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The first thing to try is to do the stereo setup routine, as described in step D-3 of the troubleshooting guide. If that doesn’t fix it then make sure the magnet is placed in the correct orientation, making sure the sticky tape is placed on the side with the black mark. You can also try to take out the sensor board (with the pot block) and use a different magnet to change the volume…move it closer and further away from the sensor as you listen to the volume. Remember the polarity of the magnet matters. That test will at least show you if your magnet is backwards and if the sensor is working properly. Other than those 2 things, I’m not sure what to tell you. I’ve never had this issue before. Hopefully the stereo setup will work.
Good luck!
-BrachPhilip,
Thanks for the questions…
I don’t recommend using a jack/socket for the piezo connections because of its very high impedance. Jack connections will get oxidized over time and that will limit its conductivity. When dealing with extremely high impedance devices, the tiniest bit of resistance on their connections will severely degrade the signal. Remember, high impedance devices have lots of voltage but hardly any current to push through impedances in the signal path. That’s why it’s always best to use hard wire connections in dealing with piezo sensors.There are some complications involved with using Plastidip to insulate the disc. The main thing you need to be mindful of is that the dip will flow away from the edges of the disc causing this area to be susceptible to electrically connecting with the copper tape. This also causes the disc to be slightly domed shaped or beveled. These days I suggest using electrical tape or Kapton tape. But to answer your question, you can use plastidip to coat the disc when it is finished. If you coat it well enough, it will be waterproof and can be used as a hydrophone.
I hope this helps. Good luck!
-BrachIs the Quaverato the only thing in the signal path or are you still using your loop switcher? I remember when we talked last, that switcher seemed to have some weird things going on with it, so you may want to test the pedal without that in the signal path, if you haven’t yet.
I highly doubt that that the Quaverato’s voltage is the cause for the distortion. It currently uses 9 volts, biased to half supply (4.5V), so the signal would have to get pushed beyond those rails (+/-4.5Vpp) to distort. Guitar signals are usually much less than 1Vpp, so it’s very doubtful that an unamplified guitar signal would run out of headroom. Most likely, (if the distortion is happening internal to the Quaverato) the problem is related to an issue with the opamp (TL074) circuit. Check the ins and outs of those opamps to see if they are biased correctly around 4.5V. The schematic in the back of the assembly manual is useful for tracking down signal and voltage problems.
If you really feel compelled to use higher voltage in the Quaverato, 12V will probably be ok to use. The only thing to watch out for is the 5V regulator. If, after a while it starts getting hot, go back to 9V….I can’t remember off the top of my head what the max voltage differential (or wattage) that regulator can stand.
Good luck.
-BrachThanks for telling me about that. That stuff seems like it would make a good grill for some other amps…future projects.
I’m glad you enjoyed the amp building experience! Hopefully you’ll eventually get into designing your own amps…that’s where the real fun is!
Take care and good luck!
-BrachRomeo,
I’m glad to hear you are enjoying your Percolator. It looks really good.
What type of screen are you using for your grill? Did you paint the screen, or did it come that way? It’s really nice looking.
Thanks for sharing your work!
-BrachScott,
Thanks for getting back to me. The resistors that make up the voltage divider are R8A (1k) and R9A (100k). They are located under the output jack, so you’ll have to remove that to access them. Look in the troubleshooting guide to see how to properly remove the jack.
If the pedal that you put after the volume pedal to reduce the volume level was set to unity gain, you might be able to measure the resistance of its volume knob to get an idea of what the voltage divider resistance values should be. Otherwise, you can make note of the signal level change by using an oscilloscope.
With tube amps, the preamp tubes don’t usually offer much tonal change between loud and soft volume levels…this is because they are able to handle a lot of head room giving a relatively clean tone. The real tone (increased odd harmonic distortion) comes from pushing the power tubes hard…but the volume level from the speakers usually gets really loud before they really start sounding good. This is why “power brakes” are used…to attenuate the already distorted tone from the loud power tubes. In your application, you were just attenuating the hot preamp signal, which has too much voltage headroom to break up nicely, while leaving the signal getting to the power tubes relatively low. Since from a tonal perspective, it doesn’t matter if the preamp is turned up or not, I suggest leaving it relatively quiet, so the signal hitting the volume pedal is lower. This way, it won’t bleed through as much when in heal down position.
I hope that makes sense.
-BrachScott,
That is correct about the gain knob, it is not a volume knob, so it doesn’t reduce volume. The circuit itself, does have a slight drop in amplitude when the gain knob is at fully the counterclockwise position, but it’s only a few dBs. The opamp circuits in the pedal are non-inverting, so they cannot lower gain, the lowest they can go is unity. But I’m not sure that lowering the volume/gain level of the pedals is going to solve your problem. It sounds like you are asking for a wider dynamic range of the pedal, which lowering the gain setting will not give you.
