Problems with phase 45

[erikl]

Hi!

Built the phase 45 univibed and with jc maillet mods on a vero board. With the mix pot in either fully opened or fully closed I get the clean guitar signal with no effect. Anythin in between gives me a tapping sound that shifts with the speed pot and no guitar. The led flashes to the tapping.
Anyone got any idea where to start debugging? I've checked everything twice. Any help appreciated.

[smank]

Do you have selected the FETs?

[RaceDriver205]

I would check the FETs first. Make sure they are the right way round, and I mean check the layout to the schematic too.
Mine works fine, but it does not have mods.

[finkfloyd]

Hi,

The fets need to be matched, then you can dial in with the mix and bias knobs, set the speed high, so you can definately notice it dial in..

Also on my first build I did, things went pearshaped, i missed a jumper ...but it refused to work, so i built it again, but there was probably no need, as the fets need to be matched to phase!!

[erikl]

I don't have matched fets. But reading from older posts I've got the impression that the problem with missmatched fets would be that the effect is weak or none. Been audioprobing and got a nice vibrato signal from G at Q2. Been swapping through my 10 pieces of 2N5458 and it's the same even without any fets in. So is could it really be missmatched fets that's causing this?

[smank]

The FET selection is very important for the Phase45, else the circuit sounds like... a treble booster. Absolutely no modulation effect!

[spudulike]

Not quite true. The FET parameter is Vgsoff thats important, and you may need to change the zener to match your FETs.

RG gave a brilliant explanation for this on another thread. Check these threads out ...

http://www.diystompboxes.com/smfforum/index.php?topic=38816.0
http://www.diystompboxes.com/smfforum/index.php?topic=40068.0
http://www.diystompboxes.com/smfforum/index.php?topic=37859.0

And remember RGs explanation

Guys, this is not an unknowable thing, nor is rumor a good way to figure it out. Consult the datasheet.

(1) JFETs have a resistive region of their operation if the voltage from drain to source is about +/-0.1 to 0.5V. There are ways to make this bigger, but the raw device does this.
(2) The JFET goes from on (that is, resistance between source and drain is Rdson, a few ohms to few hundred, depending on device) to fully off (many megohms) with a voltage between its source and gate of 0 (fully on) to -Vgsoff. For the J201, the Rdson is 750 typically.
(3) Vgsoff is unique per JFET type, and per JFET. JFET types specify the Vgsoff range. For the J201 this is -0.3 to -1.5V.
(4) For a phaser, you want the phasing resistors to be quite similar in resistance.
(5) In JFETs, very close Vgsoff is associated with close Rdson over the range of 0 to Vgsoff. Not perfect, but close enough for phasers.
(6) In a P90 style phaser, the source of the N-channel JFET is hung at a reference voltage in the middle of the "9v" power supply; the P90 was 3.0V, the tonepad version upped that to 5.1V so that more JFETs would work.
(7) You can only pull the gate down to ground in a P90 style phaser, so the Vgsoff has to be less than the reference voltage, which is the zener voltage in this circuit, so any JFET you use MUST have Vgsoff of less than the zener voltage to have a chance to work. It is possible that some JFET with a datasheet range above the zener voltage MAY work if you select specific JFETs that have Vgsoff lower than the zener voltage.
(8 ) The LFO voltage in the phaser must be able to drive the JFET VGS from nearly off to nearly on. Outside these ranges of voltage, nothing happens. So a J201 needs a small (for the 1.5V Vgsoff devices) to miniscule (for the 0.3V Vgsoff devices) LFO voltage. The P90 LFO may need tinkering to do a sweep voltage this small. The stock P90 used a 2N5292 (Vgsoff  = -1 to -3.0V).
(9) There are J201's that will work, according to the datasheet. These will be the ones in the upper end of the Vgsoff range. There are J201's that will not work. These will be the ones from the lower end of the Vgsoff range.
(10) J201 is chancy. However, so are all specific JFETs. I like either the 2N5292 or the 2N5484 or 2N5485. However there are many that can work.
(11) Your best chance is matching to a voltage within the 1V to <zener voltage> range. For tonepad's 5.1V zener, get a JFET with a specified Vgsoff of -1 to -5V, and match to about -2.5. I use the 3.0V zener and match to -1.5 to -1.8V. They have always worked right when matched that way.

[erikl]

Doh! I missed one trace cut. Third check's the charm

Matched fets using the bias-pot as described at http://www.lynx.bc.ca/~jc/pedalsPhase45.html . Got a match in the middle of the pot for 2 2n5457. The effect really isn't subtle at all. But it fades fast when changing the bias from the middle possition.

Thanks for helping me. Hope the post can help someone else.

[RaceDriver205]

Yeah, I had a short between two pads once that was so fine it was invisible. I only found it using a continuity tester (a must-have).
Glad you got it working, its only 'subtle' compared to other phase-shifters, like the bad stone (which doesnt work by the way) and the phase 90.
Sounds cool with distortion: http://www.tothemax.web1000.com/FXDemos.html