Balanced lines,Transformers

Keith Hilton · Post by **Keith Hilton** » 29 May 2000 8:33 pm

I was looking in a catalog and noticed the EBTECH Hum Eliminator, and the Furman Dual Transformer Isolator. These products all claim a "quick" cure for cancer. Sorry, I don't mean "cancer" I mean noise and hum. This suddenly got me thinking about our discussion about 10 days ago, concerning XLR connectors, balanced lines, common mode rejection, inverting and non-inverting signals, and floating grounds--ETC.
I am going to order the book Blake Hawkins suggested:"Sound System Engineering Second Edition."
Both of the noise reduction devices are obviously passive devices, meaning they operate without power, using transformers. This has made me really curious as to how the primary and secondary of the transformer, or transformers, are arranged. Also how the plus, minus, common, and ground are handled.
Has anyone looked inside one of these noise reduction boxes, or a simlar box? Has anyone used one?
I think we are getting back to "Balanced Lines" again. This is where the transformer has a center tap that is ground.
The lines coming off the ends of the transformer provide the (plus) non-inverting signal, and (minus) inverting signal. In a way, this is similar to a "Humbucking" pickup. I suppose the next question would be wheather or not tone was changed by the addition of noise reduction transformers. There are those who claim single coil pickups sound much better than the different arrangement of coils in a Humbucking pickup.
Hartley Peavey once told me--"Everything is a trade off!" So, if you reduce noise by creating balanced lines, with transformers, do you loose tone quality?
Sorry for the long post, but I am thinking!

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Mark Amundson · Post by **Mark Amundson** » 30 May 2000 5:58 am

Regarding the balanced lines game, a couple of thoughts:

One is that good audio transformers can have excellent performance if designed for the appropriate circuits. One has to keep an eye on signal levels and transformer materials and winding practices. The best transformers for pro-audio can be found at Jensen Transformer or Sescom Inc. (www.sescom.com)

Two is that balanced drivers and recievers via solid state circuits are commonly used in pro-audio. With reasonable isolation capacitors and those esd stopper diodes you use, balance operation is easy. If op-amps do not suit you, Analog Devices make dedicated audio balanced driver and receiver ICs for the purpose.

I think balanced 1/4" jacks (TRS) on steel guitar electronics are not out of the question. Plus, they are backwards compatible to unbalanced operation.

Bill Terry · Post by **Bill Terry** » 30 May 2000 6:33 am

Keith,
Before I got smart and got into the software biz (the dark side

), I worked on line drivers for xDSL systems. One of the biggest hurdles for DSL (we used DMT, discrete multitone) technology is acheiving ultra low distortion in all the gain stages of the driver. We used a transformer with appropriate winding ratios as 'free' gain to the line, since we had to have the transformer for isolation anyway. Mark is right, in selecting a transformer you have to be very careful depending on what application you have in mind. We found that good linearity was not necessarily easy to come by.

Anyway, the point is that I found Midcom to be a good source of technical information about transformers for many different applications. They have a great technical section on the their web site, Pspice models and all kinds of good stuff. I've linked it below. Might check it out...

http://www.midcom-inc.com/technology/technotes/technote_index.html

Bill

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bterry.home.netcom.com

Keith Hilton · Post by **Keith Hilton** » 30 May 2000 7:58 pm

Sounds great guys! Probably the best thing would be to contact the companies you have mentioned. I'm sure they would have some application notes that would point me in the right direction. I usually down-load application notes right from a companies web-site. If I were guessing, I would suspect the hum eliminators are designed for use with P.A.equipment and hookups. Just like most effect units are designed for Les Paul guitars, not pedal steel guitars. The only way you can get them to work with pedal steel guitar is do a lot of experimenting.
The Analog Devices active chip would be the easy way. I think it would be more exciting to experiment with passive devices.

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Blake Hawkins · Post by **Blake Hawkins** » 30 May 2000 8:01 pm

I agree with Mark...especially about Jensen Transformers.
Audio transformer building is the COMPLICATED part of audio. It is almost a Black Art.
In answer to your question about the tone...yes a transformer will change the tone. How much depends on the quality of the transformer and how well it matches the pickup and the amplifier.
You will also get a different sound if you use an active balanced line driver and an opamp.
Audio inputs using opamps need to have frequency limiting filters because their frequency response is too wide...and balanced line or no...you can have some undesirable signal ingress because of a frequency range extending to 100 KHz or more.
I also agree with your quote from Mr. Peavey.
All of us who have done engineering work realize that projects and products are all a series of trade offs.
Please, understand that my comments above are general and not specific to the products you mentioned.
It is also possible that the tone difference could be so small that you wouldn't notice it.
Blake

C Dixon · Post by **C Dixon** » 1 Jun 2000 4:50 pm

Any transformer, coil or capacitor in the circuit can change the tone. Whether it will or not has been eloquently explained in the preceding posts.

One thing is for sure, whether a given device actually changed the tone or not, some are going to swear that it does.

And therein (IMHO) lies the real dilema.

God bless all,

carl

Keith Hilton · Post by **Keith Hilton** » 1 Jun 2000 9:13 pm

What C.Dixon just said reminds me of something. Because of fear of tone changes,many guys don't install "protective" input and output diodes, current limiting resistors,clamping diodes, and many other protective devices. In time, "they" and their customers pay the price in failures. The "trick" is taking the time to do a lot of experimenting. You can add all the protective devices something needs, "without" trashing tone. If you experiment hard enough, you can even find combinations that sound even better.
I'm sure the same theory applies to adding passive coils. "YES"---you are going to have a tone change, and it probably won't be a good tone change. The trick then would be to figure out how to turn a bad tone into a wonderful tone. It might just be possible to find a combination that created a better overall tone and sound.
When I first started experimenting with my pedals, I tried thousands of combinations. There comes a point where all the formulas, and mathematical equations are thown in the trash. There is only "one" way to fine tune something, and that is to listen with a trained ear. Listen and let your ear be your guide.

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Rich Paton · Post by **Rich Paton** » 1 Jun 2000 9:43 pm

Keith, here are a couple of more reference papers, which I think are good reading on the subject of balanced-line signal transmission and receiving in audio applications. http://www.rane.com/par-b.htm http://www.rane.com/note110.htm