Just an observation about voltage swing in actively crossed-over tweeters. - diyAudio
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Old 23rd April 2014, 01:15 AM   #1
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Default Just an observation about voltage swing in actively crossed-over tweeters.

Lately, I've been ruminating on the ideal qualities I would want in a tweeter amplifier for my 4-way 24db L-R quad-amplified system. (Thanks, Rod Elliot!) It had occurred to me that the tweeter amp will be spending an awful lot of time hanging around at low voltage, in the bias zone. They will be in class-A push-pull mode there, but frequently switching to either side. That sounds like a good thing, but isn't this also where class AB amps have the most distortion?

So, I thought perhaps I should go Class-A on the tweeters. I could build an Aleph-J! And I thought that I could just build a low-power version, to avoid a lot of the excess heat generation. 18V rails should be plenty, right?

So I wanted to find out. I connected my oscilloscope to the output of my tweeter amplifier (an Adcom GFA-535), and cranked the system up to VERY EFFING LOUD with some techno music. (Yes, this is a system performance requirement.)

I saw peaks on the scope of 24 volts or so! This is similar to the peaks I saw going to the mid-range driver. I was surprised.

So, it would seem that a Class-A amp with 25V rails is actually the minimum for this situation. But now, I'm starting to think that this Adcom is actually quite well suited. It has no trouble at all with the voltage swing, and I am surprised that it does actually get to stretch it's legs. However, at lower listening levels, the scope confirms that there is a lot of hanging around the push-pull zone. I know it's Class-A there, but I know there are also issues associated with that topology. Though I don't remember why, there are those who poo-pooh class-A push-pulls.

So, which is the greater sin? An occasional clipping at high volume, or lots of push-pull class-A?

Last edited by Phloodpants; 23rd April 2014 at 01:19 AM.
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Old 23rd April 2014, 01:27 AM   #2
sreten is offline sreten  United Kingdom
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Hi,

FWIW a 4 way system with 4th order L/R crossovers is a PA
system at best and will be wrong for a proper hifi system.

Class A is best applied to the broad midrange, not the treble.

rgds, sreten.
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Old 23rd April 2014, 03:25 AM   #3
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Thanks, that's helpful. I guess I'll just rent my system out to bands.
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Old 23rd April 2014, 04:21 AM   #4
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I recall some discussion of power distribution in audio signals in threads here or on another DIY speaker forum. There have been studies (I can think of some data presented in Active Crossovers by Douglas Self) that show that the RMS power falls off above about 1kHz IIRC around 6dB/oct. But the peak power doesn't fall off. This means that your tweeter amplifier will on average be producing less heat as the RMS output will be lower, however, it still needs to have almost the same peak power (same rail voltage) as the amp for the midrange band. In this case, I think that your class-A idea is not a good approach. I hope that is more like the type of info you were interested in...
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Old 23rd April 2014, 04:51 AM   #5
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Thanks Charlie. That is interesting, and jibes with my casual experiment.

It really underscores the power advantage of electronic crossovers! That 24v peak would hit the rails if it were riding along with the bass.
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Old 23rd April 2014, 09:16 AM   #6
AndrewT is offline AndrewT  Scotland
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Default tweeter transient peak power.

I am not surprised.

I have promoted the design requirement that the peak transient SPL from the tweeter should be just as high as the mid range and bass range speakers.

Sreten, among many others, have rubbished this design philosophy repeatedly.

Average powers have little if anything to do with transient voltages and transient currents.

There is ample evidence that an optimally biased ClassAB amplifier is as clean as required at low levels, i.e. while still operating in the ClassA current region.
The push-pull transition into ClassAB is used when transients come along that require the higher current that ClassA (whether single ended or limited push-pull) cannot provide.

I built a tiny 5pair bc550/560 ClassAB headphone amplifier about 15 years ago. It sort of worked, but later I was to learn that compensation was where it fell down.
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Last edited by AndrewT; 23rd April 2014 at 09:22 AM.
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Old 23rd April 2014, 10:11 AM   #7
SY is offline SY  United States
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Quote:
Originally Posted by sreten View Post
Hi,

FWIW a 4 way system with 4th order L/R crossovers is a PA
system at best and will be wrong for a proper hifi system.

I've got some NHT M3.3s here that disagree with you.
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Old 23rd April 2014, 10:29 AM   #8
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Mmmhhh, I see from your site that those are 2nd order filters exept from the tweeter which is 3rd order.
Where the 4th order comes from ?
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Old 23rd April 2014, 10:46 AM   #9
SY is offline SY  United States
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4th order acoustic. Bass driver is 4th order electrical as well.
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