Magnetic Core Crossover Inductor Distortion Testing
Part 2
Inductor Saturation Current:
For these tests I'm increasing the drive voltage until the THD from the inductor reaches approximately 0.5%. I originally planned on using using 1% as the distortion threshold but ran out of amplifier power before hitting 1% THD on several of the inductors, though this was still the case at 0.5%. This was running sweeps up to ~1500w @ 4 ohms on my bridged iNuke 3000, at that point the amp was starting to clip below 500Hz and I actually had it trigger the thermal cutoff during a sweep.
What I've observed is the baseline THD generated by most of these magnetic core inductors (likely due to hysteresis) remains fairly constant as power rises, increasing only gradually until saturation starts to occur. Once saturation begins to take hold THD will rise much more rapidly with increasing signal level especially the high order harmonics. A change in measured frequency response seems to occur only after heavy distortion has been reached. I drove several of the inductors into severe saturation and will post the frequency response comparisons at the different drive levels for those inductors below.
Additional Note: The value of the inductor directly influences it's current handling ability, the larger the value the lower the current it can handle before saturation (all else equal) so take this into account when viewing the below measurements. I further explain and test specifically this in the 3rd part of my inductor testing. If I had identical values (mH) for each of the these different inductors I would have use them for a more equal comparison.
Besides driving the inductors until they reached a set distortion threshold I also took distortion sweeps of most of them at stepped power intervals into the 4 ohm load. These were at approximately 100w, 200w, 400w, 800w and 1200w levels (again into 4 ohms). The links to those measurements at each power level can be found directly below:
Dayton 1.0mH 18 Gauge Air Core.
Air core design so no saturation distortion is generated from the inductor.
Dayton Audio 1.0mH 18 gauge Laminated Steel I-Core.
Saturation did not occur at the amplifier limited maximum sweep level.
Dayton Audio 8.0mH 18 gauge Laminated Steel I-Core.
Threw this one in on just this test since I knew the higher mH value would cause it to reach saturation before I ran out of amp power unlike like the other normal laminated steel I-Cores.
0.5% THD reached at 34.9V drive voltage into a 4 ohm load. Taking Coil DCR (0.66 Ohm) into account this equates to ~7.5A or 260W.
Erse Audio 1.0mH 18 gauge Laminated Steel I-Core.
Saturation did not occur at the amplifier limited maximum sweep level.
Note I run this same test on the 2.0, 3.0, 4.0 and 6.0mH values of this inductor in the inductance value comparison to measure it's effect on an inductors current/power handling.
Erse Audio 1.0mH 16 gauge Laminated Steel I-Core.
Saturation did not occur at the amplifier limited maximum sweep level.
Erse Audio 0.8mH 16 gauge Laminated Steel Super-Q Inductor.
Saturation did not occur at the amplifier limited maximum sweep level.
Jantzen Audio 1.0mH 15 gauge P-Core Inductor - Permite (ferrous powder).
0.5% THD reached at 50.5V drive voltage into a 4 ohm load. Taking coil DCR (0.095 Ohm) into account this results in a current of ~12.3A or ~623W.
While the THD has reached 0.5% the high order harmonics are still fairly low in comparison at this drive level and remain low even in the higher 800w and 1200w sweeps which shows that it's not really reaching saturation at those points either. Normally when the high order harmonics start to jump up together in a single lump you can tell it's saturating. This inductor just has higher levels of 3rd order harmonics then the Erse/Dayton steel laminate I-Core designs of the same value.
Jantzen Audio 4.7mH 14 gauge C-Coil Toroidal Inductor
0.5% THD reached at 39.2V drive voltage into a 4 ohm load. Taking coil DCR (0.07 Ohm) into account this results in a current of ~9.63A or ~378W.
~1% distortion was reached at 41.5v which is 10.2A or 423W into the 4 ohm load.
~5% distortion reached at 48.7v which is 11.97A or 583W into the 4 ohm load.
~10% distortion reached at 56.6v which is 13.9A or 787W into the 4 ohm load.
Random 3.5mH 18 gauge Laminated Steel I-Core.
0.5% THD reached at 30V drive voltage into a 4 ohm load. Taking coil DCR (0.47 Ohm) into account this results in a current of ~6.71A or ~200W.
~1% distortion reached at 32.6v which is 7.3A or 238W into the 4 ohm load.
Random 1.5mH 22 gauge Laminated Steel I-Core.
0.5% THD reached at 29.4V drive voltage into a 4 ohm load. Taking coil DCR (0.55 Ohm) into account this results in a current of ~6.46A or ~190W.
~1% distortion reached at 32.1V which is ~7.1A or 226W into the 4 ohm load.
~5% distortion reached at 45.4V which is ~10A or 454W into the 4 ohm load.
~10% distortion reached at 62V which is ~13.6A or 845W into the 4 ohm load.
Frequency response comparison showing the core saturation effect on frequency response at different drive levels is also added.
Random 0.52mH 22 gauge Ferrite Core.
0.5% THD reached at 4.93V drive voltage into a 4 ohm load. Taking coil DCR (0.2 Ohm) into account this results in a current of ~1.17A or ~5.8W.
~1% distortion reached at 6V which is ~1.4A or 8.6W into the 4 ohm load.
~10% distortion reached at 10.8V which is ~2.6A or 28W into the 4 ohm load.
Frequency response comparison showing the core saturation effect on frequency response at different drive levels is also added.
Random 7.5mH 20 gauge Ferrite Bobbin.
0.5% THD reached at 4.66V drive voltage into a 4 ohm load. Taking coil DCR (0.63 Ohm) into account this results in a current of ~1A or ~4.66W.
~1% distortion reached at 6V which is ~1.3A or 7.8W into the 4 ohm load.
~10% distortion reached at 14.3V which is ~3.1A or 44W into the 4 ohm load.
Frequency response comparison showing the core saturation effect on frequency response at different drive levels is also added.
