Espoused by guitarists like Robben Ford, Sonny Landreth, and Nalle Colt, the ZenDrive by Hermida Audio continues to live up to overdrive connoisseurs. This particular overdrive is an NE5532 Op Amp based drive, and the one ending up on the bench today suffered from an op amp gone awry.
ZenDrive Gut Shot featuring the NE5532 op amp, the heart of the ZenDrive circuit.
The original complaint was that no output could be heard when engaging the effect, but when bypassed all was well. On opening the backplate and removing the circuit board I was surprised that the op amp used in this unit was seated in an IC socket. This made the repair much easier (thanks Hermida Audio!). I removed the op amp, breadboarded it within a simple buffer circuit and, sure enough, the op amp’s I/O terminals were stuck at the rails. Replacement with another NE5532 fixed the issue.
This round of Emergency Room features the 6-channel Furman HDS-6 Headphone Distribution System after a mild power surge incapacitated 5 of the 6 channels. This unit is pretty neat, it utilizes Cat 5 cabling to distribute power and channel signals to remote units (HR-6’s) which are used in the studio booth by the recording artist for monitoring the other tracks through a pair of headphones. The HR-6 can be daisy-chained with other HR-6 units, enabling recording artists the ability to locally monitor tracks while recording.
Figure 1: Showing the HR-6 Headphone Remote unit connected to the HDS-6 main board for a test-run.
I opened up the chassis and, after some visual inspection, saw one of the capacitors had blown. I replaced the cap and powered the unit back up; no good. Other than that there was an odd white discoloration of the main board around one other capacitor. It didn’t seem to be an issue, but I replaced the cap it was surrounding for good measure. There was no schematic to go by, so I traced out one of the channels and drafted it for reference (shown below). I have a more detailed schematic written out, if needed don’t hesitate on sending me a message.
Figure 2: Mock-schematic detailing the HDS-6s signal path.
This revealed two op amps utilized in the signal path. This unit has six identical channels, so the circuit above is duplicated 5 times with only the component names varying, making it easy to troubleshoot the rest of the unit.
I injected an audio signal into each of the channels and probed the signal paths. This revealed that, for almost every channel, the second op amp in the signal path wasn’t functioning. This was confirmed when measuring the DC voltages on each op amp chip. The figure below shows the voltage measurements for the three dual op amp chips U101, U201 and U301:
Figure 3: Showing all three defective op amp values. These should all be biased to 0V!
I had a couple TL082s on-hand, and replacing them fixed the issue. The TL082 has an input noise voltage of 16nV/√Hz, but if available, replacement with something with a lower input noise voltage like an NE5532 is recommended since these particular op amps are in the signal path.
Figure 4: One of the newly installed op amp ICs.
Thanks for visiting! If you have a piece of gear needing repair contact me here and please follow Mimmotronics on Instagram and Facebook. This fix was done for DMS Productions located in Ransomville, NY!
Ciao a tutti! This round of In The Spotlight takes us all the way to Verona, Italy, where we find musician, session guitarist, and instructor, Antonio Paone! One aspect of being a session player is having the ability to draw up a huge variety of tones and playing approaches. Having a diverse set of effects to draw on is an essential tool of the session player. Today, you’ll get a peak into Antonio’s approach to how he utilizes his pedalboard in ways that render it useful to his session-based profession.
[Mimmo]:Introduce yourself! What’s your story?
[Paone]:Hi! I’m Antonio Paone, a musician from Verona, Italy. In the last few years I’ve been playing a lot of Pop/Rock music, so I began a sound research that could guarantee me a range of sounds that could satisfy (almost) every sound request. My main guitar is a Fender Stratocaster American Special with some mods (a DiMarzio Fast Track 2 on the bridge, a Wilkinson bridge and Sperzel tuning pegs).
[Mimmo]:Give us 2-3 songs that allow you to utilize your pedalboard in a unique way.
[Paone]:There is a composition called “La Prova del Fuoco” (a different way to see Mozart’s masterpiece “Die Zauberflote”) by the composer Igor Bianchini (we recorded it recently in a recording studio, but unfortunately the master isn’t ready yet) that demands me a huge variety of sounds: from a crystalline clean tone, passing by a susceptible crunch, to the most aggressive heavy metal distortion that I can provide; using also some modulation effects (chorus, delay, reverb, etc.) and mixing the use of the volume pedal and the volume pot of the guitar (I use my Music Man Luke II for this particular tune).
Also a song that makes me use my pedalboard in a very creative way is “Rewind” (a song by Vasco Rossi). I made an arrangement of this tune with the band “I Folli”: with a crunchy sound, delay, reverb and chorus I recreated the characteristic sound of the keyboards that you can hear in the intro of the song, but leaving a guitar identity to the sound (also the rest of the guitar part for this arrangement is a mixture of the guitar sound and this “keyboard-like sound”, accentuating the predominant sound in the moment when it’s necessary).
