Dead Astronaut FX Chasm Reverb

Remember my Dead Astronaut Chasm Reverb PCB? I finally finished it! I left it quite a long time aside my bench, mainly because I did not have time or money to buy and build all the remaining things that needed to be done. Remember, if you want to have one, you can buy one directly from Dead Astronaut, or buy the PCB to make it yourself.

Here is my build: 

I used a prepainted enclosure, with a nice vintage color vibe, close to Surf Green color. With cream knobs of course! I just miss a cream pickguard part to have to most vintage fender look!

As I already said before, it was a fun build to make. The PCB is quite big and components are well spaced, so it is really easy and fun to build it, even for a beginner. I had absolutely no trouble at all. I did a few mistakes with the wiring, which is not a common wiring scheme as the pedal is buffered bypass. Apart from these minor incidents, the pedal almost worked immediatly, nice!

There are four potentiometers: volume (to set the output volume), mix (allows you to mix the dry signal with the reverb signal, you can go from a 100% dry to a 100% wet signal), damp (set the overall brightness of the reverb) and decay (set the amount of reverb that goes to into a feedback loop). Indeed, one of the features that make this reverb unique compared to other Belton Bricks reverb out there is that a part of the reverb signal can go through a feedback loop, allowing the reverb to auto oscillate! The switch allows to put the reverb in auto oscillation mode.

How does it sound?

I finally invested in proper recording gear (Senheiser e906 and external audio card), so I manage to record something for you! The Chasm Reverb is a deep, spacy sounding reverb, very good sounding with a delay!

The volume potentiometer is useful if you make it oscillate. At max, it is normal volume level, and you can lower it. The Mix is also quite useful, although I do not really like a too wet sound. The oscillation switch is really killer.

You can make the reverb smoother, and create "waves" of sounds, that lush for a quasi illimited amount of time! It is really awesome when combined with a delay! Perfect for ambiant stuff, and you can leave it on on the background.

Circuit analysis

Here is the circuit, from the build document:

If you have already read the circuit analysis of the Rub A Dub Reverb, you can already find some similarities. As most of the DIY reverbs, it uses a Belton Brick, an IC that allows DIYers to make reverbs without having to use a spring reverb tank.

It is divided in several parts:

Let's analyze each part of the circuit.

Power supply

The power supply is a classic one that we can find in many circuits. It provides 3 different regulated tensions:  9V, 4.5V and a regulated 5V.

The Zener diode (D6) prevents polarity inversions. R22 and C16 forms a low pass filter that will eliminate any 50Hz parasitic voltage ripples remaining from your AC outlet.

R23 and R24 forms a voltage divider that provides a 4.5V tension (VB). It is regulated by C17, a 47uF capacitor that will absorb excess of voltage. This tension is necessary for the OP amps to operate correctly

Then, there is a voltage regulator, REG1, that is a 7805. "78" means that the output tension is positive, and "05" is the output tension, 5V. The regulator is necessary to supply the Belton brick a good voltage. Unregulated voltage could result in damaging the IC that is very sensitive to higher or lower voltage drops, and so requires a regulated tension provided by this small chip that look like a transistor! You will find this kind of regulators in almost every circuit using numeric IC.

JFET switching buffer

This is a peculiar switching schematic that is very pratical here because it allows the use of a buffered bypass setting that make reverb trails possible. In a true bypass setting, the reverb would be cut abruptly when the effect is turned off, whereas here it can slowly decrease

So how does it work?

First, there is an input buffer, formed with R1, C1, R2 and the first OP amp of a TL072. As you can see, there is no resistor in the loop of the OP amp, thus it has a gain of 1. It is just used to transform the low impedance signal from the guitar into a low impedance signal.

Then, ther is the proper JFET switching. Here, JFET transistors are not used like amplifiers, but rather like "on / off" switches (like in computers!). When the JFET is turned on (by supplying 9V through the DPDT switch), it allows the signal to go from the drain to the source: the signal can pass. When a JFET is on, the other is turned off, so the signal either goes to the effect, or to the buffered output. A diode prevent any parasitic signal from the gate to enter in the signal path.

This switching scheme is nice with a reverb: it diminishes "popping" issues, and allows reverb trails, which is super nice with this reverb and its auto oscillating feature.

