TDA car-audio chip amp: purpose of output caps - Page 1
Forum section: electronics / projects

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 10, 2014, 05:19:42 am
content: Below is an typical schema from ones of the countless TDA car audio chip chip amps:(The one I'm particularly concerned with at the moment is this TDA 7372http://www.classiccmp.org/rtellason/...ta/tda7372.pdf)A mild debate has arisen elsewhere as to what function(s) the OUTPUT CAPS serve (2200uF, C9-12 in schema above).Are they to BLOCK DC and/or (as contended by another) "all push pull stages that operate only on a single rail power supply utilize a SERIES capacitor in series with the speaker, which acts a a voltage source during the negative half cycle." ?Thx!

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 10, 2014, 05:35:28 am
content: Yes, since the amplifiers are powered from a single-ended supply (+12V only), the output sits at 6V during "silence". No speaker likes having a constant DC current which interferes with its ability to reproduce the waveform. So, the most obvious function is that those are DC-blocking (aka AC-coupling) capacitors.And well, if you hold your mouth just right and squint your eyes, you could say the the capacitor forms a voltage OFFSET (not exactly a "source") during both the positive and negative waveform excursions. It shifts everything down by 6V.Note: When I use the term "12V" I actually mean whatever voltage is being supplied to the chip. As your diagram shows it could be something less than 12V all the way up to 16V or so. And when I say "6V" I actually mean half whatever the supply voltage is. "Nominally" 12V and 6V.

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 10, 2014, 05:59:51 am
content: I have seen several topologies in which a chip-amp using a SINGLE (+) supply drives, output transducer (headphones, speakers, etc) w/o ANY caps. In fact in audiophile wankdom, output caps are considered "bad". (e.g., better to use a servo, etc).Below is another TDA chip amp (this time for portable/headphone apps) ... guess what ... noelectrooutput caps ... just those two L/R 100uF caps for emergency DC protection.It's used in a modern "iPod" device:

author: ludzinc
author_rank: Unknown
timestamp: October 10, 2014, 06:09:19 am
content: Quote from: 13hm13 on October 10, 2014, 05:59:51 amUm, no, there's two 100uF decoupling caps, one for left, one for right channel...

author: Richard Head
author_rank: Frequent Contributor
timestamp: October 10, 2014, 06:15:49 am
content: Below is another TDA chip amp (this time for portable/headphone apps) ... guess what ... no electro output cap ... just that 100uF for emergency DC protection.Am I missing something? I see two 100uF caps in each output to the headphones, again for DC blocking.A bridged output stage is "better" in that it eliminates the electrolytics (or tantalums) but is a bit more complex.Another problem with capacitor coupled output audio amplifiers is the annoying click when the power is first applied. This is due to the capacitor charging to half the rail.If a bridged output is used then you avoid that issue.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 10, 2014, 07:53:30 am
content: Quote from: 13hm13 on October 10, 2014, 05:59:51 amWhat does "emergency DC protection" mean?Those are conventional DC-blocking/AC-coupling output capacitors as surely as you are reading this.

author: German_EE
author_rank: Super Contributor
timestamp: October 10, 2014, 10:35:12 am
content: A number of years ago I had a stereo amplifier that had been constructed using potted modules from a company called ILP. Something in the power amplifier module failed, probably one of the output transistors, and it applied 12v DC to a very nice pair of headphones. The first sign that something was wrong was a loud click and smoke coming from one transducer.In this case an 'emergency DC' capacitor' might have been a good idea.

author: dannyf
author_rank: Super Contributor
timestamp: October 10, 2014, 11:03:32 am
content: QuoteThere are BTL amps (bridged OTL amps) that in theory don't need the dc-blocking capacitor on the output. They are / were widely used in car amps.

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 11, 2014, 07:08:32 am
content: Quote from: German_EE on October 10, 2014, 10:35:12 amRemember: the amp outputs AC-type (sinusoidal) signal -- music or audio -- and DC protection is built into the TDA IC (see datasheet). The in-series caps are there for added protection (there are also DC coupling caps often on the INPUT of amps). Certain "high-end" designs replace caps with active servo circuits.The main point is -- and going back the OP -- that those in-series caps do not provide biasing.

author: SeanB
author_rank: Super Contributor
timestamp: October 11, 2014, 07:23:49 am
content: The only integrated stereo amplifiers that do not need a coupling capacitor are those which provide 3 output stages, using one as the way to connect the common of the headphones to the appropriate power rail. They float all three connections at half the DC supply rail, but have limited power capability.The other method uses one stereo amplifier chip per speaker, driving in a bridge configuration. This requires a separate connection to each side of the speaker, and you cannot have a common connection for any speakers.All of them have a problem where if the IC fails ( and yes, even with SOA, overtemp, overcurrent and overwhatever protection they can and do fail, often quite explosively) they often first fail and deliver the one supply rail to the output direct, and as typically only one side is monitored for current limiting ( often only the high side) and typically the high side will fail shorted first, then a very high DC current flows through the speaker until something either cooks itself into a short and blows the fuses or the power supply, or the speaker burns out.

