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- Thread starter Opie
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higher-powered amplifier, but given a hefty power supply and

heatsinking (I'd recommend a 'wind tunnel' heatsink and

forced air cooling), it should tolerate low-impedance

loads.

The output stage is series-parallel. His lower-power

amplifier should work pretty well if you parallel an

additional pair of output transistors and use

MJ21194/21193 devices and perhaps up to a +/- 75 volt

rail.

Paralleling will improve distortion at high currents

and help with the SOA, but there are limits quickly

approached, so don't push it too hard.

> I have a 3 ohm subwoofer that I would like to drive on a

> continuous basis. The reason I want to know if the amp

> could drive a 2 ohm load is that later I would like to drive

> two 4 ohm speakers in parallel.

This is what I've found out building Leach Amps, and while planning monstrous SuperAmps, but settling soon for a bit expanded version of the SuperAmp...

The original Amp can drive 2 Ohms by default. The devices are either 16A or 20A, and the protection circuit allows a bit over 10A for each. The original Leach Amp would have no problem driving your 3 Ohm subwoofer. But at 2 Ohm (two 4 Ohm speakers in parallel) you'd have a problem if the impedance of the speakers ever goes below 2 Ohms.

My friend has a Leach Amp with 6 power transistors instead of 4, and he drives happily two 4 Ohm subwoofers in parallel. The protection circuit limits current at full power if the load drops down to 1.35 Ohm. It would be "only" 2 Ohms with 4 transistors.

Cooling is no problem here. He has a 0.4K/W heatsink in it. The Amp would easily handle any techno and bass he ever tortures it with, but he has added two 119mm fans just to be on the safe side. I once tried boiling water in a 3 litre ceramic pot, using sinewaves and ceramic resistors, with the amplifier. I could use full power into 3 Ohms safely for many minutes. The 545VA transformer got once so hot I got scared, but otherwise nothing went wrong.

With music the heatsink gets barely warm, used into 2 Ohms, with full power, fans running slow, and for extended time. He gets a little more than 400W RMS into 2 Ohms. The Amp would still be fine - but not as cool - even with fans stopped.

But you were into SuperAmp as far as I understood. Basically I'd guess it would put out 20A and then limit. That means a little harder load than 4 Ohms could be used. 93Vcc, theoretically 12V lost in the power devices, we are left with 81V no-load output. With 2pcs 10 000uF for one channel, you have 64V peak into 4 Ohms. 510W RMS.

The same into 3 Ohms results in 60V peak. 600W RMS, hard to tell if voltage or current limited. 400W into 2 Ohms, current limited.

Doing the same with SuperAmp as my friend did with his Amp: 12 transistors for one channel. 30A peak current now possible. You have 700W RMS into 2 Ohm... voltage limited, surprise! With 2pcs 20 000uF in the power supply, ripple halves and you are current limited to 60V output, i.e. 900W RMS. Voltage limit would come at 64V, so it's quite balanced solution. For your 3 Ohm subwoofer (and 2*10 000uF) the figures are the abovementioned 60V and 600W RMS, only this time clearly voltage limited, current limit being quite far away. 68V and 770W RMS@3 Ohms with 2*20 000uF.

My formulas are quite demanding, just like the experiment of cooking water with Leach Amp's output power. You can get these voltage and current ratings with 50Hz square-wave (or any frequency sinewave) in the mentioned cases if you want. After the frequency of sinewave rises above a certain limit, the demand for power supply capacitance halves within an octave, and is the same above that.

-Kimmo S.

Rather than drive a 2 ohm speaker, I use a bridged Leach-amp to drive a 4 ohm load. I have also tested this on a bench supply. On the bench, I was cranking 900+ watts without limiting until I blew the 14 amp fuse... However, I have blown my share of transistors, as follows:

Output: 2 x MJ15003/4

Board: 1 x Q9 (MPS8099)

Q9 is part of the protection circuit, and when it blew, so did 2 of the output transistors. This was a freak accident due to some no-name transistor I used for the protection (at least I blame it on this). I have since switched to Motorola (On Semi).

My current amp that I use to bridge has an interesting problem that developed after abusive bridged use into my monster 8 x 12" subwoofer speaker towers that I built. In non-bridged mode (4 ohms), NO problems occur. In bridged mode, some wierd clipping distortion occurs past 600 watts or so. I haven't taken apart the one (right) channel where this occurs to fix the clipping problem. Keep in mind that this is the leachamp, not the superamp.

Summary: with a peak indicator and heatsinks on the transistors on the circuit board, I wouldn't be scared using this amp into a sub-1-ohm load.

=DJSS=

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