TRL Dude or Joule 150 MKII for Major Pre Upgrade

Update. The use of a Jensen input transformer under the brand Iso Max completely removed the buzz to a dead silence. Simply place the input transformer between the amp and preamp. Single ended preamp into the Jensen Iso Max using my RCA ic's and out from the Jensen Iso Max into my amp using xlr connectors. So the Jensen receives RCA in and xlr out. I am now able to use my amp in it's fully balanced mode.

The xlr ic's going from the Jensen to the amp must be 2 feet or under to avoid any signal deterioration.

The down side is the cost of the unit plus the cost of an extra set of ic's. Not sure as yet if then Jensen changes the sound as I am using some pro sound Mogami xlr's for now. The sound does seem a little more polite or less alive, but this my be the new set of IC's?

I will soon receive a set of xlr ic's of the same brand that I use throughout my system. I will update all as to wether the Jensen input transformer changes the sound.

The sound is very good with the inexpensive Mogami cable however. A little less top end sparkle is what I notice thus far.

I also left Jensen a message to clarify the compatability of the unit with a higher output impedance preamp. The site suggests the preamp should be no higher then 2k ohms , but other threads I have read seem to suggest this may not be the whole story.

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