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Modelling amps-genuine or not?

Started by Farthing, September 21, 2011, 09:58:49 AM

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darwindeathcat

@JMFahey: What class of chip amp were you trying to power? The reason I ask is that I've been lead to believe that class D (or T) chips are efficient enough to run off of low amp-hour batteries, but I have no direct experience with them. I don't need super high wattage, just enough to get heard on the street. I used to use NiMh's in my little pignose (which I've since sold on, since it was a one trick pony), and it was plenty loud at around 5W. It was one of the older discreet circuit ones too, but I still got several hours out it on a single set of NiMh's.

@gbono: Yeah, batteries ain't cheap! I looked up these new Lithium Ion batts he's using, and it looks like you can get a 12v pack for in the $30 range, which is similar to 12v SLA batts, but WAY less heavy (which is primo for a portable amp). The thing that always takes the price over the edge is the charger. You have to add on another $20 to $50 dollars for the damn charger! This is why I'm so keen to use NiMh. I've got tons of those already and a great charger. I use them in my cameras and other portable electronics.
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J M Fahey

1) I've powered from a single LM383/TDA2002/3 (6W into 4`ohms, 10W into 2 ohms) to TDA2005 (16W into 4 ohms, what peavy Solo and Fender AmpCan use) to my Fet powered output transformer 40 and 60W amps to 60W conventional amps (discrete or chipamp) powered by SMPS type converters.
I've been making battery powered amps for a *long* time, as you see, and I've changed as technology does.
Haven't used class D amps yet.
2) please do not get confused by deliberately misleading sales talk.
Let's straighten this once and for all.
Class D amps (class T is just a marketing name) are "more efficient" as in they heat up less, but they don't magically create power they don't previously suck from a battery.
Let's put it in numbers:
just to simplify discussion, let's talk about "ideal" amplifiers:

Class AB (the regular kind)=70% efficient
Class D=100% efficient.

If we are talking about heat, a 30W RMS AB amp will lose 30Wx0.3= 9W in heat, a sizable amount which needs to be dissipated away or will quickly burn the chip.
So it "eats" 30+9=39W from the battery.
If we have a 12V one, it means 39W/12V=3.25A.
A 7AH battery will drain in 7/3.25=2.15 Hours

Now the class D one: it will eat plain 30W`from the battery, with no self heating at all (meaning it can have a small, non heatsinked chip)
Current conbsumptrion= 30W/12V=2.5A
Battery life= 7/2.5=2.8 hours.
Extra time=2.8/2.15=1.3 meaning 30% more.
Interesting but not mindblowing at all.

The Tripath guys let you think (although they are clever enough not to say so expressly) that you will get gobs of extra battery life from their product, which is simply not so.

Well, calling their amps class T is already a marketing gimmick too.

A of NiMH AA batteries, you will need 30 x 2700 MaH ones to match a single 12V7AH lead battery.
Don't know where you live , but in Buenos Aires, 3 or 4 AA batteries cost the same as a single Lead acid one.
Do the math.
As a side note, NiMH technology is new, so you need "good brand" batteries to fulfill their promise, I'm talking Sony, Panasonic, Sanyo ones, which means expensive.
Generic is much worse.
On the contrary, lead acid is 100 Years old technology, and the cheapest generic unbranded battery performs very well, there's no mystery for it.
Being a "feet on the ground" Industrial Engineer by training (not an Electronic Engineer as many assume), cost is a "technical parameter" as important for me as any other one, if not even more so.

