Whas better dual 30w speakers or 1 120w speaker

[bountyhunter]

As for paralleling power amps, I reviewed the reference to see if my memory was correct. It is:

This configuration (ideally or theoretically) requires each amplifier to be exactly identical to the other(s), or they will appear as loads to each other. Practically, each amplifier must satisfy the following:

* Each amplifier must have as little output DC offset as possible (ideally zero offset) at no signal, otherwise the amplifier with the higher offset will try to drive current into the one with lesser offset thereby increasing dissipation. Equal offsets are also not acceptable since this will cause unwanted current (and dissipation) in the load. These are taken care of by adding an offset nulling circuit to each amplifier.

* The gains of the amplifiers must be as closely matched as possible so that the outputs don't try to drive each other when signal is present.

In addition, small resistors (much less than the load impedance, not shown in the schematic) are added in series with each amplifier's output to enable proper current sharing between the amplifiers. These resistances are necessary, without them the amplifiers will in practice fight each other and overheat.

This is what I said above as to why paralleling is not practical. BTW, adding the output series resistance (which is required to reduce the amount of current the outputs force into each other) degrades performance significantly. The output impedance has to be as close to zero as possible to give good damping factor. Also, adding series impedance degrade TIM (transient intermodulation distortion) because it affects transient response to the load. I don't think any high performance audio system would use this.

 

[audioguru]

The damping factor is destroyed in amplifiers that are paralleled and have series load balancing resistors.
An undamped speaker sounds sloppy and boomy. A well damped speaker sounds tight and does what it is supposed to do.

 

[solis365]

This is wrong, actually backwards:

...

If you parallel the outputs of two amplifiers, each one supplies half the current and sees TWICE the effective load impedance. same voltage swing, half the current, effectively sees twice the impedance. The same voltage swing across the speakers, same power delivered to the speakers, but the current is divided between the two outputs. No power gain at all.

Haha yeah my mistake. If you draw the "half circuit" for each side and connect them in parallel to make the final circuit the parallel combination of the "half-circuit load resistors" will have to equal the real load. So each amp sees 16 ohms.

thus 10Vpp / 16Ω = 0.625A --> 0.625A * 2 * 10Vpp = 12.5W
So no increase in power as you said. Which now that I take a second to think about it makes perfect sense, as each amp will just supply half the current that one amp would have to supply. 1/2 * 2 = 1 ... ta da!

The damping factor is destroyed in amplifiers that are paralleled and have series load balancing resistors.
An undamped speaker sounds sloppy and boomy. A well damped speaker sounds tight and does what it is supposed to do.

Interesting. Do you have any links that talk more in depth about the damping factor as it relates to amplifier output? (Particularly audio)
I would like to know more about the technical side of audio electronics but its not something you can look up in a standard electronics textbook...
Actually the only textbooks I have that has a discussion on damping factors are on my desk at work. I imagine their discussion of it is highly theoretical and generalized towards all amplifiers.

 

[audioguru]

The damping factor of most half-decent amplifiers is 200 to 500. Then the output impedance is 0.016 ohms to 0.04 ohms.

 

[Mr RB]

Every heavy-metal guitarist knows... 2 speakers is louder than 1 and 4 speakers are louder than 2, given the exact same total speaker impedance and same amp power. That is why they end up with 2 Marshal stacks and a total of 16 speakers.

I can only guess at the physics but i think it's got something to do with the same power into larger total cone area means less sound pressure per square inch of cone area and less energy lost in heat compressing the air right at the cone with very high pressure which happens with huge wattages and high freq rock guitars.

Maybe it's the larger total cone area more efficienctly gets the room air moving? I guess that's the same thing.

Maybe someone knows more?

 

[Nigel Goodwin]

That's news to me, but I've only been doing it for 40 years.

Perhaps you should move 40 years into the future


However, I've been doing it that long as well - and as I'm sure you're aware, even bridging amplifiers wasn't done back then.

If amps are designed to run in parallel, they would have to contain some internal circuitry specifically to make one amp the "master" and the other the "servant" so that they don't try to force slightly different voltages at the load. They don't have exactly the same gain, so they would be trying to establish differet voltages as set by their own feedback.

If you parallel the feedback networks as well they would quite simply have exactly the same gains, and follow perfectly.

However, that isn't how it's done - as I said previously, check gain-clone examples, which are most common (and based on manufacturers application notes) - heres one such:

PA100 DIY 2x LM3886 in parallel gainclone audio amplifier

I've also recently been looking at a Naim 136 monoblock amplifier, it was heavily modified professionally a number of years ago (at a cost of £1500!!!!!), and used two identical power amplifiers paralleled, with no other connections except power, ground, in and out.

If anyone is interested, the fault was a noisey (and slow) cooling fan.

 

[Nigel Goodwin]

This is what I said above as to why paralleling is not practical.

Not only practical, commonplace - you're working on an incorrect assumption.

Here's a nice example, paralleled AND bridged:

https://electronics-diy.com/electronic_schematic.php?id=674

In response to AG's next comment about damping factors, these gain-clone designs are VERY, VERY highly rated in HiFi circles.

