PWM Phaser using PIC DDS LFO and 74HC4066 analog switches as synthetic resistors

I finally got this thing working well. I am using a TL5001 PWM chip  at 48 Khz to pulse width modulate analog switches in series with 10k resistors. The average resistance seen by the all pass filter circuits is proportional to the on time of the switches. For example is the switches are only on half the time, the average resistance is 20k instead of 10k. The advantages of this approach are that there are no critical adjustments, good dynamic range, and extremely linear resistance change in proportion to control voltage. The current draw is 25mA or so…not great but better than the 50mA I started with. The LFO is a DDS sine generator using a PIC micro controller. I will include a link to the code and a working hex file. I only use a sine wave output but the code can easily be modified to support numerous wave forms and frequency ranges. The  code for this project has been modified in two ways from the DDS code described in my other blog entries. I lowered the internal clock frequency to 8 MHz from 32Mhz(saves current) and reduced the output swing of the sine wave so it did not drive the PWM chip beyond 90% duty cycle.  Originally I just use the PWM directly from the PIC with no filtering but found this created noise artifacts, so I filter the output and drive a PWM chip. Typically, phaser circuits like this use high pass filter structures at the positive input, I chose to use low pass to help eliminate the switching noise from being introduced into the output. This worked well and eliminated the need for extensive filtering. The output is very clean

One thing that is very cool about this design is that it has variable sweep range instead of depth control. So the frequency range of sweep of the phasing notches can be adjusted starting at 100Hz or so all the way up to 5KHz.


pwm phaser



Link to code:

New phaser with Homebrew 1×4 Vactrol and DDS LFO

I decided that I  should build a Phaser using my PIC DDS LFO. I designed a PWM version using CD4066 analog switches but this had issues. It just drew to much current and had some noise artifacts. I got it working but it became more complicated  than I liked. However, the design was easily modified to use a homemade vactrol. The vactrol uses four photcells glued together to make a square and then the LED is glued right in the center. The whole thing is then covered in a piece of heat shrink, electric tape or whatever.  Performance is excellent and the circuit is simple. Of course the DDS can be replaced by some other conventional LFO circuit. The main issue is operating the LDR’s in a good range to get even and adjustable notch sweep. With this design, the sweep is adjustable and you can even eliminate one notch by pushing its position sub audio. The sweep controls the level of illumination of the LDR’s and thereby adjusting the total range of phase shift. The LFO uses my PIC DDS circuit and generates a Sine wave envelope. The code can easily be changed to generate other wave forms. I am going to include a link to the a zip file containing all of my source code. This was all compiled using MikroC. The program is small enough to compile in the freeware version of MikroC or you can just use my compiled hex file.

More detail on the PIC DDS is available on this blog in earlier post.

Link to source code and  hex file:

Completed Phaser:



LDR phaser

Etched Circuit Board:

etched phaser brd

Completed Board:

phaser board

Homebrew Vactrol:

quad vactrol

The Kool-Verb

This is a simple but nice sounding reverb based on the FV-1 dsp chip. Lots of low pass filtering gives a nice washy sound. You can adjust from a small room to a large hall.  The unit provides about a 75/25(dry) max reverb. Anything more than that is useless. Check out the stomp amp demo for how it sounds…same reverb, but just with just one fixed room setting.

The switch mode power supply keeps the current down to 30mA so you can use a battery if desired. You can use a linear regulator and it will work great, but then the current consumption doubles.

The schematic:


built into a 1590b case:

kool verb


The PWM Squeazal revisited

I have enjoyed my PW modulated compressor and decided to make a feedback(the original was feed forward) version with improved attack and shorter decay response. The result is excellent – I love this version. I use a Zetex current monitor IC as a full wave rectifier and this lets me really speed up the attack. So the response is super fast, very low noise, very low distortion, and very large dynamic range.


SchematicPWM COMP1_2_4

Link to design files:

The Stomp Amp- A Battery Powered 25 watt Stomp Box Amp

This is an idea I have had for a long time and I finally designed one. The results are excellent. The amp uses a surface mount car stereo  power amp IC and can produce 25 to 30 watts with a 16V-18V supply into a 4 ohm speaker. The circuit fits in a 1590bb enclosure and has Reverb, Bass, Treble and Gain controls. The tonal response is tailored for guitar in the OP amp stages, along with the James/Baxandall tone stack. The James/Baxandall is a versatile choice because it provides boost and cut. I find it a better choice then the “beef stew” fender tone stacks. Reverb is provided by means of an FV1 DSP IC. It can be omitted easily if desired. The Power amp IC is bonded to the case to provide heat sinking when it is cranked up. I used a piece of  1/2 inch copper pipe -reshaped to be the heat sink. As I have done with some of my earlier amplifiers, I have employed negative feedback from the speaker back to a discrete stage driving the final amplifier. This is a common practice in tube amps to flatten the tonal response of the output transformers and so it is unconventional to apply it here. I find it affects the over all sound in a pleasing manner. The amount of feedback is small and could be increased or removed (this will affect the bias of the JFET Q3) All of the signal chains are low impedance (except the input) and gain distribution is such that the amplifier is very low noise.  The amp powers up when the input is plugged in. The output is not ground referenced so the output jack is isolated from the case -(which is grounded via the heat sink). All you need is a tool battery or 12-18V power supply and a speaker cabinet and you can blast away. The mosfets used for polarity protection and power switching are just high current PMOS devices and not special – lots of other devices will work here. The op amp is a low noise type with a wide supply voltage range – others will work here also.

