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Old 10th September 2011, 01:10 AM   #11
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by Bigun View Post
a) I like to keep the main power rails close together so that countervailing current flows are adjacent - minimizes area of loop for magnetic coupling.
Hooray - a voice of reason!

I'd add that in a class AB amp, the same or greater care should be taken to minimize the loop area for the half wave rectified output stage currents, but only if you want to achieve very low distortion.

A lot of folks like to "spice up" their amps with some pleasant sounding distortion, though. For example the two layouts posted earlier by Rudi are excellent examples of how to get the maximum possible even-order distortion from magnetic coupling. The second one also has the feedback take-off point offset to get more distortion another way.
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Old 10th September 2011, 01:38 AM   #12
AKSA is offline AKSA  Australia
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The problem with this approach is that the two currents, pos and neg, do not occur at the same time and thus do not cancel.

Best you can do is keep the high current regions ground plane separated from the high impedance networks, particularly the fb and input blocks.

Hugh
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Old 10th September 2011, 01:59 AM   #13
CBS240 is offline CBS240  United States
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I generally layout the high current stuff first and establish the paths and grounding scheme since these are the tracks that can create interference.
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Old 10th September 2011, 02:28 AM   #14
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Some good direction thus far, thanks much. This seems as much or more an art than a science. The last time I did an audio PCB it was bone with flm, tramsfers and pen!

Any pointers on using the toner transfer method for PCB creation? I have always used photographic thechniques but might like to try what seems a simpler method for one or two boards. Recommendations?
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Old 10th September 2011, 02:44 AM   #15
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by AKSA View Post
The problem with this approach is that the two currents, pos and neg, do not occur at the same time and thus do not cancel.
The magnetic fields won't cancel but there's a choice of whether they sum to a fairly accurate replica of the audio signal, or a full wave rectified version thereof i.e 100% distortion.
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Old 10th September 2011, 02:48 AM   #16
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PCBs are designed in the real world and are subject to constraints other than those affecting absolute performance.

When laying out a PCB the first considerations are: what are the dimensions of the available space, does the physical design call for a particular arrangement of connectors, and are there any thermal requirements such as heat-sinking.

In a PCI soundcard, for example, the PCI connector and the location of the audio I/O are pretty much a given.

If there are no such considerations, then the design can proceed purely on the basis of performance, but it is a rare circumstance in which absolute performance is the only criterion. Even amateur constructions generally require a housing of some description, and the PCB design needs must take this into account. The volume knob is frequently the largest control and often centrally placed. A symmetrical, or semi-symmetrical front-panel layout is often chosen in deference to visual balance.

Thus the final design may require iteration and compromise. Physical layout must, in some instances, give way to electrical or thermal considerations. Thru-hole components give way to SMT where double-sided boards are mandated by a paucity of real estate (or by improved decoupling). A thru-hole board will never achieve the low strays and decreased grounding-loop areas of which SMDs are capable.

Modern designs frequently call for power connectors, I/O jacks, potentiometers, displays, transistors with heat-sinks and even tubes and larger transformers to be directly mounted on the board. CAD software makes this more straightforward to achieve now than it was in the past. As designers gain in experience they will naturally wish to incorporate such features as exemplify both good ergonomics and good performance. A sophisticated design, well thought through, will frequently require less effort in construction than one assembled on an ad-hoc basis from a heterogeneous collection of PCBs of arbitrary dimensions.

Even where connectors and other components comprising the user interface are panel-mounted consideration needs to be given to exactly how the connections between these components and the board will be made.

Layouts which work for commercial designs are likely to work for amateur ones, so mimicking existing devices is not the worst strategy.
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Old 10th September 2011, 10:24 AM   #17
AKSA is offline AKSA  Australia
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Godfrey,

I was interested in your comment that I designed the FetZilla board to maximise even order distortion.

I'm not familiar with these techniques, would you like to explain precisely what you mean here?

We'll leave the argument about summing half wave current waveforms for now......

Cheers,

Hugh
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Old 10th September 2011, 05:32 PM   #18
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Default Partial PSU

Here's the start of my PSU. It will have boosted rails for IPS and VAS and will use two dual secondary transformers, 20V+10V, no centre taps. Will try to fit a capacitance multiplier for the boosted rails if space allows (still have to lay down rectifier, filter and other components for the boosted rails yet, obviously).

Comments and constructive criticism welcome.
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Old 10th September 2011, 06:32 PM   #19
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Is ther a point to keeping power rails together in a DUAL supply? isn't the magnetic flux mainly between ground and the respective rails rather?
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Old 10th September 2011, 08:46 PM   #20
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BTL loads don't use ground.

Some stereo amps run channels out of phase for more power, so most GND currents would be difference in L/R channels, majority of currents still in main rails.

Just something to consider, no rails are equal.
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