Re: About some parts on logical circuits

I don't know where all those optos are going but it makes me wonder if
each point needs an opto doing it or if on the other side of the opto
something else could be used. Like one opto driving a hand full of
FETs that are the end switch, if you see what I mean.

Not really. I was told on this NG about FET for switching. Can't remember
who proposed that though. I guess I wanted to stick to the original amp
design (I'm making a clone btw) which uses optos rather than relays. I also
took in consideration the fact that mechanical switches produce popping
sounds on operation. I read several times that there were tricks against
that (resistor accross the switch or to the ground ...) but since it wasn't
clear enough for me, I dropped the idea.

Depends on what the actual circuit is and maybe that overly
complicates things. Just tossing out ideas.


I wondered about stealing filament power.

The problem with your 7 volt is that's filtered only and not a great
idea for digital circuits. It should be regulated and that'll get you
5 volts with a low dropout 5V regulator, depending on just what the
actual AC is when that winding only has a .5A load on it, and keep the
ripple real low. Hmm, but it might drop out on low line voltage. Not
good.

Better would be to diode double it to 14 and, since 8V 3 terminal
regulators are a dime a dozen, 8V out. You *will* need to heatsink
that thing, though, and secondary AC being high on high line voltage,
plus the low load bump, with aggravate it. Could easily hit 5 watts at
.5A (and I'm assuming you go to 20mA ea on the 21 LEDs) and that's not
a dinky 1 inch stamped metal heatsink. Need a bit of oomph.

With the precisions I brought (and should have brought sooner), it
shouldn't
be that bad. What I don't understand here is why you don't want to use a
6V
or 7V regulator instead of a double the voltage output to run it through
an
8V regulator.

Because you have to have more voltage going into a regulator than
what's coming out or else it can't regulate. That's it's drop out
voltage rating and is 2 volts for the dime a dozen LM78xx family.

Ok

Well, 50mV of ripple would be OK but you must not be looking at it
with a load because there's not a rat's chance in hell a 220uF cap is
going to hold up a .5A load. Ten times that, 2200uF, will have over
800mV PP of ripple, assuming a full wave bridge. It'll be larger with
the diode doubler, roughly double.

Speaking of which, when I did the heatsink numbers I didn't bother
working our ripple numbers, so it would be less because the average DC
is less. Consider that a 'safety margin'. How much depends on how well
it's filtered.

I don't understand the 'right value' comment for the LEDs.

I meant the right value for the serie resistor for each LED to limit
current.

Getting the 14V is like generating a +- supply but with the - end at
ground. I.E. One end of the winding goes to the junction of two
capacitors and the other end has two diodes. One positive going to the
'top' cap and the negative one going to the 'bottom' cap. When a +-
supply the junction of the two caps is ground and you get +- 7 volts.
Connect the -7 to ground and you get +14 on the top side, with +7 in
the middle.

I think I've seen that voltage doubler circuit once or twice before.

If the winding is putting out significantly more than 5 volts under
low load, like your .5A, you could cut it down a notch with some
series resistance, a power resistor. Or knock it down a notch,
regardless, to cut the heat load on the regulator because it's sort of
too little or too much either way.

I have to measure what comes out of there with my voltmeter. I measured
the
6,3V filament winding and it is not above the specified 6,3V even without
a
load so...

OK. Depends on how much winding resistance and whether they bumped the
no load output to make up for winding losses at the rated load. They
got more than one way to skin a cat too and I don't know which knife
they were using ;)

That also means my numbers are approximations because, not knowing
that the winding resistances are, I can't tell how much it can charge
the filter to so it might, or might not, hit the 12.8 nominal. That
comes from just assuming 5VAC and two diode drops.

Which, come to think of it, what winding resistance are you using in
your PS calculator?

33 ohms, that's the default value and since I don't know the real one, I
assumed it's a common value that made sense.
Duncan PSU designer takes in account the load impedance (resistance
actually) and I also left it at its default value of 5k though I think it's
higher than reality. The transformer I've got (Hammond 372FX) is rated for
3A at 5V. Dunno what the winding resistance is but I don't think I'll draw
that much current from it. Sending out 0,5A should be a walk on the beach
for it.

Just did one myself. Well, sort of. I had been worrying about it but
jumped over the 'easy' solution so now I need to rip out what I wired
and redo it.

I can imagine how frustrating that can be. Not being a professional, I don't
have the time, the means and the skills to make lots of mistakes. If I could
go back in time, I think I'd do a simpler project but the no-return point
has been reached : I made the holes in the chassis :)

In the amp I'm building I use an opamp servo to automatically balance
the output tube's PP bias and, since that signal is audio, obviously,
it has to be filtered to approximate the average current. The filter
constant needs to be rather hefty so low frequency doesn't feed though
(causing low end distortion) but that also makes for a turn on delay
as the opamp takes a while to settle in, and that causes a turn on
surge in one of the output tubes because bias is not right at 'the
beginning'. I just realized I could rearrange the filters a bit,
rather than the 'direct' approach I had taken, to improve the opamp's
response without cutting into the overall filtering.

It's a little more complex than that but it illustrates the point of
having to take into account things other than 'normal operation'.

That's HiFi or guitar amp ? If it's an high-end hifi amp, don't user for
this kind of amps pick balanced tube anyway ? Even guitarists do ! :)


.

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