Hey all. I've built several MOSFET boost pedals, but the ones that I try to add a toggle that goes between two different input caps always pop. I've tried several different ways of wiring up the toggle, but can't figure it out. Someone suggested making it so one cap is always in the circuit and then adding another cap in parallel. I tried figuring this out w/ a DPDT toggle, but after 2 hours, got nothing.
So far, the closest thing has been with a SPDT. I ran a wire from the footswitch to the center lug. Each outer lug had one leg of each cap. The other legs were joined and I ran a wire from there to the perfboard. When I did that, I'd get one initial pop when I first turned on the pedal, but that would be it. There's gotta be a way, right?
If I did the parallel thing, how do I figure out which caps to use so I have the stock value of .001uf and one of .047uf?
Thanks for anyone taking the time! And FWIW, when wiring up the footswitch, I kept getting pops when I used the diagram on Dano's (beavis) site, but the diagram from JD's site (GGG) took care of this - that and a 1m resistor at the output.
Quote from: dap9 on May 20, 2009, 08:08:08 PM
If I did the parallel thing, how do I figure out which caps to use so I have the stock value of .001uf and one of .047uf?
well if .001uf is stock, and you add 0.047uF in parallel, you have 0.048uF which isn't far off enough from the intended .047uF to be noticeable. That's what I've seen, at least. If you want to double the Cap value, add an identical one in parallel, if you want to raise it by 10x, add a .01uF to a 0.001uF.
I've done this in a mod on the OCD circuit, but it was a cap mixer instead of switcher, so I didn't experience any pops. Hope this helped.
You are using pull-down resistors? If not...
It's very common practice to put a fairly large resistor ahead of the input cap. It presents a very large input impedance to the preceding pedal/guitar (a good thing for max signal transfer), it doesn't bleed more than a few nA of signal away to ground and it stops the "free" end of the cap adopting the bias voltage of the input stage of the pedal. This is what happens when you don't have an input resistor and then engage the pedal:
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/one_cap_no_pulldown_R.gif)
You can see that I used the exact same component numbers as on the schem at AMZ. I added R7 to simulate the high input impedance of the next pedal/amp. See the sudden jump up to about 2V as I engaged the stomp-switch and a wait of about 7ms before it settled down and gave me a signal at the output (the red trace... blue trace is input signal). That's the POP you get with no pull-down resistor.
OK, here's what happens when you have 2 input caps and try to switch between them. The green and pink traces are the voltages at the left hand ends of the input caps:
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/two_caps_no_pulldown_R.gif)
See how the output trace (red) jumps about by about 1.5V every time I flipped the switch? More POPS, then.
Lastly, I put a 1M resistor to ground ahead of each cap.
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/two_caps_with_pulldown_Rs.gif)
See how much better-behaved the pedal is when I flip the switch? I changed it about 5 times and the pull-down resistors kept the two caps' free ends within about 10mV of where they should have been at all times, unlike in the previous picture, where they floated around about 1V away from datum (0V).
Hope this helps. :)
that's great info.
do you think you can put the switch after a single pull down and get the same effect/solution?
Like this?
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/two_caps_with_one_pulldown_R.gif)
You can see how the right hand ends of the caps try to adopt the bias voltage and then hold it. More popping.
And a single pull-down ahead of the switch would achieve nothing except to ground the switch - the left hand ends of the caps would still float, so popping again.
i see. i was thinking the pull down resistor, then switch, then input caps, but looking again, i think we need the resister to pull the cap to ground keeping it from floating.
i think i've read that the popping can be avoided if the connection isn't broken. Instead of toggling between, use an off-on switch to add a 2nd cap in parallel to a fixed/permanent cap. I believe this is the same as a cap mixer (see input bass trims or "range" controls) but with a switch instead of a pot. Not that its any better than the extra pull down, but it might use less parts.
Like this...
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/two_caps_with_one_pulldown_R_ahead.gif)
Not quite as bad as before, but still pops quite badly, especially as you first engage.
What about a make-before-break switch? That should allow any bias voltage floating around in there to equalize before the contact is "let go", maybe?
Personally, I prefer the pulldown resistor idea ;o)
Ahh, I see. Like this, then...
