Audio through a transformer.

[TerrenceM]

Good Morning All

I am Trying to use my smartphone headset (with mic) to plug work with my landline phone.
So I created an RJ9 to 4pin aux jack converter. In theory it was all meant to be so simply, just a walk in the park...

But! You can't connect the RJ9 Mic and loudspeaker *ground together... It shorts the phone...
So I separated the *grounds using a transformer I found in a power supply.

Solved the grounding problem and now I have an obnoxious buzzz on the line.

There is my story, here is my question:
Is this buzzz from the fact that it is a power supplies transformer? Would replacing it with an audio output transformer help?

* is not actual ground but the negative side of the Mic and the Loudspeaker.

 

[BrianG]

The problem of buzz or hum is not caused by the fact that it's a *power supply transformer*. The transformer could be picking up fields from any mains wiring near where it's situated, but I suspect it's more likely that you have created some sort of a ground fault and the buzz is coming from the telephone exchange battery charging system. A drawing of what you have actually connected to the phone line would help.

 

[DanEE]

Most likely the hum or buzz is coming from the the fact that your connections have upset the longitudinal balance of the telephone line..

Since a telephone circuit can literally be miles of twisted pair hung on telephone poles approximately 10 - 12 feet below power lines that may be carrying 7200 volts or higher, each individual wire of the pair does pick up major amounts of electrical noise. BUT since the voice/data sent on the twisted pair is a differential signal, the noise cancels out, if and only if the electrical symmetry (or longitudinal balance) of the pair of wires is very high.

Any circuit connection on your end of the circuit that introduces any resistance, capacitance or inductance to ground that is not symmetrical across both wires will upset the entire circuit balance and you will experience a major increase in noise.

https://www.valuetronics.com/Manuals/wilcom_t207.PDF

http://www.nhsignal.com/pdf/NH_Analyzer_0408.pdf

 

[DanEE]

A simple way to achieve your objective would be to use a telephone whose design includes a headset jack.

For example, I use a Panasonic KX-TG7641 DECT 6 wireless system in which each handset includes a headset jack... and I use that jack to connect a Heil headset that is also interconnected into other VHF & HF radio systems.

The base unit to handset connection is wireless so your connection to the handset jack will not affect the phone line itself since there is no metallic connection. And of course with the handset being wireless, if you need to move around while talking on the phone.. well that's the whole idea of the design.

One caveat, don't try to use this (or probably any such system) while the handset is sitting in the charging cradle, noise will be introduced.. but when the handset is out of the cradle during use, (as the design intended) it works perfectly.

http://www.amazon.com/Panasonic-KX-...UTF8&qid=1412862389&sr=1-4&keywords=kx-tg7641

 

[VEBill]

Has the OP plugged into the telephone line, or into the handset connection?

He mentioned "RJ9", not RJ11.

 

[geekEE]

I don't think a power transformer, meant to handle 50/60Hz would be a good choice for telephone audio, which can be almost two orders of magnitude higher in frequency. Also, I would suspect that a power transformer doesn't need to be shielded for noise the way that a good audio transformer would be.

Glenn

 

[IRstuff]

The buzz is from the phone itself. Most phones will interfere with audio systems; place a phone close to my laptop or stereo, and obnoxious hum comes out of the speakers, unless they're disabled. Typical landline phone systems are heavily bandpassed, since the guaranteed frequency response is only about 4 kHz.

TTFN
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[MacGyverS2000]

Cellphones cause interference if placed too closely to landline systems (particularly those with speakerphones)... but that's not the same as phones in general (particularly landlines) causing interference.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[LiteYear]

I don't see why you had to short anything together in the first place. I gather the phone has a 4 pin RJ9 socket and the headset has a 4 pin aux plug? Why not just match them up pin for pin? Do you know the pinouts for both devices? A RJ9 handset or a headset is usually wired black (1) and yellow (4) for the microphone and red (2) and green (3) for the speaker. Not sure about the aux plug, but when you have 4 pins there is no need to common the two devices.

 

[VEBill]

OP wrote that he's "trying to use my smartphone headset", so it came that way.

Simplest solution is to hit eBay.

 

[TerrenceM]

Hallo All..

Thank you for all the replies, sorry about my absence.

@VE1BLL Tried that, unfortunately ebay couldn't supply what I wanted and I've invested to much money now to walk away.

@LiteYear The Smartphone jack (4 pin Aux) only has 1 ground, so Mic ground and speaker ground would have to be one, that's why I introduced the transformer.

@MacGyverS2000 & IRstuff I remember that horrible sound all to well, but this doesn't seem to be along those lines, the interference comes in even if I don't have the cellphone anywhere nearby.

@geekEE Agreed, that's why I hope an audio transformer will do the trick, but at the same time I don't want to spend more money on this and it doesn't work.

@DanEE 10:00 That is a simple idea that could of worked, but I don't want to buy a new phone for work. (if I had done that from the start, probably would of worked out cheaper)

@DanEE 9:37 Thank you for explaining the problem, this does make a lot more sense now. I get that I am disturbing the force and all that, but I really believed that the telephone would be able to isolate the line coming in from the audio going out. What if I added a buffer before the transformer. This would stop the transformer and everything afterwards from loading the telephones circuit.

@BrianG I have added a drawing.

 

[VEBill]

Plenty of telephone headset (RJ9) to 3.5mm sockets adapters on eBay, basically closely aligned to what you're attempting. Any particular one might require further audio plug adapters to make your particular smartphone headset plugs fit.

An example search:

http://www.ebay.com/sch/i.html?_nkw=telephone+headset+to+3.5mm+adapter

Obviously I'm guessing here, so if you have some unique and particular requirements, then you may be correct that there's nothing suitable available. Good luck.

 

[IRstuff]

A classical analog POTS line is a gigantic antenna that picks up noise from EVERYTHING; I used to use the telephone hook line as an AM antenna. This is one of the reasons why the voltages are so large, to mitigate the line noise.

TTFN
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