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Topic: Amplifier power + efficiency (Read 920 times) previous topic - next topic

scswift

Hey guys, I have a few questions about an amplifier IC:
http://www.ti.com/lit/ds/symlink/tpa3131d2.pdf

First, are the filters between the outputs and the speaker necessary?  I found this bit in the datasheet on page 20 which seems to indicate they are not:

Quote

EFFICIENCY: LC FILTER REQUIRED WITH THE TRADITIONAL CLASS-D MODULATION
SCHEME
The main reason that the traditional class-D amplifier-based on AD modulation needs an output filter is that the
switching waveform results in maximum current flow. This causes more loss in the load, which causes lower
efficiency. The ripple current is large for the traditional modulation scheme, because the ripple current is
proportional to voltage multiplied by the time at that voltage. The differential voltage swing is 2 × VCC, and the
time at each voltage is half the period for the traditional modulation scheme. An ideal LC filter is needed to store
the ripple current from each half cycle for the next half cycle, while any resistance causes power dissipation. The
speaker is both resistive and reactive, whereas an LC filter is almost purely reactive.

The TPA3131/32D2 modulation scheme has little loss in the load without a filter because the pulses are short
and the change in voltage is VCC instead of 2 × VCC. As the output power increases, the pulses widen, making
the ripple current larger. Ripple current could be filtered with an LC filter for increased efficiency, but for most
applications the filter is not needed.

An LC filter with a cutoff frequency less than the class-D switching frequency allows the switching current to flow
through the filter instead of the load. The filter has less resistance but higher impedance at the switching
frequency than the speaker, which results in less power dissipation, therefore increasing efficiency.


Is this referring to the LC filter on the speakers?    And what is "efficiency" referring to here?  The amount of heat generated?  Or the wattage into the speaker?   And if I leave the LC filters off, how will it impact the efficiency?


I also want to know how much power the amp will output and how much it can feed into a particular load.   I need to know how much power I will get with two speakers (BTL) and with one (PBTL) with a 6V and/or 12V supply, and 4 ohm speakers.

If I understand correctly figure 16 on page 11 shows  that with a 6V supply I can push around 5W into a 4 ohm speaker, and with 12V I can push around 20W. 

However, if I switch the amp into PBTL mode...  Will I see any benefit?  It won't raise the voltage, so I assume the load into a single 4 ohm speaker would be the same.  The only "benefit" I imagine would be that I could connect twice as many speakers to that single output channel.  But if I can connect speakers to both channels, and according to that chart each channel could output a max of 42.5W into a 4 ohm speaker w/ 1% THD @ 20V, then that would seem to jive with what the datasheet says about PBTL mode enabling up to 85W of power output.  85W into a 2 ohm load.  So two 4 ohm speakers in parallel.... versus two 4 ohm speakers, one per channel.  No actual increased power output capacity here, right?

Lastly, what's the real minimum load impedance per channel?  The recommended values indicate 4 ohms minimum in BTL mode.  But is that really the minimum?  What is the limiting factor here?  If I input 6V I can put 5W into a 4 ohm speaker, but if I input 12V I can put 20W into a 4 ohm speaker.  Figure 15 and 16 seem to indicate that if I halve the oms, I double the power, so why can't I connect 4 speakers in parallel per channel... a 1 ohm load... with a 6V supply and still have the amp put out 20W?  Is there a limit on the maximum current? 

Ohm's law says: I = P / E
So if I put in 20W and 12V I get 1.7A
and if I input 20W and 6V I get 3.3A

So I guess there is more current flowing... 

But 10W @ 6V is 1.7A.  So could I at least connect two 4 ohm speakers in parallel on each channel when running at 6V?  That would be a 2 ohm load and should be 10W.   

Hm... I just did the calculation for 20V input as well.  42.5W max with a 4 ohm load translates to 2A per channel.   That's much lower than I expected.  But it raises a new question.  With two channels, that's only 4A total.  But on page 7 under "AC ELECTRICAL CHARACTERISTICS" it indicates the overcurrent will trip at 7A.  What's up with that?  Is the 42.5W value an average and the actual power output could be nearly twice that at some points?

Caltoa

#1
Feb 20, 2014, 10:36 am Last Edit: Feb 20, 2014, 10:40 am by Caltoa Reason: 1
As far as I know, they are designed for 4 ohm minimal.
Even if the current limiter is not yet activated, I would not go below 4 ohm.

Those high frequency Class D amplifiers assume that the coil of the speaker can handle the high frequency.
They are ment to drive a coil of a single speaker. Not a speaker box with filters and speakers. It will work, but I can not guarantee that every speaker box will be okay. For example a piezo speaker inside a box for high frequencies might be a problem.

If you use wires between the amplifier and the speakers, those will transmit large amount of RF signals. Your wireless mouse might stop working. And AM radio will get noise. Even your amateur radio neighbour might complain.

I think a LC filter after the amplifier is good. Normally the LC values are different for 4 ohm or 8 ohm, but I have not read the datasheet of this chip very well. The filter will reduce the RF on the cables and make the amplifier more compatible with all kinds of speaker boxes.

The maximum power output depends greatly on the frequency. A class D amplifier is more likely to have power problems with lower frequencies than normal amplifiers.

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