Noise while switching pedals, nothing helps!!

[tss]

I am using a relay + opto-mosfet in order to bypass an effect. 1st, the opto-mosfet opens up so the output signal is shorted to ground (that is that amp's cable ground). 2nd, the relay changes position and the opto-mosfet closes.

The problem started when I kept hearing pop noises no matter what I did. I've checked ground connections on the power supply to the pedal and to the amp and everything is solid. Things got more weird when I hooked up my scope to the amp's inputs and saw about 150mV rms @ 50Hz (mains freq.) both on the tip & ring.

I consulted with the amp maker (VHT) and according to the shcematic I have it seems that chassis is grounded to the wall outlet but the circuit ground is connected to the chassis through a 10nF cap.

Today I decided to check if the earth and 0 on the wall are on the same potential (was starting to suspect the wiring at my house) but all seems fine.

Any thoughts on this?

[Mark Hammer]

Does the effect you're switching have a terminating resistor on its input?

[tss]

Does the effect you're switching have a terminating resistor on its input?

Yes, after the last op-amp there's a large cap in series and a 10k resistor to ground. Does it matter much in my setup? When the opto-mosfet is open there's a short to ground, so the resistor is parallel with a short anyway...

[Mark Hammer]

"Last op-amp" suggest a terminating resistance on the output, but not the input.  Just a typo, or are we missing something here?

Many Boss-type effects lack any terminating resistor on the input, because they assume all switching will be done within the circuit (FET switching), and not outside the circuit (true bypass).  That leaves a cap in the input "floating", and amenable to popping.

[tss]

The input has a 10k resistor in series with a 22nF cap and a 470k resistor going to VCC/2 (virtual ground), that's before the 1st gain stage. When switches the input is also shorted to ground. Why is the opto-mosfet not muting the signal without making a noise?

[Mark Hammer]

That 22nf input cap may be connected to a 10k resistor, but it is still "hanging" in the sense that until the effect is connected it has nowhere to drain off.  This is a perennial problem with Boss and similar pedals.  Their design assumes that once you plug in, the input of the pedal will never lack a path to ground for the input cap to drain off.  Once a person uses true bypass, whether stompswitch or relay to engage and disengage the effect, the input cap will gather and store charge, without a path for it to drain off, once the effect is bypassed.  Reconnect the circuit again, and you'll hear a pop or thump.  If the scenario I describe is the same as yours, what you should hear is that a few successive on/off/on/off cycles (without much time or playng in between) will result in the pop disappearing...for a little while.

[tss]

Thanks for your help Mark, perhaps I didn't emphasize the problem... Even with everything connected as is (guitar -> fx -> amp) ,and no relay switching taking place, just engaging the opto-mosfet creates a thump noise. Not a huge noise but I can hear it. When the opto-mosfet is open nothing makes a noise.

[Mark Hammer]

Then ignore my pedantry.  I guess I'm just not understanding your bypass system.

[tss]

Here's a better shot at explaining what goes on (hopefully).

Below is a terrible drawing I did of the important parts of the circuit. The input is buffered and the output is also buffered. There is a resistor to VCC/2 (virtual GND) at the input and a resistor to GND at the output. The output also goes to a TLP222G (http://www.toshiba.com/taec/components2/Datasheet_Sync/200709/DST_TLP222G-TDE_EN_4458.pdf). The input and output of the circuit goes to a DPDT relay which is wired for input grounding - much you would do with any mechanical bypass switch.

Since the relay causes a small click while switching I inserted the TLP222G to act as a output muting device before switching. So I set the MCU pin to 5V which through the 1k resistor feeds the LED enough current to make the LED bright enough to fully open the mosfets and shunt the output of the effect to ground which kills all audio signal. After this, I can switch the relay and not worry about any clicks coupling from it to the audio signal. The problem is that when I open & close the TLP222G I can hear a small thump noise as well. I don't know why is that but it kind of kills the idea of the output muting circuit since the muting makes a noise itself.

Worth to point out that the digital circuit is on a separate ground and they only connect to at a single point which is the output jack's ground pin. I don't know if this is the best choice. The ground that the TLP222G (mosfet) is connect to is analog ground while the led's ground is digital ground.

[samhay]

Have you tried putting a large capacitor parallel to the opto-coupler's LED?
As an aside, do you need the mute? Does the bypass switch cause popping without it, and if so, can this be cured by adding a pull-down resistor to the input?

[tss]

Have you tried putting a large capacitor parallel to the opto-coupler's LED?
As an aside, do you need the mute? Does the bypass switch cause popping without it, and if so, can this be cured by adding a pull-down resistor to the input?

The relay itself causes a pop. I tried this by switching the relay with the opto-coupler being closed and no LED switching. Adding a cap across the LED doesn't do anything (checked that) and PSPICE simulation also showed it does almost nothing.

[samhay]

So are we trying to fix the relay poping or the opto-coupler popping at the moment?

As Mark suggested, a pull-down resistor from the input side of the 22n input cap to ground might help with the relay popping. If that doesn't work, what do you have between the uC and the relay coil?

Try running your SPICE sim with e.g. a 10u cap across the LED. It should make the turn-on rise time a little longer, which can sometimes help with voltage spikes.

[tss]

I am trying to fix the opto-coupler popping. After it mutes the output everything else is fine. I'll run some PSPICE simulation today and post here.

[samhay]

Fair enough. For a slightly more sophisticated approach, take a look at this - http://www.muzique.com/lab/led.htm

[tss]

I deleted my LED models so I used what I could find: Vf is 1.83V @ 25mA. The LED inside the opto-coupler is 1.15V @ 10mA.

Without cap:


With 10uF cap:


So there is a slight difference... with the cap the raise time for the LED voltage is about 5ms and discharge is about 30ms, however since this is an LED I don't know how much this really changes since it only opens up at 1.15V or at 1.83V in the simulation. Perhaps this is time to take a VR and test resistance at different voltages to see how much the capacitor really affects the actual transient of the mosfets?

[tss]

So I was thinking about conducting the 2nd experiment of determining how the input voltage affects the mosfet's transition speed. For this experiment I hooked up the following circuit:


The scope's channel 1 is hooked up after the switch of the 5V source (I actually used the output enable switch on my digital power supply). Channel 2 is hooked up across the 10k resistor. The scope is triggered by a raising edge on channel 1. Yellow curve is channel 1 and blue curve is channel 2.

With no cap:


With 100uF cap across the opto-coupler's LED:


So it seems that the switching speed is the same with or without a cap, the difference is the time delta between the raise of the 5V signal and when the switching takes place (when the LED is fed a high enough voltage to open it up).