Audio test accessories for load isolation and matching

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To connect a 600Ω audio circuit to a 50Ω or 75Ω circuit or test instrument, an impedance matching circuit is required; or a transformer is required to isolate the circuit. Both methods have their pros and cons. Common transformers can achieve impedance matching with typical losses as low as 1.5 dB, provide DC isolation, and can be used in 600Ω primary circuits for balanced or unbalanced operation. The passband of a high quality transformer can be adapted to the audio range of 300 MHz to 15 kHz with minimal amplitude variations. However, transformers that match 600Ω to 50Ω or 75Ω are not readily available and can be expensive.

A minimum loss, fixed value impedance matching circuit (or attenuator pad) provides a constant frequency audio impedance conversion with only two resistors. Although a pad can provide useful impedance matching, its insertion loss is also considerable, with a loss of 14.8 dB at 600 Ω to 75 Ω and 16.6 dB at 600 Ω to 50 Ω, both of which are dynamic. The range brings unacceptable losses.

As part of a set of test accessories, this low-cost, switchable dual-impedance conversion circuit consists of a common transformer and two pads with minimal losses (Reference 1). An inexpensive common transformer can reduce the 600Ω primary input impedance to a medium impedance level of 100Ω (Figure 1). Switch S1 selects the minimum loss pad of 100Ω ~ 50Ω or 100Ω ~ 75Ω. The unit was built using point-to-point wiring that is not critical, although this design uses a Hammond 1590LB stamped aluminum housing for shielding and sealing to support three Amphenol RFX Series BNC panel mount, insulated frame input and output sockets. T1 is a 42TM031 audio transformer from Mouser Electronics with a 0.5W metal film resistor and ±1% tolerance. The total material cost is less than $20 in volume discounts.


For frequency response and attenuation verification in a 600 ohm test set, two separate components are connected back-to-back through 50 Ω or 75 Ω terminals. By halving the 600 Ω ~ 600 Ω transmission loss measurement, the measured value data for individual components is obtained (Table 1).


The insertion loss of the lowest loss pad of 100Ω ~ 50Ω is calculated to be 7.7 dB, while the insertion loss of the lowest loss pad of 100Ω ~ 75Ω is 4.8 dB. Subtracting these values ​​from the measured losses yields a transformer with a center-bandwidth loss of 1.3 dB to 1.5 dB. The insertion loss due to parasitic coupling from the selected output port to the unused output is greater than 40 dB. Combining a common transformer with two lowest loss pads maximizes the benefits of both technologies.

Low cost transformers have moderate insertion loss and provide DC isolation and good frequency response. In addition, the low frequency roll-off of the transformer helps to reduce hum and low frequency noise at 60 Hz. An electrically insulated input jack connects the transformer input to a balanced or grounded 600Ω source.