author: mikerj
author_rank: Super Contributor
timestamp: October 11, 2014, 12:52:37 pm
content: Quote from: 13hm13 on October 10, 2014, 05:59:51 amIf you have balanced supply rails, i.e. a +ve, -ve and 0v rail then you can dispense with the output capacitor. Obviously this isn't easily available in an automotive environment.The other way to do this is with a BTL (Bridge Tied Load) with two push-pull outputs which are nominally biased at half the supply rail. This gives you up to twice the voltage swing and potentially up to 4 times the output power of a single ended output from the same supply rails. Most car head units and portable audio electronics (MP3 players, phones, tablets) now tend to use this kind of output stage since it makes best use of limited supply rail voltages.Audiophile wankdom is just that however; the same people explode with joy when they listen to a tube amp that uses an output transformer.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 11, 2014, 01:54:08 pm
content: It's possible to use a charge pump built into the chip to derive the negative rail.http://www.maximintegrated.com/en/products/analog/audio/headphone-amplifier-ics/directdrive-headphone-amplifier-ics.htmlThe main strike against capacitive coupling is when OEMs skimp on the caps (or intentionally undersize them for saving space), causing the frequency response to fall off too soon. The solution I like is to add series resistance. It flattens the frequency response at the expense of volume, but as most headphone outputs are able to go way louder than necessary, that's usually not a problem. It can be done without modifying the device and also boosts SNR.http://home.comcast.net/~machrone/playertest/playertest.htm

author: N2IXK
author_rank: Frequent Contributor
timestamp: October 11, 2014, 03:57:16 pm
content: Quote from: German_EE on October 10, 2014, 10:35:12 amMost decent direct coupled audio power amplifiers have a "speaker protection" circuit built in. It monitors the amplifier outputs for DC offset, and drives a relay that only pulls in when the DC output is zero. The speaker terminals are connected to the amplifier outputs through contacts on the relay. If an output transistor shorts, one supply rail fails, or any other fault occurs, the DC output level at the output shifts, and the speakers are disconnected from the potentially damaging DC signal.Amplifiers like this usually incorporate a time delay feature, so that the speakers are initially disconnected from the amp at power up, and the relay pulls in a couple seconds after the power is applied. This eliminates the loud "thump" caused by transient voltages when the power is first applied.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 11, 2014, 04:27:49 pm
content: A more modern way of doing that (in speaker amplifiers) is to have the main and aux rails be operated independently. Then the logic and drivers power on first, after which the main rails are ramped up. Many modern designs even vary the main rails in response to the volume setting as that boosts efficiency and resolution at low volume settings. Fault protection is then just a matter of switching off the drive signals and main voltage rails.

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 11, 2014, 06:30:29 pm
content: Quote from: mikerj on October 11, 2014, 12:52:37 pmBut THOSE caps have NOTHING to do with power supply rails. In an emergency, THOSE caps will block a sustained DC current from damaging the speaker/headphone/transducer.Further, THOSE in-series output caps -- because they are IN the direct path of the audio signal -- are often NONpolar electrolytics, esp. in high-quality audio amps. BlackGate brand caps, no longer made, are popular DC blocking non-polar electros...As far as " [pos/neg dual voltage] Obviously this isn't easily available in an automotive environment. "No! Cheap n' easy for any battery powered device. You can use a rail-splitter like Texas Instruments’ TLE2426 and virtual ground. More info:http://tangentsoft.net/elec/vgrounds.html

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 11, 2014, 07:30:39 pm
content: Quote from: 13hm13 on October 11, 2014, 06:30:29 pmWhat does "emergency" mean to you? That circuit has SUSTAINED VCC/2 voltage whenever it is powered up. Even with no signal.That is called "nominal operating conditions as designed". Dunno what "emergency" means here?QuoteTypically not. In unipolar situations (as in a vehicle amp), there is no negative voltage, so there is no requirement for (more expensive) non-polar caps. The output from that amplifier is ALWAYS POSITIVE. So a simple polarized cap can be used. There is no advantage to using a non-polar cap where an ordinary polarized cap would be sufficient.

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2014, 08:47:49 pm
content: Quote from: 13hm13 on October 10, 2014, 05:19:42 amI'd say both are correct: the capacitor both blocks DC and provides a negative voltage source during the negative half cycle.

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 12, 2014, 01:53:03 am
content: For the in-series caps in question ..."...provides a negative voltage source during the negative half cycle." is incorrect!Just take apart your avg. portable (= batt. pwr'd) MP3 player, like "iPod". Mine uses an AD8397 opamp with SINGLE supply voltage (around +5V), and the output pins of the opamp are DIRECT-COUPLED to output (e.g., voice coil of headphone).Remember (for the umpteenth time)... (a) the AC (music/signal) voltage swing can happen (swing) entirely above the X-axis (domains with + 'y'); and (b) the elastic (compliance) nature of dynamic driver pulls it back ("negatively"), so no reverse bias needed.Electrostatic speakers/headphones, as noted, are another story -- they have their own NEGATIVE/POSITIVE power supply.Please blow the dust off your high-school physics textbooks , brush up on fundamentals ... and then bring intelligent/informed discussion back to this thread ... otherwise, you only embarrass yourselves and the EEV community! SIGH!LOL

author: Marco
author_rank: Super Contributor
timestamp: October 12, 2014, 04:16:57 am
content: The voltage swing doesn't have to be negative (relative to the other connection of the speaker) to pull the cone back, but so there is no DC bias. A DC bias voltage reduces the dynamic range and burns power in the coil for no sound output.As for how you can do that without a DC blocking capacitor, multiple ways have already been mentioned. If you measured the supply voltages on the AD8397 correctly then presumably your "ipod" has a virtual ground at 1/2 Vcc for the headphone jack's shared ground.

author: dannyf
author_rank: Super Contributor
timestamp: October 12, 2014, 10:59:00 pm
content: QuoteSounds like you could use some humility.

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 13, 2014, 06:48:34 am
content: Quote from: Marco on October 12, 2014, 04:16:57 amThere is no virtual ground. Jesus H. Christ!Folks ... this is the last time I'm gonna repeat this ...and shame on you degree'd EE college-boyz that a ghetto sh*t plumber (my real-life entrepreneurship biz is plungin' toilets and snakin' drains ... not shit*tin' ya ') - - and one who only dabbles in electronics as a "hobby" -- has to refresh y'all EExperts on basic electronics.The amp is outputtin' an "AC" signal to the transducer's coil lead (this can be "-" or "+" dependin' on which way you want the transducer phased). The other lead (let's say "-") is ground or sink.Electronics for Dummies:from the get go (= by design) there should be no DC present on the amp output unless somethin' is eff'd up.For those who still don't get it ... get educated for Chrissake...

author: David_AVD
author_rank: Super Contributor
timestamp: October 13, 2014, 07:08:31 am
content: So 13hm13, you're saying that an audio amplifier (using a single rail with the output @ half rail) that is capacitively coupled to the load doesn't have the load swinging above and below the 0V line?

