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Symmetrical <-> Unsymmetrical

This overview tells you what are some good ways to convert symmetrical input/output into asymmetrical, using Lundahl transformers. Also they can be used as isolation transformers to reduce hum. This is one of the areas where the Lundahl double coil transformers (See note1) are amongst the best you can choose! Still, with so many types available, it is not easy to make the right choice. This overview tells you what transformers are recommended for HiFi use.

Some challenges for the transformer are:

Keep the bandwidth
Not change the signal impedance
Have enough voltage capability
Give no loss. (Low copper resistance)

Generally you need to take care not to overdimension or underdimension 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 capabilty. Don't overdimension this one. By Ohms' law we allways 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 allways what you want. Don't overshoot in the other direction, so don't underdimension 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 allso 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 Resisance. 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 preamps), you can not 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:

1:4
Suppose you step up a pre-amplifier signal with a factor
of 1:4 (generally not recommended) here is what you get:

What do you see when you look into this transformer, from the input side?	Transformer 1:4	What is this transformer loaded with?
3k Ohms (=16k / 16) 1x primary windings capacitance 16x secondairy windings capacitance This may be too much for some pre amps	Signal step up 1:4 resulting in: Impedance step down 1:16	47k Ohms For instance the input of an amplifier

1:2
Suppose you step up a pre-amplifier signal with a factor
of 1:2 (generally no problem) here is what you get:

What do you see when you look into this transformer, from the input side?	Transformer 1:2	What is this transformer loaded with?
12k Ohms (=47k / 4) 1x primary windings capacitance 4x secondairy windings capacitance This should be possible for most pre amps	Signal step up 1:2 resulting in: Impedance step down 1:4	47k Ohms For instance the input of an amplifier

1:1
Suppose you isolate a pre-amplifier signal with a transformer
(so no step up or step down) here is what you get:

(see also the next table)

What do you see when you look into this transformer, from the input side?	Transformer 1:1	What is this transformer loaded with?
47k Ohms 1x primary windings capacitance 1x secondairy windings capacitance This is no problem for all pre amps	Signal step up 1:1 resulting in: no impedance change	47k Ohms For instance the input of an amplifier

These are the results when the transformers are used as 1:1 signal
Isolation transformer or for conversion of from symmetrical
into asymmetrical signals

Type	Core	Description	Best for	Bandwith	Maximum Signal level at 50Hz	0.1 % Distortion at 50Hz starts at:
LL1527	Mu-Metall	Classic product, widely used by many customers, for HiFi and professinal applications.	Price/Performance	10Hz --150kHz	+16dBU	6dBU
LL1527-XL	Mu-Metall	High Signal Version of LL1527	Price/Performance @ high signal	10Hz --150kHz	+19dBU	9dBU
LL1570	Mu-Metall	Extremely high bandwith	Bandwith	10Hz --200kHz	+16dBU	6dBU
LL1570-XL	Mu-Metall	High Signal Version of LL1570	Bandwith @high signal	10Hz --200kHz	+19dBU	9dBU
LL7902	Mu-Metall	Low copper resistance, finest and ultimative 1:1 transformer.	Shielding and low R-Cu	10Hz --100kHz	+28dBU	10dBU
LL1544A	Amorph	Very versatile, with many recommended configurations in the datasheet.	Users who prefer amorph cores.	10Hz --70kHz	can be wired for 14 dbU(normal) or 20dBU (very high signal)	3dBU

Note: The Lundahl transformers have a special winding technology. It means you will NOT have two coils as you may expect, one for primary, one for secondary. The true construction is, the primary and secondairy coils are sectioned into each other. This gives best possible results for audio transformers. The user must connect the sections as indicated in the datasheet, to get the transformer you need.