Leach Amps PCB Artwork

[wopo]

Hello!

Here are 3 photos of my Leachamp:


Wopo

[wigginjs]

Wow, very impressive, did you guys etch and drill your own boards?

[Tony]

yes, i used silkscreen, etched and drilled the boards my self, i have three sets stuffed with parts waiting assembly....

[wigginjs]

For sale? =)

[Tony]

yes, please send me email, tonyjtec@yahoo.com

[wigginjs]

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[Tony]

ooopss, sorry, got my e-mails mixed up, it should be tonyjtec@hotmail.com or tonytecson@yahoo.com.....

[dulel]

Q8 and Q9 in Leachamp do not have any function. Current in VAS is already limited by the maximum voltage that can occur on VAS transistor bases. This voltage is equal R11 or R12 multiplied by maximum tail current. In this case maximum VAS base voltage is 1.2k * 40V/12k = 4V. Vas current is limited to 3.3V/390ohm = 8.5 mA which is well below current nedded for Q8 an Q9 conduction (20mA).

This kind of current protection is effective if current mirror is used in input stage, and therefore maximum VAS transistor base voltage can't be determined.

regards, Dule

[slowhands]

Originally posted by dulel
Q8 and Q9 in Leachamp do not have any function. Current in VAS is already limited by the maximum voltage that can occur on VAS transistor bases. This voltage is equal R11 or R12 multiplied by maximum tail current. In this case maximum VAS base voltage is 1.2k * 40V/12k = 4V. Vas current is limited to 3.3V/390ohm = 8.5 mA which is well below current nedded for Q8 an Q9 conduction (20mA).

This kind of current protection is effective if current mirror is used in input stage, and therefore maximum VAS transistor base voltage can't be determined.

regards, Dule

You noticed that too, hun? I wrote Dr. Leach about this 3 years ago, and he replied back explaining how the current limit function worked, but missed the point. He must get tons of email, so I was glad to hear back, but he never took it seriously. I suspect this circuit has been there for decades and he changed something else to make it redundant, not realizing it.

You can leave out these VAS current limiters if you like. If the main output protection kicks in, the VAS current does get diverted to the output, so a limit is advisable, but like you I can't see what good this does because of the values selected.