Passive Reverse Phase Switch for unbalanced signal. It is possible?

[jb1]

Hi there,

I'm trying to build a phase invert circuit to reverse the polarity of the right channel on a stereo configuration. I did this in the past with a dc motor but I don't know if it works with audio. I did this drawing based on what I've been searching online. Is this correct? do I need any resistor or anything else? I'll appreciate any input.

Cheers to everyone!
JB

[MrStab]

it looks like that setup will short the signal to ground. you need to invert the phase on a constant signal path, instead of just swapping paths around like you would for a speaker or motor. the most common way to do this is passively is via. a small audio transformer (afaik). Check out Figure 2 at the following link, for a passive DI box, and see if you can adapt the principle to your needs:

http://sound.westhost.com/project35.htm

i've never passively phase-inverted myself, so hopefully someone with much more knowledge will chime in.

[FiveseveN]

Yes, you could do it with an audio transformer, but it's larger, more prone to EMI and many times more expensive than a simple active solution.

[wavley]

Yes, you could do it with an audio transformer, but it's larger, more prone to EMI and many times more expensive than a simple active solution.

To add to this, if you aren't driving the buffer with a good low impedance source you will experience loading that will kill your high end.

Personally, I love transformers, you can get inexpensive ones from Edcor for $6 or the Triad 10k:10k for three.  If I needed just a simple phase reverse and no ground isolation I would go with an active solution.  If have a ground loop, I would look at RG's Hum Free ABY at Geofex.

[Granny Gremlin]

Nope - you can't reverse the phase of a single ended audio signal by flipping the leads with a switch.  That only works with balanced signal (where a reverse phase signal already exists on the cold/TRS ring/XLR pin 3 conductor; vs hot/normal phase and ground on the other 2 conductors).  You need something to generate the inverted signal (tranny or active solution as above). 

DC motors work differently.

You can get a little tranny for pretty cheap (cheaper than the Edcors mentioned, but those are easiest to find for most that don't have a good parts store close by).  Noise issues can be mitigated to a large degree by cobbling up a Faraday shield (assuming a cheap tranny without one).  Depending on the unit it is in (any interference causing things in there?) the hammond box itself will do the job as it's already grounded (or should be).  Won't be any noisier than an Octavia.

[PRR]

> with a dc motor but I don't know if it works with audio.

It can. Usually won't.

The key is that your DC motor (or lamp, or loudspeaker) has *NO* other connection to circuit common. Either wire can be hot or common (or both hot).

Loudspeaker polarity can be flipped likewise. (Lamp polarity is usually meaningless; an exception is 2-color lamps that have one color each way.)

Most small-audio interconnects are un-balanced. One side is "ground", really "common". This is signal return AND signal shield AND sometimes Power common.

Cut a guitar cord. Cross-connect the center and the shield. This "will play", and in reverse polarity, BUT now all the outside (shield) parts of your guitar are connected to the live side of the amp input. It will buzzz horribly.

If you go all the way back to the pickup, a pickup where neither lead is also a shield could be connected in reverse polarity. This is often done in fancy switching. Still not the same both ways because there is always capacitive leakage and usually more on one side of the coil than the other, so the treble response is different when reverse-connected.

Flipping polarity between pedals, or pedal and amp, is not going to work because all interconnects assume one side is signal common and shield.

Ideal transformers would work but all real transformers suck somewhat. Between active-output pedals a 10K:10K should work, and at pedal level a low-cost transformer may do very well. But naked guitars and many popular pedals do not have the output-oomph to drive most transformers cleanly.

[tubegeek]

Bottom line: the "passive" requirement is impractical and this task is much easier & better done with active devices.

[wavley]

Yes, you could do it with an audio transformer, but it's larger, more prone to EMI and many times more expensive than a simple active solution.

To add to this, if you aren't driving the buffer transformer with a good low impedance source you will experience loading that will kill your high end.

Personally, I love transformers, you can get inexpensive ones from Edcor for $6 or the Triad 10k:10k for three.  If I needed just a simple phase reverse and no ground isolation I would go with an active solution.  If have a ground loop, I would look at RG's Hum Free ABY at Geofex.

Oops, I re-read post and realized that I said buffer where I meant to say transformer.

I do stress that you should look at RG's Hum Free ABY.  He actively drives cheap $3 transformers to get better frequency response, certainly good enough for most guitar applications.  You get phase reversal and ground isolation.  I have a set up inspired by his circuit, but I play a lot of baritone guitar and Bass VI so I needed better low end which is why I went with the $6 Edcor.  My transformers are actually separate from the active part of the circuit and sitting on top of my amps so it's the buffer driving the long run of cable and the transformer doesn't suffer from the loading of the cable, one of the transformer boxes has a phase reverse switch so that all of my amps will play nice no matter which two I choose.

Of course, always listen to PRR.

