Opinions on wonky audio amplifier - Page 1
Forum section: electronics / general

author: Guille
author_rank: Regular Contributor
timestamp: July 08, 2020, 02:06:07 pm
content: Firts off apologies for any bad grammar or incorrect terminology, Im spanishSo I was looking into this cheapo amplifier to run some big subs for a club sound system we got in the making.The price and specs look atractive but after looking at the amps power stage picture they got in their website I became weary of the whole thing.SPECSfrom the website:https://www.sfaudio.es/es/sfaudio-productos-listado/etapas-de-potencia-digital/sfaudio-sfr-14000-detail.html·2 X 3000 W RMS @4 Ohm Output Power·THD+N (20Hz-20KHz) <0.08%·Damping Factor (8Ω, 20Hz-1KHz) 400·Slew Rate 40V/µs·Amplifier Protection: short circuit, thermal, over load, clip limiter, DC voltage, soft start, ultrasonic and RF protectionPrice is 870€ or 980 USD.Now the website displays picture of one channels power stage and it looks kinda sketchy to me, was wondering what do you guys think:First thing that strikes me is how few transistors it has on those heatsinks given the power output they claim, I mean those look like just 2 transistors on the final Class D stage, also the Speakon output connector on the right is soldered to a PCB soldered to the main PCB board, how wonky is that? Another thing I noticed is that there looks like theres nothing on that middle heatskink, is it just for show?I understand this picture doesnt offer too much info but idk. The company that produced this (https://www.sfaudio.es/es/) seems to be mainly specialized in fog machines and produced only a couple of amplifiers like this one.My collegues were looking for cheap amps to power our 11 subwoofers (which are rated at 42400Wrms each) as you can see using top of the line amplifiers would get too expensive to power all of those cabinets (specialy for a couple of young adults with an unmonetized hobby like ours) but perhaps this is too cheap, they offer a 2 year warranty and the dude selling those claimed to have tested them to be up to specs, what should I ask him about those tests?Thanks a lot, Im just a newbie in this forum so if I made any mistake just tell me and Ill be happy to try and fix it

author: themadhippy
author_rank: Super Contributor
timestamp: July 08, 2020, 03:07:08 pm
content: Quoteyou can just about get 2 behringer nx6000 for that money,https://www.thomann.de/es/behringer_nx6000d.htm?ref=intl&shp=eyJjb3VudHJ5IjoiZXMiLCJjdXJyZW5jeSI6IjIiLCJsYW5ndWFnZSI6ImVzIn0%3Dwith the added advantage of having built in crossovers

author: Guille
author_rank: Regular Contributor
timestamp: July 08, 2020, 03:44:20 pm
content: Yep thats true, the NX is another amplifier that Im not so sure about its build, its got no proper heatsink and looks like carbon copy of its predecessor, the Inuke.Both the NX and the amplifier I was asking about have filters but theres no need for those since the signal is previously processed on a proper DSP

author: Mr Evil
author_rank: Regular Contributor
timestamp: July 08, 2020, 03:55:55 pm
content: Quote from: Guille on July 08, 2020, 02:06:07 pmNo need for apologies when your writing is nearly perfect. One mistake stood out though - you usedwearywhen you probably meantwary. They are two completely different words, but are commonly confused even by native English speakers because of the weird pronunciation of the wordwear.Quote from: Guille on July 08, 2020, 02:06:07 pmIf you look closely, that heatsink is raised a few mm off the PCB. Perhaps the main output transistors are underneath?

author: themadhippy
author_rank: Super Contributor
timestamp: July 08, 2020, 04:17:46 pm
content: Quotewho cares with a 3 year warranty,at that sort of price id almost class them as disposable.

author: Haenk
author_rank: Super Contributor
timestamp: July 08, 2020, 05:50:32 pm
content: The SFAudio is 3x the weight - but I mistrust stuff that has "easy to repair" printed on.Behringer is somewhat known for pretty-OK and cheap (but not crappy-cheap), so I'd rather go with this one.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 09, 2020, 12:35:35 am
content: Quote from: Guille on July 08, 2020, 02:06:07 pmWhat could possibly make a super budget amp would be a hacked Prius inverter, with an extra capacitor bank to take out the mains ripple. A STM32 or similar will work nicely as the digital filter and PWM generator. Finish it off with some inductors to take out the PWM carrier.The amount of power they can handle is truly insane:

author: themadhippy
author_rank: Super Contributor
timestamp: July 09, 2020, 02:14:29 am
content: QuoteI can imagine the venues reaction when you rock up asking for a 32A supply and a water supply for the PA

author: Guille
author_rank: Regular Contributor
timestamp: July 09, 2020, 10:02:03 am
content: Quote from: Haenk on July 08, 2020, 05:50:32 pmThat whole amplifier stage is made to be swapped out and replaced Im afraid they made some pretty cheap boards and will just replace them if you send them back. I think Ill go with Behringer or T.AmpQuote from: Mr Evil on July 08, 2020, 03:55:55 pmThanks! I appreciate this feedback tooQuote from: NiHaoMike on July 09, 2020, 12:35:35 amLol! your reply took me by surprise, looks like a real fun project but Im just a physics undergrad with an amateur knowledge in electronics, I wouldnt want to risk frying a couple thousand € in speakers with my bad engineering

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 09, 2020, 01:01:45 pm
content: Quote from: Guille on July 09, 2020, 10:02:03 amSee if you can get help from the electrical engineering students, preferably the more experienced ones.

author: Marco
author_rank: Super Contributor
timestamp: July 09, 2020, 02:59:27 pm
content: Quote from: themadhippy on July 08, 2020, 03:07:08 pmWhich are pretty much the same internally, also justtwo TO-263sto do the switching.They look incredibly similar in fact, with the sfaudio one doing away with the huge expensive DSP. It seems a bit too close to be a coincidence, did they copy past a reference design?

