What Resistor Goes on the Secondary of a 600 Ω Audio Transformer? Checking the 600:290 Rating, Winding Resistance and Line Return Loss | VOOHU

Broadcast, dispatch, conferencing and industrial intercom equipment often uses a 1:1 audio isolation transformer to separate a 600 Ω balanced line from the circuitry on the board. Many parts in a selection table are rated “600:290”, “600:301” or “600:316”, yet the turns ratio is 1:1. Engineers used to “600 Ω systems” often put a 600 Ω resistor across the secondary, and then the line fails its return loss test or the far end reports an impedance mismatch. Others switch to 290 Ω, find the level has dropped by more than 4 dB, and suspect a faulty transformer.

The confusion is about what “600:290” means. For a 1:1 part whose secondary rating is below 600 Ω, that number is not a “290 Ω secondary impedance”. It is the termination resistor the secondary should see, so that the line side looks like 600 Ω. Using the impedance and winding resistance (DCR) data in VOOHU’s audio transformer selection table, this article shows how to check the termination value, what happens when it is wrong, why the level drops, and where the resistor goes — across the secondary or in series with it.

1. The Short Answer: The Secondary Rating Is the Termination Value

For an ideal 1:1 transformer, both sides carry the same voltage, so whatever resistor you put on the secondary is what the primary sees. A real transformer has winding resistance — the DC resistance of the copper — in both windings, and it sits in series with the signal path. So in the midband, the impedance seen from the line side (primary) is approximately the secondary termination plus both winding resistances:

Line-side impedance ≈ secondary termination + primary winding resistance + secondary winding resistance

Take the WHTT4204 as an example. The selection table rates it at 600:290 with winding resistances of 141:171 Ω. With 290 Ω on the secondary, the line side sees 290 + 141 + 171 = 602 Ω — a match for a 600 Ω line. So the 290 Ω in “600:290” is the termination resistor this part expects on its secondary.

VOOHU’s cross-reference testing is done the same way. The WHTT6010 vs Bourns SM-LP-5001 report states that the WHTT6010, rated 600:412, has its insertion loss measured with 412 Ω on the secondary (pins 4-6), and its return loss specified against a 600 Ω reference.

2. What Goes Wrong: 600 Ω on the Secondary Makes the Line See 912 Ω

If you follow the “600 Ω system” habit and put 600 Ω on the secondary, the line side sees 600 + 141 + 171 = 912 Ω — about 50% higher than the 600 Ω line. Matching is measured as return loss: the higher the number, the less is reflected. Against a purely resistive 600 Ω reference:

Return loss = 20 log |(Z + 600) / (Z − 600)|, where Z is the impedance seen from the line side

The table shows ideal midband values for several ways of connecting the WHTT4204 in the receive direction, counting only winding resistance. Insertion loss is calculated with a 600 Ω source, so it can be compared directly with the selection table. The 290 Ω row shows more loss, not less; Section 4 explains why.

With the rated 290 Ω termination, the ideal return loss is over 50 dB. In practice, tolerances pull it down: with a 1:1 ±1% turns ratio, the secondary termination and secondary winding resistance seen from the line shift by about 2%, giving about 41 dB in the worst case, and a 10% error in winding resistance gives about 31 dB. Measuring lower than the ideal value is normal, and it is still a big improvement over 13.7 dB with a 600 Ω termination. With only a high-impedance amplifier and no resistor, the line side is close to an open circuit and most of the signal is reflected. Poor return loss is a common cause of complaints such as “the far end says the impedance is wrong” or “there is echo on calls”: in telephone and dispatch systems with 2-wire/4-wire conversion, an impedance mismatch directly upsets the balance of the hybrid.

3. Eight Parts in the Selection Table: Secondary Rating + Both Winding Resistances ≈ 600 Ω

The WHTT4204 is not a special case. The table below lists eight 1:1 parts from VOOHU’s selection table that have a 600 Ω primary and a secondary rating below 600 Ω. All values are taken or calculated from the selection table’s impedance and winding resistance data:

All eight land between 586 Ω and 649 Ω, within 9% of 600 Ω, and their midband return loss counting only winding resistance is above 28 dB. The two ends of the range make the point: the WHATC-05-042AR has 210 Ω of total winding resistance, a 400 Ω secondary rating and 2.3 dB insertion loss; the WHTT4202 has 340 Ω, a 287 Ω rating and 3.5 dB. The line side has to add up to 600 Ω, so the more of it the windings take, the less is left for the termination — and the more level is lost in the copper.

4. Correct Termination, Lower Level: The Part Is Not Faulty

Changing the WHTT4204 termination from 600 Ω to 290 Ω improves return loss, but the level at the load goes down. With a 600 Ω source, insertion loss rises from 2.01 dB to 3.17 dB, which falls within the selection table’s 3.3±0.2 dB.

