popping switches everywhere!

[Gaz]

i currently have an NFB switch and as well as a gain boost switch, both popping loudly when switched. here's the deal so far:

the NFB switch is a SPST with a 100k resistor across it, and another 10k resistor in series with that feeding the LTP PI. when the switch is open i get a feedback resistance of 110K, and 10k with the switch closed, shorting across the 100k resistor. i copied the arrangement from merlin's book (p.194 diagram "c."), and presumed that there would be no popping sound because the circuit is never actually broken... but there still is.

the "gain-boost" is a DPST, one side breaking the ground on a .68uf bypass cap, and the other side switching in a 'boot-strap' on the cathode follower. the bootstrapping was also taken from merlin's book (p. 131), but i thought it might be cool to make it switchable.

(for those of you unfamiliar with boot-stapping the CF: the plate resistor from the previous stage is made into an equal 'split-load' (i.e. 100k becomes 47k/47k), and the signal is taken from between the two. it is connected to the cathode of the Cf through a coupling cap.)

i've tried putting large resistances across the gain boost switch as to not ever 'actually break the circuit', but i'm still getting a loud pop. i understand this is the cap quickly charging. i've read some other threads on this, but can't figure out why the typical large-resistor-across-the-switch-trick is not helping.

thanks,


alex

[Merlinb]

Switch pop is normally caused by :

Charging/discharging caps.
DC on the switch (which goes off and feeds something else when the switch is thrown).
Massive changes in gain through the amp.

The bootstrap switch probably has some major DC on it. I suspect you will need to use two bootstrap caps in series, with the switch in between the two, and large resistors to ground (several Meg) to keep all parts of the switch ground referenced. With such a large increase in gain, some pop may still be inevitable.

The NFB switch may be popping due to DC from the LTP being on the switch, it's a little harder to say without a schem. Looking at fig. 8.19, it may help if you add a 470R(ish) resistor from the junction of R1/Rf to ground.

[Gaz]

thanks merlin! i will try those possible solutions and report back. i'm not trying to make the switching silent, just quiet enough as where no one thinks some thing's wrong with the amp when they flip a switch. thanks again.

-alex

[Gaz]

...and for the LTP, i'm using the typical arrangement with the 'scratchless' presence control, also found in your text - thanks for all the great ideas!

[Gaz]

only had time to try the resistor to ground on the NFB switch, which works flawlessly! thanks, merlin.

[Merlinb]

only had time to try the resistor to ground on the NFB switch, which works flawlessly! thanks, merlin.

Thanks for letting me know, I'll add that bit of advice to the next edition!

[Old Tele man]

Thanks for letting me know, I'll add that bit of advice to the next edition!

...your book already mentions using "pull-up & pull-down" resistors (and IC-logic analogy), although not specifically in the NFB loop.