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Class D Switching Power Amplifiers and Power D/A conversion

TI TPA3255EVM
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Old 12th February 2018, 10:44 PM   #521
DPH is offline DPH  United States
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Try throwing a mousepad cut in fourths (put it into the corners) under the subs. Looks like you've got something a'rattling. Glad to hear it's not on the electronics side.
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Old 13th February 2018, 07:48 AM   #522
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by barryso View Post
Still curious about other power supplies, though. Anyone have any experience comparing a lower current switching supply to a higher current switcher? Anyone compare a switching supply to a linear supply?
Class D demands for a SMPS, it doesn't align with a linear one *imho*.

As far as I'm informed, the TPA3255's maximum rating for supply voltage is 53.5 Vdc. So, an unregulated linear supply mustn't exceed this voltage, even at idle. As the rectified voltage will sag under load, you'll lose maximum output power. Or you'll generate excess heat in a regulated LPS.

Best regards!
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Old 13th February 2018, 11:03 AM   #523
robinlawrie is offline robinlawrie  United Kingdom
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so it looks like i might need to dive into troubleshooting my 3255 evms.. the btl channel on one (ive got 2 amps in 2.1 mode) is significantly louder than the other, and i do appear to have some distortion, although its quite hard to tell 100% by ear with sub frequencies, on drivers that are not yet in enclosures... how much is the amp, how much is the driver..? certainly seems a dirtier sound than id expect. although, as mentioned elsewhere, pure tones from a generator sound clean to my ears.

problem i have now ( apart from needing to take it all apart again for testing) is that i dont have a scope. any of you brains care to suggest what to check and any methodologies i can use to test/diagnose with just a signal source ( tone generator/music on my phone) a multimeter, and some drivers..?

mods i have done (all discussed here first) on each board are:

moved big 4700uf caps to front of board (replacing stock heatsink and rotating output inductors a bit made space)

extended one output inductor on 1.5cm legs to make room for another element.

removed redundant output caps on btl channels.

removed rca jacks

inputs on 3 pin headers

all power/outputs soldered directly to board.


thats about it..

first things to check?

heeelp!
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Old 13th February 2018, 11:50 AM   #524
barryso is offline barryso  United States
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Quote:
Originally Posted by Kay Pirinha View Post
Class D demands for a SMPS, it doesn't align with a linear one *imho*.

As far as I'm informed, the TPA3255's maximum rating for supply voltage is 53.5 Vdc. So, an unregulated linear supply mustn't exceed this voltage, even at idle. As the rectified voltage will sag under load, you'll lose maximum output power. Or you'll generate excess heat in a regulated LPS.

Best regards!
Thanks.
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Old 13th February 2018, 01:39 PM   #525
mboxler is offline mboxler  United States
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Originally Posted by robinlawrie View Post
first things to check?
If you switch drivers, does the problem move with the drivers?

With your multimeter set to DC, and no signal, measure the voltage across the driver inputs. It should read very close to zero.

Generate a 60hz signal into the amp, measure the AC voltage across the driver inputs. The output voltages should be the same on both amps.

Mike
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Old 13th February 2018, 02:37 PM   #526
robinlawrie is offline robinlawrie  United Kingdom
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Originally Posted by mboxler View Post
If you switch drivers, does the problem move with the drivers?

With your multimeter set to DC, and no signal, measure the voltage across the driver inputs. It should read very close to zero.

Generate a 60hz signal into the amp, measure the AC voltage across the driver inputs. The output voltages should be the same on both amps.

Mike

thanks for the initial pointers.. ill try that tonight.

tried a pair of bookshelf speakers, thats when difference in volume became noticeable.. distortion wasnt so obvious, but then it seemed to manifest at low frequencies, which the bookshelves are not very good at highlighting!

couple of things.. the distortion is particularly noticeable on transients (kick drums sound awful, pure sine waves sound normal) does this suggest an avenue to investigate? psu is connex 800w 48v.. distortion doesnt seem to change depending on number of channels active, so does not suggest psu problems to me..

ive been looking at a (very) cheap scope this afternoon... would something like this be useful? :

JYETech DSO Coral Pocket Battery-Powered Oscilloscope w/ Color Touch Display (DSO112A): Amazon.com: Industrial & Scientific

i need some what to measure/visualise this distortion i can hear, as im going nuts trying to work out how much is the amp, how much is speakers without enclosures. i do know that distortion improves *enormously* when i lift up subs (unblocking pole vent) but i think they are still distorting.

maybe measurement mic and laptop is the way to go, although that wont isolate the cause..
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Old 13th February 2018, 02:42 PM   #527
DPH is offline DPH  United States
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An inexpensive oscope (even the DVM style ones) will go a long ways towards helping your cause.
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Old 13th February 2018, 03:37 PM   #528
robinlawrie is offline robinlawrie  United Kingdom
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here is my amp, so far.. of course i now need to take it all apart again to troubleshoot!

i know.. the messy soldering. the shifting of the big caps to the other side of the boards was done first, and with a gas soldering iron. I then upgraded to a soldering station so the other soldering is a bit nicer!

its a 30cm x 30cm (approx) case with an 800w smps, 2x 3255 evms , a minidsp2x4HD, and an aptx bluetooth receiver. had i known how crammed it would get inside i would have got a bigger case.. i did layouts in 3d first, but didnt consider how much space fixings, cabling and metalwork take up!
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Last edited by robinlawrie; 13th February 2018 at 03:39 PM.
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Old 13th February 2018, 04:01 PM   #529
DPH is offline DPH  United States
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I remember your packaging cogitations, but yeah, that looks a bit compromised to squeeze too much into too little space. I'm honestly concerned about cross-coupling of magnetic fields, but that may be unwarranted. That's a lot of EMI floating around though, so it'll be tough knowing if you're running into a board level or package level problem.

* You could probably yank the PSU out and make a super power brick keeping the local bypass in case if you need the space.
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Old 13th February 2018, 04:26 PM   #530
robinlawrie is offline robinlawrie  United Kingdom
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yeah thats always been my backup plan. once id bought all the bits i decided to treat it as a challenge (like tetris!)

ive kept all psu related stuff inside a shielded area, and all cables with any ac or signal on them are twisted tightly and shielded (the line level stuff actually has two shields :P )

when its on with no input, i dont get significant noise on the speakers.. just a slight mains hum if i literally touch my ear to the cone.. so it works in that sense.

- i even went as far as activating the power switch with a lever/pushrod from the front button. there was no space inside shielded area directly in front of pre-drilled power button location, so i placed it where it would fit inside shield, then added the lever.. works well!


front panel controls for minidsp are hacked in, by adding a stripped down minidsp remote internally, connected to frontpanel controls. ir emitter is placed next to ir reciever on minidsp. would have been simpler if minidsp actually provided a control header on their board!

Last edited by robinlawrie; 13th February 2018 at 04:31 PM.
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