author: wraper
author_rank: Unknown
timestamp: July 09, 2020, 03:12:00 pm
content: Quote from: Marco on July 09, 2020, 02:59:27 pmWhat a surprise considering there is "BEHRINGER" written on PSU PCB.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 09, 2020, 03:36:20 pm
content: You have to remember that modern consumer electronics are designed for a price point AND a gross profit margin.Everything else revolves around those two parameters.

author: Guille
author_rank: Regular Contributor
timestamp: July 09, 2020, 03:58:13 pm
content: Quote from: Marco on July 09, 2020, 02:59:27 pmSo is the SF amplifier design close to the NX? Im no expert but it looks like the NX has a couple more output transistors and no heatsink, well perhaps its sinking heat to the lower belly of the enclosure? I never quite understood how an amp thats outputing so much power can make do with no heatsink. If I do end up buying the NX Ill probably install some extra fans on the rack since we mainly set up our rig outside

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2020, 04:10:22 pm
content: I have a "legitimate" Class-D audio amplifier that I tested under "key-down" conditions with no problems (a lower power rating). I was surprised at how little heat-sinking was required on the power devices. However, if your higher-power device can actually deliver 2 x 3000 Wmeanpower ("rms power" is a common vulgar error in the industry), that means it would need more than 50 A at 120 V or 25 A at 240 V on the AC mains. What size power cable and input fuse were supplied?

author: themadhippy
author_rank: Super Contributor
timestamp: July 09, 2020, 05:14:26 pm
content: Lab gruppen do clever stuff with there power supplys,their fp10000qs only need 16A at 240v to give out 10kw over 4 channels,and having used them regularly ive no reason to doubt that claim.

author: Guille
author_rank: Regular Contributor
timestamp: July 09, 2020, 05:17:17 pm
content: Yep for sure, I understand the whole "program power" vrs "rms power", plus FTC vs EIA ratings etc... and that marketing departments will try to slap the biggest number they can think off into this ratings but I was just curious as to why all power amps I've seen (Class D included) have a chonky heatsink inside and the NX doesnt.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2020, 07:19:31 pm
content: Quote from: themadhippy on July 09, 2020, 05:14:26 pmWow: 3840 W in and 10 kW out (or did you mean 40 kW out?)! That should solve the energy crisis, well past the stage of renewable energy.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2020, 07:20:29 pm
content: Quote from: Guille on July 09, 2020, 05:17:17 pmThe class D amplifier I was describing had the power devices mounted directly to the PCB, with only a metal pad for a heat sink. Class D can be very efficient.

author: themadhippy
author_rank: Super Contributor
timestamp: July 09, 2020, 07:56:44 pm
content: Quoteyea,around 2.5kw per channel,or 2 x 5kw in bridgeQuoteif only,there amps use magic in the power supply and the phasing of the amps to get the power they claim,and as Lab gruppen are a well respected company i'd trust their specs more than the likes of behringer,even though there now under the same umbrella company

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2020, 08:33:27 pm
content: Well, my degrees are in Physics, and I promised the University President that I would not violate the Laws of Physics after I graduated.

author: Guille
author_rank: Regular Contributor
timestamp: July 09, 2020, 08:53:11 pm
content: Quote from: TimFox on July 09, 2020, 07:20:29 pmI believe you

author: themadhippy
author_rank: Super Contributor
timestamp: July 09, 2020, 09:15:40 pm
content: QuoteNo violated laws,just clever engineering magic. from my understanding the power is switched rapidly to each amp and some clever phase shifting of the signal to avoid the peaks happening at the same time.

author: Guille
author_rank: Regular Contributor
timestamp: July 09, 2020, 09:58:38 pm
content: Maybe this is getting out of topic and mods wont like it but if they are claiming mean output power greater than mean input power that goes directly against the conservation of energy lol, if they are measuring "bursts" of greater power than mean input power (trying to simulate music or something like that) then they can achieve those ratings, in the same way you dont need a 10 Petawatt psu for a 10 petawatt pulsed laser. Energy will be gradualy stored in the capacitors and then drained during those short "music" bursts.The concept of delaying peaks between channels is weird to me because the sum wont be coherent when output through the speakers defeating the whole purpouse of gaining power... but Id rather not deviate too much from topic

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2020, 10:02:36 pm
content: There is a legitimate term in RF power amps called "Peak Envelope Power", but there is another force at work here, the magic of the marketing department in an absence of trade-description regulation.Even when manufacturers try to be legitimate, they often say "rms power" when they mean "mean power"--both terms are well defined but not synonymous.With sufficient reservoir capacitance in the power supply, an amplifier could supply a large power for a short time.

author: Haenk
author_rank: Super Contributor
timestamp: July 10, 2020, 06:24:56 am
content: Quote from: Guille on July 09, 2020, 09:58:38 pmWikipedia has a sample calculation and states a factor of about 2,25. That's just comparing Apples to non-Apples. So no free energy creation device, sorry.

author: TimFox
author_rank: Super Contributor
timestamp: July 10, 2020, 03:18:29 pm
content: My "legitimate" stereo class-D amplifier has the gain polarity of the two channels inverted with respect to each other (corrected by the color code of the output posts for correct speaker phasing). This was done so that a strong monaural signal (such as the lewd pulsating rhythm of most pop music) does not draw large current pulses from the same power rail simultaneously. In normal stereo recordings, the low-frequency content tends to be shared between the two channels; if the speaker phasing is reversed, the bass will suffer from destructive interference from the two speakers.

author: themadhippy
author_rank: Super Contributor
timestamp: July 10, 2020, 03:31:45 pm
content: QuoteThat could be what lab meant but got lost in the translation from swedish.