optimizing for efficiency

[Dwiel]

Hey guys, I was wondering if you could tell me, in general what the best combination of components would give me the best efficiency of source power. The plan is to be able to run off of batteries and I want the batteries to last as long as possible. This would be taking place outside and would probably be using it to play background music for a group say around a camp fire and maybe sometimes a little louder than background music. About the volume that you have to speak up to hear other people talking, but can still understand them. (I'm not sure how else to explain the volume I am looking for, if there is a better way, let me know)

Money is also somewhat limited as well as complexity. I would consider myself intermediate to advanced in the ways of electronics but I'd rather not make this a larger task than necessary. Most of my experience is in the field of digital electronics, not audio, so designing my own schematic is pretty much out of the question.

I have read that class D amplifiers are the most efficient, am I correct in that I should find a design using this class?

Also, am I right that horns tend to be high efficiency? If not, I think I will probably want to use speakers I already have, I don't know if I can afford buying speakers for this purpose only...

 

[paulb]

Yes, Class D designs would be the most efficient. And I think you're right about horns as well.

 

[darkfenriz]

efficiency of class D is very helpful above 100W or so.
If you need lower power there is hardly any point in building complicated switching amp.
About horns you are absolutely right.

 

[Dwiel]

Hmm, so I guess I shouldn't worry all that much about the amplifier type so much. I have been looking into horms and have found, as I expected, that they don't have a very low frequency response range. I would like to hear the bass and so I guess I will need to have a bass driver of some kind. From what I have seen, all high efficiency speakers that have the freq response I am looking for to cover what the horms can't (40 - 3,00) tend to be rather expensive. What is the best way for me to cover this range with the best efficiency and lowest cost? Sound Quality isn't terribly important, although I would like to stay away from the worst sounding ones. I was thinking that maybe some kind of car speaker would be good for me? Some have horn tweeters to cover the high range. They would also tend to be smaller, which is good because I am hoping to have a semi-portable setup in the end.

Thanks for the help!

Dwiel

 

[ilimzn]

Hm, if you were building speakers exclusively for this project, as well as an amp for them, then using 5th order ported boxes would be the way. Horns are much more efficient but need to be very large for good bass response.
Also, for efficient amps you may want to look at class G (rail commutation) amps. Sanyo actually makes some hybrid circuits that are fairly easy to use that implement this topology. Still, class D would be the ultimate efficiency-wise. There are several manufacturers (a few you can get to through this site) that offer finished modules - add power supply, input pot and enclosure + connectors, and you are set.

 

[Nelson Pass]

Try a Fostex FE206E. They are only a few bucks, and
have enough efficiency and will sound good for your application.
You'll scare the wildlife with a 2 watt amp, and a small bass reflex
enclosure will make it down to 50.

 

[amplifierguru]

Absolutely,

Use speaker efficiency ( some are 100dB wide range) with bass reflex and Q=2 , thenpower it all from a car amp recharged from a solar panel. Party AND save the planet ( and scare the wildlife!)

 

[AndrewT]

Hi,
I second the idea of using high efficiency speakers. At least 94db/watt. Be carefull to not confuse db/2.8v which can use lots of power if the impedance is low.
Fewer components in the xover helps efficiency and similarly fewer speakers helps in this regard.
A well designed classB with the driver stage optimised for low current consumption (opamp?) and low Iq should also be helpfull.

 

[ilimzn]

...fewer speakers helps [efficiency]

Not at all. Consider that two identical speakers in series produce the same SPL at half the power... which is the key to array efficiency.

 

[AndrewT]

Hi ilimzn,
same driving voltage with double the series impedance gives half absorbed power.
You say the two speakers in series will give 3db more SPL at the same driving voltage. Can you clarify that?
Dwiel,
However, I was talking about a multi passband xover with dedicated speakers to each passband. I think that the extra complexity of the xover for multi passband will waste power in the xover due to extra resistors and inductors. Adding zobels to tailor the drive unit impedance will also reduce overall efficiency.
Avoid these and I believe that the speaker efficiency will improve.

 

[Dwiel]

Thanks

Thanks for all of the great info!

I definately have quite a bit of research to do before I should decide on one design, but this is a great help.

It looks like the FE206E speaker would be like $90 - 100 or so. (shipped) Is that about as cheap as they get, assuming the high SPL goes for a full range speaker?

