TRL Dude or Joule 150 MKII for Major Pre Upgrade

Success, at long last. Outstanding! Although I guess we'll never really be able to be sure of exactly what was causing the problem, given the bizarre results of some of the experiments.

The key thing with the XLR cable is that its capacitance has to be kept very low. I suspect that you are using the PI-2RX transformer, for which the datasheet indicates a maximum acceptable load capacitance of 100 pf. That would be for the sum of the cable capacitance and the input capacitance of the amp (which is unspecified). I suspect that the main consequence of driving the transformer from an output impedance that is significantly higher than the 2K recommendation will be that the load capacitance becomes even more critical, excessive capacitance potentially affecting the highs particularly.

For most purposes the capacitance of Mogami's Gold Studio or other Mogami 2534-based cable (if that is what you are using for the time being) is reasonably low, but it may not be low enough in these circumstances. I don't know what kind of cables you are awaiting, but if they do not have very low capacitance you might want to consider say a 1 foot length of inexpensive Blue Jeans/Belden 1800F, which would be only 13 pf plus the capacitance of the connectors.

Best regards,
-- Al

Thanks Al. I will check into the cable capacitance to be sure I am doing all I can for best sound.

Hi Al, in practice 100 ohms can be quite effective. We use that value a lot. It does not seem like much in the face of the higher impedances that tubes employ, but it can make the difference between RF oscillation (resulting in hum) and absolute stability.

So right now I am thinking that grid stops are not the issue.

Its interesting that using the Jensens sorted it out. This would be a way of reducing the output impedance of the source. If the amplifier is having troubles with AC line noise at its inputs, this would be the sort of thing to shut it up. I suggested that this might be one of the explanations back on the 13th; that theory has now gained some juice.

Ralph, thanks for the info about 100 ohms. I would not have suspected that such a low value would generally be effective.

Regarding the effects of the transformer on the source impedance seen by the amplifier, though, I don't think that is the explanation. For two reasons:

1)It was reported that the problem was still present when the amp was driven by several different low impedance sources.

2)The Jensen Isomax "input transformers," such as the one whose data sheet I linked to in my previous post, do not provide a low output impedance. That presumably being a major reason that they are only recommended for use with short low capacitance cables on their output side. As you can see in the data sheet I had linked to, that particular model has an output impedance spec of 4.65K (typical) when it is driven from a balanced impedance of 300 ohms per leg. Also, the DC resistance of its secondary winding is spec'd at 1.9K (typical). When driven by a source having high output impedance, such as the Dude, the 4.65K figure would presumably be even higher.

So as I see it the mystery remains unsolved.

Best regards,
-- Al

Al, a couple of things I pointed out to Grannyring were that I did in fact speak with Bill Whitlock back in Aug, 2010, and posted about my conversation in another thread that you too were involved in. I hadn't remembered that thread, but I found it in a search. I was using a Lamm LL2 Dlx at the time, which has a rising output impedance at low frequencies and my concern was bass roll-off. My post is at the end of the thread here;
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1280795210&read&keyw&zzbill=whitlock
Bill basically told me the Jensen posted specs apply to the "worst case" scenario where the load is 10K ohms. Good news for Grannyring is that the input impedance of his Atlas amp is enormous at 940K ohms! Here is what JA at S'phile had to say;

The Atlas's voltage gain into 8 ohms was the same for balanced and unbalanced drive, at 25dB, which is lower than normal. Both inputs preserved absolute polarity (ie, were non-inverting), the XLR jacks being wired with pin 2 hot. Specified at a very high 470k ohms for the unbalanced input and twice that for the balanced, the amplifier's input impedances were at least those values at low and midrange frequencies, dropping slightly but inconsequentially at 20kHz. (With such high impedances, there is inevitably a large error in measuring the exact figure.) The Crossover inputs behaved identically to the main inputs in these respects.

I suspect with such a large input impedance, Grannyring is good to go with the transformers.