Reinventing the wheel!

mictester · Post by **mictester** » 20 Jul 2010, 06:34

I've been trying to get rid of the need for LDR or FET "variable resistors" in a phaser, and I wanted to avoid the noise and distortion of transconductance amplifiers (like in the Small Stone or Ropez). The problem with using LDRs is that you have to use quite some current to illuminate them, and if you use LEDs, the illumination against current isn't at all linear, and can give you a "dead band" where the LEDs are off unless you take steps to offset the oscillator voltage. With the FET approach, you have to match the FETs, and I can't be bothered to go through a big bag of transistors trying to find four, six or even eight with roughly the same characteristics!

A few years ago, I designed a "switched-mode attenuator" for an audio limiter (for broadcast use) that used FETs as switching elements across the audio path, and these were switched hard on and off with a pulse-width modulated squarewave. The pulse width was derived from the level of the incoming audio with a CMOS oscillator that had a mark to space ratio varying from 1 : 9 to 9 :1. This completely eliminated the need for matched components and gave great results, particularly as the audio was delayed by a time equivalent to the attack time of the limiter between the level sensing point and the attenuator. The switching frequency is well above the audio range (~40 kHz), so only a simple lowpass filter is needed after the attenuator stage.

I thought - "let's use PWM into some 4066 gates and use these as the "variable resistors" in a phaser". Two 4066s would allow eight stages! I used my same old CMOS PWM oscillator (at about 40 kHz) and a conventional chain of op-amps. It sounds great, and can be envelope swept too! I was really excited, and was about to put the thing on this board as a new way of doing things.....I then remembered the MXR Commande Phaser...... damn! There's nothing new under the sun!

However, despite my disappointment, I'll finish the PCB and Vero layouts and put the thing up here. Build it. You won't be disappointed. It's the best phaser I've ever tried!

~arph · Post by **~arph** » 20 Jul 2010, 07:38

Yes a few years ago I came across a pwm phaser in an electronics book. It was a modular one that had one audio/lfo board and could then be extended with additional boards with four stages each. I believe in the example they built a 24-stage phaser. I specifically recall that it had quite an interesting way of pwm generation. I'll see if I can find the schem. I have made some scans.

EDIT:

It is in a book called: Praktische filtertechniek, from Harry Baggen of Elektor.
It is from 1996.

I found the scan (photo actually) of the lfo section, but sadly not the phase section. It needs some work to make it more readable though.

analogguru · Post by **analogguru** » 20 Jul 2010, 07:46

Such a design has been published in the 80´s in the Elektor magazine and was sold such a phaser by MXR in the Commande series too (M-161). Schematic should be online in the web.

analogguru

Dirk_Hendrik · Post by **Dirk_Hendrik** » 20 Jul 2010, 08:25

Take a look at the ADA phaser as well. Uses this principle as well.

HEAD · Post by **HEAD** » 20 Jul 2010, 12:03

Hi,

nothing new under the sun, ok. But at least this sounds quite interesting. I'm looking forward to this as I'm realy keen in phaser.

Never heard or read about that way doing it until now. I guess now I've got some lecture to read while being burned lying at the lake.

Cheers
Helge

Dirk_Hendrik · Post by **Dirk_Hendrik** » 22 Jul 2010, 14:41

fyi:
http://www.tasteundtechnik.de/bilder/adafinalph1.jpg

borislavgajic · Post by **borislavgajic** » 22 Jul 2010, 15:22

this is how it sounds

Boris

dutom · Post by **dutom** » 23 Jul 2010, 10:00

Hello!
found this at http://www.synthdiy.com/: 6stage Audio Phaser project 447 in Electronics Today International 1976.
I´m curious about the inventions of mictesters version

Audio Phaser eti447.pdf: (390.24 KiB) Downloaded 314 times

borislavgajic · Post by **borislavgajic** » 23 Jul 2010, 11:59

I have build this one....it is nice project....

.but not PWM phaser.......
this one uses set of FET-s from 4049 chip

...some new idea from me...take 6 -8 - 12 potentiometers....dip them in canister with liquid V/2 .....move them dipper ,or get them out of liquid that is going to change resistance in every phaser stage.........it is simmilar to DeArmond tremolo pedals........

Boris

mictester · Post by **mictester** » 23 Jul 2010, 12:31

dutom wrote:Hello!
found this at http://www.synthdiy.com/: 6stage Audio Phaser project 447 in Electronics Today International 1976.
I´m curious about the inventions of mictesters version
Audio Phaser eti447.pdf

It's a completely different principle - the ETI circuit is using the 4049 as FET "variable resistors". My approach uses CMOS switches driven by a high frequency (~40 kHz) Pulse Width Modulated clock, to vary the amount of signal passed to the virtual ground at each stage. This has a number of advantages, including no concern about the accuracy of the component values, low noise (though the latest version has companding to allow it to be used in a studio), and very low distortion. The use of the FETs as variable resistors will introduce both noise and distortion.

