[Elecraft] Loudspeaker click

[Sverre Holm]

> 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. 
> This is the first thing I will improve.

I have noticed this as well, but never bothered enough to try to do
something about it. Compared to the rev A to B mods that us with the
pre-#3000 K2's have been doing, in a way this says that there aren't any
real important mods left to do with the latest K2's!

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.

Lacking that, I would start with the audio output coupling capacitor C29
on the control board and the audio decoupling capacitor C28. Both are
220 uF. Decreasing C29 might reduce the click, but with a loss in bass
response. Similarly decreasing C28 could reduce the turn-off click, but
at a risk of noise or amplifier instability (motor-boating?).

C29's impedance should be equal to the load at the desired -6 dB lower
cut-off frequency. The load is the loudspeaker or headphone impedance.
Assuming that 200 Hz is OK for the lower cut-off, then C28 now has a
reactance of 3.6 ohms. If the load is 4 ohms, it should not be changed.
In fact, this is the internal speaker impedance of the K2 and the output
spec of the K2 as quoted from the manual:

	"Audio output 1 watt max. into 4-ohm load
	Speaker internal: 4 ohm, 3 W;
	Headphones 4 - 32 ohms, stereo or mono"

If the load is always 8 ohms or greater, one can reduce C28 to 100 uF
and with a 32 ohms or greater load, it can be reduced to something like
22 or 33 uF. However, such low values would sound rather tinny on the
internal 4-ohm speaker, and could hardly justify the potential reduction
in click.

Well, just some thoughts, untried at the moment.


--
73, 
Sverre

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Sverre Holm, LA3ZA
www.qsl.net/la3za