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Old 8th October 2004, 08:19 AM   #1
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Default strengthen leach amp

i wanna build leach amp 4.5, but i want more power , i dont want to build leach super amp because of its complexness and cascode . so i want to raise rail to rail volt to +-80v, i know i should change some transistors of low Vce. so replace pre-driver transistors to mje340\mje350; driver transistors to mj15020\mj15021; output transistors to mj15024/mj15025. then ,will this modification succeed? give me some suggestion or comment, what can i do for more powerful leach amp , thanks.
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Old 8th October 2004, 09:30 AM   #2
bocka is offline bocka  Germany
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I'd select my output transistors at a view at the SOAR. In This way I'd prefer Onsemis MJL21193/21194 because second breakdown starts at 80V. The MJ15024/15025 has a lower power dissipation @80V. The drivers can be made from a MJE15034/MJE15034 as well as the pre-drivers.
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Old 8th October 2004, 10:00 AM   #3
bocka is offline bocka  Germany
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I'd select my output transistors at a view at the SOAR. In This way I'd prefer Onsemis MJL21193/21194 because second breakdown starts at 80V. The MJ15024/15025 has a lower power dissipation @80V. The drivers can be made from a MJE15034/MJE15034 as well as the pre-drivers.
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Old 8th October 2004, 11:16 AM   #4
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"I'd select my output transistors at a view at the SOAR."

john-china wants to use 80V, the MJ150024/25 are fine for that. The only way you would ever have close to 80V across the transistor would be driving a pure capacitor (a 90* phase angle), or a dead short.

If you have the MJ15024/25 use them. If you have to order them, check the price. From ON Semiconductor the MJ21193/94 are usually the same price, a better deal if the pricing is the same in your area.

The 2N3440/5416 are plenty high voltage, if not available I guess you could use the MJE340/350 parts.

"will this modification succeed? "

A friend of mine runs his 1 ohm ribbon speakers with his 80V Leach. I used 10 pairs of MJ15024/25 with another pair as drivers, and 2N3584/6421 as predrivers (probably too hard to find these days). The amplifier has been running fine since 1989, and in many respects sounds better than his Rowland Model 7 monoblocks (USD$11,000).

I used a pair of 39,000F per channel for the main filters bypassed with a pair of 200F polypropylenes. The front panel circuit breaker/power switch is the only protection, the combined output stage for one channel is rated at 5KW and I only used a 2KVA transformer (per channel). A crowbar was used for DC protection in addition to a 4 pole speaker relay (with all 4 poles in parallel) for quiet turn on/turn off.
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Old 8th October 2004, 12:01 PM   #5
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djk,

you're right. I just took a short view to the datasheets. I've overseen that the MJ15024/25 has also a higher max. junction temperature. Also I would not drive harder than 8 ohm loads with the original Leach design (3 output pairs). I would use some more BJTs (maybe 5-6) in the output, because it's safer.
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Old 9th October 2004, 03:06 AM   #6
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Default which one is better

i knew the answer is ok.

but i have another question.

mpsw42/mpsw92 and mje340/mje350, which ones are more suitable for 80v leach amp as pre-drivers .


by the way, has anyone used mj15020/mj15021, one friend of mine gave much to me, i searched datasheet of onsemi, found they are driver transistors for audio .
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Old 9th October 2004, 07:32 AM   #7
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"Also I would not drive harder than 8 ohm loads with the original Leach design (3 output pairs)."

You know I finally had a Leach blow up last year. It does 300W/4R and 500W/2R with 63V from a 1KVA transformer and a pair of 20,000F filters. This amp has 3 pair of the lighter rated MJ15011/12 and has been driving 4R since 1981, and 2R since 1989.

It ran for 23 years!

I had to re-cap it twice during that period, I think I got my money's worth out of the transistors.

You can use as many pairs as you think you need!
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Old 9th October 2004, 09:22 AM   #8
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Hi all
DJK, thanks for confirming my thoughts on amp reliability. I plan to use 3 pairs of MJL21193/4 with 1 pair MJE15032/3 as drivers running on 60v rails to give 400w into 3r as the output design parameters. I will now adjust the tranny to give 63v. ps. I do not plan to run 2r or 3r but parallel 8r plus speaker inductance must be approaching similar stresses.
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Old 9th October 2004, 09:40 AM   #9
bocka is offline bocka  Germany
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+/- 80V instead of +/-63V gives an increase of more than 60% in power and also in power dissipation of the output transistors. Well 60% more output transistors are about 5. I think djk and me are saying the same.
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Old 14th October 2004, 04:15 AM   #10
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Hello john-china,

From: http://users.ece.gatech.edu/~mleach/superamp/, and Prof. Leach;

"The transformer I used in each of my two original Double Barreled Amplifiers was the Signal 230-6. It had two center tapped 115 V 6 A secondaries which I wired in parallel to obtain a secondary rating of 115 V at 12 A. The primary had three voltage taps: 105 V, 115 V, and 125 V. I wired the AC line input to the 115 V tap. With 120 V AC applied to the 115 V tap, I got plus and minus 85 V DC on the power supplies and 270 W into an 8 ohm load. If I had used the 105 V primary taps, the power supply voltage would have increased to about 93 V and the amplifiers would have put out over 300 W."

I followed his advice on my second superamp and used the 93 V version into 75000 uF per rail. It puts out 310W per side and will drive just about any speaker you want to ear bleeding levels. It was not significantly more difficult to build than his Low-TIM version at 120W. I used the MJE15030, MJE15031, MJ15003, MJ15004 series of T03's.
When my boy and his wife moved to Hawaii, guess what they wanted to take with them?

Of course if what you want is the redesign experience, please proceed on your path.

Prosit
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