Input cap toggle on Jack's MOSFET Boost... How can I do this an not have a pop?

[R O Tiree]

Well, if he'd said that, instead of being dismissive...

However, he's still not correct when he says "toggle to shunt one or the other", because it still doesn't work in this circuit.



When C1 is shunted, it's OK - C6 has Vbias on its right-hand end and signal going in at the left-hand end and everything is rosy. However, as soon as you switch over to shunt C6, the gate of the MOSFET has a direct short to Gnd, Vbias can never recover, so no signal is passed.

aziltz came up with the right idea, though (and Vitrolin was also going along the right lines, earlier) with shunting just C1:



No pops (I switched it in and out 3 times, as you can see if you look closely), no wandering of the output signal at all, input cap value goes from 10nF to 0.91nF as you switch - you can see that the input signal (the pink trace centred around 5.5V) is attenuated slightly when C1 is not shunted, as is the output (the red trace).

[Vitrolin]

since i apperently have expressed myself poorly i will now post this image visualising what i ment earlier:

sorry but im no genius in visual arts, but you should be able to understand it

[R O Tiree]

Sorry, Vitrolin, my remark wasn't aimed at you. Here's what I found with your circuit:



Not much of a jump, here, when you move the switch to let C1 "join the party", and none discernable at all when you flip the switch the other way, to remove it. However, we're only going from 10nF -> 11nF, because we're switching the small cap in and out.

When I swapped the values around, to get 1nF -> 11nF... A big bang when you switch the large cap into the circuit and just a bit of a "wobble" when you switch it out.

[dap9]

aziltz came up with the right idea, though (and Vitrolin was also going along the right lines, earlier) with shunting just C1:



No pops (I switched it in and out 3 times, as you can see if you look closely), no wandering of the output signal at all, input cap value goes from 10nF to 0.91nF as you switch - you can see that the input signal (the pink trace centred around 5.5V) is attenuated slightly when C1 is not shunted, as is the output (the red trace).

I'm confused here...  This pic doesn't swap caps - it adds one in series to the other, no?  And the above posts put the caps in parallel w/ each other, right? 

[R O Tiree]

Remember that the total capacitance of caps in series is calculated (mathematically) in exactly the same way as resistors in parallel and vice-versa. So the total capacitance of C1 and C6 when SW1 is open is:

C = 1(1/C1 +1/C6) = 0.909090909...nF ~ 0.91nF

When SW1 is closed:

C = C6 = 10nF

There are various configurations in the schems I've posted, from parallel to series to swapping - so, there are several ways of doing it, some better than others. This one makes sure that the junction of C1/C6 is always biased at 0V, so popping just cannot occur. Whatever happens at the right-hand end of C6 is irrelevant. DC-blocking by C6 and the 1M resistor to Gnd at the junction of C1/C6 ensures this.

In previous schems that I've posted, we've been swapping caps or putting them in parallel, with variously successful results... and a series one which was disastrous! Seems to me that this one is the most effective, based on what I've found out from the simulation program.

It's been a voyage of discovery, hasn't it?

[dap9]

Remember that the total capacitance of caps in series is calculated (mathematically) in exactly the same way as resistors in parallel and vice-versa. So the total capacitance of C1 and C6 when SW1 is open is:

C = 1(1/C1 +1/C6) = 0.909090909...nF ~ 0.91nF

When SW1 is closed:

C = C6 = 10nF

There are various configurations in the schems I've posted, from parallel to series to swapping - so, there are several ways of doing it, some better than others. This one makes sure that the junction of C1/C6 is always biased at 0V, so popping just cannot occur. Whatever happens at the right-hand end of C6 is irrelevant. DC-blocking by C6 and the 1M resistor to Gnd at the junction of C1/C6 ensures this.

In previous schems that I've posted, we've been swapping caps or putting them in parallel, with variously successful results... and a series one which was disastrous! Seems to me that this one is the most effective, based on what I've found out from the simulation program.

