High output current audio driver

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?

Reply to
rickman
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something like this?

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martin

Reply to
Martin Griffith

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

Reply to
skeptic

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

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Reply to
Vladimir Vassilevsky

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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.
Reply to
MooseFET

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

Reply to
John Larkin

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.

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

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

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.

Reply to
Bob Eld

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.

--
 [mail]: Chuck F (cbfalconer at maineline dot net) 
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Reply to
CBFalconer

What if the termination is a 50 ohm resistor?

Reply to
rickman

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.

Reply to
rickman

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.

Reply to
rickman

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.

Reply to
rickman

Thanks for the reply. I didn't find "tons" of opamps that would work in this application with 12 volt supplies. I found a small handful that would do the job and had a spice model. But the problem is that I need to find a way to do this task without having to dissipate a watt with a short circuited output. I did find an audio opamp from TI that seems to be a pretty good match other than it won't drive the outputs to +-5 volts with a 50 ohm load, at least I can't tell that from the data sheet. It has a spice model, so I will be simulating it. The part even has 50 ohm series output resistors built in. I need to make sure it will be protected against shorted outputs.

I have found an alternate configuration that uses positive feedback to reduce the size of the series resistor and so the power dissipated. I was interested if there were any other options that I was not aware of.

Reply to
rickman

I looked at class-D parts for a speaker driver app once. I don't recall them working with the voltages I have available. Also, I am pretty sure I don't have the room for the inductor. But do you know if they will drive a 20 Vpp signal into 100 ohms with 12 volt bipolar supplies?

Reply to
rickman

Nonsense. The impedance of a "50-ohm cable" may not matter at audio frequencies but it will still still have a characteristic impedance of 50-ohms. Telephones don't use "50-ohm cable" which, of course, cannot be why they use 600-ohms.

--
Keith
Reply to
krw

Thanks for your replies. I am driving 50 ohm line because I am being paid to.

Well, the ultimate end customer is government so I have no doubt of that. But my specs are crystal clear.

If the end termination is 50 ohm resistive, it does not matter what the line impedance is. If the receiver has a 50 ohm resistive and I want to drive it, I must live with the physics. I was hoping that there were others here who had some insight into possible solutions.

Reply to
rickman

I would avoid positive feedback. Should your product be used to drive a load differing from the spec, and I have a gut feeling it will be *at least sometimes*, positive feedback will reinforce all effects where gain varies with impedance of the load presented to your device. Bad news if that is a loudspeaker or one with a transformer ahead of it!

See my other post - coming soon if you see this one first!

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

So far, all I know is "50 ohms", "audio", and 10 Vpp.

Sounds to me like something the LM386 does well with a 12-15 volt or so supply. If you need a somewhat lower supply voltage, then use two LM386's, and drive their inputs out of phase with each other - should work at 7-8 volts supply voltage but also higher supply voltage such as 12 volts.

This is tolerant of load impedance going significantly from 50 ohms in either direction, especially upwards.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

If the end termination is 50 ohms resistive, the a line impedance of several ohms to a few hundred ohms will be no problem if the line length is no more than a percent or so of a wavelength at the highest frequency that matters.

Since you said audio, I would guess that you only need things to be good to 20 KHz. I would allow for twice that.

Most lines at frequencies under 100 KHz have velocity factor at least .6. (Corresponding to dielectric constant of 2.777)

At 40 KHz and velocity factor of .6, 1% of a wavelength is 45 meters or about 150 feet.

If the line is a few hundred feet or more, be prepared to explain any actual ill effects in terms of mismatch between load and line. Know the "Smith Chart" and how to use one. I suspect this could come in handy if your customer gives any complaints of gain varying with frequency, especially at upper audio frequencies. For that matter, should you have to handle any complaints, you want to know the ultimate load's impedance as a function of frequency. Or at least have the customer replace the actual load with a 50 ohm resistor that is essentially a 50 ohm resistor at every frequency in question, should the actual load be different.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

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