As a side note, it's much easier to design a dumb Lead Acid charger than an intelligent NiMH one.
Let alone a Lithium cell charger; they are very complex because of the *certain* possibility of explosion and fire.
Remember Lithium cells are *forbidden* aboard planes, unless safely sitting inside your notebook, cellphone or whatever.
Just check fresh Airline Safety rules.

darwindeathcat

Thanks for the detailed response! That all makes perfect common sense. Thanks for clearing up the efficiency of class D versus others in terms of battery life. 30% extra battery life actually seems pretty good to me, though! That could be a few extra bills in the tip jar, ya know?!? :)

RE: NiMh's... The price ratio for NiMh batteries is quite a lot lower here in the states than for you down in Brazil. You can find name brand (energizer, duracell, sony, etc.) NiMh AA's for around $5 for a 4-pack if you shop at the right places, and decent chargers for around the $10 mark. So it's not SO outrageous. The cheapest decent 12 SLA batteries I can find locally have 5 AH ratings and cost about $15, plus still another $15 for the chargers... 7.5 AH rated SLA's go for about $25. So you are correct that SLA is still cheaper, but the margins are slimmer here in the states. Okay, so for fun (!), let's do some math to see what we could do with each battery for an equivalent perfmance:

So, if i can get good NiMh's for around $5 / 4, that's about $1.25 a cell. NiMh AA's run at 1.3v, so to get "12" volts you need 9 in series (9 x 1.3v = 11.7v). The ones I have (energizers) are rated for 2500MaH, so 9 batts should keep a 30watt tripath chip powered on full for at least an hour (which probably means closer to 1.5 hrs with real usage). This is 9 * 1.25 = $11.25, so we are getting real close to our SLA budget, and yes, this isn't really long enough to do much with. So, if we add a second array of 9 series NiMh AA's would up the MaH rating to 5, allowing for 2 hours straight playing at full volume (and perhaps even 3 hours of "normal" usage). While perhaps still a bit on the short side, I think we're finally getting into a decent range of use-time. SO, those 18 cells cost us 18 * 1.25 = $22.50. Let's add in the chargers: $10 for the NiMh, or $15 for the SLA charger. Okay, let's do the totals! $15 + $15 = $30 for the SLA, and $22.5 + $10 = $32.5 for the NiMh. This is interesting! For a couple of dollars more, you should get simialr performance at a fraction of the weight. What about that $25 7.5 AH SLA? Totals are: $25 + $15 = $40 for the SLA system. $33.75 + $10 = $43.75. Still about the same price margin.

Okay, let's look at other factors: It's definitely a pain that you can only charge 4 NiMh's at a time. For our 18 batts, that would mean you have to charge in 5 rounds!!! That sucks. For another $10, you can get a second charger and now do 8 at time. That's still at least 3 rounds... BUT, my NiMh charger takes only about 2-3 hours to charge up 4 cells. I think SLA chargers need like 6 or 7 hours usually, so although you have to be more proactive with the NiMh, if you have two chargers (or one that can take 8 batts), it's about the same charge time total. So now our price margin is not as slim. We've got about $15 more into the NiMh system at this point.

So, what to choose? My main reason for wanting to go with NiMh (or other standard small cell rechargeables) is for the weight reduction. SLA batteries are HEAVY man! And for a portable rig, I think it's probably worth the extra $15 or so to keep the weight down... Also, from what I understand, an advantage of NiMh is that they don't experience much voltage drop with use (ie, they stay pretty much at 1.3v throughout discharge), unlike alkalines that do drop voltage with use. The result is that performance doesn't decrease until the battery is basically drained (ie., no "low battery" syndrome). How does that compare with SLA? Do they drop voltage as they get discharged? If true, this could be another reason to use NiMh...

Anyway, it seems that either way, you are going to have to drop about $30-$40 just on the battery and charger system to build a battery powered amp that can hang for a few hours playing time at 30 watts. Now, if you decide to drop some power, and go down to, say, 15 watts (still pretty good for playing on the street), you can get double your playing time. In that case, you could potentially get at least 2 hours (potentially 3) out of your 9 2500 MaH NiMh's, and you'd only need to spend about $25. in any case, I've already got enough NiMh's to do the 9 battery configuration, and it's not TOO much of a pain to charge them using my one 4-batt charger. If I buy this amp module http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-385, I'll wire it up in dual mono as a PA, which means that if I only use one channel, I'm hoping I can get close to that 2-hours plus of battery life. The module costs only $15, and I've got an enclosure already built, which means that the most expensive piece of equipment will be the speaker. Just because I'm super cheap, and to keep this a super budget rig, I'm considering to use this low cost 10" driver (also from parts express so I can combine shipping with the amp module): http://www.parts-express.com/pe/showdetl.cfm?Partnumber=290-262. So, in terms of actual cost: I will be looking at about a $35 to $40 pay out after shipping costs. If I needed to buy everything, I could expect to spend probably about $70-$80, depending upon the enclosure. Comparable wattage commercial battery amps (like the taxi) are in the $160 range, so still a pretty good value! I have no idea about what it will sound like though!  ;D
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joecool85