[Mimmo]:What are some songs, artists, or guitarists that have influenced the way you utilize your pedalboard?
[Paone]:Oh let’s see… There are so many incredible musicians who inspire me every time I see them using their pedalboards. Steve Lukather always impresses me with his distorted sound; Mateus Asato, Mark Lettieri and Lari Basilio have a very tasty clean/crunch sound.
A guitarist that blows my mind for his sound variety is Tim Pierce. Luca Colombo also is a great inspiration for me for guitar sounds in pop culture. Recently I’m pretty much into the Vocoder because of the amazing Jacob Collier: I find his way of using it outstanding.
[Mimmo]:Are there any sounds you would like to emulate that you haven’t quite found out how to do yet?
[Paone]:I don’t have a pedal that allows my guitar to sound like a synth, so I’m pretty curious about the synth sound of Pat Metheny (the one that he used in “Are You Going With Me?”, to be clear).
After researching it, I saw that you can get a sound very close to Metheny’s with the use of the Roland GR-55, but I haven’t tried that yet.
[Mimmo]: A little off topic here, out of curiosity what is the best venue you’ve played?
[Paone]:I have a strong emotional connection to the “Teatro Bibiena” in Mantua; that’s because many years ago I saw Mike Stern (with Tom Kennedy and Steve Smith) in there and I thought “Man, what a beautiful venue to play in.” Last May I played in there with a jazz Big Band: it was really gratifying.
“The essence of the beautiful is unity in variety.”
– Felix Mendelssohn –
Where there is art, there is beauty. And according to Felix, where there is beauty, there is variety. The audio electronics world is no exception. There are multitudes of audio effects out on the market, and the common differences they exhibit renders their design an art form all its own. This blog generally deals with audio effects equipment, but the diverse nature of the audio world prevents me from dealing solely with guitar effects repairs. Today we turn our attention to a popular synthesizer from 1984, the Roland Juno-106 Analog Polyphonic Synthesizer.
This thing is a BEAST! But these days, there’s a common problem that owners of these synths should look out for: the failure of the Roland AR80017 Voice Chips. Apparently this is a super common problem with these units, and if you’re here reading this you’re probably suspecting the voice chips to be the reason for your faulty Juno.
In order to isolate the problem to the voice chips you’ll have to utilize the Juno’s built-in troubleshooting method described in EXPLORING AUDIO’s YouTube video, “ROLAND JUNO 106 : VOICE CHIP ISSUES.” You can also find the procedure on page 18 in the Juno’s service manual, found here.
Once the problem is isolated to the Juno’s voice chips the procedure described by Jeroen Allaert of Analogue Rensaissance can be used to remove them. Allaert has done some remarkable work on the AR80017A voice chip; he’s even re-engineered a replica of it which can be purchased from his website. Another replica chip can be bought through ModularAddict.com as well. So even if the following procedure doesn’t do the trick, there are replacements available.
Once the chips are removed you can start the dissolving process! Fill a glass container with acetone (100% is best) and submerge the chips. Let that sit for 24-48 hours and check the condition of the outer epoxy resin cover. You can tell when the acetone has done its job by how disassociated the resin becomes. Gently peel off the larger chunks of resin and use a small tool (something similar to a precision flat-head screwdriver) for removing the smaller bits of resin left behind.
The reason acetone is used is because its a solvent for plastics (rubbers, polyethylene, etc.). It does a great job at breaking down polymers, and the epoxy resin encasing the voice chip is a decent candidate for the dissolving reaction. In fact, excess glue can also be removed using this method.
Once most of the resin is removed you can solder them back into the board. A good pair of helping hands or something to prop the board on its side is essential in order to easily solder the chips back into place. Solder one pin to support the chip on the board, then solder the rest of the pins with relative ease.
That’s it! Thanks for visiting Mimmotronics, please follow the Instagram and Facebook pages if you liked the post. Stay tuned!
This post was done as a result of working with Cody Morse, guitarist for Buffalo, NY, psych-rock band Deadwolf. on his faulty Juno-106 synthesizer. See their feature in the Buffalo internet TV show, the Attic:
This is quite possibly the most interesting circuit I’ve worked on so far. I’ve always wondered how a digital delay works, and this late ’80s digital IC-based technology allowed the opportunity to reverse engineer the general concept. (I’ll share those findings in posts to come!) But before I could reverse engineer anything the unit had to be repaired to a working condition, as it had suffered the results of a failed board-mounted 4.6V battery:
Figure 1: Showing the Damage from Battery Leakage
There is an SRAM chip (NMC6504) used on this board to store 4 preset configurations (A, B, C & D), this is obviously being powered by the battery and was confirmed when I replaced the blown fuse, powered up the device, and no presets were able to be saved.
I couldn’t find the specs for this battery model online for a direct replacement, so I had to dig into many forums to find information and, eventually, a schematic. For legal reasons I won’t post the schematic here, contact me for more info if needed.
