving a 2r0 load and driving a 2ohm speaker?
What do you mean here? it's like asking the difference between a 4r0 resistive load and a 4 ohm speaker? amplifiers are usually designed to run speakers aren't they and if the speaker is a heavy load on an amp compared to the resistive equivalent, then so be it. the amp in question will have adequate output devices to be able to drive the rated lowest impedance according to it's specs. So in my example, we already know that just one pair of lateral fets will drive 150w into 4 ohms at +/-55v. so in theory you might expect 2 pairs to be able to drive 2 ohms. 3 or 4 pairs then, should be really comfortable on the 2 ohm load. that's my thinking anyhow.
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Old 25th September 2012, 11:33 AM   #6
AndrewT is online now AndrewT  Scotland
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confused or lost?
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Old 25th September 2012, 01:02 PM   #7
JPM14 is offline JPM14  United Kingdom
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Neither, I have been mucking about with amp kits for 20yrs and always up for experimenation. Do you have any useful input or just pointless comments?
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Old 25th September 2012, 07:42 PM   #8
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"we already know that just one pair of lateral fets will drive 150w into 4 ohms at +/-55v. "

You must mean four devices, two N and two P. One pair (one N and one P) can only drive 7A peak, about 5A RMS, or 100W at 4Ω.
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Old 25th September 2012, 07:54 PM   #9
AndrewT is online now AndrewT  Scotland
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Let's see some proper temperature de-rated SOAR predictions, instead of this guessing.
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Old 25th September 2012, 08:13 PM   #10
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Who is guessing?

My statements are based on commercial designs from B&K, Hafler, Perreax, Tandberg, Soundcraftsman, etc. that I sold, serviced, and measured.

I modified Hafler equipment and used it commercially for PA.

The main failure mode on lateral FETs is over-voltage on the gates from low impedance loads driven into hard clipping. After the gate protection zeners fry the gates of the FETs become a very low resistance from gate to source and the amplifier eventually stops working.

The second problem is heat. The RDSon is about 1.71Ω over temperature for the 7A rated devices, a huge heat problem.
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