Upgrade Low Frequency Inductors For the SDA SRS 1.2TL

I achieved very good results when I replaced the stock 16 mH SDA inductor with a Solen "perfect lay" inductor of a larger gauge and much lower direct current resistance (DCR). Of course, replacing the SDA inductor is a Polk recommended modification. It has been advised, by Polk's engineering department, not to replace the smaller inductors in SDA crossovers unless the replacement has the same DCR and inductance value. Finding higher performing inductors with the same wire gauge, inductance and DCR has proven difficult.

When I replaced the 1 mH 18 gauge (DCR = 0.7 ohm) and 2 mH 20 gauge (DCR = 1.3 ohm) inductors in the low frequency section of my SDA SRS 1.2TL's with larger gauge, lower DCR inductors from North Creek Music Systems, the results were not positive (Link: Improvements To Modified SDA SRS 1.2TL Crossover). Based on my good results with the Solen replacement for the 16 mH SDA inductor, I looked to see if Solen offered comparable upgrade replacements for other inductors the 1.2TL's crossover. Solen does not make an inductor comparable to the 22 and 25 gauge bobbin wound inductors in the high frequency section. They do make comparable upgrade inductors for the 1 mH and 2 mH series inductors in the low frequency section.

Solen's 1 mH 18 gauge perfect lay inductor with a nominal inductance of 0.47 ohm and 2 mH 20 gauge perfect lay inductor with a nominal inductance of 1.05 ohm replaced the original scramble wound low frequency inductors in my SDA SRS 1.2TL's. I found Solen's perfect lay inductors to have better sonic performance and measurably better electrical noise performance than the North Creek inductors of comparable inductance and DCR. Solen also claims that the tighter perfect lay winding technique provides better mechanical vibration abatement.

I found that the DCR of Solen's 1 mH 18 gauge and 2 mH 20 gauge inductors measured higher than what is listed on their spec sheets. In fact, the DCR values were identical to the stock inductors. With the 20% sale discount and $18 shipping, the total came to $44.26 for the four inductors.


Figure 1. Solen's 1 mH and 2 mH inductors are actually smaller (and nicer looking) than the original
inductors and make Such Good Sound.


Figure 2. Solen low frequency inductors installed.

Conditioning Regimen


Figure 3. Solen recommends a break in period of 100 hours for their inductors. The Audiodharma Cable
Cooker accelerated the break in time with a high intensity conditioning signal.

The Solen inductors were first installed in the right speaker. Improvements were immediately evident. There was more visible cone movement in the right speaker and more tactile sensation coming through the listening seat's right arm rest. Even when my preamp was switched to mono mode, where each speaker is receiving the same signal, there was still more tactile sensation on the right side.

From the listening position, it sounded like the right side was a little clearer. The increase in clarity and detail on the right side was more evident when I took turns standing directly in front of each speaker while listening in mono mode. When listening to Dave Brubeck's "Take Five", the drum kit was on the left side of the sound stage, but I felt the percussive attacks of the snare drum and kick drum more strongly through the right armrest and through floor vibrations through my right foot. Proper sonic balance was restored after Solen inductors were installed in the left speaker.

After the initial listening session, the inductors were connected in series and conditioned for 26 hours. There was a small improvement in clarity and detail after this conditioning session. The next listening session after an additional 22 hours revealed a big improvement in clarity, detail, bass articulation and tactile sensation. The next listening session was done at the 58 hour mark and slight overcooking was evident (blurred bass and diminished overall detail). The overcooking artifacts disappeared after two hours of playback.

Listening Evaluation

The overall sound was apparently louder, which was indicative of a lowered noise floor, and images were thicker and heavier. The biggest change was in the sound of drums which took on a more "explosive" quality with more slam and tactile sensation. A couple of examples follow:

Dave Brubeck Quartet: "Time Out", 180 gram limited edition LP. On the "Take Five" track, the sound of the kick drum changed from a "WHOOMP" to a "POW". The subtle "twang" of the upright bass string tones was louder. The attack and impact of the snare drum solo was faster with more tactile sensation.

Phil Collins: "Face Value" LP. On the "In The Air Tonight" track, in addition the enhanced huge drum sound near the end of the track, there was a "second vocalist" behind and to the left of Phil Collins' voice. Near the end of the track and just before the main drum solo begins, is the addition of an electronically altered overdub of Collins' vocals. I had always heard this as an enhancement surrounding the main vocal. Now I hear it as a distinct, full bodied second voice.

The degree of sonic improvement was much higher when listening to vinyl records than with CD's and SACD's.

Response Measurements

The Solen inductors closely matched to original inductors in DCR and inductance value. As expected, the speaker's overall impedance response and DCR was unchanged by the modification. There were some improvements in frequency response. It must be noted that the pre-modification frequency response, harmonic distortion and bass decay plots were taken prior to the replacement of the Power Plant Premier AC regenerator with a PerfectWave P10 AC regenerator. Therefore, some of the improvements seen in the post-modification plots are attributable to the P10. This is particularly true with regard to harmonic distortion (my P10 review is here). Room response plots were taken with Dayton Audio's OmniMic system.

It is easier to see the differences in the plots if they are downloaded and viewed sequentially.


Figure 4. Pre-modification room response.


Figure 5. Post-modification room response.

The degree of flattening of response between 150 and 500 Hz was surprising.

The bass decay plots show how long it takes the sound to decay at each frequency. As shown on the legend to the right of the decay graph, the white area extends upward to indicate when the level drops no more than 5 decibels (dB). The light blue indicates when the level has dropped between 5dB and 10dB, etc. The ideal situation is not to have some bass notes lasting much longer than others. Significant differences in decay times can cause a decrease in bass definition.


Figure 6. Pre-modification bass decay.


Figure 7. Post-modification bass decay.

The color coded lines in the harmonic distortion plots represent the following:

Black - frequency response.
Dark blue - sum of 2nd-5th harmonics.
Red - 2nd harmonic.
Purple - 3rd harmonic.
Green - 4th harmonic.
Light blue - 5th harmonic.


Figure 8. Pre-modification harmonic distortion.


Figure 9. Post-modification harmonic distortion.

Installation Notes

The inductors came with long leads. I removed the cable ties, carefully straightened the leads, and then wound them around the inner and outer edges of the inductors. New cable ties were applied and the new windings were secured with hot glue. A "brace" of hot glue was applied in front of and behind the tangential points where the inductors joined the board.

Conclusion

Wire is not always just wire. The way wire is wound, the material(s) used to make the wire and the material(s) used to insulate the wire can have a significant effect on sound quality.


Figure 10. Twenty-two year old original low frequency inductors at their retirement party.