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#1
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replacing the Oktava ML52 transformer -- some measurements
I've modified my Oktava ML52, including replacing the original
transformer with a Jensen JT-34K-DPC. Out of curiosity, I made some THD measurements of both transformers. The results should be considered preliminary or suspect, due to impedance mismatch problems with my test setup. My audio interface is an Echo Mona. I drove the transformer primaries directly from the the Mona line output (at reduced levels). I don't know what the Mona's output impedance is, but it's undoubtedly higher than the rated 31 Ohm impedance of the JT-34K's primary. I'm not sure what the effect of the mis-match might be on THD measurements. I drove the transformer inputs so that the outputs were at -35 dBu, 0.02V peak. Given the ML52's rated sensitivity of 1mV/Pa, that would correspond to 120 dB SPL with the original transformer--a high level, but well within it's operating range (max 135 dB SPL). I measured the THD at 20, 50, 100, 200, 500, 1K, 2K, 5K, 10K, and 20K Hz, using Purebits' Sample Champion software. Here are the results (which, as mentioned above, should be considered prelimary and/or suspect until I can improve the measurement system to match impedances). %THD %THD Freq Oktava JT-34K ----- ----- ------ 20 6.578 0.398 50 1.457 0.082 100 0.339 0.056 200 0.093 0.041 500 0.066 0.037 1000 0.069 0.037 2000 0.066 0.014 5000 0.057 0.012 10000 0.028 0.010 20000 0.024 0.018 The Jensen numbers are a little higher than the curves published in the spec sheet, probably due to shortcomings of my measurement setup. The relative distortion differences between transformers is probably worth noting, in spite of the measurement problems. The Jensen transformer is significantly better throughout the measured range, but particularly so in the low end (as might be expected). It is much larger physically than the original, with a correspondingly larger core. The frequency response of the Jensen was flat within 2 dB throughout the measured range, whereas the Oktava varied as much as 14 dB, rolling off on the low end, with a 2 dB rise between 5K and 10K Hz. However, this result is even more suspect (I think...) than the THD numbers due to the drive impedance problems. A few additional notes: I chose the JT-34-DPC because it has good performance (response, THD, phase) with a high turns ratio (1:37) and a low imput impedance. It also fits (barely) in the bottom cavity of the ML52. The JT-34-DPC is the same basic design as the JT-34K-DX, but the DX model is double shielded, making it larger, and not able to fit into the ML52. The DPC model has a pin interface for PCB mounting, and the DX has wire leads. The measurements were made with an RC damping network on the output of the JT-34K, as described in the Jensen spec sheet (test setups 1 and 2). This network damps high frequency ringing caused by capacitance between the secondary windings (forming a self-resonant LC circuit). One of the engineers at Jensen told me that this network should be used when the transformer is installed in the mic, as well. The JT-34K-DPC is available from Jensen for $78.51. Before I did something that necessitated a re-ribboning (oops) I did some test recordings, and compared them to recordings of the same material (drums and cymbals) made before the mods. The sound was enormously improved. I'm going to try buffering the output of the Mona with a low-noise, low output impedance amp (e.g., AD797), to better match the input impedance of the transformer. If it works, I'll make some more measurements and post them. I should also be able to measure the gain, frequency response, and phase shift. When I get the mic re-ribboned, I'll make some measurements of the its acoustic performance. I'll try to borrow an unmodified ML52 for comparison. For what it's worth, the Sample Champion software seems like a good buy. It does MLS-based impulse response measurements (among many other things), and seems relatively well-written, at $199 for the pro version. There are some additional plug-ins, and you can get a bundle with everything for $299. See www.purebits.com Cheers, Da5id |
#2
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You can improve the low freq response of any xformer by lowering the
souce and load Z. The low freq response is determined by the relationship between the xformer inducance and the source and load Z. It is not magic. I have a cheap xformer that was designed for a telphone application. It is rated at 600 Ohms and when driven by 600 Ohms, it rolls off below 300 Hz which is fine for a telephone applicaiton. I am unsing it in a wide range audio application and I drive it from 8 Ohms. When driven from 8 Ohms, the same xformer is flat to below 50 Hz. The low freq distortion is a strong function of the dirve level and the size and type of material of the core. Have fun. Mark |
#3
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I should have added, that my point is that you have to drive the
xformer with a z similar to what the mic capsule provides to make an accuracte measuremnt. This is not easy to do since the Z of the ribbon probably changes with frequency as it is also a coil. THe Z of the ribbon may be very low at low frequencies and therefore the low freq rollof that you measure may not realy happen when the xformer is driven by the ribbon. By the way, I should also have added I really applaud you for making measurmets instead of just posint something like" the mic really had a heavy solid sound to it after I made the change". Thanks |
#4
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Mark wrote:
I should have added, that my point is that you have to drive the xformer with a z similar to what the mic capsule provides to make an accuracte measuremnt. This is not easy to do since the Z of the ribbon probably changes with frequency as it is also a coil. THe Z of the ribbon may be very low at low frequencies and therefore the low freq rollof that you measure may not realy happen when the xformer is driven by the ribbon. By the way, I should also have added I really applaud you for making measurmets instead of just posint something like" the mic really had a heavy solid sound to it after I made the change". Thanks Thanks, Mark. I thought it was worth a try, but I'd like to eliminate as many shortcomings as I can from the test setup. Bad measurements aren't worth a whole lot more than the "really solid, phat sound" approach. Maybe I could measure the impedance of the ribbon at the various test frequencies (probably not easy to do accurately), and then try to reproduce those impedances with the drive amp. --Da5id |
#5
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David,
Thanks for adding to body of knowledge about the Oktava ML-52. The DIY folks over at "The Lab" would probably love to hear about your findings. There are a couple of members who are very experienced with ribbon mic design and practice. I think you'd find an interested community to dialog with. best, MJ |
#6
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David,
Thanks for adding to body of knowledge about the Oktava ML-52. The DIY folks over at "The Lab" would probably love to hear about your findings. http://www.prodigy-pro.com/forum/vie...e40084c7 15a4 There are a couple of members who are very experienced with ribbon mic design and practice. I think you'd find an interested community to dialog with. best, MJ |
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