It's been a voyage of discovery, hasn't it?

Aahh, I get it.  The original circuit calls for a .001uf at the input.  A .047 adds some nice "meet".  So in this situation all that's happening is my "meaty" cap becomes a .048uf.  Nice!

[aziltz]

unlike some other "simulation" threads i've been involved with, this one has remained quite calm, without the harsh comments i've run into before and I wanted to thank you all for that.  It can be quite frustrating when members are rude or discourteous, especially when people just throw out ideas, trying to help (which is great).

great work with the simulations R O Tiree.  What program are you using?  is it one of the Spice Derivatives?

[R O Tiree]

Thanks, aziltz - a picture paints a thousand words and a "moving" picture in colour makes things a lot clearer for me. The program is called "Circuit Wizard Pro". It's a SPICE-based program and it's really designed for schools, I guess. It has a fairly limited subset of "real" components and there's no way to "roll yer own" as you can in most other SPICE programs, because the component list is locked down tight. What it does have is a superb GUI and the ability to change values on the fly, as well as move pots, switches, etc, so you can see the effects instantly and plot them.

dap9 - Sorry, I just realised I missed a division sign out of that equation for the parallel caps:

C = 1 / ( 1/C1 + 1/C6)

[fuzzo]

I tried input cap selector on my last Jack Orman mosfet boost but I had impression to lose some gain, the sound became tinier. No more treble, just less bass.

So today, i did again a Mosfet boost for my small stone box and instead of puting a input toogle switch I wanted to that switch with the decoupling cap. So, I put it with a 220uF in parallele with the 100uf. But when the switch is engaged nothing change the sound must be fatter with more bass with two caps in parrallele (=330uF), right ?

[aziltz]

No more treble, just less bass.

well its a passive filter, so it can only "remove" frequencies, but that's still how the classic treble boosters work:  Bass Cut + Gain.

So today, i did again a Mosfet boost for my small stone box and instead of puting a input toogle switch I wanted to that switch with the decoupling cap. So, I put it with a 220uF in parallele with the 100uf. But when the switch is engaged nothing change the sound must be fatter with more bass with two caps in parrallele (=330uF), right ?

I'm a little confused by what you are explaining here.  a picture might help. which caps are you refering to?

[fuzzo]

Actually, I'm an idiot, I did a mistake, I didn't connect the postitive leg of additional cap to the Jet source so, yes, that doesn't work

I'm a little confused by what you are explaining here.  a picture might help. which caps are you refering to?

I'm talking about the 100µF cap connected to jfet source. My idea consists to increasing that cap to have a fatter sound with more bass.

Schematic I've done :



Yes, I put a 220K trim to correctly set the bias to the gate. (Also that allows to play easier with the bias)

[aziltz]

ok, I understand now.

I'm not 100% on the effect of that cap on the frequency response.  Possibly something to simulate.  You might have to go up by a factor of 10 to get the effect.

[Gus]

Look at Vitrolin post and read R.G.s again.  Get rid of the shorting wire at the switch wire a 1meg in series with the bigger cap value side of the switch.  Now the smaller cap is always in circuit and the added cap adds to the smaller value.

Small cap always wired in and added cap always wired in: However the added cap has say a 1meg resistor (to start) in series and the one cap end and the other resistor end are what are connected.  The switch shorts the series 1meg resistor out to switch the added cap in.  Also look at the Joe G. pot and added cap input control.  Also look at the Schaller wha with the added inductor schematic.

Pro audio EQs with switched caps sometimes use resistors between the switch contacts.

[aziltz]

The switch shorts the series 1meg resistor out to switch the added cap in.  Also look at the Joe G. pot and added cap input control. 

kind of like having a 1M pot that is either at Full or Zero.  neat idea!

[fuzzo]

ok, I understand now.

I'm not 100% on the effect of that cap on the frequency response.  Possibly something to simulate.  You might have to go up by a factor of 10 to get the effect.

