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Old 26th February 2005, 05:25 AM   #11
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Quote:
Originally posted by JensRasmussen


Yes, another habit of mine, I ALWAYS use fuses on every channel in an amp. This is just for extra safety, but I like to be sure that there is something protecting the amp and/or speakers, should something bad happen somewhere.

\Jens
I think Pavel used the wrong word. I think he ment if you had the fuses before the big caps => easy to break the fuses when the power is switch on. I have done the same thing in my QRP02 Gainclone, fuses first then the rest. Somehow I think it feels better to have the fuses as the first part but still I see the problem with that.

BTW, Jens: A very nice pcb job!
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Old 26th February 2005, 06:07 AM   #12
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Well I think that designing/choosing fuses is challenging in it self.

With 5 parallel output transistors and rails of about 68V the fuses have to be big, say (68-6)/4 = 15A, but on the other hand you want the fuses to offer some protection, so I think the fuses will need to be 10A slow blow types.

This size fuse will survive the surge current in the caps. I have used this for over a year on my prototypes. I remotely turn my amp on maybe 5 – 10 times every day, and have found no problem with fuses blowing.

The caps I placed on the board is only local decoupling, I have a central capacitor bank in the amp, where I keep 30.000µF for each rail. The only fuse I have before these is the fuse on the primary side of the transformer. The reason being that the current charging this much capacity will blow a 20A fuse quite often. I think it’s better to have a smaller fuse on the primary side of the transformer. The primary fuse is a major requirement anywas.

I also use a slow start circuit to limit the inrush current.

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Old 27th February 2005, 07:57 PM   #13
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by sam9
The most important think I've found is to keep the output section on one end of the PCB and the input on the other.
Better still: Use completely independent PCBs for output stage, together with its power supplies+protection circuits, and another PCB (made as compact as possible!) for the voltage gain cell with it's power supply. (i.e: independent bridge+reservoir caps.)

All grounds returned independently to star point of course...

You may still take a further step and shield the entire output stage PCB, (including it's supplies alias: output stage bridge+reservoir caps.) a la Halcro!
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Old 27th February 2005, 08:39 PM   #14
mikeks is offline mikeks  United Kingdom
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Halcro...Now...that is what i call a proper output inductor!!!
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Old 27th February 2005, 08:43 PM   #15
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Quote:
Originally posted by SM2GXN

Mr Evil what is the main reason to run the power traces on top of ground tracks or ground plane? I know that there will be some picofarads depending on trace width and type of board but for low frequency it will have little decoupling effect or is it for another purpose? Rf?
If the return path for the current going through a trace is close by the inductance of the trace is lowered.
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Old 27th February 2005, 09:04 PM   #16
sam9 is offline sam9  United States
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Quote:
Halcro...Now...that is what i call a proper output inductor!!!
Are you shure it's not really for distilling moonshine?
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Old 27th February 2005, 09:24 PM   #17
sam9 is offline sam9  United States
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Quote:
quote:
Originally posted by SM2GXN

Mr Evil what is the main reason to run the power traces on top of ground tracks or ground plane? I know that there will be some picofarads depending on trace width and type of board but for low frequency it will have little decoupling effect or is it for another purpose? Rf?

If the return path for the current going through a trace is close by the inductance of the trace is lowered.
An example of the classic dilemma. What is good practice in one case introduces other problems in a second case. In some circuits, following the rule requires you to route one or the other trace over a much longer and convoluted path than would otherwise be necessay.

A "solution" I once saw in a comercial product was to form a "Y" in the off board wiring and bring DC power onto the at two independant points for each polarity. This was with a two layer board so I think it was a deliberate strategy to keep the power traces on the board as short as possible. I've also seen something similar done to tie two parts of the ground together from distant parts of the PCB - in this case a solid core wire exites/entered the PCB at each end and formed a brich about 1/2 above the board. This one as well looked laike a deliberate strategy and not a "hack".

The nice thing about this is that it shows there is room for creative solutions, which offers some consolation for the absence of clear unversally applicable rules.
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Old 28th February 2005, 09:40 AM   #18
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Quote:
Originally posted by mikeks
Better still: Use completely independent PCBs for output stage, together with its power supplies+protection circuits, and another PCB (made as compact as possible!) for the voltage gain cell with it's power supply.
Mike,

you think that is always superior to a 1-board solution ?

What is the light stuff on the outside of the Halcro heatsinks?
(i adore chimney heatsinks)

(did anyone ask who the goodlooking on Mike's avitar is yet? )
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Old 28th February 2005, 12:33 PM   #19
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Quote:
Halcro...Now...that is what i call a proper output inductor!!!
"Almost" as big as the coils in my RF amplifiers!

Quote:
If you try to follow all these "rules", including mine, you will think an impossible task lies before you. If you try to rigorously obey all of them you will never get anything done. Just getting all the components on the board and connected without useing too many wire jumpers is challenge enough.

Quote:
You may realize by now that there are no cookie cutter rules in this, just a lot of advice some of which is contradictory. That's one thing about the Rane articles mentioned -- you realize that even the pros have diffuculties and ambiguities in this area.
Thank you sam9 your posts has been real interesting reading, Have you ever thought about writing a book
Because of those ambiguities it makes it hard to belive all you read and you shouldn't "that's what I've learned my kids". Internet is excellent but the fact is that it could even change the history if you are not able to interpret all information you read

Quote:
If the return path for the current going through a trace is close by the inductance of the trace is lowered.
Thank you relder for explaning, I didn't think of that but now I know it's a good way to lower the inductance by doing so.

I was about to start routing a board on one of Anthony Holton's Lateral FET amps but then I was caught by a real bad cold.
I don't know if I got to have Anthony's permission to put the board on Diy? He makes his living on doing all those amps so it might be better to ask.

Bjorn
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Old 28th February 2005, 02:37 PM   #20
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First, I try to be aware of:
- where some components (or mounting holes) *must* be placed
- where currents flow
- hierarchy of perturbators to victims (high to low dI/dt, high to low dV/dt) => gives good indications of where tracks should be the shortest, and which components/tracks to keep apart
- card to case coupling
- how much current flow from grounds to supply (or other card)

Second, I place&route with all that stuff in mind, with normal track width

Third, I refine (or redo), taking into account:
- min/max voltage of each net => constraint on track to track spacing (12.5mil between +/-35V rails is not enough for my taste, for example)
- hot spots (components suceptible to heat, track heating due to high currents) => for "how wide" a track/copper pour, and "how wide" the spacing around some components.
- how RF may arrive and depart from card
- how to benefit from coupling rather than fight it (ex: no problem to keep close 2 tracks that are already bonded by a cap)

Fourth, I try to adapt the schematic in order to make things easier. Sometimes, it can help to split a resistor in two, and/or move a component to another board.

Fifth, I sleep on it, then slap my face because I realize I've missed something, and rework the lot

My 2 cents...
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