Rusty Box/Traynor TS50B - are these caps backwards? + any tips

[MrStab]

still buzzing, less so when facing upright. now i'm not getting any clean, but that's probably some other problem i've picked up along the way. i have 40VAC on the V+ rail, which surely can't be good.

things i've tried:

-100uF & 100nF caps to ground on both V+ & Vbias
-attaching ground to part of metal enclosure (which it isn't in yet) and covering the circuit - very minor improvement
-hooking up volume pot and jack ground to bias for a more "true" virtual ground - much the same as the normal configuration

all opamp voltages check out. getting about 21VDC from just the bare charge pump and 15 with the circuit load. whilst i hadn't fully covered it in a grounded box, i'm doubtful that's the problem.

[psychedelicfish]

You could have a ground loop. These act as aerials and pick up hum and RF interference which are introduced through the ground of your circuit and end up mixed with your signal, resulting in lots and lots of hum. If you only get it when it's plugged in to an amp, it might be to do with the way you're powering the circuit, if you're using an AC wall adapter then it could be the case that it has a mains earth connection on it, which would be your ground loop (all mains powered amps will have a mains earth connection).

Another thing it could be is that the ground wire from your input jack has fallen off, or some kind of earth connection has come loose. It could also be a wiring mistake somewhere.

As far as your AC voltage on your power supply, are you sure you measured that correctly? You easily could have mistook 40mV for 40V, which would make sense. If you do have 40V, however, which I highly doubt seeing as you haven't reported any burning smells, exploding capacitors, etc, if you can get access to an oscilloscope you can have a look at the wave and see what frequency it is. If it turns out to be 10kHz or so, then your problem lies with the charge pump. If however, your 40VAC is at 50Hz or so, then your problem lies elsewhere.

[MrStab]

You could have a ground loop. These act as aerials and pick up hum and RF interference which are introduced through the ground of your circuit and end up mixed with your signal, resulting in lots and lots of hum. If you only get it when it's plugged in to an amp, it might be to do with the way you're powering the circuit, if you're using an AC wall adapter then it could be the case that it has a mains earth connection on it, which would be your ground loop (all mains powered amps will have a mains earth connection).

Another thing it could be is that the ground wire from your input jack has fallen off, or some kind of earth connection has come loose. It could also be a wiring mistake somewhere.

As far as your AC voltage on your power supply, are you sure you measured that correctly? You easily could have mistook 40mV for 40V, which would make sense. If you do have 40V, however, which I highly doubt seeing as you haven't reported any burning smells, exploding capacitors, etc, if you can get access to an oscilloscope you can have a look at the wave and see what frequency it is. If it turns out to be 10kHz or so, then your problem lies with the charge pump. If however, your 40VAC is at 50Hz or so, then your problem lies elsewhere.

my best guess from my limited experience is that it's not ground-related. at least, not offboard. despite the earth pin being standard here in the UK, i'm using a small 2-pin kettle plug with only live & neutral connected. it's a supply i ripped out of some other AV application and stuck some rudimentary filtering on it - i've used it live through amp/cab & PA a few times with about 5 effects hooked up to it with no issues of note. the ground to the input jack had come loose very early on, but i spotted it right away. i guess you could see my other pedals & rig generally as a control batch. my practise amp can be a bit noisy, but i know this is out of the ordinary. in regular-ground-retrofit mode and true virtual ground mode, vbias and true ground behave identically.

i checked with the 200VAC setting on my multimeter. as it's exactly double DC voltage, could that mean that I'm just getting pure AC through and the DC readings are just showing half the ripple/wave/whatever it's called? in that case - why aren't my opamp and transistors dead?! that's a weird one. in a not-so-serious effort, i tried a basic bridge rectifier made from 1n4001s  but it just gave voltage drop - given the uncertain AC or DC nature of what's going on, i wasn't entirely sure if i should connect the negative from the charge pump board like you would a bridge rectifier with guaranteed AC input. so maybe that prevented that idea from working. noise is present with a battery, but admittedly i hadn't thought to check AC reading with a battery hooked up.

yknow, i did read a very vague post somewhere saying that voltage double/triplers are AC output only - i didn't know that was implied, and i haven't found anything else on it, so i'm just not sure what's going on.

there's only straight series connection from each section/component to ground. last i tried, the charge pump ground went straight to DC input negative in parallel with the rest of the circuit, to keep out 7660 noise as per (my interpretation of) RG's notes on the matter. speaking of which i've just bridged the PNP activating power from the input ring, as it works fine, but i figure i'll just keep it out of the equation for now.

