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Old 2nd December 2011, 02:59 AM   #31
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Hi Francec
I support your suggestion, though it could be more easily viewed. I know too, that younger folk like to follow popular ideas like typical square format DX, NAP140 and other budget kits for small amps. This way, both amps might also share the heatsink and still fit in a smaller case. Perhaps though, this is not the right design on which to make such changes

Few schematics lead the eye to a wide format layout like the P3a until you get to multiple output devices. At least, similar things happen with local kid's designs here when they try to use free CAD software on simple amplifier circuits. I can only confirm that unless we do know more about design, it remains very important to follow Rod's layout, bearing in mind his challenge that it's going to be hard to beat his PCB for performance.

PS
Yep Frank, this area is like a magnet to your fellow Victorians. Many moved here for their midlife "C" change.
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Old 2nd December 2011, 03:19 AM   #32
Francec is offline Francec  Australia
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Quote:
I know too, that younger folk like to follow popular ideas like typical square format DX, NAP140 and other budget kits for small amps.
lol, they haven't worked out yet that it doesn't pay to be a sheep. Or as Brian said, in the best movie ever made, "you are all individuals" and the response "yes, we are all individuals".
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Old 2nd December 2011, 04:36 AM   #33
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For learning and understanding how to position components on a pcb, go to the Symasym designed on this forum. Look at its schematic and the pcb and understand what is going where and analyse with whatever your wisdom permits. Now u will find your pcb is resembling like that of the experts. Just give it a try.

At your age, u should be quite smart posting here.

Also remember, u have to lay the filter electrolytic capacitors as close as to the output transistors.

Gajanan Phadte
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Old 2nd December 2011, 10:57 AM   #34
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I've read Rod's description and it is indeed possible to get away with not mounting the Vbe multiplier on the heatsink. In this case anyway.

Quote:
I also included a bias servo, using a pot and transistor. This was not mounted on the heatsink, since this would cause an instant negative thermal coefficient - as the amp gets hotter, Iq will fall, potentially so far that crossover distortion will occur. This is not a good thing, and I do not recommend it. The bias servo I used was done for convenience - I had a 20k trimpot to hand (well, a bag full actually), and the transistor is a standard BC549. I know its not elegant, and the values are not worked out properly, and ..., and, ... etc, but it works.

.....

Bias current also changes with temperature, so as the amp heats up, Iq will increase. This is not serious, and will only ever cause grief if the heatsink is too small. Such grief will ensue anyway in this case, regardless of whether the bias current is stable or not.
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Hmmm....
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Old 2nd December 2011, 12:06 PM   #35
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Quote:
Originally Posted by gmphadte View Post
For learning and understanding how to position components on a pcb, go to the Symasym designed on this forum.
Are you referring to this thread?

SYMEF amplifier
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Old 2nd December 2011, 12:23 PM   #36
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Quote:
Originally Posted by Francec View Post
Here is an image of the P03A PCB from Rod's web site. The image is open to the public so I am not infringing Rod's copyright or IP. Normally, I would, and do, suggest buying Rod's boards, as I do, but a poor 15 year old needs a little bit of help.

You can use this image to use a guide for parts placement then link up the pins.
I have already given a try to copy the layout earlier, but the pic distorts really very badly.....so its difficult to analyze!


Quote:
Originally Posted by Ian Finch View Post
Hi Francec
I support your suggestion, though it could be more easily viewed. I know too, that younger folk like to follow popular ideas like typical square format DX, NAP140 and other budget kits for small amps. This way, both amps might also share the heatsink and still fit in a smaller case. Perhaps though, this is not the right design on which to make such changes

Few schematics lead the eye to a wide format layout like the P3a until you get to multiple output devices. At least, similar things happen with local kid's designs here when they try to use free CAD software on simple amplifier circuits. I can only confirm that unless we do know more about design, it remains very important to follow Rod's layout, bearing in mind his challenge that it's going to be hard to beat his PCB for performance.

PS
Yep Frank, this area is like a magnet to your fellow Victorians. Many moved here for their midlife "C" change.
Absolutely....

