Re: B40 LM386 and µB TDA2822 and that 470µF Cap

Tim Gorman


It depends on how the radio is designed. If the radio uses a
well-designed mono jack, one with a switched tip lead, it probably won't
hurt anything. When I say switched, with no plug inserted the output is
connected to a switched terminal on the jack that is connected to an
internal speaker. When you insert the plug the tip is disconnected from
the internal speaker by the action of the plug opening the switch. The
tip lead is then connected to the tip on the plug.

This is for mono type jacks. I have about three of these that I rescued
from defunct 60's style transistor radios. I plan to use one of them
for the speaker connection in my ubitx. With no plug in the jack, the
internal speaker will be activated. When the plug is inserted the sound
will be redirected out the plug.

The worst thing that might happen here is that the internal speaker and
the external unit might be momentarily connected in parallel. I doubt
this will hurt the tda2822 very much.

tim ab0wr

On Fri, 2 Mar 2018 03:09:01 -0500
"David Wilcox via Groups.Io" <> wrote:

I know that we get away with plugging and unplugging things hot on
most of our radios but as for me I will turn off the radio before
making any changes. Any thoughts on that?

Dave K8WPE

On Mar 1, 2018, at 10:49 PM, Tim Gorman <tgorman2@...> wrote:

If you insert a mono plug into a TRS jack the ring terminal of the
jack is almost always shorted to ground (i.e. the sleeve). If you
wire the output of the 2822 to both the tip and ring terminals and
then insert a mono plug there simply isn't any doubt that you are
probably going to short the output of the 2822 to ground. Bad
things will happen.

There is no reason to wire the output of the 2822 to both the tip
and the ring terminals. If you have a stereo headset then wire both
headset leads to tip of the plug and only wire up the tip lead on
the socket. Then if someone inadvertently plugs in a mono plug you
won't short the output of the 2822 to ground.

I agree that the large charging current going into the 470uf cap
could stress the 2822, perhaps only causing a failure after a
number of on/off cycles. That's something that would be hard to

I have modified my schematic to show using a 47uf cap being fed
with a 1 amp fuse from the 2822. If I ever get a chance to actually
finish up my ubitx it will be interesting to see if the 1 amp fuse
is sufficient.

tim ab0wr

On Thu, 01 Mar 2018 15:39:50 -0800
"Jerry Gaffke via Groups.Io" <> wrote:

And subsequent replies have suggested this may not be sufficient.

Some seem to fry when plugging something into the headphone jack
even if there is no short to ground involved. Or when powering up
the rig. Our best guess now is that there are significant currents
involved when that 470uF cap suddenly has to charge up to Vcc/2
And that some clone TDA2822's may be weaker than mainline
On Thu, Mar 1, 2018 at 03:31 pm, Christopher Miller wrote:

One of the first replies suggested not wiring the ring, and using
a mono to stereo adapter cable if stereo headphones are

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