Audio Line Isolation Transformers for Audio Quality Enhancement...

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Audio Line Isolation Transformers for Audio Quality Enhancement...

Audio Line Isolation Transformers for Audio Quality Enhancement and Ground Loop Elimination

System Dependencies
Minimum PowerSDR Version:
Applicable Hardware:   All transceivers 

Content provided by: FlexRadio Systems Engineering

Audio Line Isolation Transformers are used to electrically isolate one audio device from another. The advantages of using this technique is that you eliminate another ground path between the two devices that can cause hum, distortion and possibly an RFI path.

Not all isolation transformers are equal. Isolation transformers can add impedance and induce phase shift to your signal. They can also attenuate signal strength and have finite low and high end frequency cut off points. You can purchase the high end models that are used in the professional recording industry for over $100 per channel.

Isolation Transformers for the SDR-1000
Some SDR-1000 users have reported positive results using isolation transformers on the audio cards line out (I/Q transmit signal) to eliminate audio images when operating AM. Other SDR-1000 users have been able to reduce the "DC hump" that is -11025 Hz below the fundamental frequency which is the 0 Hz IF.

Note: If you decide to install isolation transformers on the I/Q audio lines in between the sound card and your SDR radio, please follow the guidelines below.  These are applicable to the SDR-1000 since the FLEX-5000's audio I/Q lines are not exposed.

  • Use the shortest audio cables you can between the SDR and the isolation transformers.
  • If you install a TX isolation transformer in I/Q audio lines, you will need to recalibrate the system. This includes the PA Calibration routine and the TX Image Rejection calibration for the SDR-1000.
  • If the TX isolation transformer attenuates the I/Q output from the audio card (SDR-1000), you may have to recheck the AC output voltage of the sound card after the isolation transformer and update the voltage value of your sound card.
  • If you install a RX isolation transformer in the I/Q audio lines the RX Image Rejection calibration and the signal level calibration will need to be run for the SDR-1000.
  • If you use both a TX and RX isolation transformer, make sure the RX transformer has the greatest bandwidth and the least amount of phase shift.
  • Pick a isolation transformer that can convert balanced to unbalanced signals. You can run balanced audio cables between the sound card (if the I/O is balanced)and the isolation transformer and unbalanced between the isolation transformer and the SDR for greater RFI protection.
  • Reduction of distortion products near the 0 Hz IF

Isolation Transformers for the FLEX-5000
Since the FLEX-5000 does not have exposed I/Q audio lines that are susceptible to RF or interfaces with a sound card for I/Q data, the only areas where they should be applied are connecting the balanced line-in (1/4" TRS) to out-board audio hardware, using the FlexWire connector for audio I/O, when connecting a PC sound card to the Powered Speaker output and connecting the unbalanced audio input (RCA connector) to out-board audio hardware.  This is applicable to the FLEX-3000 and FLEX-1500 too, but the number of AF inputs and outputs are less than that of the FLEX-5000.

When you connect external audio equipment to ta FlexRadio software defined radio, electrically isolating the two different pieces of hardware reduces the possibility of ground loops that cause hum on your audio and and eliminate a source for RFI ingress.
  • When connecting audio outboard hardware to a balanced audio connection, always use a balanced line out on the audio outboard hardware.  Lifting the ground at the audio input end will eliminate ground loops if the two devices are not at the same ground potential.
  • If the audio outboard hardware supports only unbalanced or single-ended audio connections, use the unbalanced audio input
  • Only use balanced-to-unbalanced audio cabling if absolutely necessary.  Use a isolation transformer that converts balanced to unbalanced signals.
  • Use only high quality isolation transformers that do not change the phase of the signal and have a bandwidth that exceeds that of the input transducer (microphone).

This is an EXCELLENT reference for understanding ground loops and how to eliminate them.
Understanding, Finding & Eliminating Ground Loops

Below are several isolation transformers that have been used successfully with their SDRs

Radio Shack Ground Loop Isolator (
Radio Shack part number: 270-054 - No specifications provided.

Furman IP-8 ISO-Patch Line Isolators

These are Pro (studio grade) 1RU patch panels with 8 fully isolated patch positions. Provide total d.c. block between source and load and can accommodate unbalanced or balanced input. Also makes a great audio routing and (mini) patch bay. Specifications are very similar to the Jensen transformer (1:1), typically -0.5 dB from 10 hz to 200 khz. Total Harmonic Distortion spec'ed at <0.5% 20 hz to 20 khz.

Ebtech Hum Eliminators
  • 20 Hz to 70 KHz bandwidth+/- 0.05 dB
  • Automatically converts back and forth between balanced and unbalanced lines.
  • 1/4" TRS "smart jacks" inputs and outputs.
  • Safely breaks ground loops while leaving all signal grounds in tact with no dangerous ground lifting.
  • Automatically translates signal voltages to match differences in ground potentials, avoiding clipping.
  • Less than .005% THD @ 1KHz and channel crosstalk is better than -97dB.

Jensen ISO-MAX Audio Transformers (DM2-2xx)
The Model DM2-2XX provides over 100dB of common mode isolation at 60Hz.  Transformer isolation allows any type of output driver to be connected to either balanced or unbalanced inputs without causing loading or instability problems.
  • Ruler flat frequency response preserves "Audiophile Quality" signals
  • High-Grade Nickel-Iron-Molybdenum alloy cores for extremely low distortion
  • High quality gold plated connectors for long term reliability

ProCo Sound IT-4
This unit is a 4 channel isolation transformer unit in a single rank mount complete with XLR or 1/4 inch connectors. It also includes a switch for each channel to lift the ground. Frequency response at +/- .5 db is 20hz to 20khz. the -3db response is less than 2 Hz to 85khz. Phase response is less than 3 degrees at 20khz.

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Last Modified:Monday, April 5, 2010
Last Modified By: Administrator
Type: INFO
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