SDA-1A Loud Beeping Noise

[altecman]

I am the proud new owner of a pair of mint SDA-1A's and a new member to this site - good site it is.
My problem is that these speakers didn't come with a interconnect cable so just to test that the SDA system is working I jammed some wires into the jacks making sure the the same wire went to both large blade sockets and the same thing with the smaller socket jacks. I know this is crude but I just wanted to make sure the system is working before fitting new sockets and making a new cable.
All went well for a couple of minutes then all of a sudden a very loud beeping sound was coming out of the speakers, I then quickly switched off the amplifier and am now waiting for some feedback on what would cause this. The amplifier I am using is a VTL Stereo 90. Before I go I must mention that after this happened I pull out the wires to the interconnect sockets and play the speakers as normal. The speakers played flawlessly, no probems at all.

[F1nut]

I'd have say your amp is dual mono or otherwise non-common ground and can not be used with your 1A's. You could check with Luke to see if it the negative outputs can be ground strapped, but other than that, you're out of luck with that amp.

BTW, Polk sells the blade/blade cable, runs about $35.00.

[Joe08867]

Repeat of what Jesse said.

Carry on.

[altecman]

Thanks for the reply. The VTL power supply is based on one main power transformer which branches off to seperate banks of filter caps for each channel, wouldn't this arrangement be commonly grounded or are you talking about the output transformers not be commonly grounded together. As this amp was made in the early 90's and is a tube based you would think that it would be commonly grounded. What benefit would VTL have by not doing this.

[Joe08867]

Thanks for the reply. The VTL power supply is based on one main power transformer which branches off to seperate banks of filter caps for each channel, wouldn't this arrangement be commonly grounded or are you talking about the output transformers not be commonly grounded together. As this amp was made in the early 90's and is a tube based you would think that it would be commonly grounded. What benefit would VTL have by not doing this.

If it isn't common ground they could have been going for purity in sound. Less chance of signals from one channel getting to the other. I have heard newer stuff from VTL and it is very nice. But I am not sure if it is common ground or not. My guess is yours is not.

The fact is that SDA's are really the only speakers I know of that require a common ground amp.

[altecman]

I had a look inside the VTL Stereo 90 last night, it appears that the ground from the negative speaker post is directly connected to the negative side of the input socket on chassis, this in turns goes to the signal ground of the input point on the circuit board. Shouldn't the speaker's negative wire go to chassis ground instead of signal ground.

[F1nut]

I looked up your amp in my VTL handbook, but there was no reference to it, sorry. Again, your best bet is to call Luke at VTL. I found him to be most helpful with some questions I had.

[altecman]

I think I have come up with a fix for the older SDA's. In the xover disconnect the SDA circuit from the rest of the xover circuit, treat the SDA circuit as a seperate identity. Then hook up the SDA circuit (inverted) by the way of seperate Binding Posts to the opposite channel on the amp eg left SDA speakers to right channel speaker terminals on amp, just the way the SDA principle was intended. This will cure all problems with common grounding and might prevent some channel leakage as the channels are now independant. I can't believe this hasn't been mention before..

[F1nut]

You know, maybe because it's 4:15am, but I'm not following you.

[altecman]

F1nut, don't use the common ground link between the 2 loudspeakers as you would if you used the interconnect cable, make a separate ground link instead. This is what is common between the 2 loudspeakers when using the interconnect cable - same ground. Send me your e-mail address so I can email a PDF of the modified xover to you.

[Boywonder]

I think I have come up with a fix for the older SDA's. In the xover disconnect the SDA circuit from the rest of the xover circuit, treat the SDA circuit as a seperate identity. Then hook up the SDA circuit (inverted) by the way of seperate Binding Posts to the opposite channel on the amp eg left SDA speakers to right channel speaker terminals on amp, just the way the SDA principle was intended. This will cure all problems with common grounding and might prevent some channel leakage as the channels are now independant. I can't believe this hasn't been mention before..

