TDA car-audio chip amp: purpose of output caps

[Marco]

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.

There is no virtual ground. Jesus H. Christ!

100 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.

[wraper]

...you can't do this with stereo headpones as they share a common ground, so you do need a 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.

There 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/30

Or simply Google "audio capacitor coupling".

I think you should do mod with these capacitors instead http://studiozey.com/woodencapacitor/index.html 

[salbayeng]

 regarding
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." ?
"

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 running single rail amplifiers with speakers grounded , 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. 

[salbayeng]

There 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/30

Hmm 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 parts
http://en.wikipedia.org/wiki/Madman_Muntz

[mzzj]

I think I need "DC emergency capacitor" after reading this thread 

[IanJ]

I think I need "DC emergency capacitor" after reading this thread 

With all due respect to everyone involved I gotta say this has been an entertaining thread to follow 

Ian.

[dannyf]

output (decoupling) caps are ROUTINELY removed from all sorts of devices. I've done this mod myself to an Asus soundcard:

Without 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.

[mikerj]

I think I need "DC emergency capacitor" after reading this thread 

Yup, best to fit a few of them in case of really serious emergencies

[tom66]

In some audio equipment with LINE OUT stages, the DC emergency coupling caps are replaced (and/or augmented) with muting transistors.

The 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.

[Alex Eisenhut]

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.

[wraper]

Seems to be some mysterious beast  http://paragonwiki.com/wiki/Emergency_Capacitor

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[wiss]

output (decoupling) caps are ROUTINELY removed from all sorts of devices. I've done this mod myself to an Asus soundcard:

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.

A short would be a far more reliable way to remove any DC-potential

[madires]

You might like to have a look at http://www.electronics-tutorials.ws/amplifier/amplifier-classes.html for a better understanding of single supply amps. The Wikipedia article about amps at http://en.wikipedia.org/wiki/Amplifier#Power_amplifier_classes lacks 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.

[Alex Eisenhut]

You might like to have a look at http://www.electronics-tutorials.ws/amplifier/amplifier-classes.html for a better understanding of single supply amps. The Wikipedia article about amps at http://en.wikipedia.org/wiki/Amplifier#Power_amplifier_classes lacks 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.

I 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.

[NiHaoMike]

He's just another audiofool... He should go troll other audiofools.

[Zero999]

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.

You'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.

[dannyf]

not the AD8397

There is nothing in the datasheet that would suggest that it can swing below its negative rail.

[Richard Crowley]

There is nothing in the datasheet that would suggest that it can swing below its negative rail.

Furthermore, 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"

Thanks to the fully complementary PNP/NPN output configuration the TDA7372A delivers a rail to rail voltage swing with no need of bootstrap capacitors.

The 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.

[Alex Eisenhut]

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.

If 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.

[IanJ]

There 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:

I'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.

[dannyf]

a speaker's coil in DC is a short circuit

Probably not, unless you define "short circuit" differently.

[macboy]

There 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:

A "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.

[Richard Crowley]

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.

[Alex Eisenhut]

a speaker's coil in DC is a short circuit

Probably not, unless you define "short circuit" differently.

http://www.ebay.com/itm/1-g-Yttrium-Oxide-99-999-pure-Rare-Earth-for-making-YCBO-123-Superconductor-/231358795851?pt=LH_DefaultDomain_2&hash=item35de0f104b

Happy?

[Zero999]

not the AD8397

There is nothing in the datasheet that would suggest that it can swing below its negative rail.

I 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.