Same old same old...popping cathode bypass switch.

[Mandopicker]

To anyone who is interested in offering any input. I have read a number of threads and tried the 1m suggestion to keep the unused side cap charged, but to no effect.

Here is what I have:

I've added a cathode bypass switch on a champ clone and can't seem to get rid of the pop when I switch between.

.068 uf / 2.2k
22uf / 1.4k

Switch is a SPDP (on-on)

Can anyone offer a simple solution aside from turning down the volume before switching?


Thanks in advance and my apologies for beating a dead horse.

[booj]

Make the resistor the same value for both switch positions and change one of the cap values for the same tonal effect. Maybe try a 1uf and a 1.4k. Just an idea.

[Steve A.]

To anyone who is interested in offering any input. I have read a number of threads and tried the 1m suggestion to keep the unused side cap charged, but to no effect.

Here is what I have:

I've added a cathode bypass switch on a champ clone and can't seem to get rid of the pop when I switch between.

.068 uf / 2.2k
22uf / 1.4k

Switch is a SPDP (on-on)

Can anyone offer a simple solution aside from turning down the volume before switching?


Thanks in advance and my apologies for beating a dead horse.

I found that this works better if I have the 0.68uF always in the circuit and then switch in the 22uF cap. For the resistors you can do the math to figure out what value to add in parallel to the fixed value to get the value that you want.

Even after doing all of that you may still get some switching noises because going from 2k2 to 1k4 is a bit of a jump and you are changing the DC voltage at the cathode. (I'd pretty much stick to values between maybe 1k2 and 1k8 if I am going to be switching them.)

I remember wiring up a pot as a variable resistor because I wanted to be able to vary the cathode resistor from maybe 1k5 to 10k. There was a circuit threshold that would make a whooshing noise as I crossed it- I think it was around 2k4 or 2k7.

Here's a schematic for a project amp- the top channel shows how I switched values for the Ck cap:

http://www.blueguitar.org/new/schem/...iac/sopm_1.gif

Perhaps someone here recognizes the tone stack in the lower channel?

Steve Ahola

P.S. No horses were injured in the making of this reply!

[Mandopicker]

Thanks Booj, Thanks Steve,

I went ahead with the single 1.4K and will switch between the .068 and the 22uf.

Less pop and basically the same effect as you suggested. I had done this before, but was hoping to alleviate the pop.

For now it works great and will continue to think of other ideas about the pop.

[Enzo]

You didn't show us exactly how it was wired. The idea is to put a 1M or whatever resistor in series with the cap, and then the switch shorts across the resistor to enable the cap.

And I agree with Steve, wire teh .68 in permanent, and switch the 22uf in and out next to it. The extra .68 added to the 22 won't be noticed.

[Mandopicker]

My simple drawing of how I have it set up.

Thanks for the comments Enzo.

Here is a simple drawing of how I have it wired up now.


I have tried it with the .068 uf / 1.4 k on the board and switched in the 22uf with a SPDT, but I find the pop is no different. Please feel free to advise the best way...particularly where I should insert the 100k resistor to avoid the pop.

Thanks again.

[Enzo]

Yes, this circuit will pop every time, because the caps are not charged up when they are switched in. it doesn't matter where you put them, it matters that there is no charging resistor for each cap.

You need a high value resistor in series with each switched cap. When the cap and resistor sit there, the resistor prevents the cap from having much effect. But the cap is fully charged through that resistor. Now short across the resistor with the switch. The cap is not in the circuit. The pop is reduced because there was no charge pulse into the cap.

I use 1 meg for this, but 100k is probably large enough. The simple way to do this is add the resistor across the switch contacts. Wire a resistor from the center terminal of the switch to the end terminal where the cap is. That's it.

And again, why switch both caps? Unless you have a center off switch, one of the caps is always in the circuit. SO why not make one permanent? Wire the .68uf in there for good. Now just switch the 22uf in and out - with my resistor. When the switch is open, you get the .68, and when it is closed, you get 22.68uf. Belive me, you will not hear any difference between plain old 22uf and 22.68uf.

[Enzo]

Go to Schematic Heaven and look up the Seymour DUncan 84-40 amp schematic - S-D is in the Post 70s section - and look how they wired the switched caps at the cathodes of V1b and V2a, V2b. The Enzo resistor at V1b is drawn funny, I don't know why they drew the resistor on th other side of the cathode resistor, but electrically it doesn't matter. All three of these caps have a large resistor in series to maintain charge, and the switch shorts across the resistor.

[Steve Conner]

Hi guys

If you change the resistor, it doesn't matter what else you do, this will always cause a pop, since it changes the cathode current and hence makes the plate voltage jump to a new value. You can't get round this, since the change in cathode current is presumably the very effect that you want.

As for the capacitor, the correct way as Enzo says, is to put the charging resistor across the contacts of the switch that switches the cap in.

[jrfrond]

All correct here. You need a series resistor of 1M. The pop is the initial cap charge-up. You cannot just switch in an open cap and not have it pop. The cap needs to have a potential on it. But even better, as already stated, just leave the "Marshall" cap (.68) in there and switch the "Fender" 22uF across it. There will be NO difference, especially considering the +80/-20% tolerance of most garden-variety electrolytic caps.