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Class D Switching Power Amplifiers and Power D/A conversion

how are 'cheap' amps built differently?
how are 'cheap' amps built differently?
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Old 16th January 2019, 02:04 AM   #1
NCC74656 is offline NCC74656  United States
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Default how are 'cheap' amps built differently?

i sell car audio stuff, every manufacture has their reason for being the best. what is it, on the electronics level that is different in the cheap amps? im talking about amps that can actually do what they claim but maybe at a .5 or 1ohm load vs some expensive amps that do the same powers but maybe at 2ohm and are not 1 ohm stable?

i guess im trying to understand why anyone would pay for a 2000W 2ohm stable amp when you could run at 1ohm and save quite a bit of money on amps. (just as an example)
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Old 16th January 2019, 03:04 AM   #2
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by NCC74656 View Post
i sell car audio stuff, every manufacture has their reason for being the best. what is it, on the electronics level that is different in the cheap amps? im talking about amps that can actually do what they claim but maybe at a .5 or 1ohm load vs some expensive amps that do the same powers but maybe at 2ohm and are not 1 ohm stable?

i guess im trying to understand why anyone would pay for a 2000W 2ohm stable amp when you could run at 1ohm and save quite a bit of money on amps. (just as an example)

I will be a bit provocative and state that cheap car amplifiers are mainly made as cheap copies of more expensive amplifiers such that they at best perform similar to the more expensive amplifiers. The difference in stated performance is with the marketing where power values are often heavily exaggerated, and for the cheap amplifiers much more than for the expensive amplifiers.
You mention yourself 2000W in 2 Ohm. That requires a 90V supply rail and handling of 45A peak for each channel for what we call the "clipping point". Where does the 90V rail come from with nominally 12V (actual 13V) in ordinary cars and nominally 24V in trucks? Do the amplifiers include a 2000W+ voltage booster?
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Old 16th January 2019, 01:51 PM   #3
eddperks is offline eddperks  United Kingdom
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Fauxfrench, all car audio amplifiers of significant power have a switched mode boost converter for the power rails.

Component choice allows for a huge range of prices on similar products. A single cap can vary from pennies to many pounds depending on the quality. Not to mention cheaper Chinese silicon being readily available these days.

And take all car amp specs with a pinch of salt... a 2kW continuous power output, assuming the amp is running at 100% efficiency pulls 166A. Managing that kind of real power is never cheap!
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Old 16th January 2019, 02:42 PM   #4
NCC74656 is offline NCC74656  United States
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well all amps have transformer coils, the 12v @ xxxA gets drawn up to 60/80/120/... Volts at lower amps and then goes to two rectifiers to give continuous +/- rails. not all car amps work this way but most do and certainly the cheap ones in my experience do.

what im getting at is you have a hifonics (for example) that mgiht do 1500W @ 1ohm. now its tested to do this and it really, really does do rated power. maybe the amp costs 200.00

then you have a rockford that is rated at 1500W but at 2 ohm and the amp can not run at 1ohm. this amp might be 850.00.

if we assume you will buy a sub to match the rated ohm load of the amp - why spend the extra 600.00? what do you get out of running at higher ohm load?

along those same lines, why do so many cheap amps require a 1ohm load to put out max power? the industry is moving away from 2//4 ohm as standard and going to 1//2 ohm it seems. ive seen many 2 ohm door speakers and more than 3/4 of amps that customers bring in are all rated at 1ohm (some even at .5 ohm).

i know running a 2ohm rated amp at 1ohm will decrease efficiency but if a mfg is building a 1ohm amp surly they have similar efficiency to the 2ohm variants?
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Old 17th January 2019, 10:20 PM   #5
FauxFrench is offline FauxFrench  France
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Sorry for my ignorance. I have only had very trivial (Clarion, Pioneer, Alpine, JVC, Sony) car-amplifiers typically rated at 4x45W and without a voltage booster. Without being explicit, these amplifiers assumed a speaker impedance around 2 Ohm (for 4x45W) though it was obvious that 4 Ohm would be the most frequent speaker impedance. Many customers only read "4x45W" and do not think further. Also thermally, I doubt if I could pull 4x45W for some minutes if I had tried.
I believe that for 850$ it is possible to construct a (permanent) 1500W amplifier (with a voltage booster) though I personally cannot see the need. But for 200$, I am surprised. It is an old trick to state the "power" in a very low load impedance. With less impedance, the peak power depends on the current-surge that can be pulled from the decoupling capacitors. The problem is to maintain such an output power for more than a short moment. The currents handled for >1000W in 1 Ohm are huge and hardly indicating a good reliability.
In the "class D" threads we occasionally discuss PA amplifiers with >1000W output power. Expensive items may be rather reliable but cheap items frequently suffer from reliability issues from high temperature and huge current handling. For 200$, I am surprised if a manufacturer can provide a power booster + amplifier that can deliver >1000 Weff for just 1 minute.

