Prentiss Effects

Today we're going to be looking at the Friedman BE-OD, a distortion pedal that was designed to emulate the Friedman Brown Eye Amplifier. If you're wondering how it sounds, just go check out the demo done by Pete Thorne here. While he makes everything he plays sound great, this pedal was really easy to dial in for various guitars because of the powerful EQ controls and internal gain trimmer. The gain itself is really nice, you can get a wide range of sounds from it, although you can't really get super low gain sounds unless you're rolling off the volume on your guitar. But if you're using this pedal then you're probably not looking for low gain anyway.

This pedal was built using the Thermionic Distortion PCB from PedalPCB. I really liked working with it because it's laid out really well and designed for top mounted jacks in a 125B sized box. The board version I have uses 1/8W resistors (my first time with those) but the updated one uses standard 1/4W. First, we're going to look at the theory behind the pedal and then I'll talk more about the build process and board itself.

Theory

It only seems natural to use the schematic from the Thermionic Distortion build doc, since that's the board I used, and the guys over at Pedal PCB were really cool and gave me the go ahead. I've gone and marked it up into 7 blocks that we'll be examining. While it looks like a lot, the building blocks are fairly simple. The signal path consists of 4 cascading gain stages like a tube amplifier might use, some power filtering, and various filters to shape your sound. So let's dive right in.

Power Supply

Let's talk about the power supply. This one might seem more intense than most pedals and that's for good reason. High gain pedals tend to get noisy, especially if they're not filtered properly. Here, D1 is acting as a reverse polarity protection diode in case the pedal is hooked up to the wrong power supply. The resistor that follows, R30, is a current limiting resistor that will blow if the pedal starts drawing too much current.

Then we have the voltage filtering. C20 is smoothing the majority of any DC ripple that existed in the power supply. It's followed by three 100nF caps which are used to filter the power between pins 8 and 4 of each of the ICs to prevent any noise in the signal path.

Lastly, there's a voltage divider created by R32 and R33 which is buffered by an op-amp in order to make sure each gain stage receives a constant current.

Input Gain Stage & Tight Control

This is a really simple stage. First we have an anti-popping resistor, R1, for when the pedal is switched on. The signal then goes through the input capacitor, C1, and through a low-pass filter, formed by C2 and R3, which removes any high frequency noise.

After all of that we get to the first gain stage. This gain stage is made using a non-inverting op-amp configuration which means we can calculate the gain with this equation:

2nd Gain Stage

The other interesting part is that there's a cap in series with the Rin resistor. This forms a high-pass filter with a corner frequency of ~72Hz, so all of your bass will pass through. Then there's a set of LEDs used for soft clipping and a 47pF cap to smooth out some of the treble content. Overall, this looks a lot like the tube screamer clipping section. The major difference is that it uses LEDs (a common TS mod) and the corner frequency on a Tube Screamer is much higher, cutting out a lot of bass (~720Hz). This section doesn't need as high of a cutoff frequency because the tight control in the previous stage is allowing you to choose it yourself.

3rd Gain Stage

The signal first goes through a coupling capacitor to remove any DC offset from the previous stage. This capacitor also acts as part of a high pass filter in tandem with the overall impedance of the next stage to limit the amount of bass passing through.

Next we have an inverting op-amp configuration. This is actually simpler to calculate than the non-inverting op-amp. So let's take a look at the equation.
Now, if you're wondering why we have a negative amplification, it's because we're using an inverting op-amp configuration. It's flipping the phase of the signal, not reducing the amplification of our signal. So we really have an amplification of 10 for all intent and purposes.

There's also some more clipping diodes and another smoothing capacitor like the 2nd stage has. One cool thing is the additional 22k resistor. It's not a part of the gain calculation, it's actually acting as a "knee-softening resistor". It helps to smooth out the conduction of the diodes in the feedback loop, which makes the drive more tube-like.

4th Gain Stage

For the fourth gain stage we have another inverting op-amp configuration. This one's interesting because the feedback resistor is a combination of the 10k resistor and the 100k trim pot. The math is pretty similar to the previous gain stage:
That's really all there is to this stage. Nothing real crazy

Presence, Hard Clipping, and Treble Control

The Presence control is a simple RC low-pass filter, but it's really useful. The 2.2k resistor sets up a maximum corner frequency of 7238Hz. When the control is all the way down we have a total resistance of 12.2k which translates to a corner frequency of 1305Hz. You really get a lot of tonal variation out of that control.

