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Re: Brady, Toccata & queries

Posted by gdahl (A) on November 19, 2003 at 23:05:34

In Reply to: Re: Brady, Toccata & queries posted by Dave Davenport on November 14, 2003 at 21:23:16:

I wonder if the results of your comparison of attenuator configurations might have been compromised by the film pot you used to test the traditional balanced configuration. There is a good chance that the film pot (Alps?) had some degree of tracking error between phases. Even a few percent of difference would add significant distortion, but this effect wouldn't occur with the bridging approach. As you said, AC balance is important.

As far as chasing DC balance, I suppose it only matters to the extent that the transformer is affected by it. These little amorphous beauties are devastatingly sensitive to *any* offset--Gary Pimm ran some distortion tests recently, and found that even a few microvolts messed them up. What we get in return for this sensitivity is their wonderful liveliness and low-level dynamics. But if we use them as line output transformers, we need to either accept capacitor colorations (parallel feed) or have a *really* good servo. On the other hand, iron-core output transformers such as the LL1620 aren't nearly so touchy about offset. Allen Wright's amps had those wonderful meters and balancing knobs, but I didn't notice sonic degradation when minor DC imbalances appeared on the meters.

Lynn had always wanted to avoid parallel feed in the Raven, but was unable to find an output transformer that would allow this. I think I have now found the transformer we were looking for, the LL1680/5ma. This unit can deal with residual offset, so we will be able to run it the same way we run the Amity's driver stage. The LL1680's voltage headroom will also be a plus. But of course, I'll have to build it before we can find out if the sonics come out as we hope!

I would be very curious to compare the sound of a parallel-feed line stage (such as yours) with the revised Raven. Perhaps the differences would turn out to be just a matter of small flavor variations. But we sure did like the changes in the Aurora's sound that resulted from replacing its parallel-feed driver stage with the current version.

As for the attenuator, my real hope is for a TVC that can properly split phase. John Chapman tells me that the nice people at S&B are presently working on such a project.

Gary Dahl
"So many tubes, so little time..."

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