ETI 447 Phaser questions

[acidblue]

I see on the schematic it uses 8 741's, could I use a couple of quad op amps instead? or
is there some reason to use 8 741's?

Has anyone tried adding more stages, like up to 12 stages, is there any benefit?

[PRR]

> is there some reason to use 8 741's?

The year is 1976. The '741 was WIDELY available. The dual 1458 was not so readily available. The choice of '741 made sense when the plan was developed and published. (Within a year things changed.)

Ah, but you say it is 1990 or even (gasp) 2016? Don't use '741, you'll go broke buying them and blind soldering all those pins.

IMO, use duals like TL072. Quads are too much amplifier in too small space for all the external parts an audio circuit needs. But if you find a neat phaser layout using quads, steal and adapt it.

You do know you need an UN-buffered '4049? In 1976 there was no choice. For all digital applications, buffered would be better, and sometime in recent decades they added buffering to the '4049. You can find unbuffed '4049 under a special part number which I don't recall.

I am also wondering about the specs of the 2N5485 FET and the low-limit of most opamps. You may need some front-end mods.

[StephenGiles]

This phaser had the best click of all, but I think there may have been a solution at some point over the years. I remember lurching over to a PCB business in Twickenham to buy a PCB for this phaser !

[Mark Hammer]

My recollection is that the use of a 4049 as a sextet of control elements was somewhat problematic by providing less headroom.  The end result is that you can't feed a hot signal into it without engendering clipping, such that gain is required at the end of the phase shift stages to compensate and provide decent effect/bypass balance.  It was described to me as somewhat hissy as a result.  Of course, how much of that was a result of using 741s, instead of something better, I have no idea.

At the time, the 4049 provided an econimcal and available way of obtaining 6 matched control elements.  But ask yourself: in the intervening years, how many times have you seen the 4049 used that way?  I rest my case.  There are better choices.

[anotherjim]

This might be useful....
http://www.parasitstudio.se/vero-layouts/eti-project-447-audio-phaser

[armdnrdy]

I recall reading that this ETI design is noisy as well.

ADA took a different approach in the use of CMOS ICs in a phaser circuit with the Final Phase.

Instead of matched JFETs or "vactrols", fixed resistors to ground are "switched" in and out of the phase stages.

This is the only phaser design that I can recall seeing this in.

[StephenGiles]

Did Mike Irwin have a solution to this on the old site?

[Mark Hammer]

I think so, but the question to ask is whether the effort is worth it.

[acidblue]

This might be useful....
http://www.parasitstudio.se/vero-layouts/eti-project-447-audio-phaser

Actually that was very useful, thanks.