TRANSFORMER EXPLORATION
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| LAST BATCH OF LAYERED AUDIO TRANSFORMERS |
My infatuation with audio-transformers grew drastically in the 1970’s. I hand wound a lot of prototypes along the way. When I first started making my own transformers I relied on my eyes to tell me when there was enough copper piled on to the core of my experiment. That is not a good way to design transformers.
In the ’70’s I had no money for parts nor did I know where to even buy raw transformer components, so I removed transformers from old electronics which people put out for the trash. I dismantled the transformers and carefully wound the wire on to spools so that I could reuse it in one of my hack transformer creations. Along the way I met people who helped me understand the design factors in building proper transformers.
Ultimately that led me to call Deane Jensen on the phone in the late ‘70s and discuss transformers with him. A little known fact is that Deane Jensen despised Rupert Neve. Mr. Jensen thought the transformers used in Neve products were too high in distortion.
In 1990 I started my own transformer company called Layered Audio. Layered Audio also produced transformer based handy boxes for interfacing audio gadgets. Layered Audio production ended in 2012 when I moved out of Los Angeles.
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| LAYERED AUDIO MASTERING SERIES, IRON CORE LEFT, NICKEL CORE RIGHT |
FUN WITH TRANSFORMERS
I want to keep this “look” at transformers fun rather than a science or math experiment. It’s not hard to have a good time and learn along the way but you also don’t want to blow up an output stage of a favorite piece of gear because you don’t understand all the factors.
Transformers have strange and wonderous characteristics and capabilities. Nothing beats a transformer for Common Mode Noise Rejection (CMRR). Nothing beats a transformer for electrical isolation.
NOTHING beats a transformer!
When you see “electronic balanced outputs” being touted as a substitute for a transformer that is marketing blah-blah. It’s a manufacturer saving money on not including transformers.
Transformers are the only electronic component to offer 100% galvanic isolation between two electrical circuits. No solid state circuit can do that. None!
TRANSFORMER INPUTS AND OUTPUTS
The input on an audio transformer is called the “Primary”. The output is called the “Secondary”.
Some transformers have multiple inputs, some have multiple outputs. These can be used in series or parallel to achieve some desired characteristic or design criteria.
INPUT AND OUTPUT IMPEDANCE
A simple, bifilar wound transformer, has an input coil and an output coil. They exchange power through the iron or nickle stacked in between these two coils.
The primary of an audio transformer looks like a dead short if you don’t have the right load attached to the secondary. The impedance attached to the secondary “reflects” to the primary and that is the load presented to the output stage of your gadget which is driving the transformer.
INPUT AND OUTPUT TRANSFORMERS
Transformers should be located physically next to the circuit they are coupling to. It’s not a good idea to make a box of transformers and then use long cables to connect them to your gadget or to a patchbay for “inserting transformer tone”. Noise and interference goes up as the wire length between your transformer and gadget increase.
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| MuMETAL® CAN ON INPUT TRANSFORMER |
Input transformers usually get additional noise protection through the use of a MuMetal® can around the transformer. MuMetal® is expensive so it is easy to see why a manufacturer would want to avoid that. Some input transformers are placed in little steel boxes instead of MuMetal® but this can result in over 30dB greater noise susceptibility. Aluminum enclosures are not good shields from induced hum.
Output transformers should be located near the output they are coupling.
AUDIO APPLICATIONS FOR TRANSFORMERS
TRANSFORMERLESS INPUT, TRANSFORMER COUPLED OUTPUT
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| Transformerless balanced XLR and balanced TRS connections |
A lot of our modern equipment is built using solid-state, active-balanced input circuitry. Another name for this is “differential inputs”. One of my favorite transformer applications is to use the differential input of a gadget without a transformer, and then add an output transformer to keep all the electrical ground connections separate.
UNBALANCED INSERTS - EDCOR TO THE RESCUE
Most lower priced mixing consoles have unbalanced-insert points on TRS jacks. If you take the time to connect them properly you can achieve identical performance to a fully balanced insert-point.
Lets look at trying to patch a signal processor using the unbalanced TRS insert-jacks on a Soundcraft GB series mixer. The "Tip" is the audio "Send", the "Ring" is the audio "Return" and the "Sleeve" is at audio ground and electrical ground potential.
The cable shown below has a TRS plug which feeds to two unbalanced 1/4" plugs. Many people plug the TRS into the insert point on their mixer and then use the other two plugs to connect to a signal processor.
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| TYPICAL MIXER INSERT CABLE |
Since the chassis of the mixing console is audio ground and electrical ground, it is very easy and common to end up with ground-loop hum when you use an insert point with a cable like this. If you use a typical TRS to two-1/4” unbalanced cable (like pictured above) to insert a gadget with transformerless connections (such as a dbx 166 compressor or an equalizer) you’ll get increased hum and noise. These “insert” cables connect all the audio and electrical grounds together, creating a grounding nightmare.
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| CLICK TO ENLARGE |
When you plug an unbalanced “insert” cable-end into the TRS output of a gadget, you are shorting the Ring directly to ground. Inside your processor the chip that is driving the Ring output leg is looking at a short-circuit so it turns on “output protection” circuitry inside to keep from self destructing. The chip continues to function in an impaired operating condition which does not enhance your audio quality.
TRANSFORMER TO THE RESCUE
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| CLICK TO ENLARGE |
In this illustration I’m using one of my favorite transformers from Edcor, a US based manufacturer who sells direct on the internet. Edcor manufactures a lot of models but one in particular has become a favorite of mine for interconnecting audio, the Edcor WSM 600/600. This transformer is well constructed, features iron core material and it has a frame which can be bolted to a box or chassis. This transformer is center-tapped for advanced audio applications.
The WSM 600/600 is a one-to-one, 600Ω to 600Ω coupling transformer. Technically the iron core material distorts more at very low frequencies than nickle but I don’t mind it and I love the way the transformer eliminates ground loop hum.
MAKING YOUR OWN CABLES
If you want to get the best performance from your unbalanced insert points you're going to need to learn to make your own cables. Redco would probably assemble them for you custom if you don't want to make your own. The most important takeaway from all this is you need to control your audio ground connections.
It is not a fun task to pack all these connections inside a single Switchcraft #297 TRS plug but if you use Mogami W2697 miniature balanced cable it will be much easier.
EPILOGUE
I hope you understand the schematics and figure out ways to get audio transformers working for you. Carefully used, audio transformers can reward you with better sonics and lower noise.
Thanks for reading High on Technology, Good Music To You!
©Sept 2021 Mark King, it’s not ok to quote or copy without written permission.
WHERE TO BUY - buy parts wherever you want, here are my favorite choices:
Audio Transformers, Edcor - www.edcorusa.com
Wire and connectors - www.redco.com
