Audio-test accessory isolates and matches loads
This low-cost, switchable, dual-impedance transformation circuit comprises a single conventional transformer and two minimum-loss pads.
Richard M Kurzrok, RMK Consultants, Queens Village, NY; Edited by Brad Thompson and Fran Granville -- EDN, March 16, 2006
Connecting a 600Ω audio circuit to a 50 or 75Ω circuit or test instrument requires an impedance-matching circuit or, when isolation of the circuits is necessary, a transformer. Both approaches offer advantages and disadvantages. A conventional transformer can match impedances with low typical losses of 1.5 dB, provide dc isolation, and operate from either a balanced or an unbalanced, 600Ω primary circuit. A high-quality transformer's passband can accommodate an audio-frequency range of 300 Hz to 15 kHz with minimal amplitude variation. However, transformers that can match 600 to 50 or 75Ω may not be readily available or may command a cost premium.
A minimum-loss, fixed-value impedance-matching circuit, or pad, provides frequency-invariant audio-impedance transformation and can comprise as few as two resistors. Although a pad can provide useful impedance matching, it introduces a significant insertion loss of 14.8 dB for a 600-to-75Ω transformation or 16.6 dB for a 600-to-50Ω transformation, either of which might impose an unacceptable loss of dynamic range.
Part of a suite of test accessories, this low-cost, switchable, dual-impedance transformation circuit comprises a single conventional transformer and two minimum-loss pads (Reference 1). A single inexpensive, conventional transformer steps down the 600Ω primary input impedance to an intermediate impedance level of 100Ω (Figure 1). Switch S1 selects a 100 to 50Ω or a 100 to 75Ω minimum-loss pad. Construction of the unit involves noncritical point-to-point wiring, although this design uses a Hammond 1590LB die-cast-aluminum box to provide shielding and a rugged enclosure to support three Amphenol (www.amphenolrf.com) RFX series BNC panel-mounted, insulated-frame input and output jacks. T1 is a Mouser Electronics (www.mouser.com) 42TM031 audio transformer, and the resistors are 0.5W, metal-film units with ±1% tolerances. With quantity discounts, the overall bill-of-materials cost is less than $20.
To verify frequency response and attenuation in a 600Ω test setup, connect two identical units back to back through their 50 or 75Ω terminals. You obtain the measured data (Table 1) for a single unit by halving the 600-to-600Ω transmission-loss measurements.
Calculated insertion loss for the 100 to 50Ω minimum-loss pad is 7.7 dB, and insertion loss for the 100 to 75Ω minimum-loss pad is 4.8 dB. Subtracting these values from the measured losses indicates that the transformer contributes a midband loss of 1.3 to 1.5 dB. Insertion loss due to stray coupling from the selected output port to an unused output exceeds 40 dB. Combining a conventional transformer with two minimum-loss pads takes advantage of the best of both techniques.
The low-cost transformer contributes moderate insertion losses and provides dc isolation and good frequency response. In addition, the transformer's low-frequency roll-off helps reduce 60-Hz hum and low-frequency noise. The electrically isolated input jack allows connection of the transformer's input to balanced or grounded 600Ω sources.
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feelysli - 2011-20-1 18:03:39 PST -
Germain Léger beat me to the punch - I agree.
David Signoff - 2006-22-3 12:26:00 PST -
I think there is a mistake on the circuit, probably at the drawing time. The resistors connected on the switch and going to ground should be placed on the other side of the other resistor that is in series with the input. That way those resistors would come in parallel with the input and the impedance will be match.
Germain Léger - 2006-20-3 14:49:00 PST
