2B and EARLY 1C AI-1 modifications

Schurkey Posts: 2,100
edited April 2012 in Vintage Speakers
heiney9 wrote: »
IF you have late enough serial number 1C's you can build/buy an AI-1 interphase cable which allows all non-common ground amps to be run with SDA's (mono or not)

I decided it would be better to start a new thread than to derail the other thread:

I know there's a list of serial numbers for 1Cs and 2Bs that deals with the ability to use the AI-1 isolation transformer. The nicest scan of that list that I found in a search, was provided by F1nut a long time ago in a thread far, far away.

The bottom of the instructions says that "plug adapters and some internal modification" are needed.

WHAT were the "internal modifications" needed?

First Guess: "Plug adapters" were to convert the blade/blade interconnect jacks to a pin-blade configuration. Polk used up leftover jacks from the earlier-generation speakers on the first of the later-gen speakers. ONE of the blades was actually used to transfer the SDA signal between the cabinets, and I bet the second conductor in the jack was simply un-used--not connected to anything inside the cabinet. The "modification" is nothing more than connecting the second "slot" of the blade/blade jack to the negative binding post. Polk would perhaps have supplied a plug-in adapter so a pin-blade cable end would attach to a blade/blade speaker jack.

The sum total of the Polk-approved modification is a pair of adapter jacks (or, perhaps, the proper pin/blade socket as used on the later-production speakers???) and maybe eight or twelve inches per cabinet of whatever wire Polk was using to connect the previously-disconnected blade to the negative binding post.

This could be EASILY electrically reproduced using non-stock parts. Wouldn't even have to be expensive, although I'm sure there are expensive ways to do it.

My point being that the early 1C and the 2B are completely compatible with the AI-1, with very minor changes.

What part(s) of my guess are not correct?
Post edited by Schurkey on


  • thsmith
    thsmith Posts: 6,082
    edited April 2012
    Are there schematics on the sticky under vintage speakers? If there are that may shed some light. For some reason I think it is more than that to it but I have no data to back that up. Just considering there is serial number range tells me something specific changed that allowed an AI.
    Speakers: SDA-1C (most all the goodies)
    Preamp: Joule Electra LA-150 MKII SE
    Amp: Wright WPA 50-50 EAT KT88s
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    Cables: Mits S3 IC and Spk cables| PS Audio PCs
  • Schurkey
    Schurkey Posts: 2,100
    edited April 2012
    thsmith wrote: »
    Are there schematics on the sticky under vintage speakers? If there are that may shed some light.
    Yes. Thanks for the kick in the ****. Proved most illuminating. There are two versions of the SDA 2B. Early 2B has a blade-blade socket, and according to the schematic, the second blade is not connected to anything.

    Newer 2B has a pin-blade socket, and is already AI-1 compatible.

    AT THE TIME OF PRINTING, the scan provided by F1nut posted above was correct--2Bs shipped with the older, leftover blade/blade sockets, and so "all" 2Bs (to that time) needed the modification. AFTER the above document was released, Polk began using the "correct" pin/blade socket, properly wired and suitable for AI-1.

    The situation was almost certainly the same with the 1Cs that fell within the specific serial number ranges.

    It would take MINIMAL work and expense to convert them to be fully AI-1 compatible. Parts needed would depend on the cable ends used with the AI-1-style device. Since I expect the majority of isolation transformers will be of the Dreadnaught variety and not using the "stock" pin/blade cable ends, aftermarket choices are nearly limitless. One conductor to the crossover via the original blade, or tied in using an added speaker binding post and a bit of wire--and the other conductor routed directly to the negative binding post already on the speaker. Done. $5.00 on the cheap, or sky's-the-limit for "audiophile" connectors and wire.
  • Schurkey
    Schurkey Posts: 2,100
    edited April 2012
    [EDIT to above]
    Well, technically, there are THREE versions of the 2B, but the third is exactly the same as the second except for a change to the polyswitch, and the schematic has added notes on parts quality. Inconsequential.
  • Schurkey
    Schurkey Posts: 2,100
    Seems that images posted earlier have mysteriously vanished.

  • pitdogg2
    pitdogg2 Posts: 24,924
  • voltz
    voltz Posts: 5,384
    Thanks it all make sense now!
    2 ch- Polk CRS+ * Vincent SA-31MK Preamp * Vincent Sp-331 Amp * Marantz SA8005 SACD * Project Xperience Classic TT * Sumiko Blue Point #2 MC cartridge

    HT - Polk 703's * NAD T-758 * Adcom 5503 * Oppo 103 * Samsung 60" series 8 LCD
  • befuddle
    befuddle Posts: 126
    edited June 2016
    yep that was probably it! I thought I read something somewhere sometime