QA40x and Transformer Measurements

There have been some questions on how to measure transformers using the QA40x. The transformer used here is THIS from Amazon. This transformer is designed for transforming a high-z tube output to a low-z suitable to drive a 4 or 8 ohm transformer. The specs indicate a 5KΩ input Z and a 4 or 8Ω output Z.

A 5KΩ primary and 4Ω secondary related to the turns ratio via:

\frac{{{Z}_{2}}}{{{Z}_{1}}}={{\left( \frac{{{N}_{2}}}{{{N}_{1}}} \right)}^{2}}

And so the turns ratio is the square root of the impedance ratio:


And in dB, this is

20*\log_{10}(35.35)=30.96 dB.

Now, we know the large voltage on the high-z primary will be transformed to a smaller voltage on the secondary, and so the 30.96 dB will be a -30.96 dB (since it’s getting smaller).

Let’s look at the setup to drive the transformer:

The QA403 output is driving directly into the primary. This is fine, because we expect the primary is high-Z. But we might not know exactly the impedance, and the 100Ω output impedance of the QA403 may or may not influence the exact amplitude we’re driving across the transformer. For this reason, we take the output voltage across the primary and run that into the right channel. If we specify that we’d like to use the right channel as the reference in the measurements, then the loading on the QA403 won’t matter–it will all be referenced to the right channel.

On the secondary, note a 4 ohm load is connected. The transformer needs load to do its job. For this measurement, a resistance substitution box works well because it allows you to easily play around with the load and see how that impacts the flatness.

The test setup is shown below. Note the BNC T’s on the L+ and L- output. Scope probes are using to drive the primary balanced (note that ground clips aren’t used), and the secondary goes into the resistor substitution box and is measured single-ended into the left channel.

A sanity check was made with a DVM. A 1 kHz tone at 16 dBV was generated, and the output across the primary was measured at 11.84Vrms, and the output across the secondary was measured at 258mVrms. Note these measurements were made in IDLE mode to give a constant tone the DVM can measure. This gives:

20*\log_{10}(0.258/11.84)=-33.23 dB

Returning to the measurement using the QA403 instead of the DVM, we get:

Using the reported QA403 values we get

20*\log_{10}(0.2576/12.02)=-33.38 dB

From this, we know our setup is correct. The values measured by the QA403 are more reliable than your handheld DVM. The goal in checking the with DVM is to ensure something big hasn’t been missed, like a 6 dB correction factor some place–these are easy to miss when making balanced measurements.

We can then run the Automated Test AMP Frequency Response Chirp with the following settings. we’ll drive at 0 dBV (which is actually about 6 dBV since we’re driving balanced) and we’ll plot the gain and the phase.


The resulting plot is below. We can see the output has rolled off about 3.5 dB at 20 kHz. We can also see the phase is 0 degrees at 1k, meaning we have the measurement polarity correct.

The gain at 1 kHz is -33.36 dB, which is within 0.02 dB what we measured using tones.

Now, recall above the transformer impedance winding ratio suggested the turns ratio would give a voltage transformation of:

20*\log_{10}(1/35.35)=-30.96 dB

But we measured -33.36 dB. The discrepancy here is likely the assumed primary impedance. We’ll measure that soon.


The transformer input impedance was measured using the MISC Input Impedance plugin. The same setup as above was used. For this test, an early version of a QA462 was used. The QA462 is like the QA461. But while the QA461 had a 0.02 ohm current sense resistor with a gain of 50X INA (giving 1 ohm effective output impedance), the QA462 has 0.02ohm, 0.2 ohm and 2 ohm (button selectable), followed by a 50X INA (this gives effective sense resistor values of 1, 10 and 100 ohms). The QA462 uses the new-ish INA849 from TI as the current sense instrumentation amp. The INA849 is laser trimmed, delivering killer gain matching and fantastic CMRR. It has >8 MHz of bandwidth at G=50, which improves considerably the ~70 kHz current sense bandwidth of the QA461.

Using the 100 ohm effective current sense resistor range, you can readily measure the input impedance of higher-impedance inputs, including amp input stages.

From the above, we can make a table of the input Z at 1 kHz:

Transformer Load Impedance (Ω)

Above we used the equations to determine the turns ratio:

\frac{{{Z}_{2}}}{{{Z}_{1}}}={{\left( \frac{{{N}_{2}}}{{{N}_{1}}} \right)}^{2}}

And we relied on the Amazon-seller’s claim the input Z was 5k to estimate the turns ratio. But with the impedance measured at 6.9k for 4 ohms, we can re-calc the turns ratio:

\sqrt(6.9k/4)=41.53 = -32.36 dB

Note the sign on the log gain was inverted to match the measured above. And this agrees reasonably well with with the -33.36 dB we measured above based on the impedance and turns ratio relationship.

We can also use the MISC R,L,C plugin to graph the impedance and inductance of the transformer input with a 4 ohm load:

Note from the plot above we can see the impedance at at 1 kHz is again 6.9kΩ. But with this plug-in we can also display the reactive component of the measurement, which at 1kHz is 115 mH.

1 Like

I really want to get hold of a QA462 as soon as it is available.

Hi Matt, would it be possible for those who own the QA461 to make a modification to have the performance of the QA462?

@matt thanks for a great series of posts!