Next we have a hard clipping phase. Again this is really simple, just two LEDs that run to the 4.5V bias reference (VREF). This is the kind of clipping you'd expect to see in a distortion pedal. We're using VREF instead of ground because our signal hasn't run through a coupling capacitor yet, so the AC part of the signal is still centered around 4.5V. By using LEDs the signal will clip at a higher voltage threshold which gives another character to the gain compared to some standard silicon diodes with a lower threshold. By varying the trim pot in the 4th gain stage you can change how these diodes clip. This is used to simulate the way a power amp distorts when changing the master volume.

Lastly, we have another low-pass filter that's forming the Treble control. Now since the Presence control is taming the high end before this stage, we don't really need to set up a maximum threshold. With the Treble control all the way down we have a corner frequency of 338Hz which is crazy. At this point the control is basically being used to cut off any excess treble that is not tamed by the Presence control.

Bass Control and Volume

The bass control is the trickiest part. It's active filter based on a bridged-T feedback network. Now that means something to someone, but that someone is not me. From what I've found, it involves very long equations and some complicated math that uses a bit too much greek for my liking. I'll break it down as best as I can.

A bridged-T feedback network is basically a low-pass filter mixed with a high-pass filter. It's usually a passive filter that creates a notch (cuts the frequencies) but when it's in a feedback loop it becomes a boost.

This filter is designed to boost the bass frequencies around 70hz. The two capacitors are used to form a ratio that determines how dramatic of a boost there is. The resistors are typically the same value; this is convention and helps in getting more predictable outcomes. They, in conjunction with the capacitors,help determine the frequency that is being boosted.

There's a lot of interplay between these components and it can change a lot by messing with one value. For the most predictable results you'll want to change values in pairs. For instance, changing the 22n to 10n and the 220n to 100n moves the frequency to about 150hz. If you leave those capacitors alone and change the resistors to 3.3k then you get a mid boost around 700hz (a factor of 10 higher). Generally speaking, decreasing the value the parts will shift the boost upwards in frequency and increasing the values will shift it downwards in frequency.

For a bit more information (and some complex equations) on how this works check out this site. Just know that the explanation is for a passive configuration. The active configuration is what turns it from a frequency cut (notch) to a boost. I simulated this section in LT Spice (a circuit simulation software) to figure out how it works and I highly recommend doing that before changing any part values. Simulating it will give you an idea of what to expect when you change a part and how it plays with the other pieces of the circuit

The potentiometer is used to limit how dramatic the boost is. At its lowest value it will boost 70hz the most because it's essentially out of the circuit. When it's all the way down, it's at its maximum value and is limiting how much the filter can actually do it's job which reduces the boost.

After all that complicated filtering, the signal runs through another cap and resistor and then to the volume knob which is just a standard voltage divider configuration. This is a pretty standard volume control for most pedals.

Pedal PCB Shoutout

Sources:

Jack Orman's RC Filter Calculator - http://www.muzique.com/schem/filter.htm
Op-Amp Basics - http://www.electronics-tutorials.ws/opamp/opamp_8.html
Soft Knee Resistors - http://www.diystompboxes.com/smfforum/index.php?topic=82764.0
Bridged-T Filters: - http://www.johnhearfield.com/RC/RC4.htm

Hey guys, I'm gonna be taking a look at the TC Electronic Hall of Fame 2. I was super excited when Amazon accidentally leaked this a while back. They were finally going to be adding shimmer and multiple TonePrint settings! I was already a fan of the original Hall of Fame, having used it for about the past 3 years, so these were updates I was really looking forward to. I ordered one off of amazon as soon as they popped online and two days later it was at my doorstep. If you're looking for a quick rundown of what I think and the new features then #scrollharder down to the bottom and there will be one. But now we're gonna dive in.

Algorithms

The new Hall of Fame looks like it has less algorithms, but that's not the case. Some of the less commonly used ones were removed from the front (such as the gated reverb) but are still accessible from the TonePrint editor. The original algorithms, such as Spring and Room, are mostly the same. They have subtle differences from the original due to incorporating the MASH technology. Also, they all seem to have a slightly longer decay than the original which I personally like since I get a bit more range to work with.

Let's talk about the shimmer algorithm. This is one I definitely wanted and wished the original had and now it's here. You can't say anything cliche about it because there's nothing "natural" about a shimmer reverb. Overall, I'm really pleased with how this turned out. Other shimmer reverbs I've played take too long to build into that sound and it feels uncomfortable to me, but this does not have that issue. The octaves bounce back into the signal fairly quickly and builds that nice choral sound you expect from a great shimmer. You do occasionally get a bit of modulation if you do some super complex chord voicing but it disappears in the mix when playing live. I will say that it still falls a bit short of the Strymon Shimmer that made this setting popular, but only just. That being said, I can get into that shimmer setting in the editor and tweak the small things that I want to change, but I can't really do that on the Strymon stuff (at least not that I know of). In the end, I think it's a great shimmer and easily the best at this price range.