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/two_caps_with_one_pulldown_R_ahead%20v2.gif)
Fine apart from the slight clipping from 285ms to 300ms or so. Much better behaved, though. It's hard-clipping, so it would "blat" for about 100th of a second (more, I guess if the gain was set higher than 1.5 as I have simulated it here).
Having said that, how much is a 1M resistor to give you no popping and no other weird behaviour, like clipping as the bias shifts slightly? 2 cents?
well that sets the ballot straight!
the individual pulldowns seems to be the way to go, i'd never thought of that before. For me though, if its worth having a toggle, its worth having a pot there.
carry on!
...and what would happend if SW1 shortens C1 instead of leaving it with one side disconected?
There'd be no DC-blocking with the switch closed (which is the function of the input cap, after all, isn't it?). So, if there was any residual DC from the previous pedal, then it would pop for Britain.
Edit - Turns out, when I ran the sim, that the gate of the MOSFET was hooked to ground by the shorted switch through the pull-down resistor, so that messed up the bias to the extent that no signal was present at the output at all!
DC-blocking works both ways, then. Strips off residual DC from the previous pedal/stage and preserves bias looking at things from the other side of the cap.
Holy canoli! Thanks RO!! Some of that explanation is still a little over my head, but I think after several reads and re-reads, it'll make more sense. Excellent description and visuals!
Azlitz, The pot idea intrigues me, what value would you recommend? I'm assuming a Linear pot would be preferable, right?
EDIT: And that was the ticket!!! Just made those tweaks and sure enough, that pop is gone. Thanks so much!
It's worth noting that *anything* which prevents the capacitors from "relaxing" into a different DC voltage works.
You can achieve the same result with resistors in series with each capacitor to the signal source, and using the cap selection switch to short out the series resistor for the selected cap. In this case, the resistor holds the outboard end of the cap at the signal source DC level.
Thou shalt not switch DC levels on thy signal line, lest there be pops.
Quote from: dap9 on May 21, 2009, 09:41:07 PM
Azlitz, The pot idea intrigues me, what value would you recommend? I'm assuming a Linear pot would be preferable, right?
here's a schematic
similar to a range master,
http://www.generalguitargadgets.com/diagrams/joam_sc_fb.gif
same idea as the "Bass" Control. I've used 50k before with good, smooth results. If its a linear pot, you can flip the orientation to get it to sweep backwards or not. Use whatever cap values you like, but the larger cap has to go with the pot.
Jeez Louise!
Just stick two caps in series and use a toggle to shunt one or the other. Each cap will have every opportunity to drain and pops will be gone.
Quote from: Mark Hammer on May 22, 2009, 09:31:39 AM
Jeez Louise!
Just stick two caps in series and use a toggle to shunt one or the other. Each cap will have every opportunity to drain and pops will be gone.
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/two_caps_with_pulldown_R_toggle_shunt.gif)
Errr... clearly not, then.
When C6 is shunted and you switch over to shunt C1 instead, the voltage at the junction of C1 and C6 goes instantly from Vbias+signal (5V +/-) to 0V +/-.
BANG!. Vbias drops to 0V and takes a while to re-establish - until it does, the MOSFET is mis-biased. So, that's a 3V
BANG at the output and a 45ms hiatus while Vbias is re-established (as C6 charges) and the circuit functions properly again.
When C1 is shunted and you switch over to shunt C6 instead, the junction of C1 and C6 is at 0V, which is then connected suddenly (and drastically) to Vbias (the potential of the "open" terminal of the switch). Not surprisingly, as C1 accepts this voltage suddenly applied to its right-hand end, Vbias drops to 0V once again and the pedal
BANGs once again and it takes a wee while to re-establish Vbias (only about 7ms, as the cap is much smaller and doesn't take as long to charge up).
To re-iterate what R.G. said:
Thou shalt not switch DC levels on thy signal line, lest there be pops.
i think he meant one or the other, but not both. i've been wrong before though.
what happens if C1 is permanent and C6 is either shunted or not?
Capacitors distribute DC initially in the inverse of their capacitance as the initial charge is absorbed, then they settle to the ratio of their DC leakages over the long time.
Mark is correct if the connection of the two caps is grounded through a big resistor. This removes entirely the possibility of the two caps distributing DC among them, unless the resistor is of the order of 1/10 of the leakage resistance of the cap with the lowest resistance.