Very interesting math.  Also of note, until the last year or so NiMH batteries were much more expensive than SLA.  Due to NiMH prices going down and SLAs actually going up (not a whole lot of people use them anymore), the margin is indeed quite slim.  I'm still a fan of SLAs for a lot of things though.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

J M Fahey

The math is fine.
Only very slight correction is that you need 10 NiMH in series to approach an SLA battery, since they are actually 12.6V .
Most "car" type chips need that, and many rate themselves at 14.4V (just re-read`datasheets) because that's the car system voltage when the alternator is running and charging the batteries.
Othersa split the difference and spec at 13 something volts (13.5?)

darwindeathcat

Oh, cool! Thanks, I didn't realize that... Okay, so we will need 10 NiMh's for 13v to run our hypothetical battery amp. Good to know!

Actually, and somewhat ironically, just after I posted my last post, I went over to craigslist and noticed someone selling a Pyp PB-1 (http://pyp-bomb.com/home.html) for $40 (they sell new for $150), so I snatched it up! The ironic part is that it's powered by a SLA battery!  :P

Of course, he was selling it so cheap because he had lost the wall wart adapter, and couldn't charge it. Now I need to find the specs for the missing wall wart so I can charge the sucker up (I'll start a new thread)! But the little bit of playing time that was left in it, was enough to convince me that this amp will really do the trick for me...
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joecool85

Quote from: darwindeathcat on October 05, 2011, 02:08:54 PM
Oh, cool! Thanks, I didn't realize that... Okay, so we will need 10 NiMh's for 13v to run our hypothetical battery amp. Good to know!

Actually, and somewhat ironically, just after I posted my last post, I went over to craigslist and noticed someone selling a Pyp PB-1 (http://pyp-bomb.com/home.html) for $40 (they sell new for $150), so I snatched it up! The ironic part is that it's powered by a SLA battery!  :P

Of course, he was selling it so cheap because he had lost the wall wart adapter, and couldn't charge it. Now I need to find the specs for the missing wall wart so I can charge the sucker up (I'll start a new thread)! But the little bit of playing time that was left in it, was enough to convince me that this amp will really do the trick for me...

I didn't see any specs on their site, in the manual or in the pics of it on their site.  I'd contact them and see if they can tell you.  Because it uses a "smart" charging circuit in the amp itself, you will want to make sure you get the right adapter.  Or you could bypass that, hook the 2.5mm jack straight the battery and get an adapter that will "dumb" charge it (just remember to unplug it).  If you do this you will want a 12v adapter (unregulated, should put out 15v DC with no load) that is rated for 1/10 the battery.  IE - a 2AH battery would want a 200mah charger.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

darwindeathcat

Thanks Joe Cool! I posted a schem I drew of the charging circuit in the other thread I started (http://www.ssguitar.com/index.php?topic=2318.0) plus a gutshot, so you can take a look at it over there. Yeah, I've sent a couple of e-mails to them with no response. I do have a "dumb" charger, so I think I might have to go that way in the mean time, because the battery is getting dangerously low... JMFahey is giving me some help over in that other thread, so i'll wait to hear back what he says after looking at the schem I drew before taking that measure... Thanks for all the help!

