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.
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| HP 8903B Audio Analyzer |
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| 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
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| 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
22/12/2013Product Lists
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Level 3: depends on type, size and complexity. Email us for details.
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The sky is limit of creativity.
* Our product lines are always improving and increasing. If you do not see what you want, contact us!
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