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 13, 2014, 12:12:27 pm
content: Semantics. Subjective perception. Violent agreement.

author: BradC
author_rank: Super Contributor
timestamp: October 13, 2014, 12:39:35 pm
content: Quote from: 13hm13 on October 13, 2014, 06:48:34 amI'd stick to plumbing if I were you. (And I'd have a closer look at the output stage in your iPod). Despite what you might want to call it, a capacitor in series between the amp and the transducer is there to block DC. It's there to block DC _all_ the time, not just in what you call "an emergency". A single ended amp in a vehicle does put out AC, but it alternates between 0 & 12 volts and therefore the average is 6V, so unless you have the other end of the transducer sitting at 6V you _NEED_ a capacitor in there to stop the transducer smoking.Bah, I don't know why I bother, you'll only tell me I'm wrong too..

author: Richard Head
author_rank: Frequent Contributor
timestamp: October 13, 2014, 01:29:03 pm
content: When the lower output transistor switches to ground the DC potential across the series output capacitor provides the source voltage to drive the speaker in the -ve direction.Now who's the dummy?

author: dannyf
author_rank: Super Contributor
timestamp: October 13, 2014, 01:30:03 pm
content: QuoteI agree with the rest of your post. Just wanted to point out that there are other ways to block (or eliminate) DC, aside from capacitors. It just so that capacitors may be one of the easiest ways to block DC output.

author: ludzinc
author_rank: Unknown
timestamp: October 13, 2014, 02:00:07 pm
content: Quote from: 13hm13 on October 13, 2014, 06:48:34 amSo, for no input (i.e. silence) what voltage do you think the output stage is applying to the speaker?As for snaking blocked drains, pffft, been helping my Father in Law with his plumbing business for over 15 years now, doesn't make me any smarter the next bloke.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 13, 2014, 04:32:34 pm
content: For no input (silence) the output of the amplifier is 6V.The large electrolytic capacitor quickly charges to (and holds) 6V and forms the function of a 6V LEVEL SHIFTER.At no input (silence) the speaker sees 0V (after the capacitor charges in the first few milliseconds after power-up).The speaker sees 6V less than the amplifier is outputting at any given moment.The amplifier outputs 0 ~ 12V. The speaker "sees" -6V ~ +6V.It is a mystery to me how this can be such a complex and baffling concept.Our grandparents developed the method 100 years ago.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 13, 2014, 05:02:30 pm
content: Quote from: Richard Crowley on October 13, 2014, 04:32:34 pmBut they weren't plumbers. So obviously they got it wrong.

author: mikerj
author_rank: Super Contributor
timestamp: October 13, 2014, 08:51:08 pm
content: Quote from: 13hm13 on October 11, 2014, 06:30:29 pmSeriously, you are embarrassing yourself. Why would you bother with the complexity, real estate and voltage loss of an active divider when for around the same component count you could create another channel and bridge tie the load?

author: ludzinc
author_rank: Unknown
timestamp: October 13, 2014, 09:03:30 pm
content: Quote from: Richard Crowley on October 13, 2014, 04:32:34 pmGreat explanation Richard!

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 14, 2014, 02:52:13 am
content: Quote from: ludzinc on October 13, 2014, 09:03:30 pmYeah... "GREAT ..." ... "You tell 'em" ..."You go gurl" ...Typical EE good-ol'-boyz and UNEXAMINED circle-jerking patriotic banterHumans would've colonized the Andromeda galaxy by now if the gen-pop (y'all) were not so prolific. Oh, well .. they say talent and smarts skips a generation .... bright side: your kids'll be sharp as tacks. OUCH! But I support Logan's Run type sterilizationGotta a challenge for all ya' EE's with that framedBSEE 'ploma on da wall ...Get one of the TDA7273 chip-amps in the OP ... put it in a test ckt (breadboard) and use a speaker (or heavy-load dummy resistor).IMPORTANT: don't install the emergency cap. Wire it direct from TD7273 out pin to load.Then, use a DMM or 'scope ... how many effin' DC volts do you see with NO input signal?Go on punks ... make my day====P.S.Did you punks even bother to read thedatasheet(p 8/10) that I LINKED and QUOTED previously:QuoteSO ...jackasses ... those caps ain't there for any other purpose other than blocking DC in case of emergency. Period. Full stop. The crowd goes wild. The fat lady has sung. And .. y'all degree'd EE's .... well you can go back to low-paying-job, kickstarter-milkin', credit-card-maxed, massively-in-debt, house-foreclosed lifestyles .If the concepts are elusive ... sleep on it .... or do you want mommy to kiss your boo-boos and tuck you in nighty-night?

author: Unknown
author_rank: Unknown
timestamp: October 14, 2014, 03:06:37 am
content: Well, I dunno about anyone else - but I'm getting a laugh out of this

author: ludzinc
author_rank: Unknown
timestamp: October 14, 2014, 03:22:20 am
content: Quote from: 13hm13 on October 14, 2014, 02:52:13 amA Bootstrap capacitor is used to pull up a signal to a rail (like when using old school non rail to rail op-amps). Completely different animal to an DC blocking cap.Yes, the TDA7273 has short circuit protection, but that doesn't mean it will deliver audio into a short (which a directly coupled speaker looks like).It's better to remain silent and be thought a fool than to speak and remove all doubt. Thanks for shouting it out loud 13mh13.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 14, 2014, 04:04:57 am
content: Quote from: 13hm13 on October 14, 2014, 02:52:13 amHaldol comes in grape flavor now.

