tpa3255 with 16 ohm load

Status
Not open for further replies.
Well, that's one thing about DIY - it's in the name 😉 And besides, you won't always find ready-made bespoke solutions for whatever you want to achieve.

That being said, usually you'd want the Q of the filter around 0.7 (it goes into those formulas in that app-note). That's the "ideal" compromise between peaking (ie. none at all) and the attenuation of the top end (15-20k) of the output signal. See the graph on page 6 here:

http://www.ti.com/lit/an/sloa119b/sloa119b.pdf

In fact, this whole app-note is precisely what you want / need 😉

Doing the math is a lot easier if you plug those formulas into Excel / LibreOffice, though 😀 Just mind the units...
 
i did have one semi-intelligent thought.

if the speakers are wired in series (its a dual fullrange cabinet) but one of the 8 ohm drivers has a lowpass filter on it to keep a pointsource for the high end, i assume it would present an 8 ohm load to the amp at higher frequencies?


so that might work in my favour. ..or..?
 
thanks for the pointer! i will check out that calculator see if i can figure out what i should do.

however i dont want then to create an amp that cannot work with 4 or 8 ohm speakers.. which i imagine would be the case.? i dont understand enough about amplifier theory to understand the effects. i imagine the risk is losing some of the high end frequencies?


anyone care to comment on my brainfart regarding the series speakers with the lowpass on one? how do i figure it out for speakers which present a 16 ohm load at lower frequencies and an 8 ohm load at higher frequencies? to my feeble mind that seems generally problematic.. surely at higher fequencies the lower impedance means the amp is gonna dump more power thru my delicate speaker.. ?
 
99% of speakers are not what they are. An 8 ohm speaker can be anything from 6 ohms to 30 ohms or more. Look at the impedance plot of any speaker. The filter components are to filter out the very high switching frequency of several hundred kilo hertz or more. Some class D amplifiers do not have any output filter at all and rely on the inductance and capacitance of the speaker cables, the crossover and the speaker itself. A typical coil is 15uh. a compromise for 4-8 ohm speakers. The current rating and the resistance of the coil are very important as well.
 
yeah i was just reading about that.. however the documentation goes rather over my head. is it really as simple as adding the components with the values listed in the document?

for these speakers im not going to need more than 60-100w per channel...
 
yeah i was just reading about that.. however the documentation goes rather over my head. is it really as simple as adding the components with the values listed in the document?

for these speakers im not going to need more than 60-100w per channel...

handful of resistors and capacitors?


not sure why there is a double post there..
 
The output filter on a class D (or any switchmode) amp depends on the speaker load impedance for how it will work. I think they usually design for 6 ohms, so can work well with the most common 4 or 8 ohm loads. Most speakers impedance graphs go up in impedance when you get above the audio frequency range, to at least several times the rated "nominal" impedance rating, so even if you use an 8 ohm speaker, things get sloppy in the higher frequencies where the switching noise is intended to be filtered out. If the load impedance is too high, I've heard of cases where substantial transients are generated and output filter caps have been blown out because they had too low a voltage rating. I believe this was when there was no speaker load connected at all.

A great idea IMO is to put a zobel filter across the speaker, consisting of a 0.2uF cap in series with a 20 ohm 5 watt resistor. This will have no effect in the audio frequency range, but will limit how high the speaker impedance will go above the audio frequency range, as viewed by the class D amp output filter. This should keep things safe and operating well. This is also a great idea for any tube type amp that has a transformer coupled output.
 
The output filter on a class D (or any switchmode) amp depends on the speaker load impedance for how it will work. I think they usually design for 6 ohms, so can work well with the most common 4 or 8 ohm loads. Most speakers impedance graphs go up in impedance when you get above the audio frequency range, to at least several times the rated "nominal" impedance rating, so even if you use an 8 ohm speaker, things get sloppy in the higher frequencies where the switching noise is intended to be filtered out. If the load impedance is too high, I've heard of cases where substantial transients are generated and output filter caps have been blown out because they had too low a voltage rating. I believe this was when there was no speaker load connected at all.

A great idea IMO is to put a zobel filter across the speaker, consisting of a 0.2uF cap in series with a 20 ohm 5 watt resistor. This will have no effect in the audio frequency range, but will limit how high the speaker impedance will go above the audio frequency range, as viewed by the class D amp output filter. This should keep things safe and operating well. This is also a great idea for any tube type amp that has a transformer coupled output.

would that be necessary if i add the PFFB to the amp? or would it be beneficial in any case..

i must admit im slightly concerned about using such a powerful amp with such delicate speakers ( 2x alpair 7's in series) however the offer was great, the amp is highly regarded, and the benefit is i can run that, and the sub amp (same model) from a single psu at same voltage...

i figure if im careful, i can set it up without blowing the speaker cones across the room. - both amps will be driven from a minidsp doing the crossover and eq..
 
Th Zobel filter is a part of the TI pffb implementation to increase stability. Check the example from TI documentation, it is really easy. As Bob Rickards mentions, good idea in any case.
Another crude way to lower effective impedance is to simply add a 16 ohm resistor parallell to the speaker. If you mount it to the amplifier chassis you would get away with about 30W resistor or less.
Note that a
 
ok! post filter feedback it is.

one quick query though, i notice from the measurements, that the PFFB seems to slightly reduce the power level at which the distortion goes through the roof.

not a problem for my fullrangers, but for the subwoofers, ill be likely using as much power from these amps as possible (4 ohm drivers) in this case, is it better to leave the PFFB off?
 
Status
Not open for further replies.