One thing I will say is that until this is verified I wouldn't recommend it being built by anyone who may not be prepared to do a little tweaking. The effect is slightly different from similar phasers in that it has no resistors between the drain and source of the JFETs and this may be a benefit of the 2N4303 JFETs used in the original. If you substitute these for 2N5952 or similar as mentioned on the layout, then there is a chance that the circuit will need tweaking to get it working as intended. I have left a space to add resistors if necessary just to make this easier to tweak for anyone who would like to give it a go. As usual with these phasers, a matched quad is recommended.
The original BJT transistors were TIS97's which are NPN and around 300 hfe and so I have suggested a 2N3904 or similar as an alternative to those. Maybe a BC547-550 or 2N5088 would get you in the ballpark a bit better than the 2N3904 so I'll leave that selection up to you.
The original also used 6 x 741 single opamps and so to save space I have opted for doubles. I'd make sure you socket these as you may want to audition a few to see which double hits the sweet spot for you.
One obvious mod recommendation is leaving out the Oscillator Amplitude trimmer (top) and adding an external pot to control the depth. If you do want to do that just take the wires for the pot from the holes on the board that the trimmer is connected.
I can't find any info about the original, or any clips, so the first person to build it and post a vid on youtube will have their vid added to the post! :o)
.. and the schematic:
Very nice layout. The board size is pretty much the same as in original.
ReplyDelete+m
Cheers buddy :o)
DeleteOops misplaced 150K at IC3, please get the corrected layout now posted above
ReplyDeleteSo so so awesome! Need to make it! Maybe futurely a Maestro MPF-1? :)
ReplyDeleteOk, so I thought I'd give this a go this evening and kind of as expected it's not working. I'm getting the clean signal through fine but there's no 'out of phase signal' coming through. I'm guessing that the 2k2 and 43k resistors at the output are mixing the clean and phase signals so I probed just behind them both, behind the 2k2 is the clean signal but behind the 43k the signal is there but incredibly weak, I'm guessing this is why I'm not getting the phase effect at the output. I'm going to add some resistors between the drain and source of the jfets but I'm just wondering where a good starting point might be? Looking at the Phase 90 schematic it uses 22k or 24k resistors depending on the schematic so I'm going to start there but any other suggestions would be appreciated.
ReplyDeleteYes you're right about the 2K2 and 43K mate, here is the schematic.
DeleteNow you're probably around the area I had to question when I was doing the layout. If you look at all the other stages there is a 100K resistor between inverting input and output, but on the last one to me it looked like it went from the inverting input to the junction of those two 22K resistors and not the output because there was no dot on the schematic where it passes the output. I thought about that for a while and convinced myself that is correct as per the schematic because if there was a connection to output that would make the upper 22K superfluous.
What are your voltages mate, just to see if they show an obvious unexpected reading somewhere.
Oh and I'd socket the connections but yes something from 15K to 22K seems like a good starting place.
DeleteThat'd be my interpretation of the schematic with the 100k going to the junction of the two 22k resistors but you're right it does seem odd that it wouldn't be connected to the output of the last opamp. I've had a play with a few resistors with no luck so I'm going to take a look at it with fresh eyes in the morning. I did have a look at the voltages but nothing seem out of the ordinary, I'll go over them again in the morning and write them down this time. Any idea what I should be biasing the jfets to? I've gone for 4.5v at the moment. Thanks for the help by the way, always appreciated.
DeleteNot really sure what you should be biasing them to, with phasers you would usually tweak it until you get the strongest phasing. Anyway post the details in the morning and we'll see where we can get.
DeleteI've got one of these! Anyone interested in hearing what the finished product should sound like?
ReplyDeleteYes please, that would be excellent
DeleteHey Mark, I finally saw this response. I guess i thought I'd get an email... Anyway, a sampling of this phaser is posted here:
ReplyDeletehttps://soundcloud.com/study-shuhasko/maestro-mps-2-mini-phase-test
Each pause between riffs indicates a change in the speed of the phase, starting at 0, followed by 3, 6, and 9 oclock and ending on full speed. The potentiometer indicator is kind of weird, it's upside down (ie instead of 12 o'clock being about 1/2 way through the turn, 6 o'clock is the halfway point.).
Another interesting note - the resistor missing between the drain source and the JFET is really a poor idea. Even for the 2N4303s, some tweaking is needed to line up each LFO up, and the lack of a resistor results in a strange sounding turn around in the phase sweep.
Finally, the pedal has a pretty big effect on tone. It reminds me a bit of the treble cut you'd get from an LPB-1. I think that is pretty lame for the phase effect on its own, but it really lends itself to overdrive if you're in to that.
Sorry John, I always respond on here, I can't get your email address from your Google+ page. Sounds really good to me and I'd really like to get this one verified so we could do with getting Madferret on the case to swap a few values about.
DeleteThanks for posting the clip, hopefully you'll be able to build an extra one soon :o)
why not just give a try with the original transistors
ReplyDeletesmallbear has em for a buck a piece
ReplyDeleteI am currently trying to troubleshoot of of these circuits in the original Oberheim P-100 which is the same circuit with a DC input and the Oberheim paint job on the chassis. The issue with it is that the effect doesn't oscillate. I can here the signal out of phase and I can tweek the Oscillator Amp pot and the Bias pot and hear things change, however, I cannot get it to oscillate. What should I look for here?
ReplyDeleteCheck the bias voltage of the oscillator
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