Maestro / Oberheim PS-1 _ PS-1A/B

[chaz]

Alright so it looks like this topic comes up once in a while and I have yet to read that someone has successfully cloned or created a version of this phaser.

Here's what I've done so far:

I've breadboarded both versions of the Maestro Phase Shifter floating around (one shows +/- 15 volt supply and the other shows +/- 12 volt supply). All JFETs (2N5485) are matched using RG Keen's method. PNP BJT's used were 2N4403. I cannot seem to get any kind of audible phasing.

Has anyone made this work? I have a few questions specifically in regards to the transformer used.

[chaz]

bump

[Mark Hammer]

The Maestro version/drawing of the circuit (found here: http://hammer.ampage.org/files/Maestroworld.zip ) that shows +/-12v, has a different sort of bypass arrangement than the +/-15v Oberheim version posted around (which I think may be an ancient drawing from RG Keen that was part of the original Leper's archive).  In the Maestro drawing, a single FET is used to lift/engage the phase-shifted signal from the mixing point, where it links up with the 3k (?!) resistor carrying the dry signal.  In the Oberheim drawing, you can see a pair of 47k resistors coming off the emitter of the PNP input transistor.  One goes to a mixing node where it links up with a 47k resistor carrying the phase-shifted signal, and the other goes to the other side of a different switching FET.  The bypass/effect-cancelling scheme is different for the two versions, with the Oberheim seemingly selecting between a clean buffered signal and a dry+wet effect signal, and the Maestro adding or lifting the wet to dry at the output, a technique used by countless other manufacturers.

I got the Maestro schematics for scanning, courtesy of forum member Rick Lawrence, out in British Columbia.  Rick had noticed several errors on them at the time, despite them being "factory" schematics, and I think we've collectively detected others i the subsequent years.

So, there are some seeming differences in the design particulars of the two versions, and the possibility of errors i them.

And, as I am fond of noting, despite having only 3 pins, there seems to be well over 832 ways of getting the pinout wrong on a FET.

So where do we start?

[chaz]

Great information! I appreciate the input. The pictures that I've gathered from the web also indicate a lot of discrepancies when compared to the schematics available.

The other thing that I've noticed is in the schematic do you see the -36V off of the back to back 1n4004's? Does this mean that the transformer should be a 72CT VAC?

The transformer that I've used for my builds is mouser pn 546-166F30 which is a hammond 30CT VAC / 250 milliamp transformer. The idea was to create either a +/- 15 or 12 volt signal going to the components.

The issue that I'm having in both builds is that my current method is not measuring +/- 12 or 15 volts but rather 4.5 - 6 volts when loaded. Is this correct?

I'm looking at the transformer as my problem because I feel like I should have some audible phasing at this point if my build is correct, FET's are matched, and adjustments to the bias, offset, and osc amplfier are made properly.

Thoughts?

[Mark Hammer]

The other thing that I've noticed is in the schematic do you see the -36V off of the back to back 1n4004's? Does this mean that the transformer should be a 72CT VAC?

No.  It's -36V, relative to the junction of D2/D4.  So basically, we are looking at the rectified output of a 24-26vCT transformer.  As long as it provides enough additional voltage to permit decent regulation down to +/-12vdc via whatever method you use (zeners, 3-pin regulators, etc.), you should be fine.

The transformer that I've used for my builds is mouser pn 546-166F30 which is a hammond 30CT VAC / 250 milliamp transformer. The idea was to create either a +/- 15 or 12 volt signal going to the components.

That seems to qualify quite nicely.

The issue that I'm having in both builds is that my current method is not measuring +/- 12 or 15 volts but rather 4.5 - 6 volts when loaded. Is this correct?

Normally, that's a sign that something is amiss, and forming a lower-impedance path somewhere than ought to be.

I'm looking at the transformer as my problem because I feel like I should have some audible phasing at this point if my build is correct, FET's are matched, and adjustments to the bias, offset, and osc amplfier are made properly.

The circuit, and FETs are predicated on a given supply voltage.  If you aren't providing that, yes you probably will not get proper functioning.

[chaz]

I'll try and shoot out what's reducing the supply voltage.

Have you tried building this circuit?

[chaz]

Update:

1. Voltage issue fixed (+/- 12.5 volts throughout the circuit - had to use a series of diodes to drop from 15.5 volts)
2. No audible signal passing through

I'm beginning to think I may have a faulty component.

[chaz]

Update:

- Completely cleaned up and redid breadboard from scratch. I tried to maintain the layout of the device as seen in the actual circuitboard (I'll post pictures tonight)
- Swapped BJT 2N4403 with PN3638A (seems to be direct replacement for 2N3638A)

Power supply when loaded reads +12.73 volts and -12.53 volts

I now have a very clean audible signal that is affected by the FET bias adjustment, OSC. Amplifier, and Output offset! There seems to be a crossover point in which the signal is affected the most when the bias voltage is 2.4 volts.

Note: When I matched my JFETS (2N5485) The closest ones that I matched had a Vgs between 1.88 and 1.96 volts.

What I have right now is purely a filtered signal. I do not hear any phasing going on, but I can change the frequency by adjusting the FET bias. When I adjust the FET bias, there is a hint of phasing that happens instantaneously and then goes away.

My first thought is to look for JFETS that has a Vgs in the 2.4 volt range and replace my existing with new. Any thoughts on this or what else could be happening?

[chaz]

Here's the Power section:



Here's the main circuit

[chaz]

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/DSC_00562.jpg.html?g2_imageViewsIndex=1

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/DSC_00602.jpg.html?g2_imageViewsIndex=1