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

Gordon Gibby <ggibby@...>

The capacitor is there because this are you amplifier is designed to be operated with one supply voltage, +V, and ground.

The opamp output transistor circuit can only move between zero and a positive voltage.  

To avoid keeping a physical offset displacenent on the speaker cone, at the cost of significant current through its very low DC resistance! One adds a large capacitor in series to allow the speaker cone to rest at neutral position.

In the datasheet I looked at, they were using a 1000 µF capacitor.   That’s probably give you a better low frequency response, and basically the value isn’t that critical.

On Mar 1, 2018, at 17:20, Arvo KD9HLC via Groups.Io <arvopl@...> wrote:

Someone said if you find a fence ask why it's there before you tear it down.

On the B40 there is a 470µF cap on the output, between the LM386 output and the speaker.  On the LM386's TI datasheet Figure 10 there is a 250µF cap in that spot.  It's not an unreasonable leap to surmise that someone thought, "Hmmm, a 470 would probably work there and I've got a bunch of them."  I can't find anything in the datasheet explaining the need for the capacitor.  It may be that there is some DC offset in the output that needs to be removed.

On the µB there is also a 470µF cap between the output of the TDA2822 and the speaker.  On the TDA2822's STMicroelectronics datasheet application circuits there are examples with both 100µF and 470µF capacitors.  Again, I can't find why those caps are there.

The presence of these caps in the manufacturer's guidelines makes the "too much capacitor in the µB" hypothesis much less probable.  Maximum input voltage rating of 15V in the TDA2822 datasheet imply that 12-14V supplies should not be a problem, either.

A dead short on the output should not be a catastrophic problem for the amps because the caps are blocking DC current, so any power to be dissipated is associated with how loud you've got the volume knob cranked.  So if you turn it up and don't hear anything, turn it back down!

All bets are off if we have parts of unknown provenance, ie, counterfeits.  Does HF Signals have a process in place to assure the supply of components that conform to the expected specifications?  Bad batches of parts shouldn't happen from reputable suppliers.

All that said, there should probably be a pretty plain heads up somewhere in the tribal knowledge that if you wire up the µB speaker jack per the website and plug a TS (tip-sleeve) as opposed to a TRS (tip-ring-sleeve) male connector into it, you will have a dead short across the jack and it won't work properly.

One way to avoid that trouble is to simply wire the output of the amp to the tip connector of the jack and leave the ring connector of the jack floating (unconnected).  If you plug in a mono speaker, or an old fashioned earphone, it will work.  If you plug in stereo headphones you'll only hear audio from one earpiece.

I'm leaving mine wired for stereo.

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