"Don Pearce" wrote in message
...
On Thu, 24 Dec 2009 00:28:14 -0000, "fredbloggstwo"
wrote:

Hi All,

I was laying out a pcb tonight for a line amplifier using a tool called
PCB
Wizard (a good little tool) and reminiscing to myself how I used to do it
in
the past using black tape and a scalpel onto clear film.

Despite the ability of PCB Wizard to autoroute etc., I still prefer to lay
the track out myself. As well as the obvious topological challenge, I
was trying to wonder why and came to the conclusion that, instinctively, I
avoid the use of sharp edges such as right angle turns - I always use
rounded bends and avoid the obvious stuff of high-level and low level
signals being non parallel runs, star earthing etc. that I was trained to
do
and that autorouting has no general concept of.

The line amp I am laying out is based on one from an EMI TG mastering
console where the boards were originally laid out at twice full size using
the tape and scalpel method as was the norm during the late 60s and 70s
and,
as well as the low number of active devices in the signal path and good
electronic design, modern equipment seems incapable of equating in terms
of
sound quality. The evidence of this is that these consoles are still
preferred when it comes to the audio mastering process at the likes of
Abbey
Road studios for their 'sound' as well as their flexibility.

I suppose the ultimate in this is point to point wiring, but that is not
what I want to do in wiring up a line amp.

So my question is this: is there any evidence that the layout of pcbs does
affect audio quality? If so why? The type of evidence is things like
the
use of rounded instead of right-angle corners. (For example, if I
remember
my EM theory correctly, sharp points are the most effective for radiating
an
electric field so could right angles could radiate and produce cross-talk)

Perhaps somewhat esoteric, but worth a few lines of discussion.

I would be interested in the references for hard evidence and not hearsay.

Best regards

Mike



Here's a double sided design of mine (a 50 watt amplifier) that shows
some of the things you should consider. I've omitted the silk screen
layer for clarity.

http://www.soundthoughts.co.uk/look/amplifier50W.png

The star ground is obvious just left of centre. Its purpose is to
prevent ground currents from resulting in a voltage in a sensitive
part of the circuit. You don't need much track to provide enough
resistance for this to happen. The star is not needed for the whole
amplifier, but you should include everything around the Blumlein pair
at the input.

Another very important point is the way the negative feedback connects
to the output. At centre right you can see the speaker connection from
the junction of a pair of high power resistors. The feedback is the
thin red track that emerges vertically from the last pad on the right,
which is actually the speaker terminal. It could apparently join
anywhere on the speaker output track, but currents are big, and only
by connecting at this point is a true sample taken. Measurable changes
in distortion result from this.

What doesn't show is the grounded copper areas that surround the power
supply lines top and bottom of the picture. The ground areas are top
and bottom of the board and make the power lines almost screened. This
is important as there are some very messy and large 100Hz voltages on
those lines, and they can inject hum into the input if not handled
properly.

The four mounting holes at the corners are isolated from the ground
connections. This is vital for preventing the formation of ground
loops.

Finally, the board largely obeys the "vertical one side, horizontal
the other side" rule that lets tracks cross each other at right angles
for minimum coupling.

d

Hi Don

thanks for posting that.

regards

Mike