The Reverb circuit

The reverb circuit uses the Accutronics reverb module, a great integrated circuit that I presented already in the Rub A Dub Reverb circuit analysis.

Here is a schematic of the BTDR-2H that is used in this circuit:

There are 6 pins on the brick. The two first ones are used for the power supply. Note that the power ground is supposed to be different than the signal ground. In some circuits, that is very important to separate digital and analog ground, and to combine them in only one point in order to diminish noise (especially if you combine digital chips with analog ones like MN3005 that are also in 5V).

The guitar signal enters in the third pin (signal ground on the 4th pin), and is "transformed" by the chip in a reverberated sound that goes out at the 5th and 6th pin. The reverbarated sound is not the dry sound + reverb sound. It is just the reverberated sound, so it is kind of peculiar. You have to mix it with the dry signal to make it sound like a reverb.

Here is the schematic of the reverb:

So first, there is a MOSFET input buffer, that increase a bit the signal. The signal is then split in two. A part of it stay dry (Dry signal part), and the other is treated by the Belton brick (reverb signal), they are mixed in the end with a mix potentiometer so you can set the amount of dry signal versus the amount of reverberated signal.

The dry signal just goes through this section without being modified, and goes to the mix knob.

The reverberated signal is buffered by an OP amp (TL072), with a gain of one (so no gain basically). A 100pF capacitors in the loop rolls off a bit of highs, and the signal can enter the BTDR2H brick. The signal then goes out from pin 5 and 6 of the reverb. The high are roll off by a low pass filter formed by the "DAMP" potentiometer and C5. For more infos about low pass filters, read my post about the Big Muff tonestack. Thus, you can set the amount of trebles in the reverbarated sound. Then, the reverbarated sound goes through an OP amp in a similar layout than at the entry. The signal then goes to the mix knob.

If that were the only features of the Chasm Reverb, this reverb would be a simple reverb with a tone control. What makes this reverb unique is its feedback loop. A part of the reverberated signal can go in the feedback loop and goes back to the entry of the reverberation circuit. The amound of signal going back to the begining of the circuit is set by the Decay knob and the switch that let you choose between a 47k resistor (a lot of signal goes back: oscillation) and a 100k one (less signal goes back: more a long-decay like reverb). This is really cool because if you set a high decay, a lot of signal can go back in the reverb circuit, and it can actually autooscillate! It also allows to approximately set the decay of the reverb, which is not possible with a standard BTDR2 brick.

After the dry and reverberated signal are mixed with the "mix" knob, there is another knob, that acts as a master volume knob. It is wired as variable resistor, and acts as a classic volume knob. The signal (reverb + dry) can now goes through the output buffer.

Output buffer

The output buffer is a simple buffer using a single OP amp from a TL072 chip.

A 100pF in the loop rolls of a bit of highs. If the pedal is off, the dry signal goes through it with a gain of 1 (resistor R16/R13), but if the pedal is on, it has a bit of gain (R15/R16) to compensate the loss of volume due to the Belton Brick, the mix and volume knobs. It is a simple buffer, very transparent because of the high values of the coupling caps (C10 and C13, 10uF) and the use of the TL072.

There it is! I hope that it is clear and that it was helpful! Do not hesitate to ask questions in the comment. If you like this post, thank me by liking Coda Effects Facebook page!

To go further

JFET switching (pdf by Geofex): great explanations about JFET switching, around the classic Boss / Ibanez circuit.

Accutronics BTDR2 official webpage. 

Pedalrig tips about noise, great infos too!

Chasm Reverb official webpage, if you want to buy a built one or a PCB!

Jan Ray in a 1590B with Coda Effects PCB!

Here my latest version of the Jan Ray! This time, instead of the 1590BB or the 125B version, I made a smaller 1590B version, using a PCB that I designed! 

The Jan ray is quite annoying to make on vero, mainly because of the four knobs to wire. A PCB is thus quite helpful to make it easily. Moreover, it allows me to fit it in a 1590B enclosure. Here is the finished pedal:

I used the same look as before, with vintage orange sparkle prepainted enclosure, except that it is a 1590B now :)

 Make one yourself
It is quite a simple and fun build to do

 

I also used a sticker to name it: Golden Hour (vintage overdrive). Photographers will understand! I think it gives a really nice look to this pedal. 

