High output current audio driver

[rickman]

I am designing a circuit to drive audio signals onto a cable. I have
very little board space and am having a hard time figuring out a good
solution. What makes it hard is that the load is a 50 ohm cable (with
DC terminator at end) and I have to drive it to 10 Vpp. I can find
lots of audio driver chips which will happily drive ungodly high
voltages onto 600 ohm lines. Or I can find fast, expensive, hot, DSL
driver chips that have to be compensated and eat gobs of board
space.

I was going with the DSL driver approach since that would meet the
spec of 10 Vpp into 50 ohms. But my understanding is to do that, I
need to use series matching resistors of 50 ohms and deal with a short
circuit output. Using +-12 volt supplies I was able to find a single
part that would output +-10 volts at 100 mA and came with a spice
model. Good thing I used spice too, since it showed me several issues
with my design that could have been a problem.

But when I got to board layout, the combination of high power
resistors and fat opamp chips did me in. The power resistors need to
be 0.25 Watt in normal operation. But if an output is shorted, the
power can go to 1 Watt, and this is figured with a DC blocking cap so
I used RMS voltage rather than peak. I found some 0805 resistors at
1/3 Watt and used four in series/parallel. But there are just too
many passives which combined with the thermal vias required, use too
much board space and there is no room for vias to route the board.

So I am looking for alternative approaches. The audio drivers would
be perfect, but they just won't output the voltage I need. But then I
question the need for 10 Vpp into 50 ohm cable. I will be having a
discussion about this with my customer early next week. If I can back
off on the voltage a bit, TI makes a nice, small audio driver, DRV135
that has a differential output and should do the job perfectly. It
looks like it will pump out maybe 50 mA or so at 10 Vpp, but all of
their charts are with Vs of +-18V which may be a very different
picture than +-12V.

Any other audio products rated for driving 50 ohm loads like this?

 

[Martin Griffith]

I am designing a circuit to drive audio signals onto a cable. I have
very little board space and am having a hard time figuring out a good
solution. What makes it hard is that the load is a 50 ohm cable (with
DC terminator at end) and I have to drive it to 10 Vpp. I can find
lots of audio driver chips which will happily drive ungodly high
voltages onto 600 ohm lines. Or I can find fast, expensive, hot, DSL
driver chips that have to be compensated and eat gobs of board
space.

I was going with the DSL driver approach since that would meet the
spec of 10 Vpp into 50 ohms. But my understanding is to do that, I
need to use series matching resistors of 50 ohms and deal with a short
circuit output. Using +-12 volt supplies I was able to find a single
part that would output +-10 volts at 100 mA and came with a spice
model. Good thing I used spice too, since it showed me several issues
with my design that could have been a problem.

But when I got to board layout, the combination of high power
resistors and fat opamp chips did me in. The power resistors need to
be 0.25 Watt in normal operation. But if an output is shorted, the
power can go to 1 Watt, and this is figured with a DC blocking cap so
I used RMS voltage rather than peak. I found some 0805 resistors at
1/3 Watt and used four in series/parallel. But there are just too
many passives which combined with the thermal vias required, use too
much board space and there is no room for vias to route the board.

So I am looking for alternative approaches. The audio drivers would
be perfect, but they just won't output the voltage I need. But then I
question the need for 10 Vpp into 50 ohm cable. I will be having a
discussion about this with my customer early next week. If I can back
off on the voltage a bit, TI makes a nice, small audio driver, DRV135
that has a differential output and should do the job perfectly. It
looks like it will pump out maybe 50 mA or so at 10 Vpp, but all of
their charts are with Vs of +-18V which may be a very different
picture than +-12V.

Any other audio products rated for driving 50 ohm loads like this?

something like this?
http://focus.ti.com/docs/prod/folders/print/tpa6120a2.html


martin

 

[skeptic]

I am designing a circuit to drive audio signals onto a cable.  I have
very little board space and am having a hard time figuring out a good
solution.  What makes it hard is that the load is a 50 ohm cable (with
DC terminator at end) and I have to drive it to 10 Vpp.  I can find
lots of audio driver chips which will happily drive ungodly high
voltages onto 600 ohm lines.  Or I can find fast, expensive, hot, DSL
driver chips that have to be compensated and eat gobs of board
space.