The car-audio threads may be visited by members with much better understanding of how these ultra-power amplifiers are constructed and what can be expected from them.
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Old 18th January 2019, 12:48 AM   #6
NCC74656 is offline NCC74656  United States
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its hard to talk about these things in car audio forums i feel. all too often it seems that people who dont understand the electrical side of the circuit are just talking about how amp A will do power all day long and no need to pay for amp B.

seeing as im trying to learn about how the circuit topologies differ i figured a place like these forums would be best.

so its worth noteing that when pushing amps in teh 2 or 3KW range (or more) we are almost always talking about multiple large batteries and high output alternators. while its not always the case lets set aside the power demand and just assume the car can give how much ever current is needed to power the amps.

so one thing i have not done is taken an amp and run it for any real durration. the way we often check amps at work is to put a hall effect clamp on teh power lines, play a sine wave at 30-40hz and measure the voltage output. this is over maybe a 10 second (at most) durration. then we can do the math and see power. most recent was an orion that we tested this way @ 6750W output. the amp cost about 600.00. it does this power at 1ohm.

compare this to a rockford fosgate amp i used to own - it was rated at 2500W 1/2 ohm and about 2000W 4ohm. its birth sheet was 3800w and i got about 4400w out of it at 2ohm. again i never measured it with a straight sine wave over a minute or more, i think i may try that and see how hot they get.

the rockford is a 1700.00 amp and is built to not dish out power at 1ohm but rather hold things pretty steady. where as the cheaper amp shoves power out the lower ohm you go. now its worth noteing that the orion is a class D and the rockford is a class BD
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Old 18th January 2019, 09:04 PM   #7
FauxFrench is offline FauxFrench  France
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I understand, you like to discuss how it is possible to get KW outputs with rather cheap car amplifiers. I got curious from your posting (and my own ignorance) and watched a Hiphonics (Brutus) test on YouTube. It was interesting and I may have picked up something useful.
I may use more consecutive postings as I sometimes loose what I just wrote from "cannot find server".

We (the HIFI-nerds) define an amplifier as capable of delivering an almost constant output signal in the full audio band (20Hz-20KHz). Am I right in that the Hiphonics Brutus is used to produce low bass signals up to a couple of hundred Hertz only (a sort "bass-kicker")?
For HIFI, low THD is very important and accordingly we use a separate power supply to deliver a stable rail voltage followed by a low THD amplifier operating from this rail voltage. Is my impression that your "bass-kicker" (if correct) is less focused on THD in the higher frequencies because your sub-woofer by nature will reproduce very little of higher frequency, correct?
If you only need to produce high power signals at low frequencies, the more complex design used in HIFI amplifiers may be simplified a lot such that you may use only one conversion and not two and with a much lower carrier frequency (a sort variably controlled SMPS).

If you told me to identify problems in a KW PA-amplifier, I would say heating and current handling. If you only need to produce signals up to a couple of hundred Hertz, the switching losses (heating) can be reduced importantly. The current-stress remains.

Last edited by FauxFrench; 18th January 2019 at 09:29 PM.
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Old 18th January 2019, 10:48 PM   #8
FauxFrench is offline FauxFrench  France
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Tried to look at specifications for a 1500W Rockford (T1500) and the 1100W Hifonics (BRX1116).
For the Rockford the specifications are: frequency response 20HZ-250Hz with a THD below 1% (I believe it).
For the Hifonics the specifications are: frequency response 10Hz-50KHz/10Hz-250Hz, THD less than 0.1%. The 10Hz-50KHz sounds unlikely for an amplifier that is shown to be used with a subwoofer - the 10Hz-250Hz must be valid. The THD is measured how? (I do not believe it is measured the same way as for the Rockford.)

The price for the Hifonics is 350$. What is really strange about the Hifonics specs is that they use a 60A fuse. Rockford use a 200A fuse. A 60A fuse will only allow some 750W, amplifier losses not taken into account.

Last edited by FauxFrench; 18th January 2019 at 11:00 PM.
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Old 18th January 2019, 11:59 PM   #9
tsmith1315 is offline tsmith1315  United States
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Quote:
Originally Posted by NCC74656 View Post
its hard to talk about these things in car audio forums i feel. all too often it seems that people who dont understand the electrical side of the circuit are just talking about how amp A will do power all day long and no need to pay for amp B.

seeing as im trying to learn about how the circuit topologies differ i figured a place like these forums would be best.
We have some very knowledgeable members in our car audio forum, including seasoned techs and engineers. Some with decades of experience, don't sell them short.
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Old 19th January 2019, 04:04 PM   #10
NCC74656 is offline NCC74656  United States
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hey, i think fauxfrench has learned how decieving the car audio world can be... lol.

yea, the differences in measuring specs makes it near impossible to drill down to how the amps will actually preform. this is one of the reasons why i want to learn how the circuitry works as opposed to "what specs to look for".

ive seen some car audio amps that rate at "maximum thd" instead of minimum, easy for a a consumer to miss that.

i guess ive never thought of an amp to do 20hz-20khz. in the car audio world we often use 1 amp for sub, 1 amp for mid and 1 amp for high so everythign is crossed over. its getting difficult to even find a/b amp anymore as mostly all of it is class D.



i have not been to this forums car audio side, wasnt aware there was one. ive been to other places such as SMD and the like
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