"Why was the polyswitch deleted from the SDA 3.1TL? It is the only SDA that does not have tweeter protection."
"I don't know for sure. We were having some trouble at the time with inconsistency on the polyswitches and may have decided that it was simply not necessary due to the lower power handing of the 3.1 Ultimately we determined that the trip current of the polyswitch prior to being tripped the first time was significantly higher than its rating. As I recall we then began a program of pre-conditioning polyswitches by tripping them prior to installation."
-Matthew Polk -Polk Audio Co-founder and SDA Inventor
"People complained about the protection kicking in and out. They did not see it as a benefit. The damn thing cost $1+ way back then. We came to the conclusion that it was easier to design the speaker so that the tweeter did not fail unless/until it was painfully obvious that it was playing beyond its capability. In other words, while it is possible to overpower the speaker given the limitless choices of amplifiers, the speaker will sound really distorted long before it blows - garbled almost unintelligible. SO, the limit of the loudspeaker is the point on the volume knob at which it begins to sound distorted."
- Stu Lumsden - Polk Audio VP of Engineering
The polyswitches Polk used in the SDA's and the current replacement polyswitches were made by the Raychem corporation. Doing a search on the part numbers lead me to Raychem's website (www.raychem.tycoelectronics.com) and to the websites of vendors who sell the parts. The old RDE series part numbers were superseded by and are now cross referenced to RUE and RXE series part numbers:
Part number RDE090A was replaced with part number RUE090.
Part number RDE050A was replaced with part number RXE050.
Part number RDE070A was replaced with part number RXE075.
The current polyswitches that are provided by Polk to replace the RDE parts are Raychem part number RXE135.
The RXE135, RXE075, and RXE050 are obsolete, but are still available from parts vendors. They are even sold on eBay. The RUE090 is still in production, but it is being phased out by Raychem.
Data sheets for the RUE090 and RXE series polyswitches are attached below.
The RXE050 (RDE050A) has a resistance range of 0.50 to 0.77 ohm, with a post trip resistance 1.17 ohm.
The RUE090 (RDE090A) has a resistance range of 0.07 to 0.12 ohm, with a post trip resistance 0.22 ohm.
The RUE090 (RDE090A) has a resistance range of 0.25 to 0.40 ohm, with a post trip resistance 0.60 ohm.
The current replacement polyswitch, the RXE135, has a resistance range of 0.12 to 0.19 ohm, with a post trip resistance 0.30 ohm.
I have four unused RXE135's. Their measured resistances were 0.16, 0.16, 0.17, and 017 ohm. I saw that the schematics for the SDA 1C, SDA SRS 2.3TL, and SDA SRS 1.2TL specify the RXE135 polyswitch. However, the polyswitches removed from my SDA 1C's were RDE050A's. I still have one of the polyswitches taken from my 1C's. It measured 0.85 ohm. While this is not a post trip level of resistance, it is well out of spec.
I also have five of the RDE090A polyswitches which came out of my three pairs of 1989 model SDA CRS+'s (I seem to have lost lost, misplaced, or mistakenly thrown away the other polyswitch. The 1989 SDA CRS+ uses the same crossover as the 1989 SDA 2B, yet the 1989 CRS+ schematic specifies the RDE050A and the 1989 2B schematic specifies the RDE090A. Maybe Polk neglected to update the 1989 CRS+ schematic.
The measured values of the resistances for the RDE090A's taken from my CRS+'s were 0.29, 0.22. 0.23, 0.21, 0.23, and 0.42 ohm. These values are in the range for an RDE090A that has been tripped.
While the attenuation properties of the polyswitch are relatively insignificant, the noise characteristics of the devices are considerable and audible...even when they are brand new. After a polyswitch is tripped the first time, the resistance increases from 52% to 83%. The noise from the "burned" polyswitches also greatly increases. [Quantifying the signal damage done by new and tripped polyswitches would be a good project for an interested meter pontiff.]:) Removing the polyswitches removes a tiny bit of attenuation, but removes a significant amount of noise. When noise is lowered, the signal becomes apparently louder although the amplitude (level) of the signal remains the same. It is analogous to cleaning a dirty window.
Life Without Polyswitches
I have been polyswitch free since 1990. When I consulted with Polk prior to doing my first SDA modification, I was advised that there would be an improvement in high frequency sound quality if the polyswitches were removed, but I was also advised to remove or short the polyswitches if and only if I was using quality amplification and if I was not in the habit of overdriving my amplifier.
With the polyswitches out, I heard a more natural high frequency presentation, but it was not forward or bright. It was as if a veil had been lifted off the high frequencies.
Three-fourths of the Foul Four (top to bottom): RXE135, RDE050A, RDE090A.
Change in Resistance with Polyswitch Removal
I Removed My Polyswitches