How to choose Symmetrical to unsymmetrical transformers

 


XLR Connector

The number one Question.... Can you send me a sketch of how to....

 


Cinch (also called RCA)

 

In the explanations below, we identify a major cause of hum problems. Basically these are the two solutions:

Use of "interconnect solution" cables from Lundahl, which have 1:1 transformers build inside.

Attach the tone transformer inside the equipment
examples

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
2) The ground connection (metal housings, or shielding)

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 can not 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 can not 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:

  • 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 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 data sheet. 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 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 secondary 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 secondary 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 secondary 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 Bandwidth Maximum Signal level
at 50Hz

0.1 % Distortion
at 50Hz starts at:

LL1527 Mu-Metal Classic product, widely used by many customers, for HiFi and professional applications. Price/Performance 10Hz --150kHz +16dBU 6dBU
LL1527-XL Mu-Metal High Signal Version of LL1527 Price/Performance @ high signal 10Hz --150kHz +19dBU 9dBU

LL1570

Mu-Metal Extremely high bandwidth

Bandwidth

10Hz --200kHz +16dBU 6dBU
LL1570-XL Mu-Metal High Signal Version of LL1570 Bandwidth @high signal 10Hz --200kHz +19dBU 9dBU

LL7902

Mu-Metal

Low copper resistance, finest and ultimate 1:1 transformer. Shielding and low R-Cu 10Hz --100kHz +28dBU 10dBU

LL1544A

Amorph

Very versatile, with many recommended configurations in the data sheet. Users who prefer amorph cores.

10Hz --70kHz

can be wired for 14 dbU(normal) or 20dBU (very high signal)

3dBU

 


How to add an XLR Connector to an RCA Chassis / or RCA Connector to XLR Chassis
Click for schematics here

 

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 data sheet. 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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