You can test if this by adding another pedal (with a volume control) to the effects loop after the VPM-1, and with that pedal set to unity gain, adjust its volume control lower. This will at least give you an idea of weather lowering the VPM-1’s output gain will solve your problem. If that helps the problem you are experiencing, then you could modify the voltage divider circuit on the VPM-1’s output. I can tell you what resistor to change when and if that time comes.
I think what will work better for you is if you do what i was suggesting in the previous post about setting the treadle calibration differently. If you set it to where the volume on the VPM-1 is at 25% in the heal down position, then set the preamp on the amp for a nice sound at a quiet level. Then you’ll have the entire range of treadle movement to turn up the amp. You are already using the VPM-1 in this way, where you get a sound that you like in the heal down position, but if you set up the calibration this way you won’t hear the noise artifacts at the low end of the treadle position anymore.
-BrachBob,
I’m so sorry that I missed this post until now. I’m out of town right now on vacation, so I’m a little distracted.
Anyway, thanks for asking about a pickup for your bass. The Coratado MkII kit sounds like it might be a good fit for you. You might consider modifying it by replacing C1 and C2 with bigger caps…try 12 or 15nF, to allow better low frequency response…it will pick up low harmonics better. I wouldn’t suggest getting much larger than 22nF or so or it will start sounding too big and muddy in the lower frequencies.
Also, you might want to try to add another piezo sensor in parallel to the first one (just make sure they use both the same wiring configuration or you’ll get phase issues). That way you can add one to somewhere around each side of the bridge to get a better frequency pickup range.
In my experience, this should work pretty well for your bass, as long as there aren’t a lot of things rattling around inside of it. Good luck!
-BrachFanyo,
I’m sorry about this annoying journey. In finding out that this same issue happens with another pedal, I’m thinking that there is something fishy going on with your PXL. Normally the footswitch popping occurs when there is a DC offset on one side of a bypass capacitor. When the pedal is engaged the bypass cap allows some of the DC voltage change to momentarily flow through the cap, causing the pop. Pull down resistors can be used to help keep the charge from accumulating on one side of the cap.
If you just need a Boss style buffer, they are really easy to build. You can just follow the schematic from one of their service manuals (look at the TU-2 service manual, for a very simple schematic). You just need one FET and one BJT and a few other supporting passive components. You may consider trying to build one yourself, on some perf board. It would be very small and you could even install it inside the Quaverato chassis, so you could use the Quaverato’s 9V rail to power the circuit with. Perhaps a better solution would be to find out what kind of circuit the PXL is using and modify that. You could even install the buffer circuit in the PXL chassis so this problem doesn’t occur, no matter what tremolo pedal you use.
…I’m just throwing out ideas here. I hope your Quaverato ends up serving you well, despite this annoying issue.
Take care.
-BrachScott,
I’m not sure I totally follow you, but if you want full range of motion with the treadle there is a trick you can do…
You can calibrate the pedal by pressing the first 2 or 3 points at the bottom of the treadle position (heal down) and then equally space out the other points between the bottom and the top of the treadle motion. Each point is 25% of the full range…the 5 points are 0%, 25%, 50%, 75%, 100%. So, if you put the first 2 points at the bottom of the range of motion and then space the other 3 points out equally between the heal down position and the toe down position, then the heal down position will be 25% of the volume, going up to 100% when in the toe down position. Hopefully, 25% of full volume (the heal down position) will set your amp to a level just before it breaks up. I hope that makes sense…and is helpful for doing what you want to do.
-BrachFanyo,
Well, I guess that is one solution. I wish we could have figured this out. This is still very puzzling to me. It’s also puzzling that the taper of the volume control reacts to factors outside of the pedal. I still would like to see the schematic to the PXL. I think that would tell us something about what is going on here.
Anyway, good luck using the Quaverato in this way. I hope it’s not too annoying for you.
Take care.
-BrachMatthew,
That’s correct, you shouldn’t notice any of this weirdness when using the VPM-1 as an expression pedal because it uses different circuitry for that function.
I did have some time the other day to play around with this distortion issue. I placed 10uF capacitor across the LEDs of both optocouplers and it seemed to fix the problem. That was just the only value of caps I had laying around, so it may not need such a high value, but it did seem to work. Theoretically, the bigger the cap, the longer it will take for the sound to decay when you turn the volume off in the heal down position (it works somewhat like a sample and hold circuit). But practically, even 10uF didn’t cause any noticeable delay at reasonable volume levels.
If you do this mod, you just need to be mindful that the square pads are at 5V while the round pads are close to ground potential…so if you use polarized electrolytic caps, make sure the positive lead is on the square pad. You probably could use 0805 sized surface mount MLCC (multilayer ceramic caps) that would fit directly between the leads…and they are not polarized, so their direction doesn’t matter.
