PP logo


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 [2].


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.

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 1Bourns LM-NP-1001-B1L
Turns ratio1:1 ±10%1:1 ±2%
Primary DC resistance85 Ω ±3%66 Ω ±10%
Secondary DC resistance105 Ω ±5%66 Ω ±10%
Primary inductance>8 H @ 30 Hz
( 25 H, 21 H )
Min 2.8 H @ 200 Hz
Leakage inductanceNot 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.


NTM 1:
This is typical of a small audio transformer: around 100 dB @ 50 Hz falling to around 60 dB at 20 kHz.
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.
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.


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.
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.


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

Copyright and disclaimer.

Copyright Notice.
This web-page, including but not limited to all text, drawings and photos, is the intellectual property of Poul Petersen, and is Copyright ©.
Reproduction or re-publication by any means whatsoever is strictly prohibited under International Copyright laws.
The author grants the reader the right to use this information for personal use only.
Any commercial use is prohibited without express written authorization from Poul Petersen.

The information is provided on an "as-is" basis and is believed to be correct, however any use of the information is your own responsibility.

This web-site may contain links to web-sites outside Poul Petersen domain ( www.poulpetersen.dk ).
Poul Petersen has no control over and assumes no responsibility for the content of any web-site outside Poul Petersen own domain.

Poul Petersen does not use cookies to "enhance your experience" on this website.

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

Poul Petersen notes index

Poul Petersen diy index

Copyright © Poul Petersen 2020. First published: 20200805. Last update: 20200805. Valid HTML!