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Old 3rd December 2008, 03:35 PM   #1
Ryan_Mc is offline Ryan_Mc  Canada
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Default Qestions about the Leach low-TIM amp

I'm using this design for my amp project. I'm redrawing out the board to use flat pack transistors and also a different SOA protection circuit. I noticed a couple 'oddities' in the schematic and wonder if someone could tell me why it is the way it is.

Schematic is here.

#1. Why is R51 there? If the signal grounds are going back to a HQG and the same with the power grounds, why is a ground loop intentionally made on the PCB?

#2. Why have both R25 and R26 in series with the temperature sensing diodes? Couldn't just a single 2k resistor be used?

#2b. Why is R27 there? I understand about better sensitivity but a standard single turn POT is spec'ed. Why couldn't R27 and P1 just be replaced with a 10 turn 4.7K pot?

Thanks, and sorry for the newb questions.
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Old 3rd December 2008, 10:38 PM   #2
jgedde is offline jgedde  United States
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#1. Safety. If you lose a ground, the other won't float very far away causing bad things to happen. It's a common practice (I do it often) and won't cause a ground loop if the HQG connection is very low in resistance vs the "tie" resistor.

#2 Looks like professor Leach put a lot of thought into his design... Remember the diodes are located physically away from the this part of the circuit. This balances any transmission line effects in the wiring to the diodes thus ensuring common mode cancellation of any weirdness.

#2b This again is good practice at work. This sets a minimum resistance for the Vbe multiplier adjustment. Without it, you could short the emitter to the base at full CCW which obviously would result in no bias. In other words, it's presence makes better usage of the pot adjustment range and makes it less "sensitive" when you're running the tweaker. Replacing P1 with a 10-turn pot isn't a bad idea, but I'd still leave the resistor.

Good luck! Let us know how it all turns out. I've wondered often how the Leach amp sounds...

John
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Old 4th December 2008, 12:33 AM   #3
rob3262 is offline rob3262  United States
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Quote:
Originally posted by jgedde
#1. Safety. If you lose a ground, the other won't float very far away causing bad things to happen.

I'm thinking noise isolation. Mains Earth and Signal ground are not one and the same.
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Old 4th December 2008, 01:32 AM   #4
pooge is offline pooge  United States
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all of these questions are answered by Leach on his website.
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