Phase 90 - adding MIX knob and output buffer [CLICKBAIT: pedal finish inside!]

[madros]

Hello everyone!

It's my first post here so I hope I won't violate any writen or spoken rule.

Back to the topic. Some time ago I bought a DIY kit for MXR Phase 90 because I've always liked its sound the best among all the other phasers, but for me it was lacking some knobs and options. I like to have some spare knobs and switches that I will never use once I set them the very first time I try a new pedal, so it was clear that I need to make some mods. R28 and vibrato was obvious ones and easy to manage, but I've also tried DEPTH knob, MIX knob and adding an output buffer. Well, I dont know if I managed to get first three of them right, but I actually like how they sound. To be honest DEPTH knob sound more like wave shaping (slope, wave skewness etc.), but hey - it sounds neat.

The real problems start with MIX and buffer.

First of all - tweaking MIX knob does not change sound at all. What did i do wrong? Should I go for potentiometer instead of R15?

Secondly - adding the buffer. I have followed Craig Anderton's 741 opamp buffer design but obviously I messed something up. The pedal itself works great, but when I turn buffer on (I can do that with an internal switch) all the sound just get sucked in. Im linking output of the P90 circuit with the input of the buffer one.
Can you please tell me where did I go wrong?



For real size please see: https://i.imgur.com/aUzhZxJ.png

List of elements

R0 1M R1 10k R2 470k R3 10k R4 10k R5 24k R6 10k R7 10k R8 24k R9 10k

R10 10k R11 24k R12 10k R13 10k R14 24k R15 150k R16 150k R17 270k R18 56k R19 150k

R20 3M9 R21 150k R22 150k R23 470k R24 10k R25 1M R26 4k7 RV1 C500k RV2 Tr.200k

C1 10n C2 47n C3 47n C4 47n C5 47n C6 47n C7 10n C8 15u C9 47u C10 22u C11 47n

D1 1N400X D2 1N914 D3 5v1 Q1 2n5952 Q2 2n5952 Q3 2n5952 Q4 2n5952 Q5 BC560

U1 741 U2 741 U3 741 U4 741 U5 741 U6 741

The purple one is INPUT, and I did cut the cooper lines under 741 IC.

Just to draw you attention and provide something pleasant for your eyes - this is how this whole thing looks.

[anotherjim]

Andertons circuit uses dual + & - polarity supplies, which is why the 1M resistor on the 741 +input goes to ground. You need something like the MXR U1 where the resistor goes to some voltage close to half of the + supply voltage. If the buffer is in the same box, you can use the same supply from that 5v1 zener D3. Note that the MXR U1 is also a buffer and essentially the same circuit as the Anderton.

...and yes, that is a sweet looking pedal.

[madros]

Thank you very much for your advice.

I have tried connecting 4.5V from the Zenner diode to the Andertons' circuit and I managed to get some sound (it felt to me as it was boosted a little), but only when I am hitting string really really hard.

Does that mean that the Voltage is still too low? Or should I remove/change any caps or resistors? To be honest I don't know much about op-amps so I don't really know where to tweak.

[thomasha]

I think there is something missing there... like an output cap and a reference to ground.
Take a look at the micro amp.
https://www.electrosmash.com/mxr-microamp

it has the 4.5v reference through a 10M resistor, it has an output cap and a reference to ground of 10k, you can use a pot here and see how much you need. Just ignore the stuff at the top, because you're probably looking for a buffer with unity gain, right?

In my build I changed one of the resistors at the final transistor stage. It already had a lot of gain just waiting to be used. So, there was no audible volume drop.