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Old 15th January 2002, 08:23 AM   #11
ppl is offline ppl  United States
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Geoff: thanks for the Part # Correction.
Sweede: These transistors i mentioned might just blow as yours have. i mentioned them Because In my Variation of the Leach amp the Big one 250 Watt. i used these to replace the ones you mentioned and it sounded alot better. The Breakdown voltages are better, so this just might Help in your situation. I have Driven my version very hard and not had any problems Except when one rail fuse blew it also took out one of the Input Diff amp Transistors. This only happend once and was Because i had By mastake installed a 1 amp fuse in one rail rather that the 10 amp fuse. I have had this Amp since the Mid 1980's and it has been a great Performer. I must say though that my version is almost a complete redesign and only similar to the Origenal Leach. The Basic Topology is the Same but just Refined.
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Old 15th January 2002, 10:06 AM   #12
swede is offline swede  Switzerland
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Thanks a lot again guys,

I'll try and get hold of some replacement transistors (the 21193 and 21194) and repair my amp.

//magnus
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Old 15th January 2002, 11:12 AM   #13
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just a word of warning ... there can be some very toxic materials present inside transistors so if you wish to 'pop the top' on one of them i would suggest you exercise extreme caution in doing so...
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Old 16th January 2002, 12:17 PM   #14
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Question Zobel network

What is Zobel network ??
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Old 16th January 2002, 12:39 PM   #15
Geoff is offline Geoff  United Kingdom
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JE

A Zobel network (as applied to amplifiers) is the series resistor+capacitor (commonly 10ohm+100nF) connected between the output of the power amp and earth. It is fitted to most amplifiers in order to improve stability into inductive loads.

Geoff
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Old 16th January 2002, 01:38 PM   #16
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A great treatment of All Kinds of Output Stage Problems and Page 8 or 9 of this app note deals with Snubbers http://www.elantec.com/pages/apppdf/d40954.pdf
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Old 16th January 2002, 03:04 PM   #17
swede is offline swede  Switzerland
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Hmmmm...

How picky can the amp be about the Zobel network?

Mine is two 33 ohm 2W resistors in paralell + a cap, I think it is 0.1 uF.

//magnus
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Old 16th January 2002, 04:49 PM   #18
Geoff is offline Geoff  United Kingdom
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Magnus

Doug Self carried out some test to see if the load placed on the amp by the Zobel could be reduced. He found that by increasing R to 22ohm gave some evidence of stability problems into inductive loads. Generally, the resistor is between 6 and 10ohm, give or take a little.

Geoff
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Old 17th January 2002, 12:36 AM   #19
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//magnus,

Have you checked R1 (input register)? This could be a long shot but I blew one of my speaker when I had a bad solder join for this resistor on my Super Amp MB.
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Old 18th January 2002, 04:06 PM   #20
swede is offline swede  Switzerland
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Hi again,

I got this weird feeling after the discussion about the Zobel network and checked it out again, and there it was. The 33 ohm resistors I had in parallel was not really what I expected... I accidently used 0.33 ohms in parallel...

So I'll replace them and get back to you if it still doesn't work (after replacing the transistors as well). Though my supplier of transistors were out of stock, so I have to wait for a couple of weeks.

Regards,
//magnus
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