[Elecraft] Loudspeaker click

[Sverre Holm]

There was a brief discussion of switch ON/OFF clicks of the K2 audio
amplifier a short while ago:

> Jack Leijssen PA1W wrote: 
>
> > The only small remark I have is:
> > Switching of the powerswitch gives a sharp noisy click in the 
> > headphone (and speaker). Such a high quality radio may not do this. 
>  
> Sverre Holm, LA3ZA replied:
>
> Seriously, hi-fi equipment often uses a relay to delay the 
> connection of the loudspeakers on turn-on and to disconnect 
> them instantly as power is turned off. This would be the 
> ultimate solution.
>

This got me thinking and being the kind of person I am who loves to
pursue problems until a feasible solution can be found, this is what I
came up with.

The switch on/off click is a recognized problem in the audio industry in
e.g. portable equipment, and there are audio amplifier IC's that
advertise with No switch-on/off clicks. Here are two examples:

1. LM4863: Dual 2.2W Audio Amplifier with circuitry to reduce "clicks
and pops" during device turn-on
http://eng.segyung.com/ads/download/ia/i-stb/datasheet/lm4863.pdf 

2. TDA7056; 3 W mono BTL audio output amplifier, 40 dB gain fixed. No
switch-on/off clicks:
http://www.semiconductors.philips.com/pip/TDA7056_N2.html

Both of them use a topology called BTL = Bridge Tied Load which means
that there are essentially two amplifiers operating out of phase and
with the loudspeaker connected in between them. This also removes the
output coupling capacitor (it is the charging of this C which is to
blame for the clicks). 

The disadvantage is that the loudspeaker is no longer referenced to
ground, so the output connector for the loudspeaker has to be wired
differently. The application note for the LM4863 shows how the headphone
output can remain to be referenced to ground by taking the headphone
output from only one of the sub-amplifiers.

I am not sure if this is practical to think of as a mod for the K2. It
is quite involved to replace the audio chain for what most would say is
little gain. Therefore this may be just a demonstration of an elegant
solution brought about by the consumer market and waiting to be applied
to ham equipment --- e.g. the K3, whenever/whatever that may be.


--
73, 
Sverre

------------------
Sverre Holm, LA3ZA
www.qsl.net/la3za