How to choose Symmetrical to unsymmetrical transformers
In the explanations below, we identify a major cause of hum problems. Basically these are the two solutions:
Any metal housing with working electronics inside, connected to the mains, will always have some small AC voltage on this housing. Reasons are many. First, the safety ground of your house is no "Zero" Volt connection. There is always some small, but very low impedance AC (hum) signal on it. This is coming from capacitive coupling of the "hot" AC wire, throughout the whole house. With hundreds of meters of cabling inside the walls, this capacitance is large enough to cause AC voltage on the safety ground. Now, the safety ground wire system in your house is connected to a "zero" volt connection, your electricity company supplies, and connected to the water pipes of your house, and to an additional earth grounded pin in the basement. So through "some" path in "some" way, "some" current will flow, and this is sure not very little current. The second mechanism is leakage current of all machines, and lamps and equipment that you have. These all leak to ground, either a normal portion, or in a faulty case it can be quite a lot, and you may not even know.
In the end, what we have here is a "zero" volt reference which can only be called such, for safety purposes. However, for signal purposes, this is sure no zero volt reference at all.Sill there is no other option as to connect the metal housing of equipment to this, because safety goes first.
Now comes the problem: When connecting two pieces of equipment, there are always two things you connect:
1) Electrical signal
This forces electrical paths to mix, and some residual hum may be introduced. A difficulty is, this hum can appear with a third piece of equipment! So connect the pre-amp to the power amplifier, and it will not hum. Then you connect the record player, and the record player will hum. Breaking the ground loop between pre-amp to the power amplifier will solve the hum problem, but how you can know with the power amp disconnected. Such a situation is possible, and even for experienced people difficult to analyze. Better is, use XLR connections from the beginning. Further problems come from a thing called: ground loop surface (like a single transformer winding), but this cannot be explained briefly. A patch solution, though illegal, can be to disconnect the safety ground inside the mains connectors. Alternatively use Lundahl mains isolation transformers, using those, the mains safety ground may be disconnected indeed, in a controlled way. This cannot be explained briefly here.
The solution: Signal and ground are separated, like done with XLR. That is why it has three pins. (See above picture) . So two lines are for the signal, and it doesn't need ground. One line is for optional electrical signal shielding. A ground connection of the housings is not made via the cable.
Less ideal is, to use the ground connection and signal connection are combined, like with Cinch (RCA). Here is a nice and ideal solution with tone transformers. These will interrupt the ground wire, so separate your equipment from this "dirty" line, and pass trough the tone signal still. Often referred to as "break the ground loop".
For HiFi use we have these requirements:
Generally you need to take care not to over dimension or under dimension one of the above parameters. If you do, this will go on the cost of the other parameters. What follows now are some of the design considerations, and a list of recommended transformers.
Some design considerations:
Voltage capability. Don't over dimension this one. By Ohms' law we always have the situation that a "high voltage" signal is by definition a high impedance signal too. That means if you choose the voltage capability of the transformer too high, you will end up a too high impedance transformer. Specially as an output transformer this will be of no advantage. Keep in mind that in the transformer factory, the higher voltage capability can be achieved with the same core, by using thinner wire, and more windings. This will increase the copper resistance, and windings capacity. Both is not always what you want. Don't overshoot in the other direction, so don't under dimension the voltage capability. This would result in not enough "headroom" and distortion can come from it.
Configuration. You can parallel or serialize the windings of a transformer, for getting best results for YOUR application. Also here, don't overdo it. Generally, parallel the windings will give a lower output impedance, and lower voltage which may be what you need. But... It will also give you more windings capacitance, which is bad. So paralleling windings can be done as long as you see not bandwidth problems arising. For output transformers this is more critical than for input transformers.
Copper Resistance. You will find this value in the datasheet. Copper resistance is something unwanted, but unavoidable too. Transformer copper resistance should be compared to input impedance. If the input impedance is 5k, 100 ohm copper resistance is not a problem. But if input impedance is 50 ohm (which is not very common), 100 ohm copper resistance will cause a signal loss of 2/3.
Transformer impedance. Transformers don't have an impedance of itself, but they transfer the impedance of the signal. So if you have an ideal 1:1 transformer and you transfer a signal of 3 Volt, with an impedance of 2000 ohms, what comes out is..... a signal of 3 Volt, with an impedance of 2000 ohms. The impedance of the signal is transferred with the square of the windings ratio. Transformers have a range of impedance which they can transfer. If you are confused with this item, it's best to approach the transformer as a voltage (signal) transformer.
Also, transformer impedance (no load impedance) should be reasonably high compared to source impedance. If source impedance is very high (which is the case for some tube pre amps), you cannot put a too low impedance transformer after it.
Step up the signal. This can be done, but needs good consideration. It should only be done when the source impedance is very low. The transformer transforms the load impedance with the square of the transfer ratio. This applies for the load of the transformer, and also for it's own windings capacitance. Here is a numeric example:
(see also the next table)
These are the results when the transformers are used as 1:1 signal
Note1: The Lundahl transformers have a special winding technology. The user will NOT be presented ONE input coil and ONE output coil, as with several low-tech products available on the market. The true construction is, the primary and secondary coils consist of more layers, sectioned into each other, and of most layers separate connections are given. Connecting this is less complex as you think and in each Lundahl datasheet. there is an overview of ready-to-build possibilities. This method results in best bandwidth and best waveform reproduction by the transformer.
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