~DDC
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gbono

My battery knowledge is weak but I do remember something about deep cycling lead acid batteries and lifetime/reliability. You can not deep cycle a car battery without significantly reducing its life. What about NiMh - I believe they are designed for deep cycle application - is the lead acid gel cel also a deep cycle technology?? You have to figure in the # of charges you will get from any battery technology - not cheap if it dies fast.......

joecool85

#24
SLA (sealed lead acid) doesn't like deep discharge.  It's safe to get down to about 1/2 charge on a regular though, 1/4 charge if you don't do it all the time.  NiMH is better at deep cycle but still not great.

**edit**
I guess NiMH is actually pretty good at deep cycle, I was thinking lithium-ion that doesn't love it.  Also, good ol' NiCads love a full discharge and they are dirty cheap.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

J M Fahey

Yes, NiMH and NiCads *need* deep discharge and then full charge to "clean"them and avoid what we know as "memory effect" .
Smart chargers do just that.
Lead Acid batteries can be discharged down to 11V, but need to be re-charged, at least somewhat, at once, because when discharged the electrolyte turns into concentrated sulphuric acid which "eats" everything.
When charged said acid is combined with lead, and electrolyte becomes relatively mild.

Puguglybonehead

This looks like a cool idea. I'm getting interested in the idea of battery-powered amps myself. I'll be watching this thread to see what you finally end up with as a setup. If you're not totally set on having a 10" or smaller speaker, this 12" might be a better choice. 100 dB SPL, and the price is right.

http://www.ebay.ca/itm/160665836594?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

J M Fahey

Slickly written ad.
The speaker is the cheap(est) Eminence.
The "censored " label hides the very tiny magnet.
100dB? No way !!!!!!
The real price is U$26, postage included, around $20 to 22 for the speaker itself.
That said, it may work, although I'd much prefer it in 4 ohms, what´s needed by the typical 12V 16/20W car chip amp.

Oh well, marketing !!!!

darwindeathcat

Well, since I was down at the local Fry's Electronics shopping for an unrelated item, I decided to price out batteries and charging systems so I could get an idea of what I'd pay retail here in Phoenix Arizona. First off, I looked at the SLA 12v batteries. The cheapest was $22 and was only 1.2AH. I'd say it weighed about 5 lbs. The other 12v SLA they had was 4AH, and went for $36, and probably weighed more than 10 lbs. Both batteries were "Tenergy" brand, which I've heard of. SLA "smart' chargers went for about $15, and an adjustable voltage universal wall wart that could be used as a "dumb" charger (12v, 500ma) went for $8. Best deal on NiMh AA's was again with the Tenergy brand-name, and was $11.99 for 4 AA's AND the charger. Remembering that we'd need TEN AA's to equal ONE 12v, that means we could buy two of those deals, and get a separate pack of 4 AA's for $7, or we could just splurge and get THREE of those deals so we could charge all in one go. These AA's were 2500ma. For a weight comparison I held a ten pack of regular alkalines. It was about half the weight of the small SLA, so I'd say about 2.5 lbs. If we go the AA route, we need some battery holders. The best arrangement sold in the store was to get one holder for 8 AA's ($2.49) and one holder for 2 AA's ($0.79), and wire them in series.

So, in a local retail setting the cheapest SLA set up would be about $30 for only 1.2 amp hours, and a dumb charging system, and the best SLA system would run you about $50 and you get 4 amp hours. You'd be adding 5-10 lbs of weight onto your amp. For a NiMh system, you'd spend between $40 and $45, and you get 2.5 amp hours.

Now, remember, this is buying retail. Better prices online could be found, but unless you can get free shipping (e.g., for an Amazon order above $25) or you can combine everything into one shipment for a reasonable rate, the price of shipping may be greater than the potential savings. For example, that 12" speaker listed on ebay sells for $12.99, but they want to charge you $28 in shipping!!!

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J M Fahey

I must correct my mistake.
It isn´t even a cheap Eminence (I thought so because of the characteristic stamped frame) but an Eminence CHINESE COPY.
Oh well.
One buyer posted:
   "Nice speaker made in China by american famous Co.! Thanks!"

Reallly he should have written:
"made in China FOR american famous Co."
which is not the same.