[Elecraft] Re: K3 Caution when using an external monaural speaker

[wayne burdick]

Hi David,

Lyle, KK7P (who designed the KIO3 board set) will create an application 
note and post it as soon as possible.

The short version is: put 1/8-W (or larger), 470-ohm resistors from 
each speaker output to ground. (The actual value isn't critical; 
anything from 330 to 1000 ohms would probably work just as well.) This 
can be done conveniently on the KIO3's audio daughter board, at its 
single-row, right-angle male connector.

Technical details:

The resistors hold the (-) leads of the AF amp's large electrolytic 
coupling capacitors at ground potential (RF board, C2 and C3, 470 uF). 
Thus if you accidentally ground *either* of the speaker outputs, you'll 
be changing only the AC load to the amplifier -- not the DC voltage. 
This means you won't get a big current spike backward, into the AF 
amp's output, through the electrolytic; that's what we think can damage 
the chip, because the caps can dump a lot of energy very quickly.

An astute observer would note that a similar current spike should be 
generated on power-up, if a short at either speaker jack was present at 
that time. But the AF amp chip protects itself during power-up, 
limiting current at its output pins. If it didn't, even the presence of 
a low-Z speaker might cause chip damage.

With the resistors in place, Lyle repeatedly shoved a mono plug into 
the speaker jack, under both small and large-signal AF output 
conditions. He couldn't break the amp chip.

Credit for this "bleeder resistor" idea goes to another one of our 
staff engineers, Bob, N6CM. Lyle and I both wish we'd thought of it.

73,
Wayne
N6KR

On Dec 20, 2008, at 12:59 AM, David Cutter wrote:

> Wayne
>
> I will be doing all the mods for Christmas. Hope you can give us the 
> info really soon so I don't have to take the K3 apart again.  Can you 
> just point to the circuit and tell us what you intend ahead of the 
> official mod instruction?
>
> 73
> David
> G3UNA
>>
> To this end, we're working
>> on a very simple modification to the KIO3 audio daughter board. This 
>> will be the subject of a modification document soon, and is already 
>> being phased into production.
>>
>> 73,
>> Wayne
>> N6KR
>
>

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