High output current audio driver

"Bob Eld "

** Cable capacitance of up to 0.5 uF still will be major problem for audio op-amps and the like to drive.

A termination value needs to be found that makes the line present a load that is as resistive as possible.

...... Phil

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)

In , rickman wrote (edited for space):

How about LM386 and add in series with your board's output the lowest value and wattage resistor that according to the datasheet with your supply voltage will cause no problem if that resistor from LM386 output (after any series capacitor) goes to ground?

As I said a bit earlier, I would be wary of positive feedback. If the ultimate load has impedance varying with frequency and is on a line of length no more than a few percent of a wavelength of the highest frequency that matters (I would guess about 15-20 KHz), positive feedback will at least largely reinforce the gain variations that correspond to impedance variations with frequency. Your ultimate load may be a loudspeaker preceded by a matching transformer.

If you use positive feedback or even substantial output impedance, be prepared to explain how your device meets the spec but fails to satisfy the customer. You will be better off if the customer is satisfied even if the device meets the spec. Prime example: Many, probably most loudspeakers (especially in unported enclosures) that have frequency response peaks in low or lower-middle audio frequencies have corresponding impedance peaks. Significant output impedance and positive feedback will worsen these. There are other loudspeakers with flatter frequency response, especially at lower and lower-middle frequencies, but with impedance varying greatly with frequency over some frequencies where ratio of output SPL to input voltage is more constant. Having significant output impedance will make frequency response peaks out of load impedance (as a function of frequency) peaks. Keep in mind that loudspeaker frequency response (ratio of listener SPL to input voltage as a function of frequency) is generally determined with zero or approaching-zero source impedance.

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

I thought you said before 10 Vpp. Are you counting on a same-as-load-impedance series resistor?

If so and if that is tolerable, I consider it no worse to have a scheme with a combination of current and voltage feedbacks (maybe negative voltage feedback and positive current feedback) to generate an output impedance same as that of such a series resistor - that can reduce your amplifier voltage requirement by half.

If you have a severe supply voltage requirement, consider having two amplifiers, with their inputs driven inverted from each other. Maybe or maybe-not so-good if this device has to feed an "unbalanced line", such as 50 or 52 ohm coaxial cable whose shield is a ground that your supply rails must have low AC voltage/potential with-respect-to.

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

Thanks for the insight. Actually, I am designing more than one device with the same circuit board, but different populations are allowed. The requirement for driving the 50 ohm line unfortunately is in the same configuration that requires 10 Vpp output. It does not, however, require wide bandwidth, transmitting only a 1 kHz AM signal modulated at 100 baud. The other two modes can change values of filter components, gain settings and the termination components. So if the load has a complex impedance the positive feedback can be removed and the output can be driven with the initial 50 ohm or higher resistance.

Is that any more clear? I know this sounds like an unusual application, but mine is to do or die... however I will be asking questions about this later this week. Right now I want to be able to say I have covered the bases and considered every technical approach.

Yes, for the 50 ohm load configuration, that is exactly what I am considering.

Yes, that is what I was thinking about. I have read a couple of articles about positive feedback to multiply the output resistor. This reduces the power spent in the output resistor, although not in the total circuit unless you can lower the supply voltages. But the total power is not my immediate concern. The dissipation in the resistors is since they get very large with power. The amps can be effectively heatsinked to help remove the heat, so they are better equipped to dissipate the Watt or so required.

Yes, I have been thinking about this and unless you are referring to a differential arrangement, I don't understand what you mean about the two amplifiers. I don't think I can use a differential output for the

10 Vpp, 50 ohm configuration.

"rickman" ...

High resolution? I hope it is not also high accuracy. A long cable with a 50 Ohm load will have a gain certainly less than one. And temperature dependent. How are you (is your) customer dealing with that? You are not doing remote sensing, are you?

Arie.

... snip ...

Delivering audio power down a 50 ohm terminated cable is not a reasonable use. You are better off delivering a raw signal, properly equalized for the cable properties, to a remote amplifier.

--
 [mail]: Chuck F (cbfalconer at maineline dot net) 
 [page]: 
            Try the download section.

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

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.

--
 [mail]: Chuck F (cbfalconer at maineline dot net) 
 [page]: 
            Try the download section.

Late at night, by candle light, snipped-for-privacy@manx.misty.com (Don Klipstein) penned this immortal opus:

A TDA2002 or whatever is the modern replacement should do quite nicely. As other have stated, at audio the line impedance is of little consequence.

- YD.

--
Remove HAT if replying by mail.

"High resolution" is the customer's term for better than voice grade. I am using a standard 16+ bit codec which can sample at much higher rates than what is needed for audio. That is what he essentially asked for. I have been given no details about how the voice grade mode or the "high resolution" mode will be used. The voice grade mode I can figure out easily as I have the info on their current design and voice is pretty standard. It is the 50 ohm mode that I am actually building for him. But he would like this new card to replace the old one and pick up the "high resolution" mode as well... if possible. The issues of driving cables and varying loads aside, this card will do that. The rest is up to him since I was not given info to work with.

