Leach Amps PCB Artwork

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TonyTecson

TonyTecson

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Joined 2003
m0tion said:
Have you built the amp using these PCB designs? Any pictures or perhaps a tutorial/explaination?
Click to expand...

if you are asking me, no, but i have built lots of the 'double-barreled' leach back in the mid 80's up to early 90's and i found the sound very good, in fact i find the sound better than japanese amps at the time....this amp drive many types of speakers with ease, we used this amps for mobile disco's...
 
TonyTecson

TonyTecson

Member
Joined 2003
these are the boards i used:
 

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wigginjs

wigginjs

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D

dulel

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Joined 2004
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

slowhands

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Joined 2004
dulel said:
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
Click to expand...
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.
 
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