JLH-2005 – NOTES
Background (Geoff Moss)
The object of this design was to produce pcb layouts that would accommodate all components (except the transformer and the power transistors) and which would allow for the use of both standard capacitors and higher grade types such as Black Gate and Elna Silmic.
A number of items (eg some of the 100nF capacitors, the LED indicators, the parallel output transistors, the output Zobel etc) are optional but I wanted to build in as much flexibility as possible.
Some aspects of building a JLH (eg fusing) have not yet been covered on my website and I want the new article to reflect a complete amplifier from the mains input to the speaker terminals.
It was agreed at the outset that the pcb size would be standard eurocard (160 x 100). This would give adequate space for good sized heatsinks for the on-board diodes and transistors, wide tracks for high current sections and terminal blocks at the edge of the board for all connections to/from the pcb. These were three of my fundamental requirements.
The power supply board gives the option of a ‘follower’ regulator using either a TL431 or zener reference or a simple capacitance multiplier. Two positive regulators to provide the +ve and –ve rails were chosen for several reasons:
Separate transformer secondaries and two rectifier bridges were going to be used because this arrangement is electrically and sonically better.
Only one type of power transistor is required which can lead to cost savings but, more importantly, it gives the opportunity to select the best matched pairs for use in the amplifier with the remainder being used up in the power supply.
I have read that the TL431 functions better when providing a reference voltage from its cathode than it does from its anode (as would be the case if a complementary regulator was built using a pnp pass transistor).
Only one design of regulator board is needed, which could give cost savings if the boards are manufactured commercially.
Because of the proposed power supply arrangement, the star earth point has been located on the amplifier board. It has been assumed that the majority of constructors will want to use a monoblock arrangement with separate power supplies (or at least regulators) for each channel as this gives better performance and removes the interaction between channels when initially setting the quiescent current etc.
The files are:
An overview of the amp and the power supply, showing Geoff's suggested grounding.
The schematic of the amplifier board.
The schematic of the power supply board.
A bitmap image of the copper when viewed from above the board. This file
should print actual size without any alteration though you might wish to
crop the small amount of whitespace that I left around the board outline.
The bitmap image was created from my program's print output, intercepted by
a 'graphics' printer driver. This gave a better quality than using the
'export bitmap' option and then resizing the image to print actual size. It also
enabled the board outline to be included which I thought would be useful.
An overlay drawing in gif format.
A drawing that combines the above two images.
The PCB drawing for the power supply. I do not have separate Copper and Overlay files for the power supply (nor do I have Gerber fabrication files).
Files for getting a PCB fabricated