Bottom line: the "passive" requirement is impractical and this task is much easier & better done with active devices.

And this is right too, I really only bring up the Hum Free because it future proofs you from one (of many) potential sources of ground loops.

[jb1]

Hi everyone!

Thank for everybody's input. I'm looking for a phase reverse switch AND a ground lift switch, so I check that Geofex ABY Hum free box for sure.
I'll use an Analogman buffer circuit for the input, so I guess that get me covered to having a strong signal to drive all the rest.

I'm building a junction box for my pedalboard, I want it to have an input buffer (switchable to on/off), also it will have and stereo out (switchable to mono) with a phase inversion switch and ground lift for the right channel. Also I will like it to have a switch to alternate between the "thru" signal that goes to the pedals, and straight to the left/mono out for comparison between effected and unaffected signals...

Is anybody has a circuit design for this let me know, that will save a lot of try and error. Altough I have fun with the trying thing.

Talking about transformers, does anybody has any links to buy these here in europe?, If you don't I can get them online from the web I guess...

Thanks everybody as usual
Best regards
J

[jb1]

Ideal transformers would work but all real transformers suck somewhat. Between active-output pedals a 10K:10K should work, and at pedal level a low-cost transformer may do very well. But naked guitars and many popular pedals do not have the output-oomph to drive most transformers cleanly.

The signal I'm trying to isolate and reverse phase it will come buffered and through several pedals. Can you recommend me a good transformer for that? Ideally easy to find here in Europe (I'm in Spain).

[Granny Gremlin]

Good iron costs money.  I don't know what's available over there besides Lundahl (expensive and arguably overkill) and Neutrik which has a line of modular bits for adaptor making that includes mini transformers.  Not cheap, but not incredibly expensive either.  The models relevant to you are NTE1 (no can/shield; cheaper) and NTL1 (in a can; pcb pins; better bandwidth specs; significantly more expensive than the former IIRC).  You could probably at least special order the Neutriks from any music store that sells their other stuff.

[jb1]

Good iron costs money.  I don't know what's available over there besides Lundahl (expensive and arguably overkill) and Neutrik which has a line of modular bits for adaptor making that includes mini transformers.  Not cheap, but not incredibly expensive either.  The models relevant to you are NTE1 (no can/shield; cheaper) and NTL1 (in a can; pcb pins; better bandwidth specs; significantly more expensive than the former IIRC).  You could probably at least special order the Neutriks from any music store that sells their other stuff.

thanks!. Neutrik sounds cool. I've been using their products a lot. They're solid. the NTE1 costs around 10usd here which seems decent, the other one scyrock to 60usd :S.
In case I would want to research for options, what would be the specs I'm looking for? (I'm near to zero knowledge on this): 1.1 , 10:10, watts? Impedance?

Thanks a lot for your input.

Regards
J

[Granny Gremlin]

The 2 Neutriks I mentioned are both 1:1 (turns/impedence ratio).  Any 1:1 will work, some just better than others.

These are still more expensive than the Edcors mind you, just not insane; the NTE1 is about $10 and the other one over $50 if I recall.

[jb1]

The 2 Neutriks I mentioned are both 1:1 (turns/impedence ratio).  Any 1:1 will work, some just better than others.

These are still more expensive than the Edcors mind you, just not insane; the NTE1 is about $10 and the other one over $50 if I recall.

Thanks! I found a place that has the neutriks in stock here so i'll save on shipping.
Thanks again. Cheers
J

[hymenoptera]

Since it's already in a box with other powered stuff, then a simple collector follower set up for unity gain might be the simplest (and cheapest!) option. Wire a 2pdt switch like true bypass and you're good. 3pdt and you can have an LED indicator to show that the phase inverter is engaged.

[jb1]

Since it's already in a box with other powered stuff, then a simple collector follower set up for unity gain might be the simplest (and cheapest!) option. Wire a 2pdt switch like true bypass and you're good. 3pdt and you can have an LED indicator to show that the phase inverter is engaged.

Hi there. I wouldn't know what that is, it is a circuit to invert phase? Do you have any pic, draw or something to refer to?
Cheers,
J

[hymenoptera]

Hi there. I wouldn't know what that is, it is a circuit to invert phase? Do you have any pic, draw or something to refer to?
Cheers,
J

Also called "common emitter" or with a FET it's "common source".
https://en.wikipedia.org/wiki/Common_emitter

Can also be done with an opamp in "inverting mode"
https://en.wikipedia.org/wiki/Operational_amplifier_applications#Inverting_amplifier

One of the most popular things you can do with an active device. These are some of the typical ways you'd build an amplifier with gain. The output polarity gets flipped though. That's why I mentioned it. It's usually a problem, but since that's your goal... Anyways, the resistor values determine the gain.

As for resistor values, I can't help there. Should be easy to find.