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 14, 2014, 04:15:36 am
content: Quote from: ludzinc on October 14, 2014, 03:22:20 amBut the "bootstrap" operation is precisely what is being pontificated by some of you. Go back to the OP. In it, I quoted one ofyourkind:"all push pull stages that operate only on a single rail power supply utilize a SERIES capacitor in series with the speaker, which acts a a voltage source during the negative half cycle."This, like your other other peers' arguments, are FUNDAMENTALLY wrong. IOW, you're mistake-makers.Set aside the armchair speculation and weak come-back diatribes. Do the real-wold experiment I outlined.But I suspect you won't go there because of FEAR of EMBARRASSMENT in your pEErdom.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 14, 2014, 04:18:32 am
content: Quote from: 13hm13 on October 14, 2014, 04:15:36 amWhy don't you try it? First with no load. Show your work.

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 14, 2014, 04:30:08 am
content: ludsinc defecated...Quote from: ludzinc on October 14, 2014, 03:22:20 amRUBBISH!!!From Wikipedia:http://en.wikipedia.org/wiki/Bootstrapping_(electronics)QuoteAnd this is PRECISELY what Crowley and others (see OP) have INCORRECTLY "explained" the function of those outty caps to be.Pfffft .... I'm iPaddin' this at 35k ft. And the 777's barf bags look very appealing. It ain't the bad weather ... ... the food, perhaps? ... EExperverts ... so that's what I had!

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 14, 2014, 04:41:11 am
content: Quote from: 13hm13 on October 14, 2014, 04:30:08 amYikes, if you're actually in a plane, perhaps you're one of those people that can't handle a drink at altitude. There's really no reason to go off the handle like that. It's beyond embarrassing.I see thick ty-raps in your near future...

author: sarahMCML
author_rank: Regular Contributor
timestamp: October 14, 2014, 04:47:27 am
content: Quote from: Tac Eht Xilef on October 14, 2014, 03:06:37 amI'm with you on this one - it's hilarious!I think the poor guy is confusing high side N-Channel Enhancement MOSFET bootstrap capacitor circuitry with single supply loudspeaker coupling cap. circuits.

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 14, 2014, 04:58:20 am
content: Alex defecated a stoooooopid, typical trope:Quote from: Alex Eisenhut on October 14, 2014, 04:18:32 amWho says I ain't already tired it and seenmywork.Belch .... burp .... 'scuse me.May be I'll YouTube the results. But I'm pretty lazy .... I mean, years of EEV foruming ...and all I gots to prove fer it are sixty-some lousy posts.I mean I figgers some of you EElifers, with your multi-K posts andSuper Duper Contributoraccolades, and zest for 'sperimentin', would've taken the bait by now?But we're all in touchscreen, gesture-physics, arm-chair-theorizing mode I suppose...Now get out those DMMs and proveyourselvesright ... I mean thoseBSEEs -- framed above your urinal -- denote Bachelor's of Science and not ... well ... BS, right?===Quote from TRON (1982)http://tvtropes.org/pmwiki/pmwiki.php/Film/TronI Don't Pay You to Think: Sark tells an underling, "Don't think anymore. I do the thinking around here."

author: BradC
author_rank: Super Contributor
timestamp: October 14, 2014, 05:00:01 am
content: Well this is a bit of a surprise. I honestly thought people like this were a product of bad American TV. I had no idea they existed in the real world.I'd be afraid to let him anywhere near a plunger let alone a soldering iron!

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 14, 2014, 05:04:59 am
content: Quote from: 13hm13 on October 14, 2014, 04:58:20 amIhaven't seen it and that chip is quite old. I have none.Please try it when you're not so tired and report what you see with no load first.I think I have a TDA2002 somewhere, how about I try that here?

author: David_AVD
author_rank: Super Contributor
timestamp: October 14, 2014, 05:32:18 am
content: Seems that some people will say anything to not admit they are wrong.

author: iampoor
author_rank: Frequent Contributor
timestamp: October 14, 2014, 05:52:37 am
content: Put more grease on the pipe next time you stick it up your ass 13ohm.

author: Simon
author_rank: Unknown
timestamp: October 14, 2014, 06:54:01 am
content: I think this topic has reached the stage of "enter at your own risk" but please remember that there is a difference between having a good laugh and turning it nasty.Have Fun!

author: salbayeng
author_rank: Frequent Contributor
timestamp: October 14, 2014, 06:54:13 am
content: My doctor says I need to reduce my stress, so a good laugh is helpfulI'm going to chip in here, not because I have a Master of EE hanging on the wall surrounded by Patents, (which I do)And not because I can clear a blocked septic, (and yes I have done this, and hats off to all you plumber dudes who do this for a living).But because I recently used a TDA7056B in a design , these are mono, linear class AB, BTL , 5w output, designed for 12v single supply operation, the neat thing is they have a voltage controlled volume input, just add a capacitor and no start up plops or clicks when the audio is muted. They also have an unbalanced current detection so if the speaker were inadvertently shorted to ground it would shut down. (Annoyingly if the TDA7056A is connected to a microphone, and you tap the microphone , it also shuts down). A bit of searching reveals the chip was originally the TDA7056, then TDA7056A , then TDA7056B , with subtle improvements along the way, you have to wonder whether the earlier versions had magic smoke issues.The reason for the post is the make the following observation, that seems to have been skirted around by a few:Normally with a BTL amplifier, each speaker terminal is connected to an amplifier output, but you can't do this with stereo headpones as they share a common ground, so youdo needa decoupling capacitor with headphones. So most of the TDA datasheets will show directly connected speakers , and capacitively connected headphones. So next time when you flick from page to page or datasheet to data sheet and notice the capacitors seem to come and go randomly, have a look and see if the load is a speaker or headphones.I've just looked in my resistor kit, and lo and behold , in the 13ohm drawer, there are 100 bandoliered resistors: brown-orange-black-gold-brown-red , which is exactly the same quantity as 20 years ago, so just goes to show how useful 13ohm resistors are.

