Simple buffer and phase inverter

10/16/2017

Here is a small utility board - a simple buffer followed by a phase inverter. You can take the output from either stage. It can be very useful if you are adding a clean blend to a circuit that is inverting the phase.

I recommend putting this in front of your main effect, but keep in mind that not all effects will respond the same with a buffer in front. I didn't include polarity protection, DC filtering and an output capacitor since this is meant to be used together with another effect (not stand alone).

29 Comments

Michael

2/12/2018 05:14:53 pm

Hi, I built this and it seem to work just fine. It took me two hours of troubleshooting until I read your last remarks though :) Is it possible to use this at the end of a pedal chain before a last splitter? And what value cap would you choose for the output cap?

Fredrik Lyxzén link

2/13/2018 01:40:58 am

Hi Michael,
Yes, you can put it at the end of a pedal chain. Capacitor value isn't critical (anything from 100nF upwards will be fine) unless you will put a volume pot directly after the output cap, then the cap + pot will act as a highpass filter when turning down the volume. In that case, use a large value, like a 2.2uF electrolytic cap (the negative leg going to the pot) to avoid too much lowend being cut.

Michael

2/13/2018 09:07:28 am

That was exactly what I wondered, thanks a lot!

Sebastian

7/27/2018 07:31:01 am

Hallo Michael,
i would use your circuit for my Fulltone Tube Tape Echo. There is a 440 ohm output impedance. That’s why I need a buffer. It is possible to use your circuit for that and could I operate it with 12v?
Cheers, Sebastian.

Fredrik Lyxzén link

9/27/2018 07:05:56 am

Yes, it should work fine with 12v. Just remember to add an output capacitor (any value 100nf - 10uF will work. Negative side to the output).

Marcus

2/8/2019 08:39:45 am

Hello. Thank you for this layout. Very good to find this. I have one more question. Is it possible to use your layout as dual invert buffer? because i have a stereo effect with two outputs, bouth out of phase with around 200k output impedance. Thats why i'm looking for a dual buffer with invert.
Would be great to hear from you.
Best Regards, Marcus.

Fredrik Lyxzén link

2/8/2019 11:45:32 am

Hi Marcus,
You need two of this layout to use it as a stereo buffer. One input buffer and one inverting stage for each channel.

Cheers / Fredrik

Marcus

2/19/2019 12:34:34 am

I did it. But the signal is clipping. Is it normally with this circuit?

Fredrik Lyxzén link

2/19/2019 01:03:41 am

It should not be clipping unless you have a very hot signal. In that case you need to attenuate the signal before the input with a voltage divider, or run the buffer/inverter at a higher voltage (18v)

krabasti

2/10/2019 12:52:28 pm

Hello Fredrik, thanks for your quick reply. i just built this Buffer and really like it, but I've noticed a really odd issue that I can't figure out. When I plug my Guitar and turn up the volume knob i get noticeable noise. If I turn down the volume the noise goes away. I've tried power supply as well as battery with the same results. What can i do?

Marcel

10/7/2019 03:14:36 am

Is it possible to use this buffer (tl072) with 10 Mohm resistor for higher input impedance?

Marcel

10/8/2019 01:55:37 pm

or maybe with 3 Mohm. which resistors i have to change, R1 and R2?

Wallace

4/11/2020 10:49:15 am

Hello Fredrik,

Thanks for the great work. If I was going to use this as a stand-alone circuit, how would you recommend that I add the Polarity Protection, DC Filtering, and the Output Capacitor?

Thanks!

Christopher Stelloh

8/22/2020 11:10:39 am

Hey there - question on this build - sorry if this is an old topic - but regarding the in phase and inverted phase outputs - I am planning on building this into a parallel loop mixer. I would like to make the phase inversion switchable. Would I do this with an SPDT on-on toggle, and if so, do I connect each of the phase outputs to an outer lug of the toggle, and connect the middle lug to the next circuit? Thank you for any help or insight you might have.

Chris Stelloh

8/22/2020 03:10:27 pm

Actually - I think I found a usable layout for an op amp parallel mixer that also has a switchable phase inverter in the circuit. I'll check back in if my tweaks to the design don't work out - I had to piece together a couple of different cap values since this build will be for bass guitar.

Angers

8/25/2020 09:32:45 pm

May I ask what the VR in the diagram stand for?

Fredrik Lyxzén link

9/1/2020 05:02:26 am

It stands for Voltage Reference (also usually called "Vref"). It's a voltage divider (R5 and R6) that you need when using single supply operation (the capacitor will decouple/stabilize and filter the VR rail), so that the signal can bias/center around half the supply rail (4.5v in this case) for a maximum swing.