I was going with the DSL driver approach since that would meet the
spec of 10 Vpp into 50 ohms.  But my understanding is to do that, I
need to use series matching resistors of 50 ohms and deal with a short
circuit output.  Using +-12 volt supplies I was able to find a single
part that would output +-10 volts at 100 mA and came with a spice
model.  Good thing I used spice too, since it showed me several issues
with my design that could have been a problem.

But when I got to board layout, the combination of high power
resistors and fat opamp chips did me in.  The power resistors need to
be 0.25 Watt in normal operation.  But if an output is shorted, the
power can go to 1 Watt, and this is figured with a DC blocking cap so
I used RMS voltage rather than peak.  I found some 0805 resistors at
1/3 Watt and used four in series/parallel.  But there are just too
many passives which combined with the thermal vias required, use too
much board space and there is no room for vias to route the board.

So I am looking for alternative approaches.  The audio drivers would
be perfect, but they just won't output the voltage I need.  But then I
question the need for 10 Vpp into 50 ohm cable.  I will be having a
discussion about this with my customer early next week.  If I can back
off on the voltage a bit, TI makes a nice, small audio driver, DRV135
that has a differential output and should do the job perfectly.  It
looks like it will pump out maybe 50 mA or so at 10 Vpp, but all of
their charts are with Vs of +-18V which may be a very different
picture than +-12V.

Any other audio products rated for driving 50 ohm loads like this?

You haven't mentioned how much gain you need. Have you considered
building a complementary symmetry amplifier.

 

[Vladimir Vassilevsky]

I am designing a circuit to drive audio signals onto a cable. I have
very little board space and am having a hard time figuring out a good
solution. What makes it hard is that the load is a 50 ohm cable (with
DC terminator at end) and I have to drive it to 10 Vpp

You can use pretty much any of the audio power amp ICs made by Phillips
or ST. They are protected against faults, can have differential or
single ended output, and the distortion level at 50 Ohm load will be
very minimal.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[MooseFET]

I am designing a circuit to drive audio signals onto a cable. I have
very little board space and am having a hard time figuring out a good
solution. What makes it hard is that the load is a 50 ohm cable (with
DC terminator at end) and I have to drive it to 10 Vpp. I can find
lots of audio driver chips which will happily drive ungodly high
voltages onto 600 ohm lines. Or I can find fast, expensive, hot, DSL
driver chips that have to be compensated and eat gobs of board
space.

[... details deleted to save space ...]

Take a look at the LT1206. They don't call it an audio output chip
but it has the sort of specs you need. It can easily provide the
100mA current needed. It even comes in a SO-8 package. If you can
get the heat away, it should do what you need. It is self protecting
in the case of a short, so you may not need any extra parts to handle
that case.

 

[John Larkin]

I am designing a circuit to drive audio signals onto a cable. I have
very little board space and am having a hard time figuring out a good
solution. What makes it hard is that the load is a 50 ohm cable (with
DC terminator at end) and I have to drive it to 10 Vpp. I can find
lots of audio driver chips which will happily drive ungodly high
voltages onto 600 ohm lines. Or I can find fast, expensive, hot, DSL
driver chips that have to be compensated and eat gobs of board
space.

I was going with the DSL driver approach since that would meet the
spec of 10 Vpp into 50 ohms. But my understanding is to do that, I
need to use series matching resistors of 50 ohms and deal with a short
circuit output. Using +-12 volt supplies I was able to find a single
part that would output +-10 volts at 100 mA and came with a spice
model. Good thing I used spice too, since it showed me several issues
with my design that could have been a problem.

But when I got to board layout, the combination of high power
resistors and fat opamp chips did me in. The power resistors need to
be 0.25 Watt in normal operation. But if an output is shorted, the
power can go to 1 Watt, and this is figured with a DC blocking cap so
I used RMS voltage rather than peak. I found some 0805 resistors at
1/3 Watt and used four in series/parallel. But there are just too
many passives which combined with the thermal vias required, use too
much board space and there is no room for vias to route the board.