Here is the gutshot:

It is the classical circuit of the Jan Ray, with the OP amp, and trimpo for bass content / overall gain. I just changed a few things: R4 is 1.8k instead of 3.3k. This mod allows you to have low gain settings, nice if you want to use this pedal as a clean boost, or just to modify your sounds. Playing with the pedal, I realized you can "stratify" your sound easily by adding trebles and removing bass. Sounded very good with my P90 guitar.

Another modification I did was to replace the OP amp (LM1458) with a Burr brown OPA2134. I think it "soften" a bit the saturation. A nice touch to it.

I am currently testing other mods, with the diodes (LED, compressed...etc.), treble filter (to have less trebles maybe), gain of the last OP amp (too much volume for me on this pedal)

I just made a circuit guide of it to help you understand what does what. The Big muff page has this kind of circuit guide, and I find it very helpful. I hope that you like it, let me know what you think!

If you want to experiment, socket the component you are interested in modulating, and try different values! Read the circuit analysis I made for more infos.

Sound samples !

LPB1 boost

Remember my LPB1 PCBs? I finally built one! As the PCB is quite small, I decided to make my first 1590A build. Here it is:

Simple one knob boost, with quite a lot of gain. I used a 2n5088, which provides already quite a lot of gain. It is a simple volume boost, quite transparent that can be used to make your amp saturate a bit more, or to simply increase the volume of your guitar if you use it in your amp loop.

This is my first 1590A, and everything went better than expected. I was afraid that I would not have enough space inside such a tiny box to make all the component AND the circuit fit, but it was OK.

Some advice to make it easier:

• Use PCB mount 3PDT. They are a bit smaller than "normal" 3PDT and let you a bit more space.
• Use semi-enclosed jack like Lumberg KLBM3 jacks. They are a bit smaller and easier to use than open jacks like the one I used.
• use 9mm pots.

Madbean pedals has issued a very nice guide to explain you all the tricks and tips about 1590A builds (pdf). I managed to make it, not the most impressive 1590A build ever, but nice though:

How does it sound?

Well, it is a simple clean boost. So you can either use it as a volume boost in front of your amp (if set clean), or in the FX loop, or to increase the gain of your amp if you have set it crunchy. You can also use it before a dirt pedal to increase the saturation of it.

I am planning on testing it in front of different builds. I already tried it in front of a Jan Ray build, making it basically a Tim, nice to have 2 gains settings in one pedal. I also want to try it in front of a Big Muff, like in the Musket Fuzz... I think it can fit in approximately any guitar pedal!

Circuit guide

I already did a circuit analysis of the LPB1 booster. However, I realized that sometimes, it is easier for beginners to understand the role of each component with an infographic, like the circuit guide of the Big Muff page.
Here is the one of the LPB1 booster :

Let me know if you like this kind of representations, I can try to update old circuit analysis with circuit guides like this one!

Big Muff circuit going to fab!

Here is my new Big Muff PCB!

Remember when I was telling you about PCB development? (yes, long time ago...) I finally finished one! It took me a lot of time, but now I am satisfied with my layout:

As some of you recognized, this is a Big Muff circuit! I decided to add an optional forth knob: the mid knob. I can decide whether I want to add it or not, simply by adding a jumper if not (and then, the tone potentiometer can be wired in the center of the pedal). Maybe I will add another optional feedback mod, allowing to connect Q1 emitter to Q3 emitter via a switch. It creates a nice noisy feedback, ideal for sound experimentations...

The circuit is exactly the same as a classical Big Muff, I just added the mid knob in the tone stack, and some components for polarity protection and voltage stabilization. (on the top-left part of the PCB) I will rename the components so their number will be the same as the one used on the Big Muff Page, so you can easily make any version of the Big Muff you like, or modify it with their great circuit guide.

Capacitors sizes were chosen bigger than needed, so they can be swapped with other value to fit any Big Muff model (Russian, Ram's Head, ...etc). Note the funky logo I made because OshPark is not able to use imported images...  Anyway, it is still nice!