MASH

So this is the new big thing from TC and...it's actually really cool. I was worried at first that this would be super gimmicky and not actually very useful. It turns out that it's really easy to work with and easy to operate, effectively, in a live setting. By holding down the footswitch and varying the pressure you use, it acts as an expression pedal. There's a little rubber grommet underneath the pad of the footswitch that helps you feel how hard your pressing and it's quite easy to operate. The parameters it controls also changed based on which setting your using. In the shimmer mode, it controls just how much shimmer is in your signal and the decay time of the reverb.

I know TC gets a bad wrap on their footswitches, but that's really just from one bad batch they had. This one feels really sturdy and I don't have any concerns about it going bad on me.

But the real magic of the MASH function comes into play with my favorite thing about this pedal. The...

TonePrint Editor

The editor got a fantastic upgrade in this version. In the original, you could control parameters for the modulation, the reverb, and what the decay and tone knobs controlled (up to two parameters for each). In the new version, you can assign up to three parameters to each control knob (that's one more knob guys!), and you have additional access to a bunch of parameters for the octaver. Lastly, you get to dial in how you want your MASH to work. That part is really cool because you can set it to fade things in and out depending on the pressure for cool crossover effects. It's also really simple to set up the mapping for your mash. You can pick a parameter from a drop down menu (up to three) and then scale it on a little graph to decide how it changes as you press down.

Summary

TC has made another great reverb for the money. The algorithms are really well designed and deliver where it counts in the mix. Having multiple TonePrint options makes the pedal incredibly versatile and customizable with the editor (which has gotten a nice upgrade). The shimmer setting doesn't quite get you the "chorus of angels" sound that folks like Strymon are known for, but it's still very good. Lastly, MASH function is well thought out and easy to use, rather than gimmicky, and the footswitch is very sturdy and holds up well in live use.

The 1776 Unicorn Breath Buffer is based on the buffer circuit in the famous Klon Centaur but with a little something extra that makes it a really cool/handy little project.  I'll be going over the bare bones of why you would want a buffer in your signal chain and then what makes the Unicorn Breath unique

What a buffer does...

Everyone's heard the debate over buffered vs. true bypass pedals so I won't be going over that in this post. But general consensus is that true bypass is great, unless you have a bunch of cabling and pedals between you and your amp. The extended cable lengths (through cable capacitance and signal impedance) create a low pass filter that cuts off more of your high end as the cable length increases. A buffer near the beginning of your signal chain alleviates this issue by lowering your signal impedance which raises the cutoff frequency of the high pass filter. I won't go into any more detail on that here, but if you'd like to know more then leave a comment below. On to the board!

Unicorn Breath Buffer

There's a bunch of reasons I like the Unicorn Breath. It's designed around being a DIY project so there are some aspects of the board layout that are really geared towards the builder. That being said, I'm just going to go through my favorite things about it.

1. It's really small. The board itself is only 1" x 1". This makes it perfect for small boxes like the 1590A or 1590LB. It's also a great size for modding pedals like the Ernie Ball VP JR because it fits neatly inside the faceplate. 
2. It has the LED board mounted. I like this because I can just wire in the LED without thinking. Plus I can mount my LED without a bezel if I want to. 
3. Uses a dual op-amp. Most Klon buffers call for a single op-amp. This uses a dual op-amp pinout which is a more common part that you're more likely to already have (hooray for no special order parts!).
4. Voltage buffer. Your reference power voltage is buffered. This means that if you were to use the Vref on the board in another circuit, then the current draw wouldn't be an issue. It's not strictly necessary for this circuit, but it's a nice feature and uses the other half of the op-amp.
5. Extra ground ports. I like have extra places for ground so I can wire my power supply to the board as well as my jacks rather than having to snake wires all over the place. Plus most people don't wire a buffer as true bypass, so you can do all your connections to the board.  
6. Tight layout design. There's no wasted space on the board. All the components are neatly aligned with no strange spaces in it. It's just quite nice to look at both before and after it's been populated.

As far as sound goes, it's a buffer. Because it's IC based it's going to be unity gain (transistor buffers tend to have a tiny amount of loss). If you aren't currently running a buffer somewhere in your signal chain then build one of these. You'll get some high end back that you didn't have before which may mean tweaking your knobs a bit, but it also means you'll get consistent sound regardless of cables or number of pedals.

Overall, I really like this board. It's an incredibly useful circuit and the layout is just great. It can fit in a small box that won't take up a bunch of room on your board and your getting a great quality IC buffe. Plus, it's a fun, quick build due to the low parts count and layout.