Neutrik and Bourns PCB Transformers.



	Neutrik and Bourns PCB Transformers.

Introduction
Warning
Neutrik and Bourns PCB transformers
References
Copyright and disclaimer

Introduction.

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 [2].

Warning.

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.

Neutrik and Bourns PCB transformers.

Neutrik NMT 1 and Bourns LM-NP-100x pin-out.
Fig.1: Neutrik NTM 1 and Bourns LM-NP-100x series pin-out.

NTM 1:
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.

LM-NP-100x
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.

Table 1: Specification for Neutrik NTM 1 and Bourns LM-NP-1001-B1L.
	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 )	14 mH

Numbers in italics are measured values.
You can find further specifications for the 2 types in their data sheets, but they are not relevant for this application.

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.

CMRR

NTM 1:
This is typical of a small audio transformer: around 100 dB @ 50 Hz falling to around 60 dB at 20 kHz.
LM-NP-1001:
This was a positive surprise: around 95 dB @ 50 Hz falling to around 70 dB at 20 kHz.

Magnetic shielding

NTM 1:
The transformer is housed in a quite substantial µ-metal case. It is very tolerant of nearby magnetic fields. Do not put it on top of a mains transformer though - the µ-metal will saturate.
LM-NP-1001:
This is surprisingly tolerant of magnetic fields for an unshielded transformer. It can be shielded with µ-metal, but unless you have the know-how and tools to work with µ-metal, you will most likely end up with ( very expensive ) sheet iron.
A flat µ-metal shield between the 2 transformers will improve channel separation by 10 dB to 20 dB.
I tried to shield the transformer with plain sheet iron, but distortion will increase when it gets within some mm of the transformer.

Linearity

NTM 1:
Not much to say: I can not measure it. It is within the 0.01 dB resolution ( not accuracy ) of my audio analyzer.
LM-NP-1001:
Flat to 5 kHz and drops 0.05 dB @ 10 kHz, 0.15 dB @ 20 kHz. Pretty amazing for a €2 transformer.

Some general precautions for handling signal-transformers are listed in [1], page 28.

References.

[1]	Audio Transformers by Bill Whitlock.
[2]	IA19: Transformer balanced line input amplifier.

Copyright and disclaimer.

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Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
E-mail: pp@poulpetersen.dk

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