The schematics yield a 4.6V battery, and the test point for the battery voltage is labeled +5VB after the zener. I settled on a Varta 4.8V NiMH from JLS Batteries on eBay. It’s a through-hole component, but doesn’t match the footprint of the original battery, so I soldered wires from the + and – sides to the appropriate vias. To secure the wiring to the battery’s body I used electrical tape and twisted the wires together to provide additional support. The electrical tape may also help in preventing a large amount of damage to occur in the case that this NiMH fails in the future. A square piece of Velcro was used to secure the battery to the main board.
The use of a NiMH is based on the color of the corrosion that occurred on the copper traces and component leads. NiMH batteries contain the hydroxideanion (OH-). In the presence of an electrical current and physical contact, the hydroxide anion and copper can react to form copper (II) hydroxide, which is blue in color.
Copper (II) Hydroxide suspended in a Test-Tube (WikiMedia Commons)
To reiterate, this reaction is corrosive and literally eats away the copper traces on the board. If enough corrosion takes place the board may become irreparable! (Or at least extremely hard to repair without re-engineering and rebuilding the affected circuit). Luckily, this was not the case for this unit.
Once the replacement battery was installed I powered up the Super Timeline and presets were operational! The contacts for the presets, however, were extremely unreliable. They would make contact maybe 60% of the time, so I ended up removing all of the preset button switching actuators. I tinned a thin, even layer of solder onto each contact and reinstalled them; that worked very well.
Figure 2: Programmable preset switches. The switch to far left has its’ actuator removed. This is done by applying a precision flat-head screwdriver to the plastic lip between the lug sets.
After tinning the preset contacts I took aim to the copper (II) hydroxide build-up near the power and LFO circuits. I cleaned a toothbrush with distilled water and started brushing the corrosion build-up off the leads. I quickly noticed that the component labeling was getting scraped off as if they were old stickers. From that point on I was very careful in removing the corrosive material and mainly focused on relatively open areas between components. It’s not perfectly cleaned, but the unit was working and I didn’t want to ruin the labeling scheme, as it may prove useful down the road. (If anybody has an abrasive-free solution for cleaning copper (II) hydroxide from PCBs I’d be highly interested!) To remove the distilled water I used some combination of paper towels, a hair dryer, and an air compressor.
Thanks for reading! Notes on the circuit’s operation will definitely be a topic for future Talk Theory To Me posts. Stay tuned if you’re interested!
Beware of Banshees, Sasquatches and Draugr-naughts: This month we’re turning our attention towards America’s Bayou State with Hammond, Louisiana native Nathan Heck of Rougarou Pedals! Read on to learn about Heck’s transition from the world of harmonica gear and into the realm of guitar effects pedal design.
[Mimmo]: How would you tell the story of Rougarou Pedals?
[Rougarou]: The story of Rougarou Pedals is a long one. During my senior year of high school I began working for Lone Wolf Blues Co., which produces harmonica effect pedals (this has since expanded to include amplifiers, power supplies, microphones, etc.). They were kind enough to work around my university schedule when I began my studies there, and, after I graduated, I transitioned to being the general manager. Sometime in 2015, a coworker and I got the idea to build bass specific pedals. We released the Rougarou Bass Tube Overdrive (acting as a subsidiary of Lone Wolf Blues Co.), but the project kind of fizzled out. Towards the end of 2016, my wife, Kara, and I were looking an outlet to help us cope with a personal tragedy. We took over operations, revived the brand, expanded the line and ended up where we are today.
Feast your eyes on the very first Rougarou Bass Tube Overdrive
[Mimmo]: I’ve read up a bit and I did notice Rougarou is a husband & wife operation. How does that dynamic play out?
[Rougarou]: At its most basic level, Kara (a professional graphic designer) handles all of our artwork, branding, and social media; I design the circuits. We both build the pedals, and, while my design process is largely independent, I definitely bounce sound ideas off of her all the time. She’s an avid fan of guitar driven music, and I trust her ear more than my own sometimes. People always assume there’s some sort of tension when a husband and wife work together, but it’s more like hanging out with my best friend. We listen to music and conspiracy theory podcasts (mockingly) while building pedals; it’s a good time. I’d also like to add that we have another friend, Ryan Church, that helps us by building pedals and play-testing all of our pedals.
[Mimmo]: Give us a brief overview of Rougarou Pedals’ product line.
[Rougarou]: We currently have five pedals out, but I’ll quickly run through four of them here since The Draugr gets its own question.
The Rougarou Bass Tube Overdrive is a low gain overdrive that was designed to mimic the front end overdrive of mid-power bass heads like the Portaflex. Like all of our pedals, it’s very simply laid out with just a Bite (drive) and Volume control. Overdrive is achieved by way of sub-miniature triode tube. For guitar, it works well as “thickener” at the end of a drive chain.
The Boosthulhu is a crystal clear boost that puts just a hair of brightness on top. I’m always amazed at the variety of tones people can achieve with it based on guitar/pedal/amp pairings. So far, this is our most popular pedal. I suspect the awesome Cthulhu artwork has something to do with it.