author: salbayeng
author_rank: Frequent Contributor
timestamp: October 14, 2014, 07:02:41 am
content: Quote from: 13hm13 on October 14, 2014, 04:30:08 amObviously suffering hypoxia , try travelling inside the cabin next time

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 14, 2014, 07:47:36 am
content: Quote from: salbayeng on October 14, 2014, 06:54:13 amIn some audio equipment with LINE OUT stages, the DC emergency coupling caps are replaced (and/or augmented) with muting transistors. DIY audio modders often remove these to allow a cleaner signal path. As I noted earlier, servos may also be used ... or even trimming via a variable resistor.. .This mod of an audio D/A processorditches the emergency caps (because they compromise audio fidelity) and, indeed, adds a variable-resistor topology (one simply trims this R so signalless output is as close to 0.0V as possible).Here's the schematic (You may have to register at DIYAUDIO.com to access the full schematic JPG of the mod.):This EEVBlog discussion has overstretched to 4+ pages. The query in the OP was clear yet the majority of the responses given have not been authoritative or convincing, much less topical.The main "contentious" issue being the partemboldenedin the direct quote from the OP :Quote

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 14, 2014, 08:01:57 am
content: Quote from: salbayeng on October 14, 2014, 06:54:13 amThere are countless DIY audio mod threads that discuss this issue (direct coupling vs. transformer coupling vs. capacitor coupling ... output (decoupling) caps are ROUTINELY removed from all sorts of devices. I've done this mod myself to an Asus soundcard:http://www.head-fi.org/t/587297/asus-xonar-hdav1-3-mods/30Or simply Google "audio capacitor coupling".

author: Marco
author_rank: Super Contributor
timestamp: October 14, 2014, 08:20:52 am
content: Quote from: 13hm13 on October 13, 2014, 06:48:34 am100 bucks on an ipod putting an AC voltage with no DC bias on it's output. Either through coupling caps or a bipolar power supply relative to the headphone ground.Sure you can remove the coupling caps at low enough voltages, but you're destroying the dynamic range of the speakers (ie. creating soft clipping) and heating them up. This is a retarded mod.

author: wraper
author_rank: Unknown
timestamp: October 14, 2014, 08:37:01 am
content: Quote from: 13hm13 on October 14, 2014, 08:01:57 amI think you should do mod with these capacitors insteadhttp://studiozey.com/woodencapacitor/index.html

author: salbayeng
author_rank: Frequent Contributor
timestamp: October 14, 2014, 09:03:27 am
content: regardingA mild debate has arisen elsewhere as to what function(s) the OUTPUT CAPS serve (2200uF, C9-12 in schema above).Are they to BLOCK DC and/or (as contended by another)"all push pull stages that operate only on a single rail power supply utilize a SERIES capacitor in series with the speaker, which acts a a voltage source during the negative half cycle."?"This is really arguing about the veracity of "lies to children" , after all an easter bunny is a sufficiently good explanation of the source of easter eggs.If you want to think of the speaker coupling capacitor as a 6.0v DC source when connected to a single rail 12.0v amplifier , then that is sufficiently good enough to explain the operation in broad terms. (After all if you run a circuit simulator that is exactly what it does with capacitors for a small signal analysis).C9-C12 are essential in the schematic if you intend runningsinglerail amplifiers with speakersgrounded, it is only a "debate" in the same way that King Canut "debated" with his advisors. It is obviously not the best solution, but adequate. A better solution would be to connect two of the 2200uF caps in series from +12v to gnd , and tie all four speaker common terminals to this point, A bit of clever phasing of the input signals would see low frequency AC current flowing in one speaker and out another. So, as drawn, 2200uF and 8ohms is a 10Hz rolloff , using my scheme each channel individually sees 4400uf in parallel with 2.66ohms from the other speakers, that would push the rolloff down to 5Hz. With normal musical content however , the low frequency component of all channels would nominally be in phase below 50Hz, so with "clever phasing" , the LF rolloff would be pushed down below 1Hz (determined by input coupling capacitors) . It's a bit of a moot point however , as human hearing drops off at ~ 20Hz , as does speaker efficiency.

author: salbayeng
author_rank: Frequent Contributor
timestamp: October 14, 2014, 09:23:07 am
content: Quote from: 13hm13 on October 14, 2014, 08:01:57 amHmm perhaps you have some Earl Muntz blood in you? He was notorious for walking around the electronic design lab with a pair of side cutters and cutting out unnecessary partshttp://en.wikipedia.org/wiki/Madman_Muntz

author: mzzj
author_rank: Super Contributor
timestamp: October 14, 2014, 10:14:00 am
content: I think I need "DC emergency capacitor" after reading this thread

author: IanJ
author_rank: Unknown
timestamp: October 14, 2014, 11:04:44 am
content: Quote from: mzzj on October 14, 2014, 10:14:00 amWith all due respect to everyone involved I gotta say this has been an entertaining thread to followIan.

author: dannyf
author_rank: Super Contributor
timestamp: October 14, 2014, 11:34:18 am
content: QuoteWithout a schematic and measurements before and after the mod, it is hard to see what you just did.I would agree with the general notion that if the amplifier's output sits at a substantial potential vs. ground, removing the capacitor (or shorting it as a much easier way) spells trouble for you.So capacitors may not be the only way to get rid of DC, but it sure is one of the simplest way to do so. and I totally don't undrestand the aversion to it.

author: mikerj
author_rank: Super Contributor
timestamp: October 14, 2014, 11:54:44 am
content: Quote from: mzzj on October 14, 2014, 10:14:00 amYup, best to fit a few of them in case of really serious emergencies

author: tom66
author_rank: Super Contributor
timestamp: October 14, 2014, 12:01:39 pm
content: Quote from: 13hm13 on October 14, 2014, 07:47:36 amThe two devices perform different functions. You can't replace one with the other.What is a "DC emergency capacitor"? Is it supposed to reduce the current in case of amplifier failure? For commercial applications (Hi-Fi) relays are typically used (as a protection circuit), capacitors of sufficient size are often very expensive and distort the audio. Those units either use bridge-tied load with single supply, or most commonly a split supply.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 14, 2014, 12:17:15 pm
content: Gotta say it's the first time I've ever heard them called "DC emergency coupling capacitors", as if somehow the capacitor will make a difference between just the regular old usual DC across it, and this mysterious emergency DC...Where would this emergency DC come from? The flying saucer 13hm13 is piloting?I'm still waiting for a no load measurement here.