So I am looking for alternative approaches. The audio drivers would
be perfect, but they just won't output the voltage I need. But then I
question the need for 10 Vpp into 50 ohm cable. I will be having a
discussion about this with my customer early next week. If I can back
off on the voltage a bit, TI makes a nice, small audio driver, DRV135
that has a differential output and should do the job perfectly. It
looks like it will pump out maybe 50 mA or so at 10 Vpp, but all of
their charts are with Vs of +-18V which may be a very different
picture than +-12V.

Any other audio products rated for driving 50 ohm loads like this?

There are tons of opamps that will do what you want, either 20 v p-p
with a local 50 ohm source-terminator resistor, or 10 volts p-p
without. But you can't expect that sort of power capability without
generating heat.

John

 

[[email protected]]

There are tons of opamps that will do what you want, either 20 v p-p
with a local 50 ohm source-terminator resistor, or 10 volts p-p
without. But you can't expect that sort of power capability without
generating heat.

Actually, you can, if you drive the 50R load with a class D switching
driver. You've still got to find room for a filter inductor and
capacitors to stop the high frequency componets of the switching
output getting onto the cable and generating embarassing and illegal
levels of radio-frequency interference.

http://www.analog.com/library/analogDialogue/archives/40-06/class_d.pdf

Texas Instruments also offer integrated circuit Class-D amplifiers,
but Texas Instruments datasheets have to be read very carefully.

 

[Bob Eld]

I am designing a circuit to drive audio signals onto a cable. I have
very little board space and am having a hard time figuring out a good
solution. What makes it hard is that the load is a 50 ohm cable (with
DC terminator at end) and I have to drive it to 10 Vpp. I can find
lots of audio driver chips which will happily drive ungodly high
voltages onto 600 ohm lines. Or I can find fast, expensive, hot, DSL
driver chips that have to be compensated and eat gobs of board
space.

I was going with the DSL driver approach since that would meet the
spec of 10 Vpp into 50 ohms. But my understanding is to do that, I
need to use series matching resistors of 50 ohms and deal with a short
circuit output. Using +-12 volt supplies I was able to find a single
part that would output +-10 volts at 100 mA and came with a spice
model. Good thing I used spice too, since it showed me several issues
with my design that could have been a problem.

But when I got to board layout, the combination of high power
resistors and fat opamp chips did me in. The power resistors need to
be 0.25 Watt in normal operation. But if an output is shorted, the
power can go to 1 Watt, and this is figured with a DC blocking cap so
I used RMS voltage rather than peak. I found some 0805 resistors at
1/3 Watt and used four in series/parallel. But there are just too
many passives which combined with the thermal vias required, use too
much board space and there is no room for vias to route the board.

So I am looking for alternative approaches. The audio drivers would
be perfect, but they just won't output the voltage I need. But then I
question the need for 10 Vpp into 50 ohm cable. I will be having a
discussion about this with my customer early next week. If I can back
off on the voltage a bit, TI makes a nice, small audio driver, DRV135
that has a differential output and should do the job perfectly. It
looks like it will pump out maybe 50 mA or so at 10 Vpp, but all of
their charts are with Vs of +-18V which may be a very different
picture than +-12V.

Any other audio products rated for driving 50 ohm loads like this?

Why do you want to terminate the line? Run it constant voltage with a modest
1K impedance or even 600 ohms on the far end. There is no reason what so
ever to match the far end of the line unless it is five or more kilometers
long or the frequencies are in hundreds of kilohertz or higher.

A better solution is to run it balanced 600ohms driven constant voltage,
i.e, low impedance on the drive end.

 

[CBFalconer]

No original ever appeared hear, so I am answering this quote. (Or
maybe it got lost when purging the sporge.)

Anyhow - why? That "50 ohm" cable is not 50 ohms at audio
frequencies. It is more likely something like 600 ohms. That is
why telephones use 600 ohm lines, although measurements on typical
wiring lines will show them to be close to 100 ohms.

 

[rickman]

No original ever appeared hear, so I am answering this quote. (Or
maybe it got lost when purging the sporge.)

Anyhow - why? That "50 ohm" cable is not 50 ohms at audio
frequencies. It is more likely something like 600 ohms. That is
why telephones use 600 ohm lines, although measurements on typical
wiring lines will show them to be close to 100 ohms.

What if the termination is a 50 ohm resistor?