I have used clean boost pedals for several years and never found exactly what I was hoping for until I tried the Boostlhulhu. It gives me exactly the tonal enhancement I had been looking for forever. Most boost pedals add an artificial hi and low end that I don’t care for but the Boostlhulhu adds very slight, smooth mid-range that truly makes my sound full and beautiful whether clean or with a distortion pedal. It immediately became an essential part of my sound.
– Duke Robillard (The Duke Robillard Band, Roomful of Blues, The Fabulous Thunderbirds, Bob Dylan, Tom Waits, etc.)
The Banshee Reverb is a simple, two knob spring-esque reverb. It uses the Belton reverb module that I’m sure a lot of DIY guys are familiar with. [Belton has since evolved into a merger with Accutronics, forming Accu-Bell Sound Inc.] It has a Blend control to adjust the level of reverb and a Tone control to go from dark to bright. There’s a bit of modulation when get the blend cranked. That’s my favorite part.
The Sasquash is (you guessed it!) a simple, two knob compressor. I, personally, don’t like a ton of options in a compressor because I think it makes achieving a “bad” sound way too easy. The compression stays in a reasonable range all the way up to max. This has been our second most popular pedal.
[Mimmo]: Congrats on your recent release of The Draugr Distortion! Give us a brief overview on The Draugr.
[Rougarou]: Thanks! We’re super excited about this one; she’s my baby. The Draugr is a germanium diode-based distortion that can cover a lot of ground in that grey area between overdrive and fuzz. For bass, I’ve found I like to leave it on a lower setting nearly all the time. On guitar, it does a lot of the classic big amp sounds. The Tone control is super interactive with the Drive control, and it makes this deceptively simple pedal extremely versatile. The most frequent comment we’ve gotten about it so far is that it stays super clear all the way through the drive control. If that’s your thing, you’ll probably dig it.
[Mimmo]: When you start thinking of building a new product where do you start? What is your design process like?
[Rougarou]: The ideas for new products are usually selfish in nature. With the Draugr, for example, I really wanted a distortion that played well with my board and Rickenbacker. With the idea in my head, I hand draw a schematic with the simplest possible circuit to achieve what I want to do. After that, I’ll breadboard it, see how close my brain’s understanding was to my ear’s expectations and make adjustments and additions (like clipping diodes in the Draugr; I probably tested 30 or so in the circuit.) I’ll usually build a prototype after that and gig with it for a bit to see what I like and don’t like in practical applications. Usually, that’ll result in another breadboarding phase, more fine tuning and play-testing on gigs; wash rinse, repeat. It’s a long process, but I think the results are worth it.
[Mimmo]: Are there any particular sounds you wish to capture but haven’t quite hit?
[Rougarou]: I have a very dark, bass heavy fuzz sound that is still present in the mix in my head. I stumbled across the basic idea while testing out different ideas in The Draugr development process, but I was scared of getting sidetracked in the middle of a project. I haven’t even started working on it yet, but it’s been bugging for a couple of months now. I probably should figure that out.
Artwork by Kara of Rougarou Pedals
[Mimmo]: Are there any other pedal builders for which you have a definitive amount of respect? Any builders that you simply cannot get enough of?
[Rougarou]: I feel like I’m in a kind of weird situation here. I’ve been building pedals for myself since I was just a wee young lad, so I really don’t own too many pedals by other builders. I have a few mass market things I picked up a decade ago, a Fuzzrocious Grey Stache and a Mantic ATDI-limited run reverb, but that’s about it. I also was immersed exclusively in the harmonica gear culture for a long time, so the scene is still pretty new to me. I definitely respect the Fuzzrocious duo, though. Not going to lie, we look up to them for what a successful husband and wife team can do, and I really like that they push limits. Ryan has two of Mr. Black’s reverbs, and they’re incredible. Retroactive Pedals out of New Orleans (we love locals!) has a pedal called the Designated Driver, which sounds absolutely fantastic to me in all of the clips I’ve seen, and he’s a very knowledgeable guy.
[Mimmo]: Choose a genre and build a pedalboard for a guitarist in your chosen genre. Which Rougarou pedals would you select? Which products would you select outside of Rougarou?
[Rougarou]: Hmm, I’ll go for doomy, riffy psych a la Super Snake, and I’ll limit this to six pedals.
From us, I’d have the Draugr for the big, amp-like sounds, the Banshee as a nearly always on reverb and the Boostlhulhu to punch through for solos. From other companies, I’d pick the Mr. Black Eterna because I’m in love with the octave thing it does and everyone needs at least two reverbs, the Black Arts Toneworks Pharaoh because it’s the best sounding muff I’ve ever heard and an EHX Deluxe Memory Boy because it covers so many bases as far as delay is concerned.
[Mimmo]: Popular advice for learning how to design and build effects units usually bounces between purchasing kits and performing mods on existing pedals. Is this how you started? What advice might you have for “young players”?
