High output current audio driver

[bg]

Peter Bennett wrote in message ...

You should re-check that calculation - my calculator says the lamp
only draws 0.5 watts, not 5 watts.


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

No wonder I was impressed!

 

[krw]

You have obviously never looked at real lines. Telephones use
twisted pairs, which turn out to be very close to 100 ohm lines,

Yes, so what?

and quite flat for frequencies in the megacycle range. Down at
audio (say 2000 hz) they are quite close to 600 ohm lines. The
shape of the attenuation vs frequency curve will vary seriously
with the line quality. Also delay.

Again, irrelevant.

You can avoid those problems by doing such games as modulating 10
Mhz signals with the audio, and demodulating at the other end.
Such games also allow you to pack many signals onto the one line.

Again, so?

 

[rickman]

If you can't fit an SMT quad op-amp package and 4 resistors you're TOTALLY screwed anyway ! I
can't imagine ANY other solution will take less space.

Just how much space is there ?

I don't know why you say it is *just* these parts. The point is that
I have two channels with differential outputs, so four outputs. Right
now I am using three 20 pin TSSOPs with a *bunch* of passives. I
don't get why you say there are *only* four resistors... That is
pretty far off the track! The current parts are duals, so I get both
the A and B output of one channel in each package and a third for the
two input amps used as differential to single ended converters. There
are filters, feedback resistors, decoupling caps and the list goes
on. Then the output resistors are the ones that have to be *HUGE* to
handle the power. Do the math... 0.1 Apeak into 50 ohms is 0.25
watts. If the output is shorted, it becomes 0.2 Apeak and 1 watt. A
1 watt resistor is a 2512 size X four outputs equals a lot of board
space that you can't put vias into. The opamp has to dissipate 0.35
watt normally, with a shorted output it rises to 0.68 watts. That's a
30 C rise with the optimal PCB layout which includes a large copper
area on the bottom where you can't put other parts or vias or run
traces... I can show you the gerbers of my half completed layout.
After trying to find ways to shove 10 pounds of parts and vias onto a
5 pound board, I realized that it just would not fit and now I am
looking for new ideas to make it fit.

I have opamps that will drive the required current *and* the voltage.
But the one I picked just will not fit the space available with the
current design using those huge resistors (actually four 0805s instead
of one 2512). Trying to use four opamps to do what I can do with one
opamp is not really a productive step.

But thanks anyway. Going over all this helps to keep it fresh in my
mind. When I encounter a difficult design, I tend to want to find a
way to make it work. Then I drive myself until I am totally sick of
dealing with the problem and often have started to work way too late
and am loosing sleep over it. That is part of why I came here. If
nothing else, it gives me a chance to think about the problem and not
just obsess with trying to make a bad solution fit.

Right now I am pretty happy with two possible solutions. There is a
Fairchild part that should either meet the requirements of 10 Vpp into
50 ohms with none or a small series resistor (maybe 5 ohms) for
stability. Since the part is not really spec'd in this range, I can't
be sure it will drive the full voltage, but the data given sure makes
it look good. The part is cheap, the single 12 volt supply saves a
lot of power burnt in the opamp. But it has no spice model, so I
can't check it before building it. I will have to verify everything
on the lab bench where it is much tougher to measure things like
currents.

The other solution is an audio amp from TI which *does* have a spice
model. It is also cheap and available. But it uses the +- 12 volt
supplies and will be pretty warm. It has differential outputs, but
the best thermal package is 10 x 10 mm which is large for this socket
and the thetaJA is still a wopping 80C/W, so I am not sure about the
effect of the power. It is also a fixed gain device, so would require
driver opamps to set the gain of the output (that may be a minor issue
since the two input amps can share a quad package with the drivers).
Finally, this won't drive 10 Vpp, but rather tops out at about 8.5
Vpp.

Yes, the board is really small, 4.5" x 0.85" and most of it is FPGA,
connectors and some RS-422 driver/receivers, analog switches for
isolation, ESD diodes, two power converters an oh don't forget the
codec. Worse, I am constrained by the existing design to put the
connectors, one near an end of the board and one a little past the
middle. All of the analog stuff should go between the two connectors
and the digital on the long overhang.

So there is the current solution that meets all of the requirements
other than fitting the board space. Then I have an audio amp that is
pretty hot and won't drive the full voltage into 50 ohms, but looks
fairly good in other regards. Lastly, there is a opamp from Fairchild
that looks like it would do it all and from a 12 volt supply as long
as I give up the 50 ohm output resistors, but it has no spice model to
simulate.

I do appreciate the comments and just conversation. I helps me get it
all straight in my head. I have always known that if someone can't
properly explain something, it is because they don't really understand
it. So if I can't explain it to you, I don't understand the issues
here. By trying to explain it, I learn it that much more clearly.

 

[rickman]

Most amps need only relatively small series R to stabilise against
heavy C loads.
It's easy to test on the bench, so I'd grab some of the likely opamps
and breadboard them up, and test them with nasty loads.

Since the fairchild Opamp has no thermal sense, and you are
tightly area/power constrained, I'd look at adding a temperature trip
- devices like AnalogDevices TMP35 are cheap, and small, and
could sense the output area, and so reduce the PCB area for
ressitors / Opamp and copper cooling

-jg

This is another post that has made me think a bit. I was calculating
power in the opamp considering that the current is a sine wave and the
RMS value should be used rather than the peak. But when you short the
output the current for the most part becomes a square wave of + and -
120 mA. This effectively is like driving a DC current at half the
power supply voltage and is significantly larger than the RMS case.