Things to consider when building:

The amp IC  I used was a surface mount version – and I flipped it upside so I could bond a heat sink to the ground. there is also a leaded version available

Decoupling is critical – especially for power amp – the double decoupling caps on the schematic are one set at each of two VCC pins on the power amp

Capacitors C3 and C11 weight the amplifier to tonally for guitar – these can be changed making both 10uF for example will work fine. If you want to use a Bass guitar make C3 at least 4.7 uF

The passive tone stack was chosen to attenuate gain so that the FV-1 would not be overloaded.  Other tone stacks can be substituted just keep this in mind if you use the FV-1


The Stomp Amp along with some other pedal designs of mine




Schematic Diagram



Minimized implementation of the Tube Screamer

The Tube Screamer is a classic all around great sounding distortion, even though the stock circuit is not very complex, by eliminating the JFET bypass switching and utilizing true bypass you eliminate quite a few parts. Even more, you can eliminate the input and output follower stages a number of coupling caps and bias resistors. With just one op amp and less than 25 parts, you can make a tube screamer that sounds just  great. In my implementation, I changed some values in the filter section to suit my taste and on the first stage I used BAT46 shottky diodes. These diodes have substantial reverse leakage- so you have to lower the impedance of the feedback pot and resistor that controls the the drive gain. That is why I used a 50k pot and 100 ohm resistor. I used the shottky because they start to clip much sooner and you can get a really insane amount of overdrive. Regular silicon diodes work just fine. The original stock values can be used and can be found on the web.




Demo (coming soon)

A Simple LDR Envelope Filter that is just delightful

I designed  a low pass type envelope filter using a Fliege filter topology . The result was a really easy to build filter that provides1st order low pass ..all the way up to 2nd order low pass with extreme bandpass peaked low pass(high Q, resonance…etc). I really like this pedal.


Demo Sound Clips:

Short attack, max intensity:

Mid Attack, low intensity:

Long attack, high intensity:

This filter topology has some nice features, such as:

Fixed stable gain of two in the pass band, regardless of Q

Highly adjustable Q

Uses only two op amps

Low parts count in general

Well suited for single supply operation( in my case I just direct coupled the input to a half supply biased input stage)

It also has a couple of drawbacks:

It requires three LDR’s unlike  the Mutron for example which only uses two

I could not get a TL072 to work well in the circuit. I needed to use a higher bandwidth, rail to rail type op amp, but there are plenty of amps that work well.

The OP Amps I tried were the OP213(works fine but expensive), OPA1652, and the TS922. These are all low noise, high bandwidth devices. The TL072 broke into oscillation and distorted. Maybe someone else can get it to work? I don’t have time to figure out the problem but I assume there is some sort of undesired phase shift making an oscillator out of the thing.

The LDRs can be commercial units but I just built some  from scratch – they were easy and cheap to make. I used flat faced green LED’s and small CDS photo-resistors. I super-glued them together and then just cover with heat shrink to make them light tight. It took me about ten minutes.


LED glued to photocell:


Final device:

The  parts I used are: LED digikey part # VAOL-5701DE4-ND and photocell  digikey part # PDV-P9008-ND

Circuit description:

Pretty simple – A emitter follower buffers the input and provides 1/2 supply bias for the the filter op amps. The Filter uses two LDRs for the filter sweep and one more for the intensity or Q of the filter. When LDR3 is .707 the vaulue of  the other LDR’s (1 and 2), you get an ideal 2nd order low pass characteristic. When LDR3 is higher you get bandpass peaking and this can be quite extreme. As the filter sweeps, the ratio between the LDRs must be the same, that is why you need the third one and not just a variable resistor.

The LDRs are driven by a current mode amplifier and a simple diode peak detector. I used a LM358 for this but the the OP213, OPA1652 and the TS922 used in the filter section will also work. The design provides a sensitivity control, intensity(resonance) control and an attack control. The attack control really makes it easy to dial in the sweet spot of the filter and get very snappy or very slow plodding whah whah effects.

Circuit Schematic:

Minimu board

Populated surface mount circuit board:

Link to silkscreen:

Click to access minimussilkscreen.pdf

Link to Top layer surface mount layout:

Click to access minimur.pdf

Switch Cap Low Pass Envelope Filter

Here is an easy to build  envelope filter using a MAX7410 switch Cap 5 pole low pass filter. You can only get low pass effects so its not as versatile as a mutron III, but you can get a nice range of effects from subtle wah to classic Jerry Garcia funk. Unlike a Mutron, you can adjust the attack which is a a very useful feature.

Schematic Here:

Layout for 1590B:

Click to access envelope-silkscreen.pdf

Click to access envelope-layout.pdf

(expresspcb file link:)

Audio Sample:

Updated Squezal PWM Compressor Schematic, Layout and Demo

I have completed the PWM compressor design and created a layout to fit a 1590B enclosure. The design has been simplified some and has excellent performance both in feed forward or feedback mode. I removed the full wave rectifier circuit. A simple compensated peak detector works fine. I also implemented a analog switch instead of the PFET.   It provides a little more duty cycle range and significantly more input  headroom.

The original blog post for reference:

The schematic is highly annotated – the original PFET switch can be used in place of the analog switch if desired.

Schematic Here: updated(9/12/2013)

Compressor Demos:

Link to Design Files: (Done in epresspcb single sided thru-hole)

To scale PDF image of layout for toner transfer: (you will still need to look at the expresspcb layout to see part designators)

Click to access squeazal-single-sided.pdf