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.
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/two_caps_with_pulldown_R_centre_toggle_shunt.gif)
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:
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/two_caps_with_pulldown_R_centre_toggle_shunt_C1_only.gif)
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).
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
(http://img30.imageshack.us/img30/8619/caps.gif)
Sorry, Vitrolin, my remark wasn't aimed at you. Here's what I found with your circuit:
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/vitrolin%20v1.gif)
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.
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/vitrolin%20v2.gif)
Quote from: R O Tiree on May 22, 2009, 12:42:57 PM
aziltz came up with the right idea, though (and Vitrolin was also going along the right lines, earlier) with shunting just C1:
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/two_caps_with_pulldown_R_centre_toggle_shunt_C1_only.gif)
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?
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?
Quote from: R O Tiree on May 22, 2009, 06:15:49 PM
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!
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?
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)
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 ?
Quote from: fuzzo on May 23, 2009, 10:34:49 AM
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.
Quote from: fuzzo on May 23, 2009, 10:34:49 AM
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?
Actually, I'm an idiot, I did a mistake, I didn't connect the postitive leg of additional cap to the Jet source :icon_lol: so, yes, that doesn't work :icon_rolleyes:
QuoteI'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 :
(http://www.zimagez.com/miniature/mosfetboosteramz.bmp) (http://www.zimagez.com/zimage/mosfetboosteramz.php)
Yes, I put a 220K trim to correctly set the bias to the gate. (Also that allows to play easier with the bias)
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.
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.
Quote from: Gus on May 23, 2009, 11:29:10 AM
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!
Quote from: aziltz on May 23, 2009, 11:13:27 AM
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 :D
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)
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
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.
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.
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).
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.
Right... I've spent all day messing around with this and here goes:
1. Adding all those extra components without paying attention to the input impedance, the relative impedance of the input cap and its pull-down resistor at the frequencies of interest, etc, changed the character and behaviour of the MOSFET Booster beyond recognition. What I ended up doing was to turn it into an insane Treble Booster. Oops. I went back to stock and had a re-read of Jack's blog entry and the AMZ page and found that it was designed to have a flat-line amplitude response across the entire guitar frequency band. So, having put the sim schem back to stock, that's exactly what I had.
2. I then copied fuzzo's schem of the Catalinbread into the sim. Biased it at 4.5V and observed the same behaviour (flat-line amplitude response) right up to the point where I cranked it right up to full gain. Hmmm... a different shape to the distorted waveform. I'd given them both a 100k Level pot, so they were both driving the same load.
Here's the AMZ/Orman MOSFET Booster @ 82Hz, full gain:
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/Orman%20stock.gif)
You can see the clipping is happening on the bottom of the waveform and the output cap/level pot alter the shape just a little.
OK, here's the CatalinBread:
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/Cat%20stock.gif)
Clipping is occuring at both top and bottom. This will, obviously, sound different.
OK, curiosity got me, and I decided to try to make the Orman's waveform look more like the Cat...
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/Orman%20to%20Cat.gif)
OK, that worked, by changing the bias to 4.5V and altering the output cap to 220nF. Still wasn't quite right, so I removed the little 47pF cap and voila!
Curiosity bit me again, and I thought it might be cool to turn the Cat into an Orman...
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/Cat%20to%20Orman.gif)
Biased the Cat to 5.3V or so, gave it a 100nF output cap and added the 47pF. Voila^2 !!
So, the sound would appear to be down to the bias voltage chosen (somewhere between 4.5V and 5.5V), the size of the output cap and a piddly little 47pF between gate and Gnd.
Joe Gagan addressed the issue of attempting to allow more bass into a circuit via the input cap by simply using a parallel cap in series with a variable resistance. While not exactly the same as if there was no parallel cap, using a standard-value pot, like 500k or 1M, one can fade in the extra bass via the larger cap.
So while I've got a couple of guys on the line who understand the finer points, in what ways is a variably impeded, as opposed to switched, cap better or worse than the assorted switching schemes being tried out here?
Given the input impedance of 10M (Reference: AMZ MOSFET Booster page, third paragraph (http://www.muzique.com/schem/mosfet.htm)), changing the input cap from 1nF to 100µF makes a whole 2% difference in the signal passed on to the gate! (Calculated using |Ztotal| = SQRT ( Zr^2 + Zc^2 ) and observed in the simulator.)