[Rougarou]: I learned on the job from my friend and mentor, Randy Landry. Looking back, it’s incredible how lucky I was to stumble into that situation, and I think more builders should take young techs under their wing. For anyone starting out, I recommend reading as much information as you possibly can, but also take the time to verify what you read. There’s so much misinformation out there, and there can be a substantial disconnect from theoretical certainties and practical applications. Besides that, find someone who knows more than you and learn absolutely everything you can from them.
[Mimmo]: Where can we find/contact Rougarou Pedals?
So I’ve been working on a BOSS CH-1 mod and in doing so I’ve been increasingly interested in how Low Frequency Oscillator (LFO) circuits are designed and constructed. Its worth investing time on the topic, especially since when diving into modulation effects you quickly find out that LFOs exist at every corner. Most LFOs are very simply constructed with Rate and Depth parameters, but one relatively popular DIY pedal project caught my attention – the Tremulus Lune. This DIY tremolo project was originally developed by Dan Green from 4ms Pedals and is available in kits or pre-built from their website.
The reason why the Lune was so interesting to me was the number of parameters it boasts. Not only that, but putting even more research into the circuit revealed the variety of mods that are able to be designed around it. In the coming paragraphs I try to both introduce the type of oscillator used in the Lune as well as a conceptual view into how the LFO circuit works.
Note: In order to understand the following topics you’ll need to know some basics of current flow and voltage distribution in basic op amp and RC circuits. Look at the Mimmotronics Resources page for some online information on these topics.
The type of oscillator used in the Tremulus Lune is called a relaxation oscillator. Relaxation oscillators usually produce non-sinusoidal periodic waves, like a triangle wave or a square wave. Wikipedia says it best:
Relaxation oscillations are characterized by two alternating processes on different time scales: a long relaxation period during which the system approaches an equilibrium point, alternating with a short impulsive period in which the equilibrium point shifts.
By that definition we can deduce the following:
The system needs to have 2 equilibrium points to oscillate between. The Lune accomplishes this by employing a hysteretic comparator.
There must be some way to define the relaxation time, which is the time it takes for the system to “relax” to one of the 2 equilibrium points. An RC network is employed for this task.
Note that there are other ways to build relaxation oscillators. For example, the analog version of the BOSS CH-1 utilizes a dual op amp configuration. I’ll cover dual op amp LFOs and other types of oscillators in future posts. For now, lets focus on the Lune!
The Tremulus Lune LFO
The Tremulus Lune’s LFO is modeled in Figure 1 using LTSpice. The basic parameters are: Rate, Depth, Fine, Smoothness, and Spacing.
Figure 1: Tremulus Lune Schematic Simulation
To simplify the discussion, let’s strip this circuit down to the basics. Figure 2 shows an extremely simplified version of the Tremulus Lune and Figure 3 shows the important signals we’re going to be talking about.
Figure 2: Simplified Tremulus Lune
Figure 3: Simplified Tremulus Lune Plot Simulation showing the LFO output (red), the non-inverting input signal (green), and the output of the op amp (cyan).
The op amp sees a slight input offset voltage when the circuit is provided power. This input offset voltage is amplified by the gain of the op amp and eventually the oscillations start due to positive feedback.
When our circuit is oscillating the comparator outputs a square wave. The cyan-colored signal is the output of the op-amp comparator. A square wave essentially alternates between two values. In the simulation, our square wave alternates between 7.86V (HIGH value) and 1.14V (LOW value). The amount of time the square wave is HIGH relative to its total period is called the duty cycle and is displayed as a percentage. For example, a square wave with a 50% duty cycle is HIGH half of its period. A 75% duty cycle suggests the wave is HIGH for 3/4ths of its period, and so on.
The green signal represents the voltage at the non-inverting terminal of the op amp. We can see that for each HIGH and LOW value of the comparator output there’s a corresponding HIGH and LOW value that the non-inverting terminal voltage eventually settles towards until the next step-change occurs. The corresponding HIGH and LOW values on the non-inverting terminal are 6.235V and 2.764V, respectively.
The red signal is the output of the RC network, which also happens to be the output of the LFO. In the simplified version this signal is directly fed back to the inverting terminal of the op amp. Understanding the interactions between these signals and components is best described in steps:
When the comparator output is HIGH the capacitor C2 starts charging and thus its voltage increases at an exponential rate. This voltage is also seen at the inverting terminal.
When the inverting terminal voltage starts to cross above the non-inverting terminal voltage the comparator switches its output state to a LOW value. Due to positive feedback, the non-inverting terminal voltage also switches to its LOW value, which is determined by hysteresis.
The capacitor C2 sees, through resistor R5, a LOW value at the comparator output which is now lower than the voltage across the capacitor. Because of this voltage difference, current starts to sink into the op amp’s output and the cap starts to discharge. Again, this capacitor voltage is seen at the inverting terminal.
Once the inverting terminal voltage crosses slightly below the non-inverting terminal voltage the comparator switches its output state from LOW to HIGH, and the sequence starts over.
