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Old 23rd July 2007, 02:02 AM   #1
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Default power output for mjl21194/94

How big an amplifier can you build with two pairs of mjl21193/94 BJT's? Drives both 8 and 4 ohm loads, standard class AB output stage?
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Old 23rd July 2007, 02:11 AM   #2
anatech is offline anatech  Canada
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Hi Bama Slamma,
That would put you in the neighborhood of about 100 watts continuous. A little conservative, but wise given the SOA plus lower than nominal impedances with possible large phase angles for current vs voltage. That will increase your output dissipation more than most people expect.

Can you get more out by pushing them? Yes, at the expense of a less reliable design. Don't skimp on the heat sinks either.

Also, don't forget that you may run out of driver transistor SOA. Keep these things in mind.

-Chris
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Old 23rd July 2007, 02:24 AM   #3
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Hi,
I'm getting over 500 watts into 4 ohms with 5 pairs. EQ'ed subwoofer duty, so fairly light use.
Drivers are MJE15030/31.
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Old 23rd July 2007, 02:38 AM   #4
anatech is offline anatech  Canada
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Hi MJL21193,
Quote:
I'm getting over 500 watts into 4 ohms with 5 pairs.
I'm assuming that you dropped your supply voltage to deal with it during design. Still a heavy load, but no reactive, funny crossover to mess up your impedance. I think you are pushing them as your heat sink appears a bit on the small side.

How hot does it get?

-Chris
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Old 23rd July 2007, 02:59 AM   #5
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Hi Chris,
Baby steps on startup - +/-15 volts. +/-70 volts now.
Heatsink is on the small side (homemade), and would not do for full musical program.
How hot? After watching a bass heavy movie for an hour and a half it's pretty warm, but easy to keep hold of. I had originally planned on a fan but decided against it. Unit's been in use for more than 2 years. No smoke.
Also it's EQ'ed with a Linkwitz transform, so the LFE track can push it.
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Old 23rd July 2007, 03:03 AM   #6
anatech is offline anatech  Canada
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Hi John,
Sounds good then. You are either clipping it or not running it that hard. Less heat when clipping heavily.

Now you just want a bigger heat sink!

-Chris
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Old 23rd July 2007, 03:19 AM   #7
K-amps is offline K-amps  United States
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If you are running 8 ohm loads, you can even get 200 watts per channel from 2 pairs, however as Chris says... any lower impedance or reactive loads could fry them. So for an actively crossed 8 ohm subwoofer application, I would go ahead use +/-70-80 vdc rails on 2 pairs. Any funny loads and make that 4 pairs.

A good design with a powerful PSU should yield a 100wpc amp with 2 pairs, with a skimpy PSU, you ccould do a 150wpc amp . (Because with a skimpy PSU, the rails sag much more and protect the devices).
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Old 23rd July 2007, 03:25 AM   #8
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That would be a negative on the clipping Chris. My temperature probe during testing shows 58-62 C, so heatsink is doing ok. There's more of it under the board.
Not driven hard normally, but has no problem providing enough juice to drive 2-18" subs to the near destruction of my house.

As you could probably guess from my handle, I'm fond of these BJT's.
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Old 23rd July 2007, 05:20 AM   #9
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MJL21193, that's a very nice looking amp module. How much bias are you using? Protection circuit? Used it on a full range cabinet? I guess for full range heavy use, put on a bigger heatsink, drop the supply rails down a bit, and possibly add forced cooling.
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Old 23rd July 2007, 06:25 AM   #10
Duo is offline Duo  Canada
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On the subject of the 21193 and 21194:

I was operating single pairs in a test amplifier of +/-40V into a 1ohm load and heated one of the transistors enough to cause it to de-solder itself.

I soldered the lead back on expecting a dead transistor; the same amplifier is still in use today and that was two years ago.

They are tough transistors.

EDIT: Don't ask what maniacal thing I was doing with that poor amp (of my own design), but it was very fun.
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