My approach is very economical, and doesn't use any unusual components or try to use any components in modes that they are not intended for! Using a 4049UB as variable resistors is quite a good approach, but it's a very noisy device, and susceptible to static discharges (unbuffered CMOS is notorious for getting fried by static!).

The latest version I've built uses a TL082 dual op-amp for the LFO, three TL074 quad op-amp packages (8 stages of phasing, input buffering and output low pass filtering (to get rid of any 40 kHz clock), and mid-rail supply filtering), a transistor PWM stage (a sort of clever monostable) and three 4066 CMOS ICs - two for the phasing stages and one for the bypass switching. I've just added level sensing for note-triggered phasing, using another dual op-amp, and I've also added companding circuitry for noise reduction (not that there was much to start with). The total component cost is <$30, and it's quite easily the best phaser I've ever heard!

When I'm fully happy with the design, and when I've built two or three more of them, I'll put the schematics and board layout up here.

HEAD · Post by **HEAD** » 23 Jul 2010, 12:33

dutom wrote:Hello!
found this at http://www.synthdiy.com/: 6stage Audio Phaser project 447 in Electronics Today International 1976.
I´m curious about the inventions of mictesters version
Audio Phaser eti447.pdf

Uuuh, thank you well! Nice! Maybe I'll give it a try though the use of 8 single op amps is a bit.... outdated.

Today we are lucky and are able to get our hands damn cheap on TL074 for example.

mictester · Post by **mictester** » 23 Jul 2010, 12:45

HEAD wrote:
dutom wrote:Hello!
found this at http://www.synthdiy.com/: 6stage Audio Phaser project 447 in Electronics Today International 1976.
I´m curious about the inventions of mictesters version
Audio Phaser eti447.pdf
Uuuh, thank you well! Nice! Maybe I'll give it a try though the use of 8 single op amps is a bit.... outdated. Today we are lucky and are able to get our hands damn cheap on TL074 for example.

The ETI circuit will be noisy and distorted (both functions of the way the components in the phasing stages are being used). Wait until I post my PWM phaser, and you can have low nois, low distortion and the deepest phasing you've ever heard!

littleBloona · Post by **littleBloona** » 23 Jul 2010, 13:24

No reason to wait after MXR and Nelson have done it: https://www.diystompboxes.com/smfforum/ ... ic=76747.0

HEAD · Post by **HEAD** » 23 Jul 2010, 13:43

mictester wrote:
HEAD wrote:
dutom wrote:Hello!
found this at http://www.synthdiy.com/: 6stage Audio Phaser project 447 in Electronics Today International 1976.
I´m curious about the inventions of mictesters version
Audio Phaser eti447.pdf
Uuuh, thank you well! Nice! Maybe I'll give it a try though the use of 8 single op amps is a bit.... outdated. Today we are lucky and are able to get our hands damn cheap on TL074 for example.
The ETI circuit will be noisy and distorted (both functions of the way the components in the phasing stages are being used). Wait until I post my PWM phaser, and you can have low nois, low distortion and the deepest phasing you've ever heard!

I try to, but I'm an impatient guy and I'm pretty bored by all the "yet another boutique TS" stuff looking for new approaches or challenges. I have some spare time at the moment and I would like to build something but couldn't find anything that attracts my attention until now (build so far about 60 pedals).

Cheers
Helge

borislavgajic · Post by **borislavgajic** » 24 Jul 2010, 19:42

so I was wondering......if optical phaser (fet , and ota based) are noisy and distorted comparing them to PWM.......why most famous phasers are Biphase ,mxr90,ross,bad stone,small stone...?

I can not wait to see mictesters design....

really would like to make one PWM based phaser....on YouTube ADA phaser was not that great sounding......or I am wrong...?

Boris

mictester · Post by **mictester** » 24 Jul 2010, 21:10

littleBloona wrote:No reason to wait after MXR and Nelson have done it: https://www.diystompboxes.com/smfforum/ ... ic=76747.0

"Nelson" might have a few good ideas, but the MXR circuit is really touchy, and often doesn't work properly unless to take a lot effort to select parts. I think I've done better!

borislavgajic · Post by **borislavgajic** » 25 Jul 2010, 21:22

http://www.uni-bonn.de/~uzs159/allpass.png
http://www.uni-bonn.de/~uzs159/pwmgen.png

mictester · Post by **mictester** » 26 Jul 2010, 06:01

borislavgajic wrote:http://www.uni-bonn.de/~uzs159/allpass.png
http://www.uni-bonn.de/~uzs159/pwmgen.png

That's not too far from what I've been doing. It looks like "Elektor" and I continue to think the same way!

analogguru · Post by **analogguru** » 26 Jul 2010, 06:59

yep, that´s the Elektor design from the mid-80´s I was talking about.

analogguru

borislavgajic · Post by **borislavgajic** » 26 Jul 2010, 10:42

analogguru wrote:yep, that´s the Elektor design from the mid-80´s I was talking about.

analogguru

somebody made it? how does that phaser sounds?

cheers

Boris