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Switch popping

PostPosted: 08 Mar 2014, 09:57
by crw414tele
I Know plenty has been posted over the net regarding this but I'm at a loss. I've just finished building JohnK's nobleman layout [ awesome pedal thanks JohnK ] from tagboard effects but I get an annoying pop when switching it on and off. I've tried the 1m pulldown on input and output , no change. I've tried the grounded input switch wiring , nothing . I've also disconnected the Led , still pops. So , at the moment I have grounded input and Jack Ormans delayed Led start [ 2 resistors and capacitor ] wired and it still pops. So , a few more attempts , I replaced the switch just to eliminate that , no luck. Measured the V at the circuit output at the switch and found 4.2v . So I thought it must be a leaky output capacitor so , I changed it 2x and it still pops !!!!! Do I need to swap out all caps until it stops or is there something else ???
This is the schematic i used and I swapped out 2x the 3u3 output cap.I also read it could come from a reversed capacitor but I've checked polarity and all are ok.
Please help!!!!!! going nuts :scratch

Re: Switch popping

PostPosted: 08 Mar 2014, 16:45
by Intripped
detailed pics of your build and link to layout please

Re: Switch popping

PostPosted: 08 Mar 2014, 18:23
by Manfred
The value of 150kOhms at R27 is pretty high in comparison with other screamer circuits,
10kOhms mostly. Connect a 10kOhm resistor in parallel the 150kOhm resistor for a trial.

The 3.3 uF Elektrolyte capacitor has a DC-idle current by rule of thumb formula of about 8 uAmps.
This current causes a voltage drop of 1.2 VDC at the 150 kOhm resistor.
On switching on the effect the input resistor e.q. with a value of 1 MOhm is connected in parallel,
thus the resulting DC output resistor is lowered to a value of 130kOhms, the voltage drop is 0.91 VDC now.
The reloading takes time and the voltage difference of about 0.3 Volts appears as a dynamic signal at the amplifier input
and could causes the "pop".

Mayby is the idle current is in the real world lower than the calculated value.
Let's assume the real value is ten times smaller than the signal would be 30 Millivolts
and still audible using a sensitive amplifier input.
I recommend to substitute the 3.3uF elektrolyte capacitor for an e.q. WIMA MKS2 3.3uF/16V foil capacitor.

Re: Switch popping

PostPosted: 08 Mar 2014, 19:40
by Intripped
crw414tele wrote:Measured the V at the circuit output at the switch and found 4.2v


what about this?

Re: Switch popping

PostPosted: 09 Mar 2014, 00:26
by electrosonic
The 3.3 uF Elektrolyte capacitor has a DC-idle current by rule of thumb formula of about 8 uAmps.


I realize caps aren't perfect but I have never seen that rule of thumb - are you saying you would expect 8uA of leakage? Reference link?

I would also look at the input, this circuit doesn't need an input cap if the feeding circuit is well behaved. I would include one anyways in case the preceding pedal is leaking DC (bad output cap from preceding pedal?)

Andrew.

Re: Switch popping

PostPosted: 09 Mar 2014, 00:31
by Intripped
check also that R27 is connected to ground and not to Vref

Re: Switch popping

PostPosted: 09 Mar 2014, 02:06
by IvIark
Yes he had R27 to vref on an earlier version of the layout. Check his latest one.

Re: Switch popping

PostPosted: 09 Mar 2014, 02:20
by crw414tele
Thanks guys , I spotted the 150k to vref after posting here and have snipped it off the board an put it on the output to ground on the switch ' V down to 4ma so the pop is barely there. I might try some lower value resisters today

Re: Switch popping

PostPosted: 09 Mar 2014, 12:25
by Manfred
electrosonic wrote:
The 3.3 uF Elektrolyte capacitor has a DC-idle current by rule of thumb formula of about 8 uAmps.


I realize caps aren't perfect but I have never seen that rule of thumb - are you saying you would expect 8uA of leakage? Reference link?

I would also look at the input, this circuit doesn't need an input cap if the feeding circuit is well behaved. I would include one anyways in case the preceding pedal is leaking DC (bad output cap from preceding pedal?)

Andrew.


Hi Andrew.

my rule of thumb formula is:
I_leakage = (C x U)/2 where is given I_ leakage in Microamps, C in Microfarads and U in Volts.

You can find a more exact formula , have a look at page 5, at this page:
http://www.amelec.ch/tl_files/amelec/downloads/capxon/capxon_general_screw.pdf

You can find a leakage current chart for electrolyte and tantal caps at this page:
http://www.tpub.com/celec/54.htm

Manfred