TA4100A issues

[nertog]

I've been designing a 2-channel TAA4100 based amp in the last couple of months and I just finished the first prototype. It's almost interely based on the application schematic provided in the datasheet. The only changes are the input capacitors (1.5 F foil instead of 0.47F elco) and the input are clamped with 5.2V zeners. The power supply is a 20V 120W SMPS.

Upon first testing I discovered a few problems. First of all: the transition from mute to unmute is not quiet at all. I get a pretty large "thump" followed by +/- 0.5 seconds of white noise (oscillations?).

The little amp sounds very nice at low power, but once I ask for more than a few watts of output power I get some strange sounds: at a certain power output level you can hear a slight "ticking" sound through the speaker..when you raise the volume some more this "ticking" goes over in white noise (again: oscillations?). When the heatsink and chip is warm (let's say 45C) this noise will be more prominent and sometimes won't stop even when you turn down the volume. If you play, for example, a pure 25hz sinus tone, there is a ticking or crackling sound even at lower power output (speakers are ok, I tested this with a few different pairs). At higher frequencies (80-100hz) I can get a low more power out of the amp. The problem seems to be concentrated at lower freqs....

I triple checked all soldering, schematic and board layout...and everything seems to be ok. I have the feeling this has something to do with a grounding issue...or maybe something at the inputs: when I touch the RCA jacks (metal, signal carrying part), the amp stays quiet, as it should. However, when I touch the input pins on the tripath chip (or anything after the input caps) there's a loud hum. Even stranger: if I touch input nr. 3: 50hz hum + white noise. The other inputs: just a 50hz hum.

FYI: Power ground (= - pole of the SMPS input jack) and signal ground are connected at 1 point on the PCB.


Any ideas what this could be? Maybe a faulty component? Or a major design fault?

Please have a look at the schematic, board layout and pics of the board.

Many thanks,
Wim

[V-bro]

Hi, Jan would probably know best, but I fear your inductors are not quite up to the task.

The core material needs to be low perm. What you're using looks like power supply chokes. They are typically made of high perm (low winding count for high inductance) and may suffer from saturation causing the amp to oscillate...

You are dutch aren't you?

Nice looking design!

------X-max. can be several meters on any driver, too bad it can be done only once...------

[nertog]

Hallo!

Well, here's the datasheet of the inductors: http://www.farnell.com/datasheets/89249.pdf

I don't know which core material they used, but the saturation current is 14 amps. Tripath suggests inductors with a saturation current of at least 9 amps so I should be safe. Also, when they saturate they should heat up, right? Well, they don't...

Should I contact Jan by email or does he read the forums?

Groetjes,
Wim

PS: I'm Belgian

[V-bro]

Hmm, when they stay cold I guess there should be no problem...

I do know the core material used in tripath amps is sintered iron instead of ferrite. This should ensure them to work well with the extreme high frequencies.

Jan does read the forum now and than, but he has very little time, so maybe you can mail him.... I hope he can answer...

It is very hard for me to say what's wrong from this distance..

Jan does have a file here on the downloadpage on how to troubleshoot the 41hz amps. It includes a lot of info on what can be wrong when certain sounds appear....Maybe worth a glance...

Succes en groetjes!

------X-max. can be several meters on any driver, too bad it can be done only once...------