BJT Phaser Idea

mac · June 28, 2008, 07:25:33 PM

This is the BJT version of this simple FET phaser:

http://www.diystompboxes.com/smfforum/index.php?topic=67167.0

I started using FETs but BJTs are more consistant.

It now uses a single 10mm yellow bright LED facing the LDRs, and an external rate indicator.

I removed the regeneration path from the output transistor to the base of Q3 since when in phasing mode it works nice, but in vibrato mode, Mix pot at one extreme, it resonates too loud. The minimun resistor needed to prevent resonance when in vibrato mode is quite high to notice a significant regeneration when in phasing mode.

mac

GibsonGM · June 28, 2008, 11:25:59 PM

That looks really neat, Mac, thanks! So you are just bundling the 1 LED with 4 LDR's, getting a "4 in 1 Vactrol"? I'll try this one out.

mac · June 29, 2008, 12:03:39 AM

Yes. Better control over sweep, and it reduces current draw which is 7.5ma when leds are fully on. Near 1.5ma go to the BJTs, the rest to the opamp. I'll try another IC.

About how to make the "vactrol", I hope the graphic in the schematic is clear.

mac

GibsonGM · June 29, 2008, 08:27:34 AM

Yes, very nicely drawn. For singles I use heat shrink; I'm thinking maybe a small piece of plastic tubing (1/2" to 3/4") would be good for this setup.

frokost · June 29, 2008, 09:42:37 AM

I have never quite understood the need for light isolating homemade vactrols. After all, it's dark inside an enclosure, right?

petemoore · June 29, 2008, 01:18:47 PM

Not dark enough if it can be darker...
try to argure with that ^ so keep it to yourself...

I have never quite understood the need for light isolating homemade vactrols. After all, it's dark inside an enclosure, right?

Right.
And also...when is the resistance so high it doesn't matter that it go higher tonewise.
I found low light and no light to be really close, and opted for lower light than low light in the EZVibe...ie sealed case is just fine.
Now for highlights, I like white or shiny inside the vactrol isolation, getting the resistance to the low side seems to be the extreme what a photocell might can't do so well as to make it do abso-max of what it's capable of.

mac · June 29, 2008, 03:26:21 PM

As the "vactrol" is on the breadboard I have to isolate it. Yes, inside the box is possible that no isolation be needed.

I was trying to buy a 4 LDR vactrol but I could not get one. It would be easier for making a layout, and save space.

I'll record a new sound clip, but the fet version one is quite similar.

mac

mike_a · June 29, 2008, 05:52:43 PM

Again, great job mac!

I would really like to hear how this sounds with univibe caps.
Also, did you try darlingtons (like MPSA14)?

mac · June 29, 2008, 10:17:09 PM

QuoteI would really like to hear how this sounds with univibe caps.
Also, did you try darlingtons (like MPSA14)?

I guess I could record a sample with a univibe config, or similar.
To be honest I had only 2 ksp13, too lazy to go out and buy some more, so I used what I had.

mac

GibsonGM · June 30, 2008, 08:34:54 AM

Do you suppose this would work with 2N5088's? Or is the hfe too low?
Another good reason to isolate the 'vactrols' is mechanical...keeping them aligned etc. You never know what's going to happen to a stompbox on stage or when you're fiddling with it, LOL.

mac · June 30, 2008, 01:55:38 PM

I guess 2N5088 have enough gain to work here. If gain is lower, like a 2N3904 @ hfe=200, Vc would be near 7V, which is ok.
Actaully I have the fet version working at Vd = 7.8V using 2SK246 Q1-4
So 2N5088 and most 2N3904 will work. If gain is below 200 maybe reducing 4M7 to 3M9 or 3M3.

QuoteAnother good reason to isolate the 'vactrols' is mechanical...keeping them aligned etc. You never know what's going to happen to a stompbox on stage or when you're fiddling with it, LOL.

... or an angry jealous girlfriend is going to throw it at your head! Believe me, it happens!

mac

GibsonGM · June 30, 2008, 06:35:22 PM

Hey, I've had that happen too!! He he he
Cool, I'll play with the 5088's, after I finish my insane 2 Tube Screamers in 1 box project...

dschwartz · July 02, 2008, 08:16:58 PM

i found this surfing on the net..

it may help..
http://www.freeinfosociety.com/electronics/schemview.php?id=306

mac · July 15, 2008, 02:54:14 AM

New sound sample at:

http://guitarfxs.tripod.com/index.html

I used a RM and a FF (not at the same time) before the phaser. I had to supress CRT noise so you may hear some strange things in the high content.
First 3 takes fet v.3, the rest bjt version.
Home recording at low volume, sorry.

mac

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BJT Phaser Idea