Yeah I could simulate that, one I tried with SwitcherCad III   and I gave up , so complicated 

Also there's a switching pop in mine even with the 1M Resistor at input.

Anyway, I've to order missing part and I'll see that but there're any reasons to don't work.

In waiting I'm using Gus NPN boost (really great that one)

[Gus]

R.G. posted what I posted before me I added a little more, that is why I posted reread R.G.s posts.  1 meg is a starting value, things depend on the circuit.  If you are using a sim program try different values

[Mark Hammer]

Of course you folks realize that, much like a tree in the forest with nobody to hear it, if the pedal is in bypass mode, there is NEVER any audible pop from other switches.

I'm not being dismissive here.  Just pointing out that there are areas where people do tend to switch caps while the effect is engaged, whether with a stompswitch or toggle, and those are the things that one needs to learn more about in order to avoid audio nastiness. But there are a whole lot of things that probably make a pop which nobody will ever hear if you do the switching while the pedal is in bypass.  And those are the sorts of things that it isn't worth driving yourself nuts about.

In the particular example illustrated in this thread, one wonders what the likelihood is that the user would either footswitch the extra cap in, or toggle it while playing.  If the user sets the input cap before engaging the pedal, then the popping is simply academic.  Certainly worth understanding (and Mike's pictures have been very interesting and informative), but not necessarily anything to want to wrestle to the ground in this instance.

[Gus]

Mark I disagree.
  It is something to understand and do correctly.  This is good design practice and NEVER underestimate what a buyer of a product will do with it.  Someone will have the guitar volume at max and the amp up loud and switch that switch on purpose or accident, you might have a blown speaker and a bad web rep that can hurt a business.

This and other thing like small toggle switches with no protection from being stepped on and broken and other things on effects sold make me chuckle.

[R O Tiree]

I hear you, Mark... in a gig situation, one is unlikely to want to switch that cap in and out. However, what about rehearsal? Or recording studio?

"Any chance of fattening up the bass on your guitar a bit?"

"Sure..." clickBANG...

Oops. I've heard it said that perforating your sound engineer's eardrums makes your nose bleed...

Edit - Gus just said that as well while I was typing...

Even geniuses have cognitive failures. And idiots have them all the time. Probably best to try to make these mods genius-proof, then, so "wrestling this problem to the ground" (very apt) is probably worthwhile?

Thanks for your kind words about the pics, BTW, and I apologise for the snippy remark. They take ages to do because, like all SPICE programs, it takes a while for all the caps to "fill up" while the circuit reaches its operating point - that's why the source cap in my diagram is only 10µF, not 100µF as called for on the schem as the 100µF was taking about 10 minutes to fill up and every time something changed, here came another couple of hundred µC into the cap and it took another couple of minutes to settle again...

...which leads me to another point. fuzzo - I've done a bit more simulation with that source cap and, at 100µF  and running an 82Hz signal into it (low E string), that's all the fattening you're ever going to get. Adding a 220µF in parallel with it does nothing. If you use a 10µF, then bass is attenuated, but this would then be a very tinny and fizzy pedal, I think. Remember that the way this circuit works is to attenuate the bass freqs going in (small input cap) and then boost everything. The simplest way to get more bass is, therefore, to allow a bit more bass through in the first place (slightly larger input cap). Clearly, if you let all the bass freqs through (large input cap ~ 100nF or so?), then this just becomes an utterly insanely-powerful distortion pedal (output voltage of up to 5V peak-to-peak).

[fuzzo]

Ok thanks.

It's weird. I did the stock Orman's schematic but I didn't really like the way it sounded , so I changed some values (catalinbeard values) : 470uF instead of 100uF with the fet source and 220n output cap. I really heard a difference, so if you say the 100uF is enough to allow all bass (82Hz),why there's a hearable difference ?

the output cap that changed to 220n ? I think it makes a filter with the drain resistor.