author: wraper
author_rank: Unknown
timestamp: October 14, 2014, 12:23:47 pm
content: Seems to be some mysterious beasthttp://paragonwiki.com/wiki/Emergency_CapacitorCrafted at Basic WorktableSalvage Required Human Blood Sample Salvage, Mutant Blood Sample, Spell Scroll, Source CodeORStabilized Mutant Genome, Mutant DNA Strand, Spell Ink, Holographic MemoryORHydraulic Piston Salvage, Pneumatic Piston Salvage, Demonic Threat Repor, Chronal Skip

author: wiss
author_rank: Frequent Contributor
timestamp: October 14, 2014, 12:51:55 pm
content: Quote from: dannyf on October 14, 2014, 11:34:18 amA short would be a far more reliable way to remove any DC-potential

author: madires
author_rank: Super Contributor
timestamp: October 14, 2014, 01:17:53 pm
content: You might like to have a look athttp://www.electronics-tutorials.ws/amplifier/amplifier-classes.htmlfor a better understanding of single supply amps. The Wikipedia article about amps athttp://en.wikipedia.org/wiki/Amplifier#Power_amplifier_classeslacks the most important diagrams unfortunately. The most important point is, that you don't want to have any DC at your loudspeakers. Good audio amps got a DC protection circuit which switches off the speakers if the DC becomes to high.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 14, 2014, 01:53:17 pm
content: Quote from: madires on October 14, 2014, 01:17:53 pmI don't think he cares or understands. He constantly mixes up signal level with power level and then goes into hysterical rant mode/irrelevant example mode.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 14, 2014, 05:27:43 pm
content: He's just another audiofool... He should go troll other audiofools.

author: Zero999
author_rank: Super Contributor
timestamp: October 14, 2014, 07:35:13 pm
content: Quote from: 13hm13 on October 12, 2014, 01:53:03 amYou've forgotten that the first post in this thread discusses the TDA7273, not the AD8397 or anything else. There are plenty of other ways of driving a speaker from a single supply which don't involve capacitors but in the case of the schematic in the first post, the output capacitors are both blocking DC and allowing the output of the amplifier to swing below the negative rail of the power supply.

author: dannyf
author_rank: Super Contributor
timestamp: October 14, 2014, 07:51:51 pm
content: QuoteThere is nothing in the datasheet that would suggest that it can swing below its negative rail.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 14, 2014, 08:16:13 pm
content: Quote from: dannyf on October 14, 2014, 07:51:51 pmFurthermore, the datasheet (http://www.st.com/web/en/resource/technical/document/datasheet/CD00000159.pdf)explicitly states"MINIMUM EXTERNAL COMPONENT COUNT" and"NO BOOTSTRAP CAPACITORS"The TDA7372A is not capable of outputing any voltage beyond its supply rails which are 0V and VCC. It delivers "rail-to-rail"QuoteThe TDA7327A outputs VCC/2 at the quiescent-state with no input signal.That is why it is shown with DC-blocking capacitors at each output, AC-coupling the output signal to the loads (the speakers).The amount of audiophool rubbish around here is diminishing the value of these forums.Clearly some people have no working concept of the difference between filter, bypass, bootstrap, coupling, and blocking applications of capacitors.And that doesn't even get into the resonant, frequency-dependent, RF, etc. applications.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 15, 2014, 01:22:25 am
content: Quote from: Richard Crowley on October 14, 2014, 08:16:13 pmIf you're talking just about 13's meltdown, that isn't so much "audiophool" as just "jackass" stuff.I'm still amused by his "how much DC do you measure when you attach the speaker directly" claim. Well sure, a speaker's coil in DC is a short circuit, what do you expect to measure? You aren't going to get much of an output signal either!Probably enable the chip's short-circuit protection too.

author: IanJ
author_rank: Unknown
timestamp: October 15, 2014, 05:59:25 am
content: Quote from: 13hm13 on October 14, 2014, 08:01:57 amI've had to do the exact opposite. I bought a wee Alesis MultiMix4USB mixer to interface a mic to my Asus MB soundcard and connecting the line-outs sends the L-R channel LEDs on the Alesis up to 70%. Scoping it I see a large DC offset occuring.........the fix?.........a couple of series caps on the line-outs internal to the Alesis and the job is a good-un.Ian.

author: dannyf
author_rank: Super Contributor
timestamp: October 15, 2014, 09:58:08 am
content: QuoteProbably not, unless you define "short circuit" differently.

author: macboy
author_rank: Super Contributor
timestamp: October 15, 2014, 02:20:04 pm
content: Quote from: 13hm13 on October 14, 2014, 08:01:57 amA "coupling capacitor" is not a "decoupling capacitor". Completely different. Those are coupling capacitors; they couple the AC signal from one DC domain (offset) to another. It may seem like a minor nomenclature difference, but if you really understood and respected what these capacitors are doing, you would simply never call it a "decoupling capacitor" as it makes no sense.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 15, 2014, 02:38:18 pm
content: Yes, he has already demonstrated that he understands almost nothing about the various roles and applications of capacitors. If I were a moderator, I would remove this entire thread as useless and misleading. Perhaps the various uses of capacitors would make a useful blog episode.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 15, 2014, 03:09:11 pm
content: Quote from: dannyf on October 15, 2014, 09:58:08 amhttp://www.ebay.com/itm/1-g-Yttrium-Oxide-99-999-pure-Rare-Earth-for-making-YCBO-123-Superconductor-/231358795851?pt=LH_DefaultDomain_2&hash=item35de0f104bHappy?

author: Zero999
author_rank: Super Contributor
timestamp: October 15, 2014, 05:14:23 pm
content: Quote from: dannyf on October 14, 2014, 07:51:51 pmI don't believe anyone did say the AD8397 can swing below its negative rail. There are a other ways of driving headphones from an op-amp such as the AD8397 without using AC coupling capacitors but that wasn't the original topic of this thread.

