I wonder if both transformers are connected to each channel or one connected to 2 and another to 3.

I'd assume that one transformer is connected to two channels and the other to three channels. I would assume that the two channels are the front l/r channels. I guess that it's possible for two transformers to be connected in that manner, but this would effectively act like one large transformer to each channel. I don't think that this would be wise because each transformer would have to have the same number of windings, which wouldn't provide a space benefit. Also, there would be ten sets of input/outputs (or plus/minus), which would be expensive.

Perhaps you'd want to send an e-mail to the Outlaws to get an answer for this.

It brings up an interesting point, though, that all five (or seven) channels may not be created equal. I'd always assumed that they WERE equal, and that there'd be no difference to connecting your fronts to the channels marked "surrounds" if for some odd reason you felt like doing that.

The other possibility is the transformers are rated to drive 3 channels and they are using only one to its capacity.

I have wondered the same thing myself. They went to dual transformers to save space (allow the amp modules to be longer) as a single transformer would have been thicker.

Acurus made some of their home theater amps stronger for the front L/R channels. How to best do that, however, is open to some debate. I'm not sure I'd put both front channels on one transformer.

I've designed audio power amps (I'm an audio engineer) and I'd be tempted to split each front L/R channel onto its own transformer. That way the amp acts more like a true dual monoblock unit in stereo mode. You will have less interaction between the stereo channels and more power available. In this scenario, one transformer would be burdened by the extra bias/quiescent current of the extra unused amp module but I don't think that would really affect things much.

In 5 channel use, the center channel usually works the hardest. So you'd ideally want it on the transformer with only two channels. So I'd probably put say the front L and Center on one transformer and both surrounds and the front R on the other.

If you run your system in say a 5 channel "equal power" music mode, the three channels that are sharing one transformer are going to clip slightly sooner than the other two. Still, under most conditions, we're probably talking about less than 1db of peak power differential regardless of how you split the channels up (as long as the transformers each have a high enough VA rating to stay out of saturation running three channels with fairly normal speaker loads). With really reactive low impedance speakers (which place far heavier demands on the power supply), you might run into some audible problems, however.

The whole debate about sharing a single transformer (like ATI does) or using a transformer for each channel (like Sherbourn does) is a tough one. The more transformers you have, the more likely you are to have some mechanical noise/hum. Also, one channel cannot borrow from the current reserves of another (beyond sharing the 120 volt primary winding of the transformer) so you typically have lower dynamic headroom.

The advantage to individual transformers, of course, is the amp behaves pretty much as a bunch of monoblocks. You get better channel separation and virtually identical power from each channel regardless of the load on the other channels.

So... hopefully that sheds some light on things? If someone wants to pop the top off their 755, it should be fairly easy to trace the wires out of the transformers and see how they split the channels up. You can then connect things up however you'd like.

As I know of, I think there is only one lucky bastard to have a 755 right now, and I think he's had the top off. So what say you, RAF?

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Matthew J. Hill
matt@idsi.net

Interesting question and one that I was able to talk to the designer himself about, thanks to an Outlaw connection. I'm using the 755 for the three front channels (L/C/R) and the two side surrounds (my Marantz MA700 monoblocks are powering the two rear surrounds). I was curious about which channels were connected to which toroid. My feeling was that if I used the bigger toroid (the configuration is 3/2 with one transformer having 3 secondary windings) for the front soundstage, then when I listened to 2 channel stereo I'd have a more robust transformer working. I was also concerned about any power steering (one of the reasons that some people like monoblocks because that doesn't happen with no shared components.)

The point of all this is that the engineer told me not to worry about which amps used which transformers since it would never show up in any audible way. The 755 and 770 have been designed so that they have so much reserve power or capacity (or whatever term is correct here) so that power steering never becomes a problem. And, since Audiophile Mike brought up an interesting point, I think I'll make sure that the two front channels (L/R) are using amps from separate toroids, just to be extra sure for 2 channel listening. This appears to make more sense than my original idea of choosing the three amp toroid for the front soundstage, although it's most likely a real world tossup. Besides, banana plugs make the switch a breeze!

Incidentally, the major reason for designing the 755/770 with two transformers instead of one was, as someone mentioned, to define the size of the case. Not only would the 755/770 case have been deeper but, more importantly, it would have gone over 8" tall and only fit in custom shelving. Even at ~7.5" + ~1/2" for the feet it's a tight fit in my Sanus stand. At least the feet keep the air moving near the bottom and there's a little air flow on top (remember, there are no fans in the 755). Also, power regulation is completely separate for each amp as are other components. Only the transformers are shared (according to the designer.)

I hope some of this information is helpful.

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RAF

My HT (latest update 02/14/02) Now includes Outlaw 950

[This message has been edited by Robert A Fowkes (edited March 15, 2002).]

Are we sure they don't both just charge the same cap bank? I mean, why would they even need to be split at all? My impression was it was often a question of PHYSICAL size and dimensions than electrical considerations.

Charlie

Thanks for the added info RAF...

As for why they don't "feed the same cap bank" I believe the Outlaw amps are designed with self contained amp modules. This means each one takes in AC from its own transformer winding, rectifies it to DC and uses it's own filter capacitors.

The 750 used a single transformer with 5 sets of outputs to feed each channel. There are some distinct advantages to this approach--the most significant being less cross talk and interaction between channels. Even better is using a dedicated transformer for each channel so they don't share a common primary winding.

You could wire the two transformers in the 755 in parallel and and let them feed a common DC supply shared by all the amp channels. The advantage here is that if only a few channels are working hard they have huge surplus current reserves to draw on. So the "one channel driven" power figures go up substantially as usually does the dynamic headroom. Carver and others have called this "power steering" but there's no active circuitry doing any steering in most such designs. The downside is you get more interaction between the channels and generally the amp won't sound as good.

In 98% of audio power amps (including the Outlaws), the main power supplies are NOT regulated. A 200 w/ch amp might have plus and minus 70 volt DC supply rails at idle. If you drive the amp to it's rated output, they will typically drop down below 60 volts. When playing loud music that means you can have 20 volts (peak) of "signal" riding on the DC power supply rails. Good amplifer designs tend to reject such power supply modulation, but only to a point.

So if you have multiple channels sharing a common supply, one channel will suffer from what the other is doing. This is another key reason why discrete power amps (which often have isolated power supply schemes) sound better than receivers (which usually don't).

This is what I like about the SAE, opps , I mean ATI 1506! Each transformer/ 3 in all, handle a stereo pair which makes bridging a snap. I have considered the 755, but like the idea of bridging pairs of channels to get you thank huge power I need to drive my Walsh 5's. My 380/channel Carver cant feed these speakers hunger for watts and, mostly, power supply. Has anyone tried bridging the 755??