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egagdesigns 12th August 2012 03:11 AM

TDA7386 Problem
Hey all,

I'm currently working on a project that consists of taking a line level input from something like an iPod and play it out some speakers. I'm having a problem with the subwoofer and woofer, whenever there are really low notes they must short or something causing the power supply to current limit. I have concluded that it's the woofer/subwoofer because without them connected it works fine.

As for the circuit, I have the line level input coming into 4 sallen-key active filters. For the subwoofer, there is a lowpass filter with a 3db cutoff point @ 200Hz, tweeter has a high pass filter with 3db @ 5KHz, Mid-Range has a band-pass (high pass + low pass in series), and woofer has a band-pass (high pass + low pass in series) with 3dB cutoffs @ 100Hz - 1KHz.

The filters look very good when viewing through an o-scope and scanning through the frequencies. There is no distortion and the 3dB cutoff points are perfect.

From the filters, the signal travels into a TDA7386 chip which is a 4 channel 45 Watt amplifier which then goes out to the 4 respective speakers (35W rated, 4 ohm). I'm not sure if I don't have enough decoupling caps or if I'm doing something wrong with the amplifier chip.

I am following the exact circuit layout from the datasheet, with one exception which is pulling the mute and st-by to Vcc. When everything is working well, the power supply gets roughly 0.5A (at a more than reasonable volume) pulled. I'm using a 12V power supply (benchtop lab power supply) that has a current limit of 5A which keeps activating when it's operating (only during loud bass notes it seems).

Any help would be very much appreciated.

AndrewT 12th August 2012 12:41 PM

I never recommend a regulated supply for a power amp.
I think you are hitting the very reason that I refuse to use regulated or current limited PSU on any of my power amps.

The problem is that fast signals into speakers demands very high instananeous peak currents from the PSU to the speaker.
A 5A PSU is almost certainly collapsing when the peak demand hits it's limit.

I recommend you build a linear non regulated PSU for your 4 channel amplifier.
Buy a transformer + bridge rectifier + some smoothing capacitors. That's all you need to get your amp working.

abraxalito 12th August 2012 01:18 PM

What AndrewT says. If you add up all your amp wattage it comes to 180W RMS or 360W peak. Your PSU is 60W peak. You might get away with a 60W PSU if you had a very large electrolytic capacitor on its output (say 47,000uF). Safer though to go for a PSU giving 15A.

egagdesigns 12th August 2012 01:38 PM

Alright, appreciate the help gentlemen. I'll give that a try and let you know.

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