The Spacing Parameter
In the description above the non-inverting terminal voltage is used as a reference. Not only that, but its reference voltage alternates between two values and is done using a technique called hysteresis.By way of hysteresis it’s apparent that resistors R1, R2, and R3 determine the HIGH and LOW voltage levels on the non-inverting terminal signal. If we’re able to alter one of those 3 resistances the hysteresis voltage levels can be varied.
So…what’s this Spacing parameter do? Let’s look at the simplified schematic, but with the Spacing parameter added. (Figure 4) You will see that R1 from the previous simplified schematic now has another resistance “SPACING” on the same branch, which we will alter below.
Figure 4: Simplified Tremulus Lune LFO with Spacing pot
And let’s also take a look at some plots. Figure 5 shows the non-inverting terminal voltage, comparator output voltage, and LFO output voltage when the SPACING pot is set to 0Ω. Figure 6 shows the same signals when SPACING is at 250kΩ and Figure 7 shows them at 500kΩ.
Figure 5: Simplified Lune with Spacing at 0. HIGH reference voltage is 6.832V. LOW reference voltage is 4.111V. Difference in reference voltages = 2.72V. 66.4% Duty Cycle.
Figure 6: Simplified Lune with Spacing at 250k. HIGH reference voltage is 6.235V. LOW reference voltage is 2.764V. Difference in reference voltages = 3.47V. 50% Duty Cycle.
Figure 7: Simplified Lune with Spacing at 500k. HIGH reference voltage is 5.981V. LOW reference voltage is 2.191V. Difference in reference voltages = 3.785V. 42.4% Duty Cycle.
The most obvious property that changes is the Duty Cycle of the comparator’s output signal. How does this happen? The RC network needs both charging and discharging current paths. When the capacitor is charging it sinks current primarily from the output of the op amp, but some current is supplied through the hysteresis resistor R3. The amount of current supplied through R3 is determined by the voltage difference across R3. The Spacing resistor controls this voltage difference via setting the voltage references on the NonInv node.
When we set the Spacing resistance to a lower value we effectively increase the reference voltage at the non-inverting terminal-side of R3. As a result, when the comparator output-side of R3 is LOW there is a larger absolute voltage difference across R3 compared to when the output is HIGH (see Figure 8). The larger voltage difference suggests more current is sourced through R3 and the capacitor charges that much quicker. On the flipside, when the output is HIGH the amount of current sinking through R3 is proportionally decreased. In this way the Spacing resistor can be used to change the duty cycle without majorly changing the output frequency of the LFO.
Figure 8: Spacing is set to 0. The voltage difference across resistor R3 (red) and the current through R3 (green) is shown. (A positive current flow is defined as being from the Comp_Out node to the NonInv node)
The Rate & Fine Parameters
To understand how the Rate and Fine Parameters work you’ll need some background on RC circuits. Check out this page for a quick overview. After that, take a look at the simplified schematic with the Rate and Fine pots added in (Figure 9).
Figure 9: Simplified Schematic with Rate and Fine pots maxed.
The main function of the Rate and Fine pots is to adjust the frequency of the LFO. The frequency of the LFO is dependent on the rate by which C2 charges and discharges via the RC network resistance (made up of R5, RATE, and FINE). By simulation you should expect a frequency range of 416mHz to 40.5Hz. If the goal is to modify the Lune’s frequency range, then the RC network’s resistance is where your interest should be.
The Smoothness Parameter
The Smoothness pot acts as a cross-fader control between the RC network output and the voltage signal at the NonInv node. The NonInv node boasts some high-frequency transient properties. Therefore, when crossfaded with the LFO output signal, the output becomes more jagged and less smooth. Figure 10 shows the simplified schematic with the Smoothness parameter added as two resistors.
Figure 10: Simplified Tremulus Lune schematic with Smoothness control.
The Depth Parameter
The main oscillating section of the circuit (the RC network & hysteretic comparator) acts in a closed system with voltage and current levels that need to be maintained. The node that we’ve been using to plot the LFO output is not suitable for driving an LED since that requires a relatively large current draw that could “weigh down” the oscillator (see Loading Effect). As a result we need to employ a voltage follower which is going to be able to supply the necessary amount of current to an LED driver circuit without loading the oscillator system too much. This can be seen in Figure 11 along with the Depth potentiometer.
Figure 11: Simplified Tremulus Lune schematic with Depth control.
The OUT_SIMPLE node is used to apply the LFO voltage to an LED which, in the Lune, will be used to vary a photoresistor in the pedal’s signal path. The Depth parameter acts as a Level pot for the LFO. Figure 12 shows the output with the values in Figure 11. Figure 13 shows the output with DEPTH_TOP = 1 and DEPTH_BTM = 1k.
Figure 12: Depth_Top at 1k, Depth_Btm at 1.
Figure 13: Depth_Top at 1, Depth_Btm at 1k.
