MXR Micro Amp Phase Inverter

[rthryhorysak]

I would like to be able to invert the phase coming out of the micro amp. Since it is using a single op amp, TL061, can I just set up with a dual op amp, TL072, and the second section as a inverting amp with two 10k resistors for equal gain at the end of the signal? I am fairly certain this will work, I just want to be sure so any input it appreciated.

[Cozybuilder]

Seems like it should work:

[Mark Hammer]

That should work.  I recommend reading this ancient, but still relevant, Craig Anderton article in DEVICE, from 1979:  http://hammer.ampage.org/files/Device1-8.PDF

Noise levels for different kinds of op-amps can vary with the resistances around them, even when one is merely aiming for unity gain.

[PRR]

> can I just set up with a dual op amp...a inverting amp with two 10k resistors

Why not?

What does it cost to try?

> Noise levels for different kinds of op-amps can vary with the resistances around them

10K is a fine choice here. A1 output hiss is already at the 56K level. A TL0xx's hiss is maybe a bit higher, so there's no point in heroically low impedance around A1. 10K around the A2 inverter adds "negligible" hiss.

As your (Craig's) table shows:



TL0xx is "BiFet". There will be some variation among the many part #s and factories, but the trends will be consistent. Hiss resistances zero to 10K are all the same (JFET hiss) and resistance to 100K causes only slight increase (low JFET input current).

This also suggests why TL0xx are good choices for guitar inputs. Guitar can be 5K lows and mids, but peak over 100K in treble. This peak is in the ear's most sensitive range. While other amps are better below 10K, they are worse at 100K.

[karbomusic]

If you mean something like this then yes. Built a 3-way buffer/splitter awhile back where each output has a phase invert switch.

[rthryhorysak]

If you mean something like this then yes. Built a 3-way buffer/splitter awhile back where each output has a phase invert switch.

So is the switch stopping voltage from VA?

[karbomusic]

Yea and when that happens VA now comes from the output of UA1 through the 10k resistor. The switch which causes it to flip from the non-inverting to the inverting input and vice versa. I was going to share the link that discusses this method but I can't find it. I can say it works extremely well. IIRC there is a tiny difference in output between inverted/non-inverted but appears to be completely negligible in most practical uses. I was able to get a hi-z buffered input with three buffered outputs with polarity switches for all three outputs in a single TL074.

I updated the picture above to the full schematic just so you can study the inversion stuff and rip out what you need, if anything there fits your needs.

[rthryhorysak]

What about this? So instead of changing the output from an inverting to non inverting I am just bypassing the inverting stage and vica versa.

[karbomusic]

Where is VRef (+4.5V) for TL072.2 coming from?

[PRR]

> What about this?

[Mark Hammer]

Better, no switch-popping.  Also better because the TL072 has better noise specs than the TL061/62.  The 61 requires less current, but with everyone powering their pedalboards with external supplies these days, "battery life" is much less of a concern, and lower noise takes center stage.

[rthryhorysak]

Ok thanks guys, I'll try them both. I do like karbomusic's method and it seem more practical than bypassing it all together. So is VA in the buffer curcuit +9V?

[PRR]

> is VA in the buffer curcuit +9V?

Va comes from a subcircuit lower left. Two resistors across 9V and zero, Va tapped at mid-point.

It is unlikely to be 9V, or zero, but something in between.

Since the two resistors are equal, and (I tell you) the loading is negligible, the figuring is easy.

[maoriente]

If you mean something like this then yes. Built a 3-way buffer/splitter awhile back where each output has a phase invert switch.

Hi,

Hope it's ok to post on such and old thread.

Do you get a pop when switching phase with this setup?

I built Craig Anderton's version from DIY and there is a noticeable pop when flipping the switch. I then modified it similar to what is posted above, only using +9/-9 and switching + op amp input to ground rather than to vbias, but still have the pop.

[ElectricDruid]

I'd expect a pop, yes. Unless the signal is exactly at the "zero" midpoint level, when you invert it, it'll change a little bit. For example, 20mV becomes -20mV. That's a jump of 40mV that you hear as a pop.

HTH,
Tom

[karbomusic]

Just did a quick test, but was in a hurry so if there, it's pretty small. That wasn't a concern for the design though as isn't something I'd ever switch during performance.

[Mark Hammer]

If the circuit maoriente shows is used to feed any true-bypass pedals, there will be noticeable switch-popping.  This is because the 22uf caps on each output are "floating", without any means to drain off.  An e-switched pedal, like Boss, DOD, Yamaha, etc., will leave the connection between those caps and their input intact after you plug in.  A true-bypass pedal, however, will momentarily disconnect and reconnect those 22uf caps EVERY time you switch to off or on.  They should be terminated by a resistance to ground that allows them to drain off.  I would imagine 100k would be in the ballpark.

[karbomusic]

I surely don't doubt what mark and tom explain but notice there is no footswitch on that unit since it wasn't designed or intended for live use, it's used in my studio for a specific signal splitting purpose (such as a DI + 2 amps when recording). FYI, those are 2.2 caps just for accuracy sake. On a side note, it has served me well but again, if live switching is what is needed, and any chance of a pop when doing so is a concern... this isn't the design to use. Not that I wouldn't add the 100k resistors (good call).

However, if you want buffered, split signals for recording with selectable impedance and polarity switching, it might be the one to use. YMMV.

[maoriente]

Interesting, thanks everyone.

I have my Phase Switcher after the effect.

Mine is also not intended for live use, I just find any pop annoying :0)

Sorry I don't have a schematic drawn up, but my setup is 3 Craig Anderton designs.

From DIY Projects for Musicians: A Buffer feeding an effect loop, with the effect's return feeding the Phase Switcher, who's output is connected to the (Electronic Projects for Musicians) Mixer's input level control.

So it's Buffer, Effects Loop, Phase Switcher, Mixer. 

No modifications, although I did omit the Phase Switcher's 100k output resistor as I have it's 2u2 output cap connected directly to the Mixer's 10k input level pot. I thought it would be redundant to have the 100k paralleled with the Mixer's input pot. (I've since tried adding the 100k resistor - just because. No improvement)

Another thing I don't get, even with the effect loop's return jack tied to ground (when no effect is connected), and the loop's level control turned completely down, meaning both the input and output of Phase Switcher circuit are both tied to ground, (edit: My apologies, I had inadvertently broken the ground connection to the pot)  I still get a pop when changing phase.  Where is the pop coming from?  (The pop is mild)