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 16, 2014, 03:40:54 am
content: Quote from: Hero999 on October 15, 2014, 05:14:23 pmIt doesn't need to swing negative to work ... It Doesn't Need To Swing Negative to Work! ... IT effin' DOESN'T NEED to effin' SWING effin' NEGATIVE to effin' WORK!!!!Duhhhhhhhhhh!!!Dave went over this in his opamp video,EEVblog #600 - OpAmps Tutorial - What is an Operational Amplifier? (around 28:00):

author: David_AVD
author_rank: Super Contributor
timestamp: October 16, 2014, 04:28:39 am
content: So you're proposing to drive the speaker on the positive half of the waveform and let it "spring back" for the negative half?

author: wraper
author_rank: Unknown
timestamp: October 16, 2014, 07:21:30 am
content: Quote from: 13hm13 on October 16, 2014, 03:40:54 am

author: BradC
author_rank: Super Contributor
timestamp: October 16, 2014, 12:04:03 pm
content: Quote from: David_AVD on October 16, 2014, 04:28:39 amDamping? What Damping?

author: dannyf
author_rank: Super Contributor
timestamp: October 16, 2014, 12:24:37 pm
content: QuoteYou don't have to shout.To you point, how do you propose to get an ac current through the speaker cone if is is subject to a dc signal?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 16, 2014, 12:40:59 pm
content: Quote from: wraper on October 16, 2014, 07:21:30 amSigh. "there's". Indeed there's no hope.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 16, 2014, 01:31:11 pm
content: Quote from: David_AVD on October 16, 2014, 04:28:39 amI believe that was the gist of his informational animated gif/rant.It's hard to tell; he always seems to answer the question he heard rather than the question that was asked.

author: BradC
author_rank: Super Contributor
timestamp: October 16, 2014, 01:43:42 pm
content: The Gene pool is green beyond Chlorine I'm afraid. Does this rag smell like Chloroform to you?

author: mikerj
author_rank: Super Contributor
timestamp: October 16, 2014, 04:29:35 pm
content: Quote from: 13hm13 on October 16, 2014, 03:40:54 amYou are quite correct, the output of the amplifier itself does not need to swing negative, but the signal applied to the speaker absolutely must swing negative; the net DC component applied to the speaker needs to be kept to very small values, preferably zero. It is the job of your "emergency"caps to remove the DC bias from a conventional power amp run from a single supply rail.I appreciate you are finding this entire topic very difficult to understand, but the capacitor effectively converts a 0 to Vcc swing to a -Vcc/2 to +Vcc/2 swing.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 16, 2014, 05:22:18 pm
content: Quote from: mikerj on October 16, 2014, 04:29:35 pmYa know, I wonder if he has speakers with a weird crossover network that has a cap in series with the woofer anyways, which is why removing the cap from the amp works?I don't know enough about passive crossovers but I've always seen them with a LC lowpass filter with the cap in parallel at the woofer.

author: Zero999
author_rank: Super Contributor
timestamp: October 16, 2014, 06:42:03 pm
content: Quote from: 13hm13 on October 16, 2014, 03:40:54 amIn order to drive a speaker properly, the positive terminal of the speaker needs to both swing above and below the negative terminal of the speaker. The negative terminal of the speaker may or may not be at the same voltage as the negative supply voltage.Again, I repeat, this thread is about the TDA7273, which uses DC blocking capacitors and the positive terminal of the speaker does indeed swing below the power supply's negative rail.

author: dannyf
author_rank: Super Contributor
timestamp: October 16, 2014, 08:01:23 pm
content: QuoteIt would be odd for a woofer to have a serial capacitor (only). If the serial capacitor is in parallel with an inductor, then the same problem persists.Serial capacitor (only) for a tweeter makes sense, however.QuoteThe speaker would defeat the capacitor in parallel.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 16, 2014, 08:14:40 pm
content: Quote from: dannyf on October 16, 2014, 08:01:23 pmEh wot? "LC lowpass filter with the cap in parallel at the woofer"That's the way they've always looked to me.

author: 13hm13
author_rank: Frequent Contributor
timestamp: October 17, 2014, 03:19:24 am
content: mikerj urinated ...Quote from: mikerj on October 16, 2014, 04:29:35 pmTRYIN'TO PATRONIZE? ..... PFFFFTTTTTT!!!Quote from: mikerj on October 16, 2014, 04:29:35 pmpssst ...the caps are there for emergency DC protection (the amp/speaker will work, with single supply, w/o those caps)....the caps are there for emergency DC protection (the amp/speaker will work, with single supply, w/o those caps)...the caps are there for emergency DC protection (the amp/speaker will work, with single supply, w/o those caps).Oh, and uh, one more thing...

author: BradC
author_rank: Super Contributor
timestamp: October 17, 2014, 03:26:14 am
content: Quote from: 13hm13 on October 17, 2014, 03:19:24 amYou say it will, we know it won't. You made the assertion, now would be the appropriate time for YOU to prove us wrong. Put up or shut up.

author: ludzinc
author_rank: Unknown
timestamp: October 17, 2014, 03:50:55 am
content: Quote from: 13hm13 on October 17, 2014, 03:19:24 amShout it out as loud as you want.You're still wrong.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 17, 2014, 04:12:33 am
content: Quote from: 13hm13 on October 17, 2014, 03:19:24 amSimply make the breadboard and measure the output with no load. Or send me a chip.Or describe a situation that would qualify as a DC emergency.