I receive the first batch of prototypes:

It is a very compact build! It can fit a 1590B enclosure (well, I hope so...), or at least a 125B with top mounted jack. For now, I ordered a test batch of 3 PCB in oshpark, and I plan to test it as soon as possible! If it works correctly, I think I will produce a small serie of PCB.

As you may know, PCB are cheaper if ordered in large quantities. If you are interested in buying one PCB (or more!), send me an email, so I can adjust the number of PCB I will order! Price will be around 5-8 euros for the first batch (depending on the number of people interested), with a 1 euro shipping cost for France, and 3 euros abroad.

I first designed this PCB for personal use, but it can be a win-win situation if people are interested.

For now, I am thinking about the logo I will use, I thought of this:

I noticed that the "Pi" symbol, looked like a lot a Dolmen... Which is also convenient for a Fuzz that has been used by many stoner guitarists, including ones disguised as druids (Sunn o))), anyone ?). Maybe I will have it laser cutted in black or red (maybe purple for a "violet era" rams head), in order to have 2/3 different version. A bare aluminium version with the red logo that will be the "vintage correct" version, following a classic 73 ram's head fuzz schematic, and a black logo on a black or dark green (russian) box for a "doomy" version, very bassy and gainy.

Let me know what you think about it, post a comment!

Black Keys's Big Muff: dealing with mids frequencies

Another Big Muff clone! This time, I was inspired by the Black Keys (if you do not know this band, go check it out, it is awesome!). Dan Auerbach, the guitarist/singer, uses a lot of fuzz effects, and especially a green russian Big Muff, and a Earthquaker Devices Hoof (a Big Muff variant with some interesting modifications as we will see later).

So I decided to basically mix these two variants in one Big Muff! Here it is, a 4-knobbed Big Muff:

LPB1 mini PCB !

The LPB1 (Line Power Boost 1) is a boost pedal that was commercialized in 1968 by Electro Harmonix. It is the first boost pedal using a silicon transistor. Indeed, previous boosts like the Dallas Rangemaster Treble Booster were using germanium transistors. Another novelty with this pedal was that it was boosting all frequencies and not only trebles, making it the first clean boost available!

 

The circuit is really the most simple one you can imagine for a boost:

First, there is a 0.1uF coupling capacitor that prevents parasitic DC current from the guitar to go in the circuit. With the R2 resistance, it forms a high pass filter : by changing its value, you can modulate the amount of bass going through the circuit. If you increase C1 value, more bass will go through, and vice versa.

Then, there are two resistors forming a voltage divider (R2 and R1), to provide a certain voltage to the base of the transistor. Here it is : R2/(R2+R1)x9V = 43/(43+430)x9=0,81V at the transistor's base.

The silicon transistor is a 2n5088 (originally a 2n5133 - same transistors that were used in the Big Muff later), wired as a common emitter. R4 and R3 will define the amount of amplification. If you increase R4, amplification will be larger. If you increase R3, there will be less gain.
A second 0.1uF coupling capacitor that prevents DC current from the battery to go out of the circuit. Finally, a 100k potentiometer wired as a variable resistor defines the final volume.

If you look carefuly, you can see that the last stage of the Big Muff circuit is exactly the same! A LPB1 circuit is used to increase the final volume.

Indeed, this circuit can be used in a lot of different effects to boost the entry or final level. A fun thing to do is to add a LPB1 boost before a saturation effect to increase the gain. It is very fun on a Big Muff (like in the Musket Fuzz or Supercolider fuzz), or on a Tube screamer (like in the Fulltone Fulldrive)!

In order to be able to add this little circuit when I want, or to test it on several pedals, I decided to make a small PCB that allows you to add a "boost" knob on every pedal you want. There is a "in" and a "out" pad that allows you to place it anywhere in a circuit:

I will test it on a Big Muff or a Jan Ray! I could also try to see if it can fit a Hammond LB enclosure (super small squared enclosures like the one I used for my Strymon Favorite switch) Note that there is no voltage stabilization or polarity protection system on this board (too small, not enough space for it!), so do not hesitate to wire a 100uF capacitor between the two lugs of your power supply input.

I should receive all the parts to test it soon...