Someone else mentioned using a 2W amplifier. I am not entirely sure how jokingly this was said... It seems that most amplifiers are at least 50W even in cars... Will a 2W amp really cover it or was he just being sarcastic? Assuming this is the case, I could probably use a nice OP AMP design... In the case that I do use opamps (I like this idea because the amplification is already packaged for me = easier) what would be a good low Iq?

Thanks for all the great help!

 

[B Cullingford]

He was not joking, 2W into a 96dB speaker is 99dB output,
into a typical 86dB effiecient speaker this takes 20W

(typical?? - well I it makes the math easy as there plenty of speakers around this value some less some more)

Then you could go with horns - 100 or 110dB efficient - 10W in and you are talking paaaaaaaain! (google for example Electrovoice MH4020A - about 110 dB 1W/1m - no idea what they sound like, designed for stadiums - you know there is that /1m in the spec and people in stadiums are many meters away)

I had a friend who had some electrovoice drivers connected to a homemade horn - had to pad them down to match some effiecient 15" bass drivers in big reflex(?) boxes - his 300B hardly had to work at all - nice sounding system, there seems to a certain 'ease' to effiecient systems and they can be very dynamic.

Rant mode - why do people think that adding more drivers can make a speaker more efficient? You don't get something for nothing. You can of course 'couple' it to the air more effieciently (horns for example) or 'beam it' (as in acoustic lens used on some horns) where the same sound power is concentrated in a smaller area and is therefore louder, or ... I'm probably missing something. AND talking about efficiencies but not using power as the reference!???!!! as in 92dB/2.82V/1m - yes that is 1W into 8 ohms but it is 2W into 4, and efficiency is about power! Of course it is easier to measure voltage and not worry about power - and ... this is going to get to long. end rant.

Bill

 

[ilimzn]

Hi ilimzn,
same driving voltage with double the series impedance gives half absorbed power.
You say the two speakers in series will give 3db more SPL at the same driving voltage. Can you clarify that?

Like you say above, same driving voltage with double the impedance (for two series connected speakers) gives half absorbed power but the same output SPL.
Similairly, two speakers in parallel absorb twice the power (3dB) but produce 6dB higher SPL.
Sounds strange but has to do with acoustic impedance matching - larger cone area is preferable to larger excursion, and with two membranes you get that preference.

Regarding zobels, you don't really get better efficiency by not using them unless we are talking current drive. If you are looking at a frequency response measured in a DIN baffle, it is done (for woofers) using voltage drive, so any impedance you would put in parallel with the speaker would change nothing. Obviously, this would be the default for a voltage driven full range speaker. The problem is when it's not voltage driven (i.e. large source impedance), a zobel will obviously gobble up power, but then, without it will get a rising frequency response. That being said, there are speakers speciffically designed for this kind of drive, and they indeed are usually full range.

 

[ilimzn]

Rant mode - why do people think that adding more drivers can make a speaker more efficient? You don't get something for nothing... or ... I'm probably missing something.

Multiple drivers emulate larger cone area and this is precisely about coupling. How do you think arrays acieve such high efficiency
Of course, it is not something for nothing - you can only couple it so much so if for practical purposes area approaches 'infinite', the 'gain' from increasing it diminishes.

AND talking about efficiencies but not using power as the reference!???!!! as in 92dB/2.82V/1m - yes that is 1W into 8 ohms but it is 2W into 4, and efficiency is about power! Of course it is easier to measure voltage and not worry about power - and ... this is going to get to long. end rant.
Bill [/B]

Actually, it would be better to express efficiency as driving voltage as the grand majority of speakers is intended for voltage drive (or at least the parameters are declared for this kind of driving mode). Otherwise, you would need to adjust driving voltage vs frequency to get constant power because the impedance changes with frequency. In other words, you need to have the impedance diagram at the very least.
In the vast majority of cases, howeber, when a manufacturer declares dB SPL/1m/1W the 1W is whatever voltage it takes to get 1W at the RATED impedance (This is actually the whole reason behind the rated impedance figure!). This is because most speakers are measured with constant voltage drive anyway. In order to calculate crossovers (as a first approximation well before tweaking by ear) you normally need to express sensitivity as XdB/1m/X Volts or it becomes extremely convoluted to calculate padding, especially if you are using zobels and different driver unit impedances.