Sda-1c Common Ground Problem?

francis1967

Ok, you probably know that I am now the proud owner of a pair SDA-1C's and two Rotel amps.

One polkie told me that I could bridge my Rotel amp to run the SDA-1C's while another told me that I would run into the common ground BLOW YOUR AMP crap if I did this.

What is correct and what do I need to do? If anything, please use laymen terms as this common ground issue isn't getting across easily for me. I looked around for the issue here but I couldn't find something specific to bridging the Rotel 976 into 3x150watts into 8ohm and use two of those channels.

If this is a huge problem, will the Rotel run the SDA-1Cs with "just" 60 watts per channel?

I really don't want to blow up anything or melt anything so any advice is greatly appreciated.

F1nut

You can not use a non-common ground amp, bridged amps, dual mono amp or mono block amps with your speakers unless you use the special AI-1 interface cable. The cable is no longer available, but you can make one and there are some threads here that explain how to that. They are some exceptions to the above. With some dual mono and mono block amps you can tie the negative outputs together, but check with the manufacturer first. You can not do that with bridged amps such as your Rotels.

You can power your SDA's with 60 wpc, but it's really not enough power to drive them properly.

francis1967

Hey F1nut,

I don't understand how this works....

The SDA-1C studio speakers have only one set of inputs and in addition the proprietory Polk pin cable that goes between the speakers. I have just seen a common ground cable and it has banana plugs on each end. Where the heck does this cable go? I might be missing something really simple here. I am completely in the dark.

Zen Dragon

francis1967 wrote:

I have just seen a common ground cable and it has banana plugs on each end. Where the heck does this cable go? I might be missing something really simple here. I am completely in the dark.

Please explain on the cable you are referring to. I do not know that there is a specific "common ground" cable.
It is a little confusing to a new SDA owner, but it has to do with the way the Polk's cross feed part of the signal between them to acheive the SDA effect. You will damage a non common ground amp and possibly the speakers.
The only solution is to use amps that have a common ground, which yours will not in mono configuration, or to modify the SDA cable with an isolation transformer.
Another option is the speakers can be run off your bridged amps as long as you do not hook up the SDA cable. The speakers will sound good, but you will get no SDA imaging and no output from the SDA drivers. This at least allows you to set them up while you decide what you want to do. If you decide to set them up this way in the short term do not try to connect the cable. You would likely damage the amplifiers.
Your only options for utilizing all the drivers in SDA mode in the future are to either
- run the SDA's with the cable and only one amp at 60WPC. A little lean powerwise for them
- do the required cable modification if you have some electronic knowlege
- find someone to do the modification for you
- choose a different amplifier for the system.

F1nut

Some folks have made an upgraded cable with banana plug ends and RCA inputs installed in the speaker.

Zen Dragon

Here is a thread on someone who did the mods to the cable
http://www.polkaudio.com/forums/showthread.php?t=23442&highlight=stancor

If you want to find more reading try searching for isolation transformer in the search function.

francis1967

I found this photo of a cable made a couple of years ago and sold to several polkies. Supposedly this cable solved the issue I have now. I still don't understand how this will work because there just isn't any input for it to plug into...or am I just not seeing the forest for all the trees? Is there a difference if the SDA-1C is a studio model or a standard model? Having not seen a standard 1C, I don't even know the difference except that SIG-1 says they can't be biwired or biamped.

francis1967

I will make this real easy.

If anyone has a spare cable that we are talking about, I would be interested in buying it. Looking at the threads covering this issue, I can tell you now, there is no way I am going to be able to use my 10 thumbs and get this right.

francis1967

Could this be a viable solution in order to bypass the whole problem:

I bought the HK AVR 7300 this afternoon and it has 110watt high current to offer to a pair of mains.

If I use the Rotel bridged for my newly purchased CS400i's speakers, then the AVR7300 won't have to use any muscle to power the centers, which leaves just the mains and some smaller surrounds like FX500i's. I would think that if the receiver only has the mains to worry about, it should have serious power in reserve for this purpose, yes?

Can this solution make my SDA-1C's sing to 95% of their potential? If so, then I will just solve this whole issue by not using the bridged Rotel for the SDAs at all and let the AVR7300 power them.

Cheaper, simpler and safer. That is, if there is enough ooomph in this HK to drive the SDA's to a level that is, say, 90% of what I would get if I went with my separates. After all, my separates aren't huge groundshaking units like the Adcom GFA555II, Rotel RB-1070 or McIntosh MC206.

Could it be as simple as this?

Zen Dragon

francis1967 wrote:

I found this photo of a cable made a couple of years ago and sold to several polkies. Supposedly this cable solved the issue I have now. I still don't understand how this will work because there just isn't any input for it to plug into...or am I just not seeing the forest for all the trees?

This cable is used in place of the SDA interconnect. If you look at the photo closely you will see one of the leads is a flat blade connector.

F1nut

The cable in the picture is a home made AI-1 (isolation transformer) with the pin/blade ends, the same ends that your standard cable uses. You use the AI-1 instead of the standard cable, so it plugs into the same connector on the back of your speakers. Their is no difference between the Studio and regular version of the 1C's other than the exterior.