I've been pondering this for some time also, and wondering why Polk went with the IC cable instead of just wiring each cabinet from each set of speaker terminals, with the dimensional drivers inverted in polarity. According to Polk's patent disclosures, the dimensional drivers are fed a left minus right (or right minus left) signal as discussed in the early Polk SDA patents. Here is part of the abstract from Polk Patent #4489432:

A right sub-speaker and a left sub-speaker are respectively disposed at right and left sub-speaker locations equidistantly spaced from the listening location, and further from the listening location than the main speaker. The left and right channel outputs are respectively coupled to the left and right main speakers. A left channel minus right channel difference signal is coupled to the left sub-speaker and a right channel minus left channel difference signal is coupled to the right sub-speaker. In one embodiment, the main and sub-speakers for each channel are respectively incorporated in a common enclosure to fix the spacing therebetween. A technique for determining optimal spacing between the main and sub-speakers and between the various speakers and the listening location is set forth.

So it sounds like the dimensional drivers need an "difference" signal, not just an inverted polarity signal.

[Boywonder]

Here is another Polk SDA patent abstract that mentions just inverted polarity dimensional drivers.....from Polk Patent 4497064

The left and right channel outputs are coupled to the left and right main speakers, respectively. Inverted left and right channel outputs are coupled to the first right and first left sub-speakers, respectively. Additional sub-speakers can be provided for each channel and fed the same signals as the first sub-speakers. Alternatively, a second right sub-speaker and a second left sub-speaker can be fed other signals, such as the left and right channel outputs, respectively. In one embodiment, the main and sub-speakers for each channel are respectively incorporated in a common enclosure to fix the spacing therebetween.

So, are the dimensional drivers fed a L-R (R-L) difference signal or just an inverted polarity signal?

[Joe08867]

I believe they are fed a L-R (R-L) signal. That is crossed over so the bass is in polarity or you will have that out of phase bottomless sound.

[Boywonder]

I believe they are fed a L-R (R-L) signal. That is crossed over so the bass is in polarity or you will have that out of phase bottomless sound.

Good point, but how do you control the dimensional bass with a mid-woofer and PR? That probably explains the 16mh inductor on the dim drivers on the SDA-1C schematic.....

[F1nut]

F1nut, don't use the common ground link between the 2 loudspeakers as you would if you used the interconnect cable, make a separate ground link instead. This is what is common between the 2 loudspeakers when using the interconnect cable - same ground. Send me your e-mail address so I can email a PDF of the modified xover to you.

I don't see how that is going to work. Have you tried it yet?

[Joe08867]

Good point, but how do you control the dimensional bass with a mid-woofer and PR? That probably explains the 16mh inductor on the dim drivers on the SDA-1C schematic.....

Boywonder, this is exactly why the inductor is there.

This is from another post about Erse inductors. A Letter from the man himself.

A letter from Matt and Stu

In this generation of the SDA's the stereo drivers were nominally 6.5 ohms and the SDA drivers were nominally 3.5 ohms. The plus terminals of the SDA drivers on each side were connected to the plus amp terminal on that side via their cross-over network. Then, the negative terminals of the SDA drivers on one side were connected to the negative terminals of the SDA drivers on the other side via the interconnect cable. This causes the SDA drivers on the right to produce an R-L signal and the ones on the left to produce L-R. Since bass is pretty much mono in most recordings, if the SDA drivers both get full-range R and L signals they would cancel at low frequencies and the SDA drivers would just sit there acting like badly tuned passive radiators. So, we added an inductor in each speaker between the SDA driver negative terminal and the amp negative terminal on that side. At low frequencies that diverts the signal back to ground instead of through the interconnect to the SDA drivers in the other speaker. As a result, at very low fequencies the Right SDA drivers produce only right channel bass and vice versa, while still producing the R-L and L-R signals at higher frequencies. The transition occurs between about 50Hz and 150Hz.

[altecman]

Thanks everyone for their thoughts on this and yes it doesn't work. At least I've tried and it failed, as they say, there's no harm in trying (as long as nothing blows up). Now I have to go back and modify the VTL Stereo 90 now, whilst I'm at it can someone provide me with the details for obtaining a Large Blade / Small Blade Interconnect Cable for the SDA-1A's because in Australia this will be impossible to find or buy.
Thanks!!

[BrettT1]

Polk sells the blade/blade cable, runs about $35.00.