Burn in Step up transformer with ipod.

07-26-13: John_tracy
As to the possibility of damaging a solid state phono pre by inputing a 10Vrms signal into a unit that is turned off, get real. If someone can come up with a real example of this, show me the circuit and I will eat my words. A well designed unit should not be damaged even turned on. If that signal some how shocked the input stage into some type of destructive oscillation then the unit was probably crap to begin with.

John, take a look at the schematic shown on the last page of the manual for the Hagerman Bugle2 phono stage. Note that the input signal goes through a low value resistor into an OPA2134 op amp, which uses a FET input stage. Here is the datasheet for the OPA2134. Note in the Absolute Maximum Ratings listed on page 3 ("stresses above these ratings may cause permanent damage" as stated in note 1 below the listings) that the absolute maximum input voltage is specified as being 0.7 volts beyond the power supply "rails" (voltages) that are supplied to the device.

That kind of absolute maximum specification of input voltage, specified relative to the supply rails, is very common in the case of op amps and other analog integrated circuits.

When the phono stage is turned off the supply rails are of course zero. Therefore at the positive and negative peaks of a 10 volt rms sinusoidal input that absolute maximum rating will be EXCEEDED by (10 x 1.414) - 0.7 = 13.4 volts. (The 1.414 factor converts rms to peak, of course).

And even when the device is powered up, its supply rails are approximately +/- 10 volts in this phono stage (as indicated on page 7 of the manual), and so the absolute maximum rating of its input will be violated by 3.4 volts at the positive and negative peaks of the waveform.

Will that damage the device, or degrade its long term reliability, if it were done for a prolonged amount of time during a burn-in process? I don't know. But it certainly strikes me as a significant risk, and as very bad practice.

That was the kind of situation I was envisioning when I offered my previous comment, not an oscillation scenario. And also in the back of my mind was the fact that bipolar transistors, which might be used in other designs, commonly have very low absolute maximum voltages specified for their base-to-emitter junctions in the reverse direction, in the neighborhood of 5 volts or so.

Regards,
-- Al

"BWT, it is a myth that prolonged core saturation will damage a MC transformer"

How so? Transformers can only handle so much wattage before they saturate. Extended saturation causes heat issues. Heat issues cause problems. What am I missing?

IMO and experience, it is never a good idea to use anything far outside its intended design parameters.

I am happy to report that no damage has been done to my step up. I have been feeding it with 10% to 60% of the volume of the ipod for approx 80hrs. Normal music signal was used.

Results: When i listened after burning at 10% volume around the 24hr mark, i could not really hear much difference. But now WOWWWWW, complete transformation. Bass is really deep and thunderous, mids and top really opened up. At the same thing it sounds very relaxed. I also find that it sounds louder at my previous volume level. This could be due to the Step up sounding a lot more dynamic.

I will continue burning in unit in the following weeks.The MC step up is the New York based Audio Labs T20. I also have a Lyra Erodion that i have been using for the past year, but which i will also burn in.

Thanks to al that have contributed to this thread.

If we analyze what a transformer is, it is a length of signal-conducting wire (s) that are wrapped around a permeable core. As a purely passive device, it has less to go wrong than with an active amplifier. The possible points of failure due to too-exuberant burn-in would be the core, and the wire.

Unlike a phono cartridge, a stepup transformer does not need to maintain a permanent magnetic field, therefore temporary saturation should not cause problems. However, I wonder how effective saturation will be in the burn-in process, since saturation will remove signal amplitude variations that the transformer core "sees".

If the current pushed through the signal wire is too high for its diameter, the wire will melt. But the wire diameters used in a transformer are certainly bigger than in a phono cartridge. It isn't so likely that a line-level signal will damage the wire.

If you use the output of a power amp to drive the transformer, or connect the transformer to the AC mains, certainly the signal wires could be damaged.

As long as you make sure that the current level used in the break-in process doesn't exceed the current-handling abilities of the transformer's internal signal wire (which will be affected by load / termination as well as signal level), I doubt if anything nasty will occur.

BTW, a transformer is a bi-directional device. Normally feeding a signal into the primary windings (normal input) will cause a stepped-up signal of equivalent power (higher voltage, lower-current) to appear at the secondary windings (normal output). However, the transformer can also be connected back-to-front and used as a step-down transformer - if you feed a signal into the secondary windings, you will get a stepped-down signal output from the transformer primaries (output signal voltage will be the inverse of the transformer's stepup ratio).

Needless to say, the above can be utilized when burning in an MC transformer (in which case, don't forget to load the primary windings).

hth, jonathan carr