As Jack says on that page, "NOTE: The frequency response of the AMZ Mosfet Booster is flat and extends down low enough for both bass and guitar use. There is no need to mod the design to add more bass; it will not make any audible change."
There's no point, in this particular circuit. As to variably impeded as opposed to switched in other circuits, I'll have a bash with a BJT circuit tomorrow (Si Fuzz Face?) and see. That would probably have a lower input impedance?
Not that I want to heap any more on your plate than you've already got, but one circuit where changing the input cap makes a very interesting difference in the qualitative properties of the resulting tone is the Jordan Bosstone. The stock unit has a 100ok attenuator pot on the input followed by a .022uf cap. Change that cap to a higher value, like .33uf and you start getting weird sub-octaves and unusual ghost tones.
i don't know if the Catalinbread uses a trimer to set the bias. I did in mine 'cause I didn't have 62K R I though it will be easier to have the bais with a trimmer.
Anyway , Why not changing the 100uF cap to lower value to have a so-called "treble booster" like Mark said in this thread (http://www.diystompboxes.com/smfforum/index.php?topic=58563.0) instead of using a toogle input cap switch ?
I think I'll try that, like this way I could use the Switch on my small stone box. I'll put a 1uF or 2u2 with the 100uF actived via the switch.
Quote from: Mark Hammer on May 24, 2009, 09:00:42 PM
Not that I want to heap any more on your plate than you've already got, but one circuit where changing the input cap makes a very interesting difference in the qualitative properties of the resulting tone is the Jordan Bosstone. The stock unit has a 100ok attenuator pot on the input followed by a .022uf cap. Change that cap to a higher value, like .33uf and you start getting weird sub-octaves and unusual ghost tones.
Well, that was a fun day...
1. First point, it makes no difference whether you use a large (470k) pot to "bleed" another cap in or switch it in the Jordan Bosstone - the pot needs to be 470k otherwise you get increased signal getting through. Phase angle difference either side of the input cap network is not affected by this pot, bizarrely. What
does affect it is the input impedance of the transistor. I did some scratchings on the back of a cigarette packet and, with about 38nA or so going through Q1's emitter, that means the Shockley resistance times hfe (I guessed at 500) in parallel with the 150k from base to ground of Q1 gives an impedance of around 100k. This gives a phase-lag of around 60 degrees or so on the low E string, improving at higher frequencies. (If it was going into an opamp, then the phase-lag is about 2 degrees max at low freqs and almost zero the higher you go).
So, to answer your question, whether you switch or "bleed" with a pot, the results would appear to be the same, with no phase-angle changes due to the "bleed" pot. I found that (in the sim) switching/bleeding to a 330nF cap at the input just passed all frequencies of interest and made a fairly boring square-ish wave with about an 80% duty cycle with single frequencies. Much more interesting seemed to be dropping it to 10nF instead. However, when I hooked up a couple more signal sources and "played" it an open E5, I got all sorts of interesting outputs at all sorts of values from 10nF up to 330nF. As you said, octave-down and other seemingly unrelated tones as well, not to mention the harmonics generated when the diodes clip... Not for the fanit-hearted, then.
Now, OT, perhaps, but this pedal is full of weirdness. Whether it's "musical" weirdness or not is in the ears of the listener... I thought I'd share.
2. When I first sparked it up, the output waveforms were quite bewildering... huge oscillations and much weirdness in the output waveform compared with the voltage at Q2's emitter:
(http://homepages.tesco.net/~michael.jdcastle/jordan/weirdness.gif)
Every time the trace at Q2's emitter (blue trace) "kinks", dV/dt in C4 is changed, affecting the current flowing. Because this cap can charge/discharge very rapidly, even the slightest kink causes radical changes in the output waveform (red trace). Once the diodes start clipping, then things get very spiky indeed. Changing C4 to a 47nF smoothed some of the rough edges off, though.
3. I think the kinks in the waveform at Q2's emitter are due to having it connected to Q1's base, and I think C2 has enough charge to re-bias everything when both transistors switch hard off but I'm going to think about it for a day or so. It seems to be a sort of elastic-band push-me-relax-you thing going on.
Quote from: R O Tiree on May 22, 2009, 06:15:49 PM
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?
