Leach Amp bias/star ground questions?

hard times

Member
2002-08-01 8:11 pm
CA
Howdy all, new guy here. I'm 97% done with building two dual channel Leach amps. It's my first attempt at something like this. All I can say is ---WHEW!! I'm doing my best not to feel like I'm in over my head! This experience reminds me of the old adage-- "How does one eat an elephant"? -------- One bite at a time-------
My first question is about the bias pot P1. Prof Leach states on his web site that you should set P1 to max resistance before power-up. Hmmmm- now would that be full counterclockwise so that the base of Q7 sees the full value of R27 and P1, or, would that be full clockwise so that the base of Q7 sees only R27? Just when I think I know what he is talking about, I can see the exact opposite clearly also. (I'm using Prof Leach's PCB's and the exact Bournes pot as called for in the parts list.) So gents, will that be full CCW, or full CW??
My second question is in regards to the general concept of "star grounding". I am modifing the original Leach plans by adding two extra PS filter caps per channel, and giving each pos and neg rail it's own bridge rectifier. So, should I run separate wires to central ground from each of the PS filter caps or bridges, or can some be tied together first, then have only one wire run to ground? I guess the question can be "Is it best to "daisey- chain" multiple alike components with only one feed wire to ground, or is it best to take each component to ground with it's own wire?
I thank you all in advance to take the time to relpy to these simple questions. I will value any and all answers.
Good Day.
 
I too am building a multi-channel Leach amplifier. In my case four channels, each with their own transformer, bridge, and resevior caps, in one chasis. How should the grounding be handled in this case? Should there be a single central star ground for all four channels or should they all have their own independent star grounds? If independent, how is the ground from the AC mains handled?

As for the initial setting of the bias pot, counter clockwise will provide the maximum resistance.

Let's hear from others who have built amps with variations on the single power supply for a stereo amplifier.
 

traderbam

Member
2002-01-23 2:00 pm
Earth
stars

You guys can figure this out for yourselves. You need to understand why star grounding is used sometimes. Often things are daisy-chained and this is correct too in the right place.

If wire had zero resistance star gorunding would be unecessary. Electricity flows in circuits (at low frequencies) and the current causes small voltage drops along wires. Obvious. What is not always obvious is when a single ground wire is being used by more than one circuit at once. In this case the circuit currents add and you get a voltage drop along the wire that is related to the sum of the two currents. This can be a problem in some cases - and it is in these cases that the two circuits must have separate ground feeds.

In power amps a common mistake is to have the i/p ground attached to the speaker ground at the pcb, then a common wire from pcb to psu caps. The big speaker currents cause a voltage signal to appear at the i/p gnd - which in effect is a signal into the amps input. Suddenly you are feeding the amps output into its input by mistake.

The process is to look at your schematic and identify all the individual loops of current. Then look for where loops share ground wires (or any wires for that matter) and decide whether this matters or not. If it does then give them separate wires back to some reference point. When you do this you will end up with several ground lines from the amp pcbs back to a star point. You will also find that paralleling psu caps does not require starring because they are part of the same circuit.

BAM