Transformer Measurement

Transformer Measurement

How do you measure a transformer? Here are some equipment (some are REALLY expensive!) that you can use (not a comprehensive but basic list). 

• Impedance analyzer – measure transformer impedance as a load with respect to frequency, ~20 Hz to 20 kHz or more.
• Oscilloscope / spectrum analyzer – monitor or measure input and output waveforms integrity, frequency response and etc.
• Function generator – generate waveforms to be send to the DUT (device under test)
• Audio Analyzer – distortion, frequency response, THD, IM, SNR and various other parameters.

Important tips from our gurus:

• If your chassis is small, remember to orientate the transformer gap to face AWAY from the tube or sensitive circuit to prevent magnetic coupling. This applies to all the cores that have gaps, like EI core, C-core, double C-core, R-core and such.

One of the basic setup is to measure the amplifier output waveforms using oscilloscope. Here is the setup:

• Function generator (or sound card that can output signals)-> DUT (amplifier)-> 8 Ohm load (or actual speaker) -> oscilloscope

The function generator will generate sine / square / triangle waves to be sent to the DUT for amplification, before being monitored on the oscilloscope display. One can measure the load to observe the gain, frequency response, signal integrity and noise of the amplifier as a function of time using this method. 

User can view the response of the amplifier or transformer from 20 Hz to 20 kHz or more if the oscilloscope is capable of higher bandwidth. Usually we would need the scope bandwidth to be at least 5x the intended to be measured frequency. Let’s say we want to measure up to 100 kHz. We will need the oscilloscope to have at least 500 kHz bandwidth. This should not be a problem with modern oscilloscopes. I’m using a 60MHz 1G/s Tektronixs oscilloscope that is more than enough for this purpose! 

If the oscilloscope has more advance mathematics function, it can even cover the functions of a spectrum analyzer like the FFT (Fast Fourier Transformer).  

Some low end manufacturers only provide the test results based on 1 kHz at 1W output. This really does not show how good the gear is performing. We would want to measure the gear from frequency of 20 Hz to 20 kHz, the full audio spectrum, or more. On top of that, we would want to measure it at rated output, say 3.5W for a 2A3, or 8W for a 300B amplifier. 

1W corresponds to 2.83V on an 8 Ohm load using sine waves. 

• P = V-square / R
• 1W = square(2.83V) / 8 Ohm
• 3.5W = square(5.3V) / 8 Ohm
• 8W = square(8V) / 8 Ohm
• And so forth.

So, now you know what to demand for when you look at the measurement data. Showing you measurement results of just 1 kHz at 1W is not sufficient and it does not reflect the real usage scenario. Ask for more! 

Square / triangle waves will be calculated differently and we shall not cover here in depth.

Apart from observing uniform waveforms, one will have to look for ringing, overshoot and undershoot, as that shows whether the amplifier is having issues on the frequency responses, as well as oscillation problems. 

For those technical savvy users, we would like to measure the amplifier with an oscilloscope to see whether the amplifier is really performing up to specifications or not. There are times where we can’t even see a proper square waveform on the scope at both frequency extremes (~20 Hz or ~20 kHz). Some can’t even make it to 100 Hz or 15 kHz! No joke! 

The reason why is that square waves consists of many different frequencies of sine waves combined together, where some of the frequencies need to be very high or low to properly form the square wave. The frequency response of the amplifier actually needs to exceed the 20 Hz to 20 kHz specifications to generate a nice square wave. Therefore, we always like to design the output transformer to exceed the audible audio frequency spectrum to faithfully reproduce all the signals in the music we listen to.

Other than that, one can use LCR analyzer to measure the primary impedance, phase angle and primary inductance of the output transformer at various frequencies. A good set of starting points will be from 20 Hz to 20 kHz (again!).  One shall see whether the impedance varies a lot over the frequency range that is in direct relationship with the frequency response. You can see the “dB” difference with this formula of “10 log (based 10) measured impedance / intended impedance”. We would want to see at least 20 Hz to 20 kHz tolerance of +/- 3dB. 3dB is the smallest increment in loudness that common (not those with golden ears) human ears can differentiate. 

Theory or “BS” aside, measurements aside, transformers making still requires skilled and experienced winders that really have the know-how to wind excellent transformers. Like what we always say, transformer design is an art. Our master winders have gone through all the hard work to arrive at such status. They’ve paid the tuition fees on behalf of you. So, use them with confidence!

Some might measure OK, look nice, but will fail not long after being in use or does not sound as good as it measured. Therefore, sometimes, don’t be too stuck up with measured data or text book figures and bandwidth. If it sounds good, it sounds good! If it measures well, that’s even better, a big bonus! Tube amplifiers come with 5% or more measured THD and very low SNR, but who’s complaining? They sound great! 

For our transformers, one of the most excellent characteristics is our phase shift. We manage to keep the phase shift as low as humanly possible (0 ~ 10 deg for critical range). Phase shift is done by careful design, implementation and experience as it is almost impossible to get it right without proper workmanship. Phase shift is determined by the combination of inductance and capacitance of the winding. Both will need to act together to align it at 0 deg. If one is larger than the other, then the phase shift will increase either to positive (inductive) or negative (capacitive) quadrant, and therefore the real frequency response will be out.

• Inductive impedance = 2 * pi * F * L
• Capacitive impedance = 1 / (2 * pi * F * C)

Just imagine how hard it is to match both to get 0 deg where frequency varies from 20Hz to 20kHz or more! The variation is just too huge!

Our quality is for the expert users but our price is still at the beginner level. Walk the talk, that’s what we REALLY do! 

J&K Audio Design

20/12/2013

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