Let’s take a look at the comparator output signal in Figure 3. There we see that there are voltage spikes occurring at the edges of the square wave. To mitigate these spikes and to prevent them from propagating to the LFO output we need to introduce C3 and R6, shown in Figure 14.
Figure 14: Introducing C3 and R6 for suppression of high-frequency transients.
Capacitors generally let AC signals “pass through” them, but “blocks” DC signals from passing through. The voltage spikes are, by their nature, high-frequency transient AC signals. When they occur at the Comp_Out node they make their way through C3 and into the Inv node (instead of through R5, because of the “apparent” electrical short created by C3.) At this point these signals see the resistance of R6, which forms an additional low-pass filter with C2. The cut-off frequency of the low-pass filter formed by R6 and C2 is so small that the voltage spikes are almost entirely attenuated at the output. If these components were left out the player would hear a “popping” sound in sync with the frequency of the LFO.
Welcome to the first In The Spotlight session! This new category of posts will highlight individuals in the effects pedal community, mostly encompassing guitar players and effects pedal builders. Today we’ll be taking a look at Matthew Fantini’s board and how he utilizes it with the Buffalo-based band Space Junk.
Matt Fantini of Space Junk
In The Spotlight Interview:
[Mimmo] Q: What’s your story? How did you end up playing with Space Junk?
[Fantini] A: Space Junk began as a 3 piece live EDM/Jam band that I formed in college with Kevin Rogers (drums/keys/DJ) and Will Thompson (Bass) in 2004 at SUNY Fredonia. In 2015 we added keyboardist Cary Meehan to complete our current lineup. We are all huge fans of experimental, electronic, and jam music but we also incorporate elements of blues, jazz, and funk.
[Mimmo] Q: Give us two Space Junk songs that allow you to utilize your pedalboard in a unique way.
[Mimmo] Q: Are there any sounds you’d like to emulate that you haven’t quite found out how to do yet?
[Fantini] A: I am always working on getting analog keyboard sounds to cut through my mix better. I have an original EHX micro synth in addition to the reissue. When I push them a little hard with a boost, I sometimes have clipping issues. Pairing with a compressor does help, so maybe I need to put them on a loop together. Is there a pedal that will make me sound like Jimmy Herring?!
Q: Time for Quick-Fire Favorites! Favorite Overdrive/Distortion/Fuzz?
A huge thank you to Matt Fantini for participating on In The Spotlight! Catch Space Junk at their next show by following them on Archive.org and Facebook. Their sound emulates a wall of pure psychedelic energy that toys on experimentalism, yet manages to firmly keep itself together by way of masterful musicianship.
Introducing: the iSP Decimator Noise Reduction unit! This is quite probably the best noise reducer I’ve played through thus far. There’s only one problem..it pops! You had one job Decimator!
In any case, it’s a good thing the Decimator is in our hands. What’s happening is the pedal works correctly when set to -30dB and below. Problems are only seen when disengaging the pedal with the potentiometer past the 12:00 position. Engaging the pedal is fine but once we decide to disengage, it lets out a noticeable “pop” in the output. Turning the potentiometer clockwise from 12:00 makes the popping sound even more noticeable.
So we see two things here: (1) The popping intensity is dependent on the position of the potentiometer and (2) the popping only occurs when disengaging the pedal. Keep these in mind as we continue.
Okay, so now we have some clues. As an aside, when pedal-popping complaints arise it is usually a result of a DC current snaking its way into the signal path and eventually into the pedal’s output signal. Now I will admit to not knowing all the ways this can happen, but the primary cause is a build up of charge. This charge build-up only results in pedal pop when it quickly affects the output signal. The quicker the discharge, the “shriller” the pop. The slower the discharge, the “duller” the pop.
The same can be said on how filtered the pop is. If a pop is filtered enough it can be transformed into a “thud”-like sound. You can read more about that here.
After some careful thought the next thing to do is take a look at the schematic and try to pin-point possible areas where charge build-up can occur. But first…we need a schematic. I was pointed to Fred Brigg’s La Revolution Deuxblog and found a schematic for an iSP Decimator model similar to what I had. Along with the schematic it’s nice to have the datasheets handy for any ICs or transistors onboard. This Decimator only uses 3 types of ICs:
Reading through some of the datasheets I suspect the THAT chip is intercepting a DC input voltage. Though, this is ruled out since there happens to be an input coupling capacitor which should thwart any chance of a DC voltage popping up at the input of the VCA.
Eventually I take another look at the schematic and realize the switching circuitry for the model I have isn’t conveyed. This is in fact directly related to the original problem, so maybe the solution lies in that area of the board…
I pull flex my reverse engineering muscle and trace out the Decimator’s switching circuit. This is important since it reveals a 2N5484 JFET transistor used to switch the pedal on and off. It’s important to understand conceptually how this switching circuit interacts with the rest of the circuit. I’ll explain the inner workings of how the pedal switches ON and OFF. (I’ve made a mock-schematic for you in Figure 1).
