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Re: Brady, Toccata & queries

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Francois G

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In Reply to: Re: Brady, Toccata & queries posted by gdahl on November 13, 2003 at 12:54:06:

Gary,
Regarding volume control -- have you tried "bridging" the primaries of the input transformer with a single potentiometer instead of using the dual control in each side of the primary? If I understood correctly Dave and Kevin listened to various options in the K&K Raleigh line amp design and preferred the bridging method, in spite of the fact that it changes the loading on the input transformer at changed volume settings.
I shall appreciate your comments, and Dave's too.
Francois

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Re: Brady, Toccata & queries - Dave Davenport 21:23:16 11/14/03 (4)
Hi Francois,
Gary is correct in what he says about the balance and constant reflected input impedance of the configuration he is using. I know that Lynn has used this configuration for a while but I had not asked them about it before. That is why I was interested. Kevin and I have compared three different differential attenuators. The first is that which Gary is using, the second is the simple bridge and the third is a hybrid, which is a bridge with a center tap ground. Like Gary, I like the technical characteristics of the one that he uses; however I prefer the sonic characteristics of the simple bridge. I must say that it is only a slight difference, but nonetheless a difference. To me, I will almost always go with what sounds best even if I think another approach is technically superior. I used film pots for my comparison, but I think it is an apples-to-apples comparison. To digress for a bit, sometimes something will sound better at one stage of incrementally improving a design (a puffy way of saying "tweaking") and later after several changes, that way that used to sound better no longer does. But getting back to the attenuator, I use the bridge because to me, it sounds better.
Now, I am going to open a can of worms with some speculation. I'm not sure how to say this... but I think chasing balance is the wrong approach. Balance is important only because you have a balanced transformer. For example, in the output stage, I agree that the balanced push-pull parafeed is not the best sounding choice between several alternatives. If you look at the output circuit on the schematic for my line stage on the K&K Audio site you will see that it is a differential circuit but the transformer primary (although there are two sections in series) is really a single, non-balanced circuit. The two halves of the primary are not pushing and pulling, but rather acting as one like in a single-ended circuit. Now, the important thing with this is that there is a single signal loop rather than two signal loops like in push-pull. So with a single signal loop, there is nothing to balance. If one side were a little weaker than the other, it would still work okay. Now taking this same thought to the input, I think the single unbalanced loop has the same advantage. If this is true, and I am speculating here, than perhaps this could be why the bridge sounds better.
Dave
- Re: Brady, Toccata & queries - gdahl 23:05:34 11/19/03 (3)
  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..."
  - Re: Brady, Toccata & queries - triodenuts 02:10:33 01/01/04 (0)
    Hi Gary Dahl!
    Do you know how can I get in touch with John Chapman from Bent audio? Their website is not exist anymore. How can I get in touch with him?
    Thank you!
  - Re: Brady, Toccata & queries - Dave Davenport 20:08:23 11/21/03 (0)
    It could very well be the case that my comparison of attenuators was compromised by slight imbalances between the potentiometer sections.
    Thanks for the insight and I'll have to think about this some more.
    Dave
  - Re: Brady, Toccata & queries - Dave Davenport 20:07:28 11/21/03 (0)
    It could very well be the case that my comparison of attenuators was compromised by slight imballances between the potentiometer sections.
    Thanks for the insight and I'll have to think about this some more.
    Dave
Re: Brady, Toccata & queries - gdahl 15:47:05 11/14/03 (0)
Hi Francois,
I haven't tried the "bridging" method. As for Lynn's method, it works especially well with the DACT CT2 because of its tight resistance tolerance. The resistance sections of typical film pots don't track accurately enough to ensure equal attenuation of both phases, but the DACTs do.
With the bridging method, if I understand it correctly, the input impedance decreases as the volume is turned down. The result is a lower reflected seen by the source component through the input transformer. Using the DACT makes it possible to present a consistent load to the source component at varying volume settings.
I wouldn't be surprised if Kevin and Dave preferred the sonic results of the bridging method over the results obtained using two decks of film pot per channel, but I don't know if they compared it with a DACT in true balanced configuration. If they did, it might just be a question of a choice of sonic flavor. My preference is based on my feelings about the technical aspect of the issue; as I said, I didn't try it both ways.
Kevin and Dave obviously have good taste and technical expertise, so I don't doubt that the bridging method works well too.
Gary Dahl
"So many tubes, so little time..."

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