Quote:
Originally Posted by gmphadte View Post
For learning and understanding how to position components on a pcb, go to the Symasym designed on this forum. Look at its schematic and the pcb and understand what is going where and analyse with whatever your wisdom permits. Now u will find your pcb is resembling like that of the experts. Just give it a try.

At your age, u should be quite smart posting here.

Also remember, u have to lay the filter electrolytic capacitors as close as to the output transistors.

Gajanan Phadte
I have no idea what you are talking about, I tired searching 'symasym' but i didn't find anything pertaining to this topic...Please Elaborate!!

Quote:
Originally Posted by ingenieus View Post
Are you referring to this thread?

SYMEF amplifier
No, i'll starting reading it now.....


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Old 2nd December 2011, 12:25 PM   #37
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Quote:
Originally Posted by ingenieus View Post
I've read Rod's description and it is indeed possible to get away with not mounting the Vbe multiplier on the heatsink. In this case anyway.
As far as I know this is always the case with CFP/Sziklai output stage amps.
According to Self, only the drivers' temperature influence the quiescent current, and the multiplier should be on the same (small) heat sink as the drivers.
This cannot be the same heat sink as the output devices.

I have a p3a clone in service for about a year.
The drivers do not have a heat sink, but the multiplier is located just next to one of the drivers on the board, almost touching.
If I recall correctly, quiescent current is relatively stable after 15 minutes or so.
For sure there is no thermal run-away.
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Old 2nd December 2011, 01:00 PM   #39
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Quote:
Originally Posted by discrete View Post
As far as I know this is always the case with CFP/Sziklai output stage amps.
I'm not sure about that. The Vbe multiplier does not need to be on the same heatsink as the output drivers, but it still needs to track the temperature of the pre-drivers. Randy Slone explains this rather better than Douglas Self - I have just checked. Not mounting it on any heatsink can work, but not every time.

There are other considerations with Rod's circuit.

Here is the circuit (apologies if I transgress a forum rule about other people's circuits).

Click the image to open in full size.

Rod describes it as follows:
Quote:
The circuit for the bias servo (actually the whole amp, with some of the other mods I have mentioned elsewhere) is shown in Figure 1a - notice that I left the diodes in circuit as a fail-safe, since the servo I used will go open circuit if the pot wiper becomes disconnected (I strongly suggest that you do the same). In practice this works extremely well, and I can set bias current to anything I like.
Those diodes in parallel with the bias servo (a.k.a. Vbe multiplier a.k.a. amplified diode) changes the picture a bit.

But wait, there is more:

Quote:
Variations in supply voltage will have an effect on Iq as well. I hadn't actually considered this much (I have never had one of these amps self destruct, and normally don't even bother measuring the quiescent current). The variation is caused because the Class-A driver current is not derived from a true current source, but is a simple bootstrapped circuit. Since the current must change with voltage, so must the voltage across the diodes (or bias servo).
If there was a current source instead of the two 2k2 resistors the Vbe multiplier, things changes again. With the current through the bias servo constant, something has got to give.

The mathematics says that the relationship between transistor current, voltage and temperature is

Click the image to open in full size.

where
  • IE is the emitter current
  • VT is the thermal voltage kT / q (approximately 26 mV at 300 K ≈ room temperature)
  • IES is the reverse saturation current of the base–emitter diode (10−15 to 10−12 ampere)
  • VBE is the base–emitter voltage
We can now start analysing this thing to death, but let us not confuse the OP. The circuit works well as it is and as long as it is not changed everything will be fine.

Last edited by ingenieus; 2nd December 2011 at 01:08 PM.
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Old 2nd December 2011, 01:14 PM   #40
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Quote:
Originally Posted by AndrewT View Post
Here's a list of the symasym threads opened during 2011.
....

No symef among them.
Thanks for the answer.

Just typing 'symasym' into the forum search brought up a list of seemingly unrelated threads. The SYMEF one is the only one that comes even close. It does go through a PCB design exercise. That's why I asked the question.

I should have used the advanced search.
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