Smaller Leach Amp V1

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#621
Pooge,

I just reread the original webpage - it turns out that you are right.... It seems that the resistors are there to isolate the VAS from any cap to gnd on the long wires.

I have however not had problems with stability on the amps I have build. I know I have only made and tested 6 channels, but I think I would have seen oscillations if anything was present.

\Jens
 
#624
Would you care to teach us mere mortals the ways of the force ?
I can "barely" calculate some values only using Ohm's and Kirchoff's laws but advices from experienced people would be mostly welcomed.

I know a universal formula is not practical but something rather empirical may be usefull.

Best wishes,
Jonny
 
#625
I'm not really sure what you mean by "decouple any capacitance from the leads of the diode string" I think Mr. Leach placed the extra resistor in order to make the layout job easier
Click to expand...
Personally I have made some "desensitivation" of this temp sensing. It comes from a rather high-ohmish node. R25 and R26 may be inserted in this purpose.

Prof. Leach writes this:
Many amplifiers have the VBE multiplier transistor on the heat sink with the output transistors. This eliminates the need for the diodes. The wires which run from the circuit board to the transistor exhibit capacitance to ground which can affect the high-frequency response of the second stage. At worst, this could cause oscillation problems. With the diodes on the heat sink, resistors on the circuit board can be used in series with the wires to isolate this capacitance from the second stage. These resistors are R25 and R26 in Figure 10. If Q7 were mounted on the heat sink, isolation resistors could not be used because they would affect the voltage regulation between the collectors of Q8 and Q9.
Click to expand...

I'll guess he was aware of the problem we are discussing.

EDIT: Ooops, didn't read Jens' post..
 
#628
Re: I doubt Leach would have specified a resistor where a jumper

loek said:
It is not much effort to cut a track and insert a resistor here(if neccessary).....
Loek
Click to expand...


Not necessary here. If used, it could be used as a lead to the diode board, and the other resistor reduced.

The wires which run from the circuit board to the transistor exhibit capacitance to ground which can affect the high-frequency response of the second stage. At worst, this could cause oscillation problems.
Click to expand...

Another possible problem capacitance could be paracitic capacitance to the ground plane. I think I read somewher about pulling the ground plane back from the VAS transistor to avoid this. Hopefully, that is not a problem, or if it is, one that is easily correctable by adjustment of the compensation cap.
 
#631
Re: Zener & R3 (R22) dissipation

AndrewT said:
Hi,
would this spreadsheet help anyone to calculate the values of R3 & R22 and check the dissipation when different values of Zener and Vrail are used?
Click to expand...

the board has component #'s right? Why not use them instead of "LTP tail current" (what's this??) why not identify the R and say "current through R__" or bias through R__

Also, zener??? what about ZD1 or whatever it is in the appropriate spots?

Nice spreadsheet though.

(I get Vrail)
 
#633
If everybody is going to mess around with different parts, how much documentation is needed for this project. I’d hate to spend a lot of time getting parts lists and a webpage together if most of the builders are going to experiment with alternative parts from day one. Is a schematic and placement plan enough to get people going, or should I take the time at put together a webpage with building information for this PCB?

\Jens
 
#634
Actually there are too many opinions in this case😕 Personally I believe that anybody could do whatever he want himself. Mr Leach is very clear in his webpage, and always anyone can be helped by this forum. However, with much respect
and appreciation to Jens's time and work, a webpage with building information would be valuable, mostly for the members that they are not professional ee or similar. As for the "experimentation panic" maybe we can leave it for later, at least nobody have a working pcb until now :att'n: except Jens of course😉


Spiros:smash:
 
#635
I agree. I will build mine on spec first, then play with upgrades and tweaks later.

If the parts list is the same of very similar to the original (with obvious exceptions of output devices), then most of the parts have DigiKey part numbers in Dr Leach's building guide. That might save at least some time in putting together the BOM.
 
#636
JensRasmussen said:
If everybody is going to mess around with different parts, how much documentation is needed for this project. I’d hate to spend a lot of time getting parts lists and a webpage together if most of the builders are going to experiment with alternative parts from day one. Is a schematic and placement plan enough to get people going, or should I take the time at put together a webpage with building information for this PCB?

\Jens
Click to expand...


I would love for you to put that together for us. I plan to follow your design. I want a working model 1st time. 😀

Blessings, Terry
 
#639
Hi Lgreen,
I pulled the component numbers straight off the PDF dated 12-05-2005. Or have I misunderstood the point you are making?
Recently some contributors have asked what power of Zener (D1 & D2) they should use and what power and/or value of feed resistor (R3 & R22). This allows them to check for themselves without having to generate the maths. It is not intended to divert anyone away from the main aim - to build a Leach Clone.
But a justification for it's being would be a change in transformer secondary voltage (recent requests for info this past week) leading to a change in conditions around the Zener controlled bias current and power dissipation in associated components.

Jens,
A recipe would be excellent & I second the thought of right first time.
 
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