[MrStab]

boxing it up has helped a lot, although since somehow screwing up my charge pump, i've been testing on 9V/4.5Vbias. still a bit of hum, but much closer to acceptable limits, i'd say. almost no buzz at all with gain on full without boost activated, buzzy with full gain + boost on, but seems to just be the characteristic buzz of distortion pedals pushed to the max.

nasty pop when using the boost switch - maybe a FET/PNP would help with that. or can the typical pulldown resistor be used in this situation (i usually associate those with switches passing signal, not DC)?

[psychedelicfish]

That's strange, it shouldn't really pop because the only capacitor that gets disconnected by the switch (C3 in my schematic) has a reasonably low value resistive path from one end to the other. Try a 1M resistor across the switch.

[MrStab]

everything almost works now, but since trying with the charge pump back in action, there's a weird old-radio/R2-D2-like whine on the clean channel. it's only present if the Gain pot is turned up a little bit, and you can make cool Artoo sounds by turning it. in clean mode, the gain pot weakens the sound the more it's turned up, but works as expected in Boost mode. weird. so something's up when there's no vbias going to the boost section of the board (the 7660S has pin 1 connected to V+ to theoretically prevent audible whining, plus the problem disappears in Boost mode, so i doubt it's that). so tempted to use NPNs or a 4053 i have lying around to just kill the gain pot in clean mode lol. i'll have a look at the original switch on the schematic.

pop seems to have disappeared as well. fingers crossed it doesnt come back, but i'll bear the resistor idea in mind if it does.

[MrStab]

i take it back - it probably is the charge pump. upon swapping out the IC for a known defective one, the sound changed. and it seems there is a whine present for just a small part halfway up the gain pot.  i can predict trying all sorts of decoupling/isolation/filtering solutions that don't change anything. doesn't seem to be affected by reducing proximity to other components.

at this point, i'm feeling it would be so much easier to try and get cheap 18V regulated supply, seeing how i'm only getting 15V under load on the charge pump anyway. laptop chargers are usually up there. hmm...

[psychedelicfish]

at this point, i'm feeling it would be so much easier to try and get cheap 18V regulated supply, seeing how i'm only getting 15V under load on the charge pump anyway. laptop chargers are usually up there. hmm...

Or you could change some components around and adjust the amount of gain to give you similar levels of distortion running the circuit on 9V. You said you were making this for a friend, so having it run off a battery or a standard pedal power supply would be far more convenient for them. We're changing the supply voltage from ±15V, or 30V to 9V (let's say 10V to make things easier), so we should reduce the gain of the op amp stage by a factor of 3. This can be done by replacing the 1.5M resistor labelled on my schematic as R7 with a 500k resistor, and changing the gain pot for a 250k one.

[MrStab]

replacing the 1.5M resistor labelled on my schematic as R7 with a 500k resistor, and changing the gain pot for a 250k one.

that sounds like a plan. i guess that 250k would still need to be a log pot? all i have in the way of 250k is a linear, although i might have a 500k linear somewhere i could taper down (although that usually doesn't work out well for me). a bit more distortion then the original wouldn't be such a bad thing, as it's meant to be like the Rusty Box but not necessarily identical.

i just remembered something from a couple pages back:

My only worry is that opamps scale up and down with voltage quite easily.  Transistor do not.  Transistors in the differential amp configuration (and also the buffer) may also not like going from a bipolar supply to a single supply.  If you aren't going to use the 30 volts and/or a bipolar supply then you might have to adjust the biasing of the differential amps to work off of a lower voltage single supply.

it doesn't seem to sound that different besides more distortion on 9V, but think there might be any ground to Bill's concern?

thanks again!

[psychedelicfish]

replacing the 1.5M resistor labelled on my schematic as R7 with a 500k resistor, and changing the gain pot for a 250k one.

i guess that 250k would still need to be a log pot?

Linear should be fine, unless your friend likes to rapidly turn the knobs while playing 
Even then it wouldn't be too hard to swap it out later.

i just remembered something from a couple pages back:

My only worry is that opamps scale up and down with voltage quite easily.  Transistor do not.  Transistors in the differential amp configuration (and also the buffer) may also not like going from a bipolar supply to a single supply.  If you aren't going to use the 30 volts and/or a bipolar supply then you might have to adjust the biasing of the differential amps to work off of a lower voltage single supply.

it doesn't seem to sound that different besides more distortion on 9V, but think there might be any ground to Bill's concern?

