Neutrik and Bourns PCB Transformers.
The Neutrik NTM 1 is a compact microphone transformer while the Bourns series LM-NP-100x is designed for telephone line applications.
The audio measurements below are with the transformers installed in my IA19 amplifier .
Audio transformers are not designed for and should never be used as mains-insulation transformers.
Even a transformer with an insulation rating of 6.5 kV ( like the Bourns type ) do not have sufficient insulation distances for use as mains-insulation transformers.
Fig.1: Neutrik NTM 1 and Bourns LM-NP-100x series pin-out.
Pin 1 is the core, static ( Faraday ) shield and µ-metal case.
Pins 6 and 8 are the ends of the primary.
Pin 7 can be used for phantom power ( the transformer is designed for microphone input ).
With a common mode signal on pins 6, 7, 8, the CMRR at 20 kHz is 36 dB with GND on pins 1 and 2 while it is 58 dB with GND on pins 1 and 4.
It would have been nice if Neutrik had included this information in the data-sheet.
Pins 3, 5 and 9 are not connected internally. Grounding these improves CMRR by around 1 dB.
Pins 1 and 8 are in phase as shown ( for some reason Bourns do not mention this in the data-sheet ).
The secondary connection is not important. Of 5 samples I tested there is around 1 dB difference in CMRR with pin 5 or pin 8 on GND.
|Neutrik NTM 1||Bourns LM-NP-1001-B1L|
|Turns ratio||1:1 ±10%||1:1 ±2%|
|Primary DC resistance||85 Ω ±3%||66 Ω ±10%|
|Secondary DC resistance||105 Ω ±5%||66 Ω ±10%|
|Primary inductance||>8 H @ 30 Hz|
( 25 H, 21 H )
|Min 2.8 H @ 200 Hz|
|Leakage inductance||Not specified|
( 2.0 mH, 1.8 mH )
Note that all measurements have been made with the PCB on a table with no electrical shielding at all.
Performance will most likely improve when the boards are mounted in a chassis.
Some general precautions for handling signal-transformers are listed in , page 28.
|||Audio Transformers by Bill Whitlock.|
|||IA19: Transformer balanced line input amplifier.|
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