Breadboard a Simple Phaser

I like watching That Pedal Show on youtube from time to time. Their video What Guitar Pedals Should I get First? is suggesting that my first three pedals should be some sort of distortion, modulation and time based pedals. I should’ve said – spoiler alert … too late now. As suggestions go, they are matter of personal taste, but this seems like a good advice as any ?.

If you go to articles page there’s a bunch of gain based pedals – boosters, fuzzes, distortions and overdrives. So I have distortion pedals covered (and boosters!) Let’s move on then with modulation and start with a widely used effect – phaser. In this post I’ll cover a phaser based on MXR’s Phase 45 (first issued in 1974 … wow, two more years and it’s going to be its 50th anniversary! … or it may have already been, depending on when you’re reading this)

I’m a great fan of Pink Floyd and David used phasers a lot in some of theirs most famous songs. But phaser can be found in all genres of music so I think this is great one to do. I chose Phase 45 because it is supposed to be the simplest one ?.

Here I cover the basic schematic for the circuit, cover some interesting details, there’s even SPICE simulation if you want to do some analysis yourself. I’ll move on to put the effect on a breadboard and try it out. I also added bill of material (of components that I used for breadboarding).

Schematic

The schematic is not exactly Phase 45, mostly due to components I had available a bit (in reality it’s a cross between Phase 45 and Phase 90).

Here’s my version of the schematic:

The Differences

It’s funny how I put there “Phase 45 like clone” on the schematic ?. Most of the changes I added are due to components I had around and I used Phase 90 schematic as the guide as well. So it’s mostly Phase 90 with phase shifting stages from Phase 45. The changes should not affect the sound though.

It is important to realize that this is the first take on the schematic. Arguably, I could’ve done full clone, but I don’t have the exact components and realistically, it’s going to be though to find exact components for any 50 year old pedal.

The most important change in all reality are JFETs. I put J113 on the schematic, and for this first take I opted for ones that I know will be in the range I needed them to be. Later, when I experiment with the pedal I’ll see if I can get better sound or control with others.

Interesting Bits

For in-depth analysis go to Electrosmash, I can’t do anything better ?. I’ll just point out some interesting things about the circuit, and will point out the differences because the aforementioned analysis is for Phase 90.

Also, if you want to see more detail – watch my video. I think I may have spent too much talking on it.

Input/Output Stage and Reference Voltages

Input stage – I just have a buffer there, nothing special with added pull-down resistor for helping with switch popping.

The output stage is common emitter amplifier with PNP transistor. PNP is used probably since it’s easier to bias it in this configuration. The amplifier is also used as a mixer, mixing wet and dry signal in 50-50 ratio (R10 and R11 are making sure of that).

The section providing reference voltages is slightly different from what we have encountered before. Zener diode is providing our reference voltage for op-amps (Vref on the diagram), but there’s also a trim pot that allows us to adjust biasing point of JFET transistors (Vbias).

Anyway, nothing fancy so far, this could work in any pedal build. The next couple of sections are the meat of the pedal.

LFO

Low-Frequency Oscillator – LFO, is what drives our sweeps:

I spent a lot of time on video going through this. There are couple of interesting points though that I forgot to mention. I took this from Phase 90, and there are slight differences between Phase 45 and 90. R18 resistor is 150K in Phase 45. That might affect how much of a sweep the LFO does.

Also, I think I mentioned it in the video but let me cover other differences. I did not have 4M for R15 and I used 3M resistor instead. That might affect biasing a bit actually, that might affect which transistors can or cannot be used in the circuit.

I used R16 there to limit the maximum frequency of the oscillator – 4.7K was used in P45. Also, I did not have 15uF electrolytic cap handy so I used 10uF. That might cause difference around max/min frequency.

More I think about this … more this becomes Phase 90 with two stages removed ?. Anyway, I’m not gonna go into huge amount of detail just yet. All components mentioned above, I’ll see if it’s worth experimenting with them. Now that I think about it, only R18 might affect sound (how much of the sweep happens), the rest of them only affect “controllability” (max/min frequency).

Phase Shifting Network

The main part of the effect that is shaping the tone is this:

It consists of two stages of all-pass phase shifting networks. What this really means is – the signal is not filtered or adjusted, only the phase of the signal is shifted. If we only listen to the wet signal, before it’s mixed with dry signal, we should not notice much of the difference (since we, humans, don’t really perceive phase change much).

To get some change we need to mix this phase shifted signal with the original input. This is because if the signal is phase shifted 180^o, when mixed with original signal, wet signal and dry signal will cancel each other out.

That sounded slightly non-sensical. The above would be correct for a pure sinusoidal wave. For guitar sound, which is more complex, this means that a portion of the signal that corresponds to the frequency that has been shifted by 180 degrees will be cancelled out. This way we still get a lot of signal, only certain frequency is filtered out (hence the famous notch on the frequency domain plot).

Now imagine moving that notch up and down (that’s what those JFETs are used for), you get the phaser sound. See my video, I go into slightly more detail there. Phase 45 has 2 stages and one notch, Phase 90 has 4 stages and 2 notches, Phase 100 has 10 stages ? (but slightly different implementation). More notches, more dramatic effect. I’ll cover this more in my next post.

SPICE

Here’s a handy SPICE circuit diagram you can play with:

It has all the parts I covered in the video in case you want to play around. I deliberately did not include any diagrams here, I’ll go into more detail once I start experimenting with the circuit. But that does not prevent you from playing with this ?.

Breadboard Diagram

Let me prepare a breadboard diagram for this. As usual, I used DIY Layout Creator. Here’s a possible layout:

There’s a lot of components there, I wanted to make sure I get this right, that’s why I planned it out. Also, I wanted it to be as neat as I can possibly get it so I can replace components when I play around with it (in one of the next posts).

Bill of Material

Here’s the list of components I used for breadboarding:

Breadboarding It

Let’s do the actual breadboarding and hear what the effect sounds like. Here’s the final version of the breadboarded effect (looks the same as the diagram above, right?):

That was a success I thought. Next thing to do is do some experimenting with the effect and see if there are changes/actions we can do to improve the effect.