The voltage drops more than the insertion loss, for two reasons. First, the load goes from 600 Ω to 290 Ω, so the same power across a smaller resistor means about 3.2 dB less voltage. Second, the smaller termination draws more current, the roughly 312 Ω of winding resistance takes a larger share of the power, and the load receives about 1.2 dB less — the increase in insertion loss from 2.01 dB to 3.17 dB. Together, the load voltage is about 4.3 dB lower than with a 600 Ω load. Separately, with 290 Ω the load voltage is about 6.3 dB below the voltage at the line terminals: about 3.2 dB comes from the impedance step from about 602 Ω down to 290 Ω, and about 3.2 dB is loss in the winding resistance.

So the reason to use the rated termination is to make the line side look like about 600 Ω and meet the return loss requirement; the cost is a bit more level lost in the windings. A drop of just over 4 dB after correcting the termination is normal, not a sign of a bad part. Make up the receive gain from the measured voltage difference, not from the insertion loss figure. If level margin is tight, choose a part with lower winding resistance: in the table above, the WHATC-05-042AR loses 2.3 dB, 1.2 dB less than the WHTT4202 at 3.5 dB.

5. How to Terminate Parts Rated 600:600

The selection table also has parts rated 600:600, and these take 600 Ω on the secondary as rated. The line side then sees more than 600 Ω — the excess is the winding resistance. For example, VOOHU’s WHTT4001 vs HanRun HR228434 report states that the WHTT4001 has its insertion loss and return loss specified with 600 Ω on the secondary (pins 6-4): 2 dB max insertion loss at 1 kHz, and 10 dB min return loss at 350 Hz, 1 kHz and 3.5 kHz. The same report gives maximum winding resistances of 140 Ω (primary) and 170 Ω (secondary).

Using those maximum winding resistances, the WHTT4001 with 600 Ω on the secondary shows about 910 Ω on the line side, about 13.8 dB midband return loss and about 2.0 dB insertion loss — consistent with the 10 dB min and 2 dB max in the report. In other words, the WHTT4001 meets its own limits with 600 Ω, but the return loss is not high. The midband return loss of each part with 600 Ω on the secondary, counting only winding resistance:

If the system needs better return loss, there are two options. One is a part with lower winding resistance — the WHTT4006 still gives about 23 dB with 600 Ω. The other is to change the termination to 600 Ω minus both winding resistances: for the WHTT4001, about 290 Ω. Return loss improves sharply, at the cost of insertion loss rising from about 2.0 dB to about 3.2 dB. After changing the termination, measure insertion loss and return loss again; the limits given for a 600 Ω termination no longer apply. Before you change anything, confirm what the system specification requires for return loss and level.

6. Where the Resistor Goes: Across the Secondary to Receive, in Series to Transmit

Receive direction (line → transformer → amplifier): put the termination resistor across the secondary. The amplifier input impedance should ideally be at least 30 times the termination (about 9 kΩ or more for 290 Ω); the parallel combination is then only about 3% below the termination value. If the input impedance is lower, include it in the calculation. For example, with only 2 kΩ, 290 Ω in parallel becomes about 253 Ω, the line side sees about 565 Ω, and return loss falls to about 30 dB. Use about 340 Ω instead, which gives about 290 Ω in parallel with 2 kΩ.

Transmit direction (amplifier → transformer → line): an op-amp output is close to 0 Ω, so put the resistor in series between the op-amp output and the secondary. Use the secondary rating as its value; for a 600:600 part, use 600 Ω, and estimate 600 Ω minus both winding resistances only if you need better return loss. With the WHTT4204 and 290 Ω in series, the line sees a source impedance of about 602 Ω. If the op-amp drives the secondary directly with no series resistor, the line sees only the winding resistance, about 312 Ω, and return loss is only about 10 dB. A DC-blocking capacitor is usually added between the op-amp and the secondary as well, so the op-amp’s DC offset does not bias the core.

For polarity, dot convention and dual-channel checks, see How to Wire an Audio Transformer. If the wiring is correct but there is no output at all, see Audio Isolation Transformer: Signal Goes In But Won’t Come Out?.

7. Return Loss Is Best in the Midband: Primary Inductance Limits the Low End, Leakage Inductance the High End

All the return loss figures above are ideal midband values. At the low end, the primary inductance sits across the line side, and its reactance falls with frequency, pulling return loss down. Take the WHTT4001, whose primary inductance is 1.7 H min in VOOHU’s report: re-terminated at about 290 Ω and estimated at 1.7 H, its return loss is about 36 dB at 1 kHz but only about 25 dB at 300 Hz. At the high end, leakage inductance and winding capacitance move the impedance away from 600 Ω.

