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Old 3rd February 2010, 12:30 AM   #1
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Location: Norlane; Geelong: Victoria: Australia
Default Aussie kit amp query

I have a Jaycar kit amp ( No I didn't build it myself; too complicated )
The "Playmaster Pro 300"

I am building new speakers and intend to use Rod Elliots ELF system for the bass, I need to know if in this application whether or not the amp can handle the low DCR load of 3 * 7.8 woofers in parallel

I am told the design of this amp runs the power MOSFETS very close to capacity.

FS and therefore impedance peak is at 32 Hz on average with the spread being 29 -> 36 over 6 drivers and I didn't test them myself, had help from a friend who is now too far away.
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Old 3rd February 2010, 12:57 AM   #2
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from ETI-466 Amplifier Technical Review

“the Electronics Australia Playmaster 300 [ref. 23] used as standard three MJ15003/ MJ15004 pairs for the same power output rating. Even then designers J. Clarke and L. Simpson noted that "typical 4 Ohm reactive loads are likely to cause the protection circuitry to operate when driven hard and thus reduce the maximum power delivered. This compromise is absolutely necessary. While a six transistor output stage of this sort may appear very rugged, it cannot be allowed to drive 4 Ohm loads without protection circuitry”

maybe you can reduce the supply voltage (+- 70v)?
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Old 3rd February 2010, 01:10 AM   #3
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Or redesign for use with C5200/A1943 which are rated to 12A and 230V and have superior SOAR for reactive loads. I use these devices in all my amps and have never had a problem either on the testbench or with customers using a wide variety of speakers in domestic settings. They are very fast devices however, (ft of 30MHz) so you'd likely need to examine the compensation on the amp to ensure it's unconditionally stable, and this might be a bit much for a hobbyist without a good CRO and a lot of angst.

Alternatively, without too much change, you could rejig for MJE21193/94, which are very rugged devices and a similar speed to the original, so likely no compensation changes required.

At these powers, it's unlikely you'd really get into trouble in a domestic setting with speakers down to 3R, but you might consider softening the settings of the protection circuitry a little. The 15003/4 are widely used in PA/sound reinforcement amps, pretty tough devices.

Hugh
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Old 3rd February 2010, 01:31 AM   #4
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Default Sorry my mistake

My amp is the Pro Series 3 designed in 1994
uses 2 * 300 VA toroidals and only two (2) of the Hitachi MOSFETS per channel.

It has no protection or soft start circuits built in, it pushes a solid 4R load
( Kicker sub ) with no problem
While it is rated IHF 380 into 4R loads I always assumed that rating was optimistic and the clipping light comes on when pushed ( + 90% on the pre-amp but that is LOUD )
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