Re: uBITX - U1 Getting Fried - possible cause #ubitx

Tim Gorman
 

I'm not sure even a 100uf cap is needed. For communications purposes
you should be able to get by with a 47uf capacitor. A 47uf cap in
series with an 8ohm speaker should give you a low freq 3db point of
about 500hz. I realize this is a kind of a paper napkin calculation
because a speaker doesn't exhibit true resistance but it should still
be close.

You can get an 8ohm, 4" CB replacement CB speaker for about $5 on
Amazon.

I personally like the idea of a fuse in the speaker lead. That's
probably what I will do. If it is a current spike of several amps that
is blowing the 2822 then a fuse should protect it. And you don't have
to worry about wasting half your audio power in a resistor.

tim ab0wr

On Thu, 01 Mar 2018 12:16:35 -0800
"Jerry Gaffke via Groups.Io" <jgaffke=yahoo.com@groups.io> wrote:

I think you're right.

The output pin has a quiescent DC voltage of around half the supply
voltage, or about 6v, so the 470uF cap will have a 6v charge on it
when operating.  A significant bit of energy for the TDA2822 to
provide, and it tries to do so almost instantaneously in the case of
a short. A series resistor seems prudent. 

Perhaps the 470uF cap could be reduced to 100uF.
At 500hz the 100uF would have an impedance of 1/(2*pi*500*100e-6) =
3.18 ohms, which seems borderline for good sound quality when using
an 8 ohm speaker.

I vote for the resistor.  Something like 4 or 8 ohms, 1/2 Watt.
But worth experimenting if somebody has a stash of cheap TDA2822's on
hand.

Configuring the TDA2822 for Dex's  bridge configuration does away
with the cap charging problem, but now the headphone jack has to be
floating.  The bridge would be ideal for an internal speaker.

Jerry

On Thu, Mar 1, 2018 at 11:47 am, Jim Sheldon wrote:


My take is that huge 470 uF electrolytic in the output of the
TDA2822M and the speaker gets a huge surge at turn on and during
any loud noise or POP in the audio chain.

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