Figure 1 – Mock Schematic for Conceptual Use, Final Mod is in Red (discussed later)
When the pedal is disengaged, the gate terminal of the JFET is too low of a voltage to allow current to pass through the source and drain terminals. This effectively severs the dependency of the input signal from the VCA’s operation, leaving only a virtual ground (4.5V set by an on-board potentiometer) applied to the Ec+ control pin (pin 2) of the VCA. This DC voltage is constant and, at 4.5V, the pedal sees virtual ground. (This is because the ground, pin 6, of the VCA chip is also seeing 4.5V, so in the chip’s perspective it’s ground.)
If we take a look at Figure 2 we see that when the chip sees a control voltage of 0V (in relation to it’s ground pin) the VCA gain is set to 0dB. This means the chip doesn’t amplify or attenuate. Whatever signal you put into it should also be received at the output.
Now let’s understand what’s going on when the pedal is engaged. Take yet another look at Figure 2. If we decrease our control voltage the amount of gain we apply to the input signal proportionally decreases. Essentially, when engaged, the circuit varies the DC voltage at this Ec+ control pin and this controls the amount of gain the VCA applies to the input signal.
Figure 2 – Note that the control voltage is in relation to virtual ground, so in this case our virtual ground and the control voltage are equal at 4.5V.
The control voltage is a function of two things: (1) the amplitude envelopeof the input signal and (2) a user-controlled threshold voltage set by the potentiometer. If the input signal’s amplitude envelope is below the threshold voltage the circuit decreases the control voltage accordingly and the VCA attenuates the signal. If the input signal’s amplitude envelope is above the threshold voltage the circuit increases the control voltage such that the VCA eases off with the attenuation.
So how does the circuit increase or decrease the control voltage? By way of current! Firstly, both the input signal amplitude envelope and the threshold voltage are sent to a differential amplifier(which we can think of as a comparator for ease of discussion). This component increases or decreases it’s output voltage based on the comparison of its two input signals. This voltage is seen at the cathode side of a diode. The anode side sees the drain terminal of the 2N5484 FET. If the voltage across the diode (which is based on the comparison above) is positive enough, current starts to sink through the FET and resistor R37. This current produces a voltage difference across R37 that pulls the control voltage lower than virtual ground. This in turn provides the attenuation and, hence, “noise” reduction based on the plot in Figure 2.
Now for the main take-away: This control voltage is only there because the FET is allowing current to pass through it. When the pedal goes from being engaged to being disengaged (i.e. when the gate voltage of the FET is suddenly hit with 9V from the switching circuit) the current providing the drop in control voltage ceases and the control voltage increases (or decreases) back to it’s original virtual ground of 4.5V.
This self-correction of control voltage is sudden. So sudden that this popping sound could be originated by it’s occurrence. If a fast voltage transition is the suspect, why not find a way to slow down this voltage transition?
Take your thinking cap out: what causes the voltage transition? In this case, it’s the step change generated by the switching circuit applied to the FET gate terminal. This type of step-change signal is depicted in Figure 3. All we need to do is smooth out this transition (Figure 4) while making it last long enough to not hear a “pop” in the output. We can achieve this by constructing an RC circuit at the gate terminal while choosing component values that allow a slower voltage transition.
Figure 4 – Same signal as Figure 3 showing the effect of applying an RC circuit
The circuit already utilizes a 1M-ohm resistor at the gate terminal. What needs to be added is a capacitor between the gate terminal and ground (0V). I tested the pedal’s operation with a 103 ceramic capacitor branched between the gate terminal and ground. This allowed a softer fade out of the current through the FET which ultimately eliminated the “pop.” Awesome!
All that for adding a little capacitor! We got a bit technical this time around but nonetheless I hope you enjoyed this post! This one’s for Deadwolf guitarist and Afterglow Studios’ owner/producer Cody Morse. Look out for Deadwolf at Buffalo, NY’s HerdFest, June 17th at Dreamland!
Another Small Stone on the bench! This time, the Nano. The problem? The LED would turn on but sometimes, when engaging the effect, the signal would entirely drop out. Other times it worked great. The first thing I like to do when an intermittent issue like this comes up is look for any mechanical devices that are directly involved in regenerating the problem. In this case, the 3PDT switch was the first thing to inspect.
Sure enough, the switch was the issue! But to be sure enough, I regenerated the problem. The signal was dropped out yet the LED indicator was on. Applying pressure to the shaft of the footswitch resulted in reviving the signal. On intuition, there is probably a loose connection internal to the switch that was shaken up from excessive use.
The Small Stone Nano sporting a new 3PDT switch
In the end, I replaced the 3PDT switch and the pedal was phasing once again! This is one area where it’s great getting to know the people you’re fixing things for. The Small Stone is heavily used in the band, which is important information for deducing the problem to an overused mechanical device.
This fix was done for Cody Morse of Afterglow Studios & Buffalo band, Deadwolf! Watch out for them, they’re coming up strong!