Other than socketing the transistors, I think you'll be alright. If you have a few PNP transistors perhaps you could try a few out and find a combination that sounds good.

Since you're building for a friend you might want to let him/her play through the circuit before you box it up to make sure it sounds good.

[MrStab]

just as i tested the circuit after changing the 1.5M resistor, the whining is back, without the charge pump, just fainter. happens with a battery, too.
it only happens after the halfway point on the gain pot, so maybe i need to tone that back.


screw this, i'm going to bed.

[psychedelicfish]

At this point it seems like it's your layout and/or wiring which is causing the circuit to oscillate. Basically, you have the input of one gain stage (probably the op amp) too close to its output. I've just been looking at your layout, and at this point I'm pretty sure that's your problem. You have a big long jumper from the -input of the op amp to R4, R5, R6, etc. which passes right through the middle of the transistor "ladder", which is a later gain stage. The capacitance between the transistor "ladder" and your jumper makes a positive feedback loop which causes the circuit to oscillate. To fix this, I suggest you replace the jumper in question with a shielded wire, you may be able to get away with using a small length of guitar cable for this. Connect the shield to the trace going to pin 4 (DO NOT connect the other end to anything, or you'll make a ground loop) and use the centre wire as your jumper.

[MrStab]

that does make sense, and i'll either get that jumper shielded or moved when i wake up later. but there's something i want to bring into the equation first - there seems to be some connection with the treble pot, as though a 10k fixed resistor creating a minimum resistance at the end of the turn would prevent the noise altogether. then i remembered i'd used a 10k pot for Mid instead of the prescribed 15k. a small difference, but not an entirely random theory - you don't think that could be causing anything?

right, i really am going to bed now. lol

[psychedelicfish]

Seeing as the tone controls are all capacitors and resistors in the "oscillator" changing their values would also change the oscillation. You might find that turning the tone controls changes the oscillation frequency slightly.

[MrStab]

in that case, one quick question: in terms of functionality, should i add a fixed resistance to the lower or upper end of the Mid pot (to increase its theoretical size)? if a 4.7k on one side of that doesn't work (or 2.2k on either side), i'll try the treble pot. i wonder why the mid pot being wrong make treble seem like the culprit, but not itself.

turning the pots do change the sound's frequency - i've had full conversations out of the droid.

[MrStab]

well... a piece of shielded guitar lead in place of the opamp inv. input - C4 jumper, with one end tied to ground and the other open, has made it usable at 9V (or so it seems). charge pump still makes crazy whine. sucks, as the circuit seems to lose some fidelity at 9V. but then, it could be psychosomatic and i could be expecting the feel of a guitar pedal.

[MrStab]

clearly i'm the worst driller on the planet, but FWIW:


problems went away after using a shielded wire, so thanks, Edward, and ofc Mr. Faraday! (the whine comes back with the charge pump, but it sounds fine on 9V). i restored the 1M log pot for more gain but retained the 470k resistor i replaced R7 with. there's a bit of interference right at the end of the gain pot turn on Boost mode, but i explained this trade-off to my friend and i'll see what he says.

it sounds more guitar-ish than bass-ish to me, but it sounds crunchy in a nice, organic way. i may get a clip at some point, but i'm a bit busy lately so dunno if i'll be bothered to set up my USB interface and a camera might not do it justice.

[psychedelicfish]

I'll knock together a PCB layout soon. Perhaps one of us should start a dedicated project thread for this?

[psychedelicfish]

i restored the 1M log pot for more gain but retained the 470k resistor i replaced R7 with. there's a bit of interference right at the end of the gain pot turn on Boost mode, but i explained this trade-off to my friend and i'll see what he says.

If you want to get rid of that interference, I would try replacing C2 with a 220pF (or the closest value you have on hand), which would give you a high frequency roll-off of ~30kHz as opposed to ~150kHz (a normal guitar amp will roll of the high end somewhere around 5kHz, so no treble will be lost).

[duck_arse]

mrstab, seeing your picture, and being blinded by your leds and you saying charge-pump whine has me thinking. not necessarily a good thing. are you running your leds from before or after the charge pump, or have we covered this already? what value series resistor are you using for your leds?

or did you say that whine was constant? I have read this thread as it happened, but can't remember where we are up to ......