Notice this uses the exact same chip as the OP is asking about, one of the 'gain-clone' chips.

 

[audioguru]

The amount of required speaker damping (from the extremely low outpout impedance of an amplifier) depends on the speaker's spec's. Old speakers designed to be driven from high output impedance vacuum tube amplifiers sounded pretty good without much damping.

Modern speakers are designed to be driven from the extremely low output impedance of a modern speaker and even the resistance of a long thin speaker cable affects their response near resonance. The different response can be measured and heard.

One of the paralleled amplifiers has an output impedance of about 50 milli-ohms and the other has about 240 milli-ohms. I think the difference will be audible with some speakers.

 

[Nigel Goodwin]

One of the paralleled amplifiers has an output impedance of about 50 milli-ohms and the other has about 240 milli-ohms. I think the difference will be audible with some speakers.

Like I said, these are VERY highly thought of amplifers, people are replacing extremely expenive commercial amps with them.

I suspect the effects of damping are very over exaggerated - and my Trio/Kenwood amp has a stupendously high damping factor - but I can't say it sounds any better than ones with lower factors?.

 

[bountyhunter]

Perhaps you should move 40 years into the future

Whatever, dude. I posted the reference explaining why paralleling amps is a bad idea that yields poorer performance, but if you think that it's progress then I assume further information would be a complete waste of time. Of course you will claim damping factor is unimportant to support the claim these amps are "high end".

If you parallel the feedback networks as well they would quite simply have exactly the same gains, and follow perfectly..

Exactly the same gain? If you actually believe that, it shows you have no understanding of offset voltage or offset current among other things which cause gain errors in amplifiers. maybe you should brush up on them.

However, that isn't how it's done

Not interested in trying to brute force power amps in parallel. And, the reasons have already been posted in this thread.

 

[bountyhunter]

In response to AG's next comment about damping factors, these gain-clone designs are VERY, VERY highly rated in HiFi circles.

That doesn't surprise me. I still remember Julian "Tin Ear" Hirsch and others. I am rarely impressed with what audiophiles claim since A-B blind tests routinely show they can't hear the differences they claim.

Anyway, if you want to strap power amps together, go ahead. I disagree with the claim it is "common" since about 99.99% of the power amps built today use standard output stages.

 

[bountyhunter]

LM4780 Gainclone Amplifier

.

Truly ironic..... I worked at NSC for 18 years and knew the guys in the audio group. I even got the parts from them to build the amp that drives my small TV center channel speakers.

Did you really look at the circuit? It's what we used to call "an expensive solution in search of an application". In other words, the standing orders at NS were to come up with apps that use as many expensive NSC parts as possible even if the design is a dumb idea which can be done more easily in other ways. This is vintage NS apps.

Exactly why would any high quality audio maker use this Rube Goldberg mess? In case you are not aware, most modern high performance power amps use switching converters to generate internal power rails as high as they need, which means they are not "rail limited" so they have no need to bridge to get the power they want. A bridge amp is only advantageous if your supply rails limit voltage swing and you are forced to use a bridge to get enough power. And they surely don't need to parallel amps, they just throw more silicon at the power stage and get the current capability they need to drive whatever the design impedance is.

This isn't the dumbest looking application I have ever seen from NS, but it is kind of funny.... and it surely is not what anybody would call an optimal high-end design topology.

 

[solis365]

Did you really look at the circuit? It's what we used to call "an expensive solution in search of an application".

Well perhaps the *best* solution is to design a better amplifier using those other methods you mentioned in your post however...

this project is a DIY thing, and my guess is that its cheaper to buy twice as many $5 chips for the average DIYer than it is for them to design a complete amplifier by themselves and then have no one to fab it.

does national (or anyone else) sell an IC that can output that much power (that setup looks like at least 240W/channel) for less than $20?

And by all reports, these chips sound *really* good. I know what you are saying about audiophiles and their blind tests, so take what they say with a grain of salt, but you and I both know that there is a difference between a "bad" amp, a "good" amp, and a "great" amp. Maybe the difference between a "great" class A and a "great" class A-B cant be heard in a blind test, but the difference between either of these and a "good" or "bad" amp of any sort can probably be heard by most people. And the community seems to think that these chips sound at least good, if not great.

 

[Nigel Goodwin]

In case you are not aware, most modern high performance power amps use switching converters to generate internal power rails as high as they need, which means they are not "rail limited" so they have no need to bridge to get the power they want.

Hardly 'most', an incredibly tiny number of chips do this, almost exclusively ones for in-car use - it's blindingly obvious which ones do, based on the output power and the supply voltages.

Many chips are also made as bridged amplifiers, and can't be used as single ended ones - again, normally low voltage designs, in order to produce decent output from a low voltage supply.

Personally, I wouldn't start off planning to bridge an amplifer - it's the sort of thing you do as a 'last resort' when you don't have the correct impedance speaker. With two speakers, and two amplifiers, you get the same power, and it doesn't go quiet if one amp dies