 

[Phil Allison]

"Bob Eld"

"rickman"



Why do you want to terminate the line? Run it constant voltage with a
modest
1K impedance or even 600 ohms on the far end. There is no reason what so
ever to match the far end of the line unless it is five or more kilometers
long or the frequencies are in hundreds of kilohertz or higher.

** Really ????

50 ohm co-ax exhibits 100 pF per metre of capacitance, if not terminated
with 50 ohms - or 0.1 uF per km.

The -3 dB point for a 600 ohm source driving 0.1 uF is 2,650 Hz - ie less
than telephone bandwidth.

For a cable run of 100 metres or more, termination is likely to be very
desirable to pass the full audio band.

A better solution is to run it balanced 600ohms driven constant voltage,
i.e, low impedance on the drive end.

** If the run really is 1000s of metres long, doing that will not give
audio bandwidth with readily available twisted pair shielded cable.



........ Phil

 

[rickman]

Why do you want to terminate the line? Run it constant voltage with a modest
1K impedance or even 600 ohms on the far end. There is no reason what so
ever to match the far end of the line unless it is five or more kilometers
long or the frequencies are in hundreds of kilohertz or higher.

A better solution is to run it balanced 600ohms driven constant voltage,
i.e, low impedance on the drive end.

Thanks for the thoughts. Unfortunately, I don't get to pick the specs
I am designing to. I am building a board, not the system. I may go
back to my customer to ask for a change to the specs, but this is what
I have. When they spec'd the design I guess they have to account for
any number of situations. I know that there are three different
applications for this design which will only differ in the line
impedance and the frequency range. Some are voice, some are "high
resolution" and the main use is for IRIG-B. My spec is for 50 ohm
line with 50 ohm resistive termination. It's the resistive
termination I find difficult to deal with.

 

[rickman]

something like this?http://focus.ti.com/docs/prod/folders/print/tpa6120a2.html

Thanks for the lead. I think this part requires too many components
to support it. My current problem is that I am out of board space.
Otherwise I have a workable solution.

 

[Phil Allison]

"rickman"

My spec is for 50 ohm
line with 50 ohm resistive termination. It's the resistive
termination I find difficult to deal with.

** For heaven's sake man - the answer was posted ages ago.

http://www.electronica.ro/audio/TDA2030.shtml





....... Phil

 

[Eeyore]

I am designing a circuit to drive audio signals onto a cable. I have
very little board space and am having a hard time figuring out a good
solution. What makes it hard is that the load is a 50 ohm cable (with
DC terminator at end) and I have to drive it to 10 Vpp.

Why are you driving audio into 50 ohms ? It's absurd.

Graham

 

[rickman]

You haven't mentioned how much gain you need. Have you considered
building a complementary symmetry amplifier.

No, I have not considered that. I have a single ended source with
about a 2 Vpp output which must be amplified by 10 in my current
configuration. One alternate approach I am considering is using
positive feedback to allow a smaller series matching resistor to be
used which would make the resistor power and size much smaller. But
an article I read on it showed poor results when the load had no
termination. But I am going to simulate it to see how well it might
actually work in my application.

 

[Eeyore]

I can find lots of audio driver chips

I think you mean op-amps.

which will happily drive ungodly high voltages onto 600 ohm lines.

Simple. Just add a discrete complementary emitter follower stage on the
end of one.

Beware of power dissipation issues into 50 ohms though.

Graham

 

[Eeyore]

Any other audio products rated for driving 50 ohm loads like this?

NO-ONE (in their right mind) uses 50 ohms for audio hence NO. Even 600
ohms is mostly irrelevant.

Why are YOU using 50 ohms ? I suspect you've made some misunderstanding
somewhere.

Graham

 

[Eeyore]

My current problem is that I am out of board space.

So how are you going to deal with the power dissipation issues into 50 ohms ?

Graham

 

[Eeyore]

Anyhow - why? That "50 ohm" cable is not 50 ohms at audio
frequencies.

It's basically open circuit.

It is more likely something like 600 ohms.

No it isn't. *Telegraph* lines are 600 ohms.

That is why telephones use 600 ohm lines,

They don't actually.

although measurements on typical wiring lines will show them to be
close to 100 ohms.

At ADSL frequencies.

Graham