inverting guitar phase with an Op-amp

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Gamedojo
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inverting guitar phase with an Op-amp

Post by Gamedojo »

Hey there,

I ran into some issues using my EP3 echoplex into the second input of my Marshall since I found it flips the phase of the signal 180. Normally never an issue, but because I split the signal to input one and then to the EP3 then input 2, there tends to be phase cancellation.

Sooo... I built an Op-amp phase inverter in a little box. Works great! Did what I wanted. BUT, I'm not up on my circuit knowledge and only designed this with the bare minimum of parts.

I'm wondering if anyone would have any suggestions of additional circuit changes that might improve the fidelity or performance of the circuit. Such as coupling caps, or such. Thanks!

[img:1024:768]http://www.tylergrund.com/images/opamp_ ... r_v1.0.jpg[/img]
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Cliff Schecht
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Re: inverting guitar phase with an Op-amp

Post by Cliff Schecht »

Yeah you would benefit from a few changes. Input and output coupling caps are usually a good idea, especially because you don't need to carry over the 1/2 supply offset from this design (and in your case don't want to). Aside from 0.1uF and 10uF caps decoupling the op amp itself, the rest is probably fine. With bipolar op amps you sometimes add a series resistance to the "+" input to make the two pins have a more equal impedance (helps with input offset issues) but with your TL082 you can omit this without any issues.

You don't want any level controls or anything?
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ChrisM
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Re: inverting guitar phase with an Op-amp

Post by ChrisM »

I get that it's unity gain and it's inverting.

Whats with the 10K and 1mF caps though. I don't understand how you configured that. The 9V supplies the Vcc+ and Vcc- but whats the point of the resistors and caps? Just trying to understand...
Gamedojo
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Re: inverting guitar phase with an Op-amp

Post by Gamedojo »

Cliff Schecht wrote:Yeah you would benefit from a few changes. Input and output coupling caps are usually a good idea, especially because you don't need to carry over the 1/2 supply offset from this design (and in your case don't want to). Aside from 0.1uF and 10uF caps decoupling the op amp itself, the rest is probably fine. With bipolar op amps you sometimes add a series resistance to the "+" input to make the two pins have a more equal impedance (helps with input offset issues) but with your TL082 you can omit this without any issues.

You don't want any level controls or anything?
So a .1uf right after the input and a 10uf right before the output? What are these doing to benifit the circuit?

No level controls. I don't want it to amplify, I'm trying to build a transparent unity gain phase inverter.
ChrisM wrote:I get that it's unity gain and it's inverting.

Whats with the 10K and 1mF caps though. I don't understand how you configured that. The 9V supplies the Vcc+ and Vcc- but whats the point of the resistors and caps? Just trying to understand...
I'm not 100% of the science, but I read you can't just hook a 9v battery to the pos and neg of an Op-amp. You have to create a 4.5v Pos and Neg or something. Most pedals use a bias methode of dealing with the 9v, but I read this was a good simple way of supplying the power. It works, so I'm not too concerned.
Last edited by Gamedojo on Sun Jan 23, 2011 7:08 pm, edited 1 time in total.
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Structo
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Re: inverting guitar phase with an Op-amp

Post by Structo »

The only other thing is maybe a better opamp.

There are higher fidelity units.
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Gamedojo
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Re: inverting guitar phase with an Op-amp

Post by Gamedojo »

Structo wrote:The only other thing is maybe a better opamp.

There are higher fidelity units.
What would you recommend? I did this pedal on a whim buying parts at radioshack. All they had was a TL082. I built in a socket so I can easily swap out.
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ChrisM
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Re: inverting guitar phase with an Op-amp

Post by ChrisM »

The input and output coupling caps block any DC in your signal. You want ones big enough that they pass all frequencies in your signal and don't block any, you wanted this to be "transparent".

I understand you want your bias point to be half your V+. So 4.5V for 9V units, then your operating in your linear region and you have +/- 4.5V to +/- saturation. The 4.5V is usually generated with a linear voltage divider.

Try an OPA op-amp. They sound pretty good to me compared to your run of mill op-amps.
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Re: inverting guitar phase with an Op-amp

Post by Cliff Schecht »

Sorry I wasn't more clear in my post. The 0.1uF and 10uF caps go from VCC to ground, the larger one close to the battery and the smaller one as close as you can get it to the +VCC pin the for highest effectiveness. It shunts any noise or other crap from your VCC to ground as well as providing that extra bit of current during transient situations (hitting a power chord full blast).

The input caps can be anywhere from 0.1uF to about 0.022uF without too much attenuation. The output will be fine with a 0.1uF cap and some people also add a 1 Meg pulldown at the output, although that probably isn't necessary here.

The 1/2VCC trick is common in pedals and there are lots of ways to get the same offset. What you are doing will work fine, as you found.

You're probably fine with that TL082. Guitar is by no means a high fidelity instrument.
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Structo
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Re: inverting guitar phase with an Op-amp

Post by Structo »

WHen I was trying to improve a solid state effects loop I had it came with a TL072.

So I ordered four different higher fidelity opamps.

I settled on the OPA2134PAG4

http://www.mouser.com/Search/ProductDet ... PA2134PAG4
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FUCHSAUDIO
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Re: inverting guitar phase with an Op-amp

Post by FUCHSAUDIO »

I would raise up the resistors to 470-K 470-K. Still unity, much higher input impedance. The guitar is seeing 10-K if I recall my op amp theory right...yes, coupling caps on the in and out too.
a
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Gamedojo
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Re: inverting guitar phase with an Op-amp

Post by Gamedojo »

Thanks guys! Great info!

So...after the suggestions given. Does this look right?

[img:1024:768]http://www.tylergrund.com/images/opamp_ ... r_v2.0.jpg[/img]
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Re: inverting guitar phase with an Op-amp

Post by Cliff Schecht »

FUCHSAUDIO wrote:I would raise up the resistors to 470-K 470-K. Still unity, much higher input impedance. The guitar is seeing 10-K if I recall my op amp theory right...yes, coupling caps on the in and out too.
a
The guitar is seeing a 100k input impedance which is fine. Definitely no smaller though.
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FUCHSAUDIO
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Re: inverting guitar phase with an Op-amp

Post by FUCHSAUDIO »

Most guitars like to look at 1-M do they not ? I think 100-K is a little on the low side imho..
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oldhousescott
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Re: inverting guitar phase with an Op-amp

Post by oldhousescott »

You could do something like this, since you're using a dual opamp. The switch will let you flip the phase as desired.

[IMG:1506:613]http://i51.tinypic.com/ohmwps.jpg[/img]
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Merlinb
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Re: inverting guitar phase with an Op-amp

Post by Merlinb »

Gamedojo wrote: I'm wondering if anyone would have any suggestions of additional circuit changes that might improve the fidelity or performance of the circuit. Such as coupling caps, or such. Thanks!
As mentioned, the circuit would benefit from higher input impedance. However, you should not just increase the resistances you already have, as that results in major Johnson noise. The professional way to do it would be to use two opamps (you have two already anyway!).
Run the first opamp as a non-inverting buffer, which can have high input impedance without adding resistance in series with the signal, then use that to drive the second opamp which is inverting, using smaller resistors (10k say). The TL082 is fine, there would be negligible improvement with a special-quality opamp in a setup like this.

As well as input/output caps, it is a good idea to add 100R resistors in series with the input and output to protect against RF and load capacitance, respectively.

(Why not add a boost pot while you're at it!) :D
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