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Old 24th October 2002, 02:04 PM   #1
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Default Leach Superamp vs Leach 4.5

Can anyone comment about the sonic comparisons between the Leach Superamp and the Leach Low TIM Amp Version 4.5?

Are there any stability issues with the Superamp?

How many MJL21193/21194 (Plastic package) devices should I use in the Superamp to operate reliably into 2 ohm loads at continous full power? This is for serious PA applications.

Thanks in advance,
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Old 25th October 2002, 11:25 AM   #2
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There are several issues here.

No meaningful answer may be given without knowing the power level and B+ required.

Generalizations that can be made: Plastic is a waste of time and money for big amplifiers. Assuming realistic transistor case operating temperatures you will need twice as many MJL21193/94 as you would MJ21193/94 because of the maximum junction temperatures. Then there are thermal cycling issues, at the same junction temperatures the metal case style has 10X the life span as the plastic (except aluminum, but I don't see those much anymore).

Series vs parallel? If driving 1KW at 2R you need 22.5A RMS, 32A peak, and +/- 95V or so (typical brute force unregulated supply). Eight pair of metal will do for parallel, but because of the high current you will still need eight pair in series (four in parallel with another four in series per half). Sixteen total per channel either way. With older transistors you would have needed twice as many in parallel because of the high voltage SOA limitations, but the MJ21193/94 has twice the SOA at 100V that the MJ15025/25 does.

Consider using the MJL1302/3281 as drivers for this big of an amplifier.

One last thought about plastic and series connected outputs: If you use a tiered power supply the increase in efficency would allow you to use the plastic parts.
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Old 25th October 2002, 01:25 PM   #3
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djk, thanks for the reply. Very informative.

Does the same rule apply to Mosfets also, ie., Metal can (TO3) being able to handle more power than Plastic (TO3P, TOP264 etc.,)??

Although this query is away from the subject of this thread, I am asking this question, since I have almost ordered for Plastic package Exicon Lateral Mosfets(200Volts, 16Amps, 250Watts). A wiser decision would be to order for the TO3 version with identical parameters.

Any answers? Thanks again.
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Old 26th October 2002, 06:34 AM   #4
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In the case of the Motorola parts the TO3 case parts are rated at 25% more power at room temperature, plus there is a 50*C difference in the maximum allowable junction temperature.

The Xicon/Magnetec/Semelab parts are rated at the same power in either case style, plus the TO3 case has the same maximum temperature as the plastic. I still like the TO3 package over plastic though. I have not seen any thermal cycling data on these parts either. The data from International Rectifier on their FETs actually showed the TO3 as being worse than their plastic for reliability (the number of samples may have been too small to be statistically correct). Data from Harris (RCA) and ON (Motorola) show the stainless steel cased TO3 to be way out front of plastic.

One important note about the double die Xicon/Magnetec/Semelab parts : they may have oscillation problems because there are no gate resistors internally. Not to mention that they are very expensive on a SOA watt/$ basis (4X as much based on 100 lot pricing and 100V SOA. When you take into consideration the maximum junction temperature this spread only gets worse for the FET).
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Old 26th October 2002, 09:20 AM   #5
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Thanks again djk. I wonder why the IR parts are the only exclusion/contradiction to an otherwise accepted fact that the TO3 package is consistently better than Plastic cases (TO3P, TOP264 and ofcourse, TO220) in terms of max.Junction Temperature and Dissipation characteristics; perhaps, it may have to do with the internal structure.

P.S. One of the moderators of this forum could probably link this thread in such a way that a Forum Search for "TO3 vs Plastic case" would show up djk's very concise, informative posts.
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