Comparator vs. OpAmp: pros & cons?

[Joseph Hynes]

I'm actually ok with the idea of "predicting" when a peak will come before it does.

I would put the audio signal through a "differentiator" (a series capacitor with a buffer) to get the first derivative of the signal. That's the slope at which it's climbing.

Then mix the absolute audio level with the derivative in the right proportion and use it to drive the comparator / commutator.
The idea is that the faster a wave is rising, the more likely it is to continue rising for a while.

And yes, if the commutation is to be made smoothly, that requires additional forewarning.

Joseph

[Eva]

Joseph:

I think that you don't have analysed the problem in the right way. Considering the maximum slew rate of a 20Khz sinewave, your switching may be delayed several microseconds and still be right. The trick is to trigger it when the output waveform is still several volts away from the supply rail. Also, the trick to reduce output spikes is to do the switching with some controlled voltage slope, that ideally would be the slope of a rail to rail 20Khz sine wave.

If my quick calculations are right, a +-150V 20Khz sine wave requires just a 10V/us slew rate. Also, if you detect the switching with 10V of margin, then you have 1uS of advantage (I think that half of that would be enough). Note that +-150V are already 1400W rms into 8 ohms, this is an insane power level, but I think that it's reachable with standard components (and six 50V rails).

[poobah]

Eva,

As always, you are right... As Chris pointed out earlier, the need for more power is about the low frequencies, where more time is available.

If we focus on frequencies below 300 - 500 Hz it allows much more time for the "decison" to take place. You have no need to use use full power at 20 kHz... that is not music. The power distribution over frequency is what this "trick" relies upon.

[Eva]

poobah:

You are absolutely right, I was trying to analyse the worst case. A lot of relaxation on switching times is allowed if true music signals are considered. Full swing capabilities above 400hz are probably never required.

[poobah]

Eva & Joe,

This is very interesting to me, I would like to run some calcs... This is not so much an easy issue.

[Christer]

Originally posted by lineup

A parameter often seen in comparators is 'settling time'
I have only a slight thinking of what this may be,
but do not know exactly

Settling time is also often specified for op amps, and some regard this as a very important parameter for audio op amps. For instance, LC Audio used to claim the AD825 was so superior because of its very short settling time (until they started using some other op amp ).

When specifying settling time, you must also supply an accuracy figure. If an op amp has a settling time of 100 ns to 0.1 %, this means that if you feed it a step pulse, the output will have settled to its final output value +/- 0.1 % in at most 100 ns. If the step response is overdamped, it simply means it must have reached 99.9 % of its final value within 100 ns. If it is underdamped you will have oveshoot and som decaying oscillation. In this case it means the oscillations must have decayed so much in 100 ns that the output will stay between 99.9 % and 100.1 % and not go outside that bound again. Settling time thus says something about the relationship between speed and accuracy. Read the datasheets carefully, though. Settling times are not always comparable, since they may be for different accuray. One op amp may settle to 0.1 % in 100 ns and another to 0.01 % in 1 us. It is not obvious which one is the better.

[poobah]

A very good and very thoughtful point.

[Workhorse]

Originally posted by Eva
Full swing capabilities above 400hz are probably never required.

I dont agree, systems comprising of Bi-amping/Tri-amping usually employ high power Class-H/G amps even for seperate Mids and even high with power easily exceeding 1KW in professional setups...

I have checked various pro-amps QSC RMX4050 , Crest PRO5200...all these are Class-H amps and are fully functional to Swing 20KHZ upto near the highest rail voltage in to their rated load....there isn't any limiting based on frequency criterion...

Besides this , I have also built a prototype Class-H amplifier with all N-channel mosfets in output and switching..which is capable of swinging 20kHz upto its highest rail..

K a n w a r

[poobah]

OK then,

What is the exact criteria for switching to (and from) the higher rail voltage?

And what special measures are required to avoid noise from the rail switching process?

[Workhorse]

Hi Poobah,

Lets take QSC RMX amp example...
It has 2 Tiers +-45V, +-90V
In idle conditions the output devices SEES 45V rails...but eventually whenever the signal at the output peak voltage reaches 38V...the BUS is switched to +90 and remains switched on until the signal reverts back to 40V, thereupon reverting back to lower rails at +45V

From above example it is clearly seen that there is always a margin of 6 volts above the output signal.....This keeps the output from Pre-Clipping....in lower tier....Therefore, it is wise to keep up some margin between the output voltage threshold and rail voltage..so that the output device never saturates permaturely...

Secondly, When Switching during High Frequency signals...the delay of switching must be minimised in order to get seamless transistion from one rail to another....One way to accomplish this is using a comparator triggered from the VAS of amp rather than output....In QSC..The voltage at VAS are sensed and used to carry out appropriate switching..It uses LM311 Comparator + IRFZ44N + MUR1680 Diodes to carry out requisite operation....RC snubbers across the Switching Bus diodes were used to eliminate switching noise during switching transitions......Snubbers across output & rails terminals of output device also serves to reduce switching noise, spikes...

Also the Hold up time for the mosfet-Comparator combination must be enough to accomodate large dynamic low frequency signals to pass seamlessly...this is often accomplished by using bootstrapped drivers to turn-oN the mosfets and hold their state till the signal is way down towards its reverting back to another polarity only when lower tier is just crossed......


K a n w a r