TRL Dude or Joule 150 MKII for Major Pre Upgrade

I sadly passed on a "like new" Dude this week because I couldn't make the high output impedance work with my amps. Although the amps have 100K ohm input impedance, they have balanced inputs only. I have to use transformers to convert rca to XLR since using a basic rca/XLR adaptor causes hum. The transformers have an input impedance of only 23K ohms, which would not work with the Dude. I will need a balanced version if I want to try one, or own one. I would like the opportunity to see and hear one sometime before taking the plunge to purchase one.

Almarg and Atmasphere, thank you for the comments. I did indeed try Cardas adaptors this week when I heard the buzz. I also own another generic set of adaptors I can try. If I can