Dreadnaught Avel-Lindberg 1000VA Transformer Group Buy?

Sent you an email Neil. I'll have payment out in the morning when I get off work.

DarqueKnight wrote: »

Polk should order about 50 of these to avoid broken hearts in the future.

You know, if Al Ballard was still there he would have made that happen. Don't hold your breath now though.

And also Larry make that 2 cases for me with speakon. If I can't swing 2 of the 1000 I'll settle for an 800

scubasteve4sq wrote: »

And also Larry make that 2 cases for me with speakon. If I can't swing 2 of the 1000 I'll settle for an 800

im assuming that the 800 would fit into the same case since the 1000 is larger correct?

I can wait

Worth the wait in my opinion. I'm sure there are other people who will want to get in on this.

Especially considering it's been almost 2 years since the last buy.

Here are some quotes from Matthew polk to consider

"The original isolation transformer was successful at eliminating the issues of SDA with non-common ground amps but at the cost of a significant compromise of the SDA sound stage. We later figured out how to do this right with the transformer incorporated into the SRT controller.

The transformer is a 1:1 ratio and allows the L-R and R-L SDA signals to be derived while isolating the grounds of the two amp channels. The inverted portions of the SDA signals only need a bandwidth from a little below 100Hz to 1.6kHz. It's helpful if the transformer has more bandwidth on the top end since the high-end cutoff is controlled by a low-pass in the cross-over. At the low end, however, it's best if the transformer windings have a very high inductance as measured with the secondary open, around 14mh, and a very low DCR.

In the mid-range where SDA operates, the SDA drivers on one side operate in series with the SDA drivers on the other side with the signals passing through one set of SDA drivers then through the transformer and then through the set of SDA drivers on the other side. However, at low frequencies the positive part of the signal passes through the SDA drivers and is shunted directly to ground by an inductor in parallel with the transformer. The result is that the SDA drivers operate in parallel with the stereo drivers at low frequencies but operate in series with the other side to produce the SDA difference signal in the mid-range. If the DCR is too high, as it was in the original isolation transformer, it will reduce the level of the SDA signal and alter the bandwidth. If the inductance isn't high enough, also the case in the original, it will raise the low-end cutoff of the SDA signal further reducing the SDA soundstage and allow the SDA drivers to produce too much upper bass thereby muddying the sound.

The transformer in the SRT controller had a DCR around .3 ohms and an inductance around 14mh as best I recall. Current carrying capability before saturation is also an issue and was not adequate with the original. I think it was only about 3 amps which would be about 60 watts to the SDA drivers and not enough. I don't recall exactly what it was in the SRT but seem to recall something like 10 amps which would deliver about 600 watts to the SDA drivers.

As a practical matter we found the only way to get a low enough DCR was with a 1:1 toroidal transformer. These are available off the shelf but expensive. Because of the efficiency of the toroid the cores must be much larger to avoid saturation.

Hope that helps.

-msp

Yes, I did mean 5 amps. The transformer will have to handle nearly the entire amplifier output for any signal that is primarily in one channel or the other. The impedance of the sda drivers in series is around 6.5 ohms. So, a 5 amp rating will deliver up to about 160 watts to the sda drivers before saturation. The inductance determines the frequency at which the transformer will shunt low frequencies to ground allowing the sda driver to produce the same low frequency signal as the strereo driver in that cabinet. 14mh puts that point well below 100 Hz. That means that the sda signal will be preserved at full strength down to 100 Hz which is important for best imaging. Higher DC resistance just attenuates the sda signal and the bass. I think the transformer in the SRT system had a DCR of 0.3 ohms, 14mh inductance and saturation current around 7.5 amps.

-msp

The face plates are drilled out for a D size chassis connector, but there are many choices and they are not all the same. This is the one you should use, http://www.neutrik.com/en/speakon/speakon-chassis-connectors/nl4mp-uc

It's sealed (very important at the speaker end) and high current rated unlike the rest. Make sure the cable connector is also rated for high current, such as this one, http://www.neutrik.com/en/speakon/stx-series/nlt4fx-bag

You will also need M3 (metric) screws, lock washers and nuts to properly mount the sockets. I recommend stainless steel. They are available from Fastenal.

To mount the transformer use either a brass or stainless steel bolt, fender washer, lock washer and nut.

recommending that the SDA interconnect cable be the same as the stereo speaker cable.

Only makes sense to me, but not required.

I also remember hearing or reading that some people shield the SDA interconnect cable.

Speaker cables are generally not shielded. I believe you are thinking of the original AI-1, which sounds a lot better wrapped in ten layers of aluminum foil, aka the baked potato.