I should take some ritalin... I totally skipped over the first part about how caps in parallel are calculated... I just assumed they're like resistors. I've since educated myself and get it. But what I can't figure out (if it's even doable) - how do I go about sticking to the stock setting of .001uf and then having the other cap be a .047uf? My initial question got answered earlier on in the thread, but I'd like to try this other idea of having two caps parallel just to improve my knowledge. The sad thing is, IIRC, this is some basic math where you have two of the values you need, and just need to figure out the third value. Sadly, I've forgotten how to do it. Thanks to anyone who takes a shot.
The point is, with this particular circuit, that Jack Orman, the designer, figured out the correct values (on a scruffy piece of paper - check out his blog (http://www.muzique.com/news/amz-mosfet-booster/)) that would give an absolutely flat-line response as you go up in frequency. This means that, from 41Hz (bass guitar's low E) up to as high as you can get on an electric guitar, the signal gain is constant within a couple of percent. It also happens that it doesn't matter what value you put in front of the circuit, you won't get any more bass gain at all. As I said in an earlier post, you could stick a 100µF cap in there and there'd be absolutely no difference at all.
OK, with that out of the way, go back to my very first reply and you'll see a simple way to go from 1 -> 47nF. If you really want to go from 1nF to 48nF (1 and 47 in parallel) then you'll need to just switch in and out the 47nF, but make sure you put a LARGE pull-down resistor to ground in front of the 47nF, otherwise it's pop-city again.
Quote from: R O Tiree on May 28, 2009, 04:47:23 PM
The point is, with this particular circuit, that Jack Orman, the designer, figured out the correct values (on a scruffy piece of paper - check out his blog (http://www.muzique.com/news/amz-mosfet-booster/)) that would give an absolutely flat-line response as you go up in frequency. This means that, from 41Hz (bass guitar's low E) up to as high as you can get on an electric guitar, the signal gain is constant within a couple of percent. It also happens that it doesn't matter what value you put in front of the circuit, you won't get any more bass gain at all. As I said in an earlier post, you could stick a 100µF cap in there and there'd be absolutely no difference at all.
OK, with that out of the way, go back to my very first reply and you'll see a simple way to go from 1 -> 47nF. If you really want to go from 1nF to 48nF (1 and 47 in parallel) then you'll need to just switch in and out the 47nF, but make sure you put a LARGE pull-down resistor to ground in front of the 47nF, otherwise it's pop-city again.
Ok, thanks, man, I'll go back and re-read everything. This has been (and continues to be) a great learning experience. One thing I'm still trying to wrap my head around is the below diagram. I haven't had a chance to breadboard it, but the way I see it, the 1nf cap is never out of the circuit. In the position of the picture, doesn't the signal still go through the cap? Or does it "go around" it? Thanks again for all the info you've provided!
(http://homepages.tesco.net/~michael.jdcastle/mosfet_boost/two_caps_with_pulldown_R_centre_toggle_shunt_C1_only.gif)
Yes - when the switch is "closed", the signal just takes the easiest path and bypasses the 1nF cap completely.
Bear in mind, when you bread-board it, what I said earlier in the post at the top of this page, about relative impedances and how I'd messed the circuit up and turned it into an insane treble booster...
The pull-down resistor (R8) doesn't need to be huge (100k?) as long as it's ahead of the cap. Do bear in mind, though, that with another circuit where changing the input cap does make a difference, you must pay attention to the relative impedances of the pull-down resistor and the cap at the frequencies of interest and this diagram has a drastic effect on the amount of signal passed. A better way of doing this would be to gave a pull-down cap at the front and then use a 470k pot to bleed the other cap in parallel.
The source bypass cap in the diagram above is too small. As I said above, it takes ages for the sim to "fill up" all the caps and reach a steady state, so I made that cap smaller. Mind you, that doesn't seem to make much difference either - you need 100µF to get an absolutely flat-line response, but 47µF will get you about 95% at very low freqs and 10µF will get you about 85%. Any more than 100µF just doesn't do anything extra for you.
The reason it turned into a treble booster was that the 1M resistor I had behind the 1nF cap in this switching arrangement just wasn't anywhere large enough and that messed up the impedance that low frequencies "saw" and they were allowed to bleed to ground.