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 17, 2014, 05:07:30 am
content: Quote from: Alex Eisenhut on October 17, 2014, 04:12:33 amYou are wasting your time. 13hm13 clearly doesn't even understand the question, so he would be unable to respond coherently.People who don't know what they don't know think that the rest of us are just ganging up on them to be mean.They don't have any useful understanding of the underlying concepts they think they are discussing.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 17, 2014, 05:19:59 am
content: Quote from: Richard Crowley on October 17, 2014, 05:07:30 amI think you're wrong. Once someone uses 36 point font, you know you're wrong and they're right. Now it's just a question of getting some measurements...

author: David_AVD
author_rank: Super Contributor
timestamp: October 17, 2014, 05:20:30 am
content: Dunning Kruger effect

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: October 17, 2014, 05:59:15 am
content: Quote from: David_AVD on October 17, 2014, 05:20:30 amYou might be right David, but my internet spidey senses tell me 13ohm is just an old fashioned troll and a rather expert one at that. He knows just what buttons to push. Simon should have locked this thing down.

author: IanJ
author_rank: Unknown
timestamp: October 17, 2014, 06:01:49 am
content: Clear yet?Ian.

author: wraper
author_rank: Unknown
timestamp: October 17, 2014, 07:54:06 am
content: I'm really amazed how you guys still can answer him seriously. I wouldn't even try to do that after a few of his posts. It's just Sisyphean labor regardless if he is a troll or just non smart. Even if he will understand finally, he does not deserve your effort because of his altitude.

author: wraper
author_rank: Unknown
timestamp: October 17, 2014, 08:04:58 am
content: I even would say that he deserves a lesson in a hard way - burned speakers and amplifier.

author: BradC
author_rank: Super Contributor
timestamp: October 17, 2014, 09:05:06 am
content: Quote from: Alex Eisenhut on October 17, 2014, 04:12:33 amFire at the Whitehouse?

author: macboy
author_rank: Super Contributor
timestamp: October 17, 2014, 01:01:54 pm
content: Quote from: dannyf on October 16, 2014, 08:01:23 pmSpeaker voice coils have (commonly) 8 ohm impedance, which for a woofer, really does measure as roughly 8 ohms resistance. The purpose of the parallel capacitor (after the inductor) is to have a lower than 8 ohm impedance above the crossover frequency, reducing the current that goes into the speaker, thus providing an additional pole to the filter.

author: wiss
author_rank: Frequent Contributor
timestamp: October 17, 2014, 01:10:57 pm
content: Quote from: macboy on October 17, 2014, 01:01:54 pmAnd to ensure that the driver always sees a low impedance, this will reduce out of band distortion compared to a single series inductor.

author: Zero999
author_rank: Super Contributor
timestamp: October 17, 2014, 08:26:07 pm
content: Quote from: 13hm13 on October 17, 2014, 03:19:24 amNo, the capacitors are necessary. Take a look at the schematic when there's no audio signal. The outputs of the TDA7273 are biased at half the supply voltage. Without the output capacitors, the current through a 3 Ohm speaker will be a steady 2A, causing it to overheat. Hopefully, the IC's internal protection circularity will kick in and shut it down before any damage is done to either the speakers or amplifier.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 19, 2014, 06:16:52 am
content: Quote from: macboy on October 17, 2014, 01:01:54 pmThis is true! I looked up some woofer datasheets and checked the wiki for the T&S parameters, turns out voice coils are designed to have a DC resistance (Re) of 80% of nominal impedance. How little I know!Call it 6 ohms, so with a 12V TDA sitting at 6V, without that "emergency" cap, you'd get about 1A at 6V = 6W static power dissipation, which is just about the limit of that chip anyways. So while not quite a "short circuit", it also means there's no useful headroom for actual musical signal there, which is pretty much a practical short circuit. And you get a hot speaker for no audio output.This has been a weird and wonderful thread.

author: David_AVD
author_rank: Super Contributor
timestamp: October 19, 2014, 08:12:24 am
content: Quote from: Alex Eisenhut on October 19, 2014, 06:16:52 amThe troll is probably safely back under his bridge now.

author: anman
author_rank: Contributor
timestamp: January 22, 2017, 03:13:52 pm
content: A good boardTDA7293for car audio.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 22, 2017, 06:23:36 pm
content: Quote from: anman on January 22, 2017, 03:13:52 pmSure, if you use a PAIR of chips for each speaker, you can use them in "bridged" mode. Where BOTH sides of the speaker float at 6V during quiescence. But that was not the original question.

author: salbayeng
author_rank: Frequent Contributor
timestamp: January 23, 2017, 03:26:50 am
content: What Richard said, andI don't think it is necessarily a "good car audio board"(a) They have fitted smaller electros than the footprint on the PCB, so they are probably cheap 85C 1000hr caps, that will deteriorate in a year or so with a typical car parked in the sun. They should really be 105C 5000hr rated, and operating at < 60% of rated voltage.(b) They have 250V rated film caps , wtf! , this is to appease the golden ears brigade, seriously a 63v polypropylene cap would suffice, but doesn't have that valve audio look!.(c) The TO-220 regulators? have no mechanical support, so typical car vibrations would fatigue the legs and they would fall off.(d) This board requires a split power rail of +/-35V so not useful with a car's single 12v supply. You would need some kind of auxiliary power supply.(e) to get the purported 350W out you need several 1000uF caps scattered around, that need to have ripple current ratings of several amps each, the caps on the board don't look big enough to do the job.(f) The green connector with the output (in middle of PCB) is labelled "IN GND" , not "SPKR+ SPKR-" , if they can't get the simple things right, the rest of the design is suspect.

author: Unknown
author_rank: Unknown
timestamp: January 23, 2017, 06:13:57 am
content: Quote from: salbayeng on January 23, 2017, 03:26:50 amWhat, you expect good design sense from someone who re-awakens a necro-thread with a thinly-disguised Aliexpress referrer link? That's a spammer...