6 volts x 120 mA - 5 ohm x 120 mA**2 = 0.65 watts! The thetaJA of
these parts is not so good at 155 C/W. That might be a good reason to
go with two of the quads, rather than three of the duals. Still, at
140 C/W that is a 90 degree rise above ambient! Normal operation is
only 0.17 watts and is only a 25 degree temperature rise. None of
these parts have a thermal pad, too bad.

 

[Eeyore]

Delivering audio power down a 50 ohm terminated cable is not a
reasonable use.

I couldn't agree more.

Either the client has some specific equipment they want to connect
that's only available with a terminated 50 ohm input ot they're
requesting an unrealistic specification.

Graham

 

[David L. Jones]

I don't know why you say it is *just* these parts.  The point is that
I have two channels with differential outputs, so four outputs.  Right
now I am using three 20 pin TSSOPs with a *bunch* of passives.  I
don't get why you say there are *only* four resistors...  That is
pretty far off the track!  The current parts are duals, so I get both
the A and B output of one channel in each package and a third for the
two input amps used as differential to single ended converters.  There
are filters, feedback resistors, decoupling caps and the list goes
on.  Then the output resistors are the ones that have to be *HUGE* to
handle the power.  Do the math... 0.1 Apeak into 50 ohms is 0.25
watts.  If the output is shorted, it becomes 0.2 Apeak and 1 watt.  A
1 watt resistor is a 2512 size X four outputs equals a lot of board
space that you can't put vias into.  The opamp has to dissipate 0.35
watt normally, with a shorted output it rises to 0.68 watts.  That's a
30 C rise with the optimal PCB layout which includes a large copper
area on the bottom where you can't put other parts or vias or run
traces...  I can show you the gerbers of my half completed layout.
After trying to find ways to shove 10 pounds of parts and vias onto a
5 pound board, I realized that it just would not fit and now I am
looking for new ideas to make it fit.

Then seriously consider vertical riser boards if there is room. A long
thin board only say 10mm in height can hold a surprising amount of
parts. Likewise, low height profile SMD connectros or whatever can
gain you an extra daughterboard to almost double your PCB area.

If you are really out of space for the vias then perhaps consider
blind/burried vias as well.

Dave.

 

[John Devereux]

building is approached through "Washington
Place," an alley, at the entrance of which one encounters a sign, "No
White Men Admitted Here, Only Chinese." This notice, which has been
put up at the entrance of Oriental brothels in Chinatown, has been
ordered by the Chief of Police, it is claimed, to prohibit Americans
associating with Orientals in vice, so as to prevent demoralization
and race quarrels. We do not dispute the motive, but the _effect_
is, that those who would work for the rescue of slaves are kept at a
distance, and no one who is likely to make a complaint against abuses
and law-breaking can approach the place without permission from
the police, which gives ample opportunity for getting everything
objectionable out of sight. As far as prevention of the commingling
of the different races is concerned, that may be hindered at certain
points, but American men are on the inside track here, as to making
money through these slaves. The building has been erected and is
owned by Americans, and one man of European name is a partner in the
immediate management of the place. On our first visit to this building
we were informed on reliable information that there were 125 Japanese
and over 50 Chinese girls in the place, a

 

[SoothSayer]

innocent little things at all realized the
fate in store for them. In one place we saw two very old women in
the front room. In another, a woman knelt before the idolatrous
shrine engaged in her devotions. At one point there was a very
large boat brilliantly fitted up for music, dancing, smoking
opium, and feasting. At the far end of the street was a
'kitchen-boat,' from which supplies of food, ready cooked,
could be bought. All the way along we saw little girls with the
unmistakable signs of their destiny upon them. Our interpreter
said the girls were usually made to stay upstairs during the day
time, but at night the whole place was illuminated and alive; then
they were brought down and to the front. Occasionally we would see
one of these huge house boats full of painted girls, floating down
the middle of the stream, for they move about from place to place
at will.

"At Canton, February 18th, 1894, we met and conversed with a
missionary lady who had just come from a station in the interior.
She had travelled from her station on a Chinese boat, which had
been chartered by her adopted son for his use going up, and for
hers coming down the river. When she was about to embark, she
required that the men should search the boat, and down below, in
the very bottom, were a lot of little girls--_child slaves_--being
smuggled to Canton for the trade of a vile life. She made the men
take the children off the boat, but with great difficulty. They
resisted, but she stood courageously, and saw her commands
executed. After she had accomplished this, and started down the
river, all alone, so far as any English-speaking person was
concerned, the men, who were still deeply enraged at being
defeated in

These are the times that try men's holes...

 

[Rich Grise]

Well, I see it's an equal-opportunity gibberish-bot. ;-)

 

[John Smith]

John Devereux wrote:

[...]

I get it!

It is that time honored game of "Plonk a Pig!"

JS

 

[neon]

INPEDANCE 50 ohms audio where did you get this cable. try any cable any inpedance type at some point in its lenght there will be [0] inpedance that is why carefull cutting of tv cable is inportant but for audio the lenght for 1/2 wave wlenght is sooo great that nobody even care. you have no idea of what you are even thinking do you>?