-BrachThank you for trying to be so clear in your communication….sorry for my lack of understanding very well. I’m not sure what is going on here. Somehow the Quaverato is interacting with the PXL in a weird way, seeing that when it’s not in the signal path the issue doesn’t occur, no matter what pedals are in the signal path. I would love to find the schematic to the PXL to see what buffer circuit they are using.
You were very clear that the pedals that have been part of this issue contain opamps (“an actual physical DC-coupled electronic voltage amplifier with a differential input; also known as an OPAMP”). But you also described the buffer in a non-activated Boss pedal as “an opamp”. From the Boss service manual schematics we can see that Boss pedals use common collector discrete buffers, so I’m a little confused about what you are calling an opamp…are you just calling an active pedal, despite it’s particular circuitry, an “opamp”?. Either way, you said the jump does occur when a bypassed Boss pedal is powered on downstream of the Quaverato, so I’m assuming this volume jump also happens with discrete, non-opamp based pedals like a big muff, or LPB-1, or mosfet boost are on downstream of the Quaverato…is this correct?
One thing this might be related to is the phase of the Quaverato signal. If you look at the Quaverato schematic, you will notice that the output signal is out of phase from the input signal. But I’m not sure why only the Quaverato would cause a jump in the volume and not other pedals that flip the phase (like the LPB-1 set to unity gain, for example…which I suppose is something to check out). There is a way to mod the Quaverato to have in-phase output…it requires a lot of trace cutting and jumper wire running.
Another thing that crossed my mind is that the “silent switching” circuit on your pedal may not be operating correctly…possibly causing impedance issues. You can check this by removing C17, which will remove that circuit from the signal path. You’ll need to use a solder sucker to clean out the PCB holes (heat it up from one side of the board, suck from the other side of the board).
That’s all I can think of for now. Good luck!
-BrachFanyo,
Thanks for the detailed info. I don’t feel defensive about my design work or the Quaverato. I’m constantly learning and I would probably even design the Quaverato somewhat differently if I was to do it over again these days (after all I’ve learned in the years since I designed it). I’m always wanting to learn and be a better engineer, so thanks for letting me be a part of this troubleshooting process.
One thing that I’m curious about is to remove the looper from the equation…have you tried using the Quaverato and other pedals together outside of the looper? According to the user manual the PXL live does have “buffered bypass.”
I know you said you tried every conceivable combination of pedal orders and it happened on all of them…but If the Quaverato is the very last pedal in the chain (hooked up to the looper) before the amp, does it still cause this issue? What about if it is the last pedal in the chain after the looper, where the looper output is plugged in to the Quaverato input and the Quaverato output is going into the amp, does the volume jump happen then, when other pedals are turned on?
Have you tested the Quaverato when it is just plugged into the amp and your guitar? I’m curious if the boost you hear is really the normal volume level of the Quaverato, and the volume level is being reduced when it is in the pedal chain.
So far, from the list of facts you gave, none of them disprove my original hypothesis of it being a frequency sensitivity issue. I don’t know how you can say this is definitely not true. This type of thing can be experienced with an overdrive or distortion pedal that has the tone know all the way up…there is often a large boost associated with that adjustment, most of the time at least 20% boost. Most of the time, when modulation effects have this issue it is at a much lower volume level than what you are experiencing here, but it doesn’t always have to be. The same principles could be at work here. Unfortunately, it will take more than a multimeter to test it…you would need at least an oscilloscope and signal generator. You could experiment with the crossover switches, changing the frequency to see if that has an impact on the boosted/jumped signal. If it does, that’s a clue pointing to what is happening.
Let me know what you find when you have the opportunity to check these things out.
Good luck!
-BrachMatt,
Thank you for the video. I hear what you are talking about.
The volume not cutting out all the way tells me that you have a lot of gain after the volume pedal. On the VPM-1 the volume is guaranteed to get reduced to something like -70dB or so (I can’t remember the exact specs). So the signal is never completely gone, but just reduced by a whole lot. This is the way all volume pedals work, even the EB Jr, but in the case of passive pedals the volume is reduced much more, because it gets really, really close to ground level…so it would take tons of gain to make it audible again. With active volume pedals, depending on the volume reduction technology, the volume can only be reduced a finite amount. So in this case, the reduced signal is getting boosted back up again by the following pedals and amps in the chain. We mention in the manual to only use as much gain on or after the pedal as necessary. Unfortunately, because of the gain structure of this pedal, it’s not an exact drop-in replacement for the passive EB pedals…some signal chain gain adjustments may need to be made for it to operate optimally. It’s one of the limitations of the particular technology, I’m sorry about that.
After hearing it, I think the distortion sound is caused by what I was speculating in my previous post. This happens on the very first of the 256 volume level steps. This happens on all VPM-1s but most of the time it’s inaudible because the gain structure is lower. I think a possible mod to help it be less audible is to add a small value capacitor across the LEDs of the optocouplers. I’ll try to look into this when I get a free moment. But I’m sorry for the annoyance and trouble with these issues.
-Brach