My only real concern with the Fairchild part meeting the spec for all of these modes (which can have different build options, btw, so resistors and such can change) is driving a capacitive load. They say to use an output resistor to isolate the capacitance. But if I use a very large one, it will reduce the voltage level. That is why I would like to simulate it, there are just too many variables to try to get it right the first time without simulation.

On a sunny day (Tue, 22 Jan 2008 08:29:40 -0800 (PST)) it happened rickman wrote in :

My personal experience with 'audio' is, that they drive the cable hard. I once did a 200 Ohm (think it was) audio distribution amp, and had

200 Ohm resistors. They did not terminate the other end, so the levels were 2x too high. So better have no resistors, and then that requires a short circuit proof amp, and some way to dissipate the heat. Maybe you can protect things by foldback current limit in the supply.

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

Yes, so what?

Again, irrelevant.

Again, so?

--
Keith

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.

in it, so we will content ourselves by saying that during late years physicians of prominence from every part of the world have assembled twice at Brussels for Conferences in regard to this matter. These physicians are in large numbers Continental doctors, the very ones who have had most to do in enforcing such measures. Each time the number of opponents to the Contagious Diseases Acts has rapidly increased, after listening to the testimony from all sides as to their inutility; in fact, the whole force of opinion at each of these Conferences, in 1899 and 1902, was against State Regulation, though there was a division of opinion as to the substitute for it.

In 1903, the Minister of the Interior of France, the country where these Acts originated, nominated an extra-Parliamentary Commission to go thoroughly into these questions. This Commission held its numerous sittings in 1905, and in the end by almost a two-thirds' majority condemned the existing system of regulation in France, and furthermore rejected the alternative proposal of notification with compulsory treatment, by sixteen votes to one. In reporting on the Conferences held in Brussels, the _Independence Belge_ said, in a leading article: "Regulation is visibly decaying, and the fact is the more striking because the country that instituted it (France) is at present the one that meets it with the most ardent hostility."

CHAPTER 4.

MORE POWER DEMANDED AND OBTAINED.

In 1866 the Governor of Hong Kong, Sir Richard Graves MacDonnell, determined upon the repeal of Ordinance 1

W.H. Marsh, of Hong Kong, learning that there was a likelihood of the Contagious Diseases Ordinance being disallowed by the Home Government, wrote to the Secretary of State for the Colonies:

"It is the unanimous opinion of the Executive Council that the laws now in existence have had, when they have been properly worked, a most beneficial effect in this Colony ... in putting the only practical check on a system of brothel slavery, under which children were either sold by their parents, or more frequently were kidnaped and sold to the proprietors of brothels. These unfortunate girls were so fully convinced that they were the goods and chattels of their purchasers, or were so terrified by threats, that they rarely if ever made any complaints even when interrogated. It was very seldom that sufficient evidence could be obtained to punish such nefarious traffickers."

A document enclosed in this letter to the Colonial Secretary at London, signed by the Acting Colonial Secretary at Hong Kong, the Colonial Surgeon, and the Registrar General, states: "Perhaps the strongest argument in favor of the Ordinances is the means they place in the hands of the Government for coping with _brothel slavery_." From the moment Mr. Labouchere put this false claim to the front it has been the chief argument advanced by officials eager for the Contagious Diseases Ordinance as a method of providing "clean women," in order to win to their side the benevolent-minded.

On this point the Commission reported: "In regard to the only result worthy of a moment's consideration, viz., that referred to by Mr. Labouchere's dispatch, of putting down the virtual slavery of women in brothels, the conclusions of those in the best position to form

construed to be slaves, or persons intended to be dealt with as slaves. Hundreds of persons are held in such servitude as pledged or pawned in Hong Kong, and not one of the parties to such transactions has ever been proceeded against under these Acts."

"In addition to the above-mentioned Acts of George, William and Victoria, there is also the Imperial Act, entitled The Slave Trade Act, 1873, which consolidates the laws for the suppression of the Slave Trade, and which is in force in Hong Kong by its own authority. We have also the provisions of the Local Ordinance 4 of 1865, sections 50 and 51, and 2 of 1875."

"Offenses against the provisions of these Ordinances, so far as they relate to women or children, are still very common, and are growing more numerous every day, and until the system of prostitution which prevails in this Colony, and the system of breeding up young girls from their infancy to supply the brothels of Hong Kong, Singapore, and San Francisco, _is declared to be slavery_, and is treated and punished as such in Hong Kong, no stop will ever be put to the kidnaping of women and the buying and selling of female children in Hong Kong. This buying and selling is only an effect of which the existing system of Chinese prostitution is the cause. Get rid of that, and there is an end of kidnaping."

Again the nail had been struck on the head. _Licensed brothel slavery_, as it exists at Hong Kong, was put forward by the C

"There is another matter connected with the brothels, licensed and unlicensed, in Hong Kong, which almost daily assumes a graver aspect. I refer to what is no less than the trafficking in human flesh between the brothel-keepers and the vagabonds of the Colony. Women are bought and sold in nearly every brothel in the place. They are induced by specious pretexts to come to Hong Kong, and then, after they are admitted into the brothels, such a system of espionage is kept over them, and so frightened do they get, as to prevent any application to the police. They have no relatives, no friends to assist them, and their life is such that, unless goaded into unusual excitement by a long course of ill-treatment, they sink down under the style of life the