Vim

3/21/2021 08:17:56 am

Hi if I want to use 15 v positive and negative can it be used. I have a centre tap too so how can that be used. Thanks

Per-Olov Fredrik Lyxzén link

3/22/2021 05:48:57 am

Yes, it can work with some small modifications to the layout.

You simply need to power the op amp with +/- 15v, so the positive voltage input is unchanged, but you need to put the negative voltage where the ground is connected now (pin 4 of the op amp).

yYu can omit the voltage reference (the two 10K resistors that forms a voltage divider at IC pin 5 + the 22uF cap). Instead you want your center tap (which is your ground reference) as the input for pin 5 of the op amp.

The last change is that the 1M resistor at the input needs to be changed so that it goes to ground instead of the negative voltage rail, so just move the bottom leg up two rows.

I hope that makes sense. / Fredrik

Vim

3/25/2021 01:43:57 am

Hi fredrick much appreciated any chance of a small picture please😁

Jens Wellendorph

8/22/2021 09:34:42 am

Hi Fredrik. Thanks for your great work, hope you get by in these Covid-times. Is it it a simple matter of changing 1 or 2 resistors to get the phase inverter boosting signal with 10 - 16 db ? I need that to level up active magnetic PU's with Piezo in a mix-of-2-worlds guitar I've modified. Thanks :) ((I've left a few bucks as donation, I lived from being soundman for 20 years and can really see the bad effect on that branch))

Fredrik Lyxzén link

8/23/2021 11:01:29 am

Hi Jens!
Yes, increase the value of the feedback resistor (R4) to increase the gain of the inverting op amp. Exactly how much you gain in DB's depends on the input signal that you start off with, but the gain formula is very simple. Gain = Feedback Resistor divided by the input resistor (R3). So for example. If you want a gain of 4, just use a 40K resistor in the feedback (or 39K which is the closest common value) and so on... Best regards / Fredrik
and Thanks you alot for the kind donation. <3

Jens Wellendorph

8/23/2021 12:53:56 pm

Thank You, Fredrik. If I get it, then I put a 100K resistor as R4 to get 10db? But only on the reversed output - will the in-phase output then be 0? Or can I put 10k in front and 100k in feedback and get +10db, the same as reversed output?

Fredrik Lyxzén link

8/27/2021 07:12:43 am

Yes, if you put a 100K resistor as R4 you will get a gain of 10. Just be careful that you don't clip the signal with too much gain. The in-phase output will still be at 0 gain (unity). The non-inverting opamp doesn't work the same way as the inverting opamp however, so you can't put a 10K in front and 100K in the feedback. I recommend that you check out the RST academy youtube video on opamps to understand how to set up the gain for a non-inverting opamp. It's not difficult, but too much to explain here. Cheers

Chris Wilson

9/3/2021 12:44:35 pm

Hi,

Is it possible for this circuit to split both in the same phase ie like a switch to invert a signal?

Fredrik Lyxzén Parasit Studio link

9/6/2021 01:25:31 am

Hi Chris,
I'm not sure what you mean. If you want to use it as a phase inverter that you can toggle between in-phase and out-of-phase, simply use a SPDT switch that toggles between the two op amp outputs (each output to a outer pin of the switch, and the middle switch pin is your output). Just remember to add a capacitor and pulldown resistor at the output. I hope that was what you meant (maybe I misunderstood?). Best regards / Fredrik

Chris Wilson

9/13/2021 03:26:49 pm

I meant could you have both outputs in the same phase, not one in and one out. I've sourced another design since though that does it, thanks anyway.

Vincenzo

9/25/2021 05:45:46 am

Hi Fredrik, thanks a lot for this circuit! I wanted to build this to add a parallel fx loop to my pedalboard: http://tagboardeffects.blogspot.com/2012/02/split-n-blend.html and, as you'll see, this one behaves strangely with pedals spotting reversed polarity. That's why I wonder if your buffer 'n phase inverter can be daisy chained to the parallel efx loop in a single enclosure as follows:

gtr --> buffer/inverter input --> in phase out / phase reversed out --> SPDT toggle switch --> toogle switch out --> 10uF capacitor + pull down resistor on caps' positive leg --> split 'n blend input --> output(s).

Would that work? Also, which value would work for the pull-down resistor? Thanks in advance!

Björn

12/8/2021 12:23:32 am

Looks neat! If I want to build a clean blend pedal with the possibility of phase inversion, do I connect the input of your circuit to the send of the blend? Or how would I connect the two circuits? Thanks in advance

Simple buffer and phase inverter

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