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Saturday, December 21, 2013

Single Ended Output Transformer Measurement

Single Ended Output Transformer Measurement

Usually we would use the LCR Analyzer (Hameg 8118 or GW Instek LCR-821) to perform impedance measurement or actual machine for measuring the output waveforms with oscilloscope with FFT function. Both these measurement would portray the performance of the output transformer sufficiently. 

If one is still not satisfied with the above method, they can go for the Audio Precision tool, or get an audio analyzer from Agilent or the other famous test equipment manufacturers. Hewlett Packard, in the olden days had this famous HP-8903B Audio Analyzer that is quite powerful and useful for such applications. If you have enough dough, you can of course choose the new version, which is better and more advanced in features and technology. We would be happy with just the HP-8903B Audio Analyzer. 
HP 8903B Audio Analyzer

Hewlett Packard 8903B Audio Analyzer

In order to plot the frequency response graph, one would need additional GPIB card as well as a formula or program to do so through the HP-8903B Audio Analyzer.

Above picture is taken from Tango X-2.7S output transformer datasheet. It shows you how to connect the audio analyzer and how to setup the system for testing output transformer. 2.7K is the primary impedance of the transformer. A capacitor is inserted between the signal generator (HP-8903B in this context) to prevent DC from going to the analyzer. DC of 100mA is supplied to the primary winding using a laboratory power supply in current mode.

It’s simple, isn’t it? As a standard, Tango is using 4V bias. For correlation, we should setup as 4V bias too. Then, on the DC current side, one should set the DC generator to generate the intended operating current for the output transformer (assuming that we are testing a single ended output transformer). 

If you have the opportunity to perform both impedance analysis and frequency response analysis, you will find that the results are the same. They correlate. Well, of course! Therefore, if you have a LCR analyzer that can be programmed to do a sweep, have DC bias, you can do the same measurement in impedance perspective. 

On LCR analyzer, one should monitor the impedance versus frequency, not forgetting the phase difference too that has to be kept as minimum as possible (maximum acceptable on desired range is <30 deg). 

Our Precision Series of 300B single ended output transformer measures ~3498-3499 Ohm at 1kHz, only 1~2 Ohm different compared to the set goal of 3500 Ohm. It deviates samples to sample but it stays within 3495- 3505 Ohm, which is ~ 99.8% accuracy! Now that’s why we call it as Precision Series. This level of precision is not just being met by chance but by experience and know-how on output transformer design. 

Below are some reference points for our 300B single ended output transformer:

  • 8Hz –       1.75k Ohm, phase 46 deg,       -3dB
  • 20Hz –     2.6k Ohm,   phase 29.5 deg,    -1.3dB
  • 1kHz –     3.49k Ohm, phase 2 deg,         0dB
  • 10kHz –   3.43k Ohm, phase 2.5 deg,      -0.1dB
  • 50kHz –   3.6k Ohm,   phase 14 deg,       -0.1dB
  • 100kHz – 3.33k Ohm, phase 32.2 deg,    -0.2dB

300B Single Ended Output Transformer

Now this is what we call a good single output transformer. If you do not believe, just order one and try it yourself. Also, get some output transformers in the open market for comparison. You will be surprised. 

Sometimes users are too obsessed with rolling tubes or purchasing exotic expensive tubes. They’ve forgotten that the output transformer plays a vital role to the sound quality and omitted that. No matter how good the tube is, if the output transformer is not up to the task, the real character of the tube will not be reveal and they’re just spending unnecessary, and wasting the tube’s lifespan. 

If you use Western Electric 300B tube, or even the NOS Western Electric 300B tube, you’ve got to ensure that the output transformer can really reveal the tube’s character faithfully. Don’t judge a tube before you have a good output transformer installed on your amplifier.

J&K Audio Design
22/12/2013


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* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.

* Our product lines are always improving and increasing. If you do not see what you want, contact us!

* Please email for volume discounts, distributor and OEM pricing.

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