STA540 - works, but clips with bass heavy sources - diyAudio
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Old 27th April 2011, 11:52 AM   #1
diyAudio Member
 
Join Date: Jan 2011
Location: Newcastle/Durham
Default STA540 - works, but clips with bass heavy sources

Hi,

I've recently finished building a couple of kits which I'm using in a self-contained Ipod-dock style cabinet.

Firstly I've used a TDA1524A based kit as a basic preamp tone/gain control.

I've also used the ST Micro STA540 kit from Sparkfun for the amplifier stage.

These power a pair of Infinity Kappa 4" car-audio speakers, rated at 45w rms.

The setup works (which I surprised myself with!), and when fed with ouput from an Ipod dock connector both the tone controls and amplifier volume work well - I set the preamp to around 25-30% gain. It will go *very* loud

However, whenever the audio source contains significant bass notes the amp immediately starts to clip and the peak LED flashes on and off. The amp has to be power cycled or the gain on the preamp turned right down, at which point the clipping subsides after a few seconds.

I'm running the amp from a 18.5v / 3.5A laptop style PSU - the tone controls are powered from a 7812 regulator from the same supply but I don't think it has any impact as that kit is only rated to pull 150ma or so. In addition, if I use the preamp to bias the input to the amp to one channel or the other, the volume and gain can go slightly higher before clipping occurs. Changing the input voltage from 18.5 to 12/15/16v reduces the level at which the clipping occurs and changes the clipping frequency to be slower (down from ~5hz to 3-4). Upping the voltage to 20v increases the level slightly and increases the clipping frequency (to around 6-7Hz).

I am *not* massively experience with electronics, but I can follow instructions and the kits went together as directed and *do* work well at low to medium volumes. I'm just trying to figure out whether it is lack of power that is causing the clipping or something else.

Any ideas would be gratefully received!

Cheers,
John
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Old 27th April 2011, 06:43 PM   #2
diyAudio Member
 
Join Date: Jan 2011
Location: Newcastle/Durham
I've done a little more investigating this evening and have found that if I drop the PSU output right down to the lowest setting of 12v then the amp is much more tolerant of bass-heavy audio - I still have to be quite careful with the preamp gain (especially the bass tone control), but I can now get quite a reasonable level of volume (ie rather loud!) without hardly any clipping.

My PSU must, therefore, be right on the borderline of providing enough current at its lower output voltages (the specs state 12v @ 4A, dropping to 3.25 by 18.5v, but who knows what the real deal is), and clearly nowhere near good enough at my intended supply voltage of 18v (required for decent volume from the seperate mono amplifier powering a down-firing bass driver in the bottom of the cabinet).

I'm going to presume that the kit and other supporting components are assembled correctly and see if I can find a supply that can output a genuine 5A or so at 12-18v - as it's an external PSU, it's no big deal to change it over when the project is completed.
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Old 27th April 2011, 08:14 PM   #3
DF96 is offline DF96  England
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Join Date: May 2007
If you pull the supply below about 15V then this will feed through the 7812 regulator to the tone control stage - the 7812 needs a few volts spare in order to work. My guess is that you are overloading your power supply.

Do the maths. 18V at 4ohms (I assume 4ohm speakers?) is 4.5A peak. Double for stereo - 9A. Convert to average - about 5A? I doubt if your PSU can provide 5A at 18V. It will dip, and this is your problem. Try 8ohm speakers, or get a bigger PSU.
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Old 27th April 2011, 08:41 PM   #4
diyAudio Member
 
Join Date: Feb 2011
" laptop style PSU"
that is Your problem i guess, those are not ment for loads that change swiftly.
It is unable to provide the current Your amp needs, DF96 is absolute correct.

You should go to an electronics supply store, there are nice transformators sold for 12 V halogen lights. They are absolute perfect for the task. You will need a diode bridge (shoot for one that is rated to at least 30 A) and grab a ~ 10,000 @ 25 V rated cap, along with a smaller (100 nf will do fine, film type) to smooth out the psu.

The transforamtor should be able to handle more than 100 VA, a common size to see -here- is 100 and 150 VA ones.
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Old 27th April 2011, 08:55 PM   #5
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Join Date: Jan 2011
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Thanks folks - I was leaning towards thinking that it was the PSU.

I'm not sure if I trust myself to go down the route of anything more elaborate in terms of power supply - I did track down an external power brick that can deliver 6A at 20v though - that might be an option.
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Old 2nd January 2012, 10:12 PM   #6
diyAudio Member
 
Join Date: Jan 2011
Location: Newcastle/Durham
Sorry to necro-post, but if anyone else is searching for info on the STA540 IC then I solved the clipping/power cycling issue with a much larger PSU - this IC does appear to be very picky about what power supply you feed it with (similar issues reported on the Sparkfun forums).

I'm now using a 12v, 12.5A Mini-ITX style power brick (kinda the same size as the one for the Xbox 360). The amp is much happier with this and will go as loud as my ears can handle!
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