So measure return loss at both ends of the band your system specifies (300 Hz to 3.4 kHz is common for telephone and dispatch lines), not just at 1 kHz.

8. Complex Reference Impedances, DC on the Line and Non-1:1 Ratios

First, this article uses a purely resistive 600 Ω reference. Some telephone line specifications use a complex reference impedance made of resistors and a capacitor. In that case, replace 600 Ω in the return loss formula with that complex impedance and build the termination as a matching RC network rather than a single resistor.

Second, DC on the line needs separate treatment. DC flowing through a winding biases the core and increases low-frequency distortion. If the line carries 48 V phantom power, see Audio Transformer on 48 V Phantom Power for which side of the feed point the transformer should sit on, whether DC flows in the windings, and whether a center tap is needed.

Third, for a part that is not 1:1, “600 minus both winding resistances” does not apply directly. With a turns ratio of n:1 (primary:secondary), line-side impedance ≈ primary winding resistance + n² × (secondary winding resistance + secondary termination), so the secondary termination ≈ (600 − primary winding resistance) ÷ n² − secondary winding resistance.

9. Four Steps to Check the Termination, Plus a Prototype Checklist

Step 1, read the rating: for a 1:1 part with a 600 Ω primary and a secondary rating below 600 Ω, terminate the secondary with the rated value; for a 600:600 part, use 600 Ω.

Step 2, check the math: for a part with a secondary rating below 600 Ω, confirm that the rating plus both winding resistances comes to about 600 Ω. The actual termination on the secondary, including the amplifier input impedance, should equal the rated value; do not change it to 600 Ω minus both winding resistances. Only when a 600:600 part needs better return loss should you estimate a new termination as 600 Ω minus both winding resistances.

Step 3, decide where it goes: across the secondary to receive, in series to transmit, and include the amplifier input impedance.

Step 4, measure: at the frequencies your system specifies, measure return loss and insertion loss, and measure the voltage at both the line terminals and the load. Set the receive gain from the voltage difference (about 6.3 dB for the WHTT4204 with 290 Ω). Use the table below as a prototype checklist:

10. FAQ

Q1: Why is a 1:1 transformer’s secondary rated 290 Ω instead of 600 Ω?

Because the impedance seen from the line also includes both winding resistances. For the WHTT4204, 290 Ω plus about 312 Ω of winding resistance comes to about 600 Ω.

Q2: It still works with 600 Ω on the secondary. Do I have to change to 290 Ω?

It depends on whether the system has a return loss requirement. With 600 Ω, the line sees about 912 Ω and return loss is only about 13.7 dB, which is often fine for short point-to-point links. For long lines, multi-drop connections, or any system with a return loss specification, use the rated 290 Ω.

Q3: After changing to 290 Ω the level dropped by more than 4 dB. Is the transformer faulty?

No. Halving the load resistance and losing more voltage in the windings makes the load voltage about 4.3 dB lower than with 600 Ω, which is expected. Set the receive gain from the measured voltage difference, not just the insertion loss.

Q4: If an op-amp drives the secondary directly, do I need a series resistor?

Yes. Add a resistor equal to the secondary rating (290 Ω for the WHTT4204) and a DC-blocking capacitor. Without the resistor, the line sees only the winding resistance as its source impedance, and return loss is only about 10 dB.

Q5: Measured return loss is much lower than calculated. What could cause it?

Check the test frequency first: primary inductance limits the low end, and leakage inductance and winding capacitance limit the high end, so return loss is lower there than in the midband. Then check the turns ratio and winding resistance tolerances, whether the amplifier input impedance has been included, the termination value and tolerance, and any extra series resistance or filter capacitors on the PCB.

Q6: Can a 600:600 part replace a 600:290 part?

It can be evaluated, but the termination has to be recalculated. With 600 Ω on the secondary, a 600:600 part gives about 14 to 23 dB of midband return loss (counting only winding resistance), depending on how high its winding resistances are. To match the return loss of the original 600:290 part, change the termination to 600 Ω minus both of its winding resistances, and measure insertion loss, return loss and level again. Also check the footprint, pinout and Hi-Pot (withstand voltage) rating.

Need Samples or Technical Support?

If you are tuning return loss or level on a 600 Ω line, send us the line specification, the source and load impedances and the required frequency band. VOOHU FAEs can help you calculate the termination and recommend audio isolation transformer samples with suitable winding resistance and insertion loss.

——— VOOHU Engineering Team

Technical support: fae.thorne@voohu.cn | Tel: 400-1048-018

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