Amp Switching Circuit Popping

[8Len8]

I've get a Phaez amp that has 2 individual inputs, one for each channel. I decided instead of using an AB box that I'd install a switching circuit to route a single input to either channel.

Originally the amp had each channel's preamp tube connected to its own input jack.

I got a Granger optoFET channel select board:
http://grangeramp.com/shop/index.php?main_page=product_info&cPath=37&products_id=493

I powered it off a wall-wart and hooked it up like this (all connections via coax):



My only problem is that I hear a slight pop when switching from the clean to dirty channel. I've read that FET based channel select circuits should have quiet switching. Is there a pull-down resistor or something that I'm missing that could be causing the pops?  Even after I added the RC high pass filters the pop was still there...

[kagliostro]

Give a look to the way Sluckey do the commutation on his latest amp

no optocoupler, a simple DPDT switch

http://home.comcast.net/~seluckey/amps/dual_lite/Dual%20Lite.pdf

K

[8Len8]

My mistake.  Here's the correct schematic of how the circuit is wired:

[kagliostro]

I would like to try a pair of 1M resistors across the switch (and maybe remove the 470k resistors ?)



K

[8Len8]

I'll try that, but don't the tube inputs need a DC path to ground?

[kagliostro]

I wrote

(and maybe remove the 470k resistors ?)

the question mark is because the presence of the in series 0.1 cap confuses me

K

[8Len8]

I added the 0.1 uF caps (the the 470K resistors) as a suggestion from another forum where I posted this problem.  Before then I just had the Granger Opto module directly connected to each preamp tube input pin.

[shooter]

I think if your search the forum for "channel switching" or "channel pop" you will find something about the 1M K is referring to.  FWIW here's a schematic with relays but the I think the 100k's and big caps are for switching type spikes, pops.

[kagliostro]

Try with the 1M resistors and without the cap and the 470k resistors


---


If the previous didn't solve, try also this configuration






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I really would like to know if using a simple DPDT switch the pop is present

K

[8Len8]

Try with the 1M resistors and without the cap and the 470k resistors

If the previous didn't solve, try also this configuration

I really would like to know if using a simple DPDT switch the pop is present

K

I'll try the 1M resistors next week when I can get my hands on some.  I'd need a pretty quiet DPDT switch to try out your recommendation so I wouldn't be confusing the switch noise with the amp circuit noise  :).

[8Len8]

Try with the 1M resistors and without the cap and the 470k resistors


---


If the previous didn't solve, try also this configuration






---

I really would like to know if using a simple DPDT switch the pop is present

K

I tried the top mod (1M resistors) and the pop is still there.  I'm not sure removing the caps would do anything.

[kagliostro]

I don't say it can solve, but try to remove the cap


a charged cap when switched on give a pop


K

[2deaf]

Those dirty channels generally have huge gain making the input very sensitive.  I always move the switching down the line someplace where it isn't near as sensitive.  I used to use H11F3's, which is an infrared LED controlled bilateral FET.  The things could handle a 30Vp signal, so it was possible to use it after the first stage.  I don't know what your device will handle, but I'd bet that it is similar.

By the way, I quit using H11F3's because they added a subtle sound to the amp even when only used to short the signal to ground.  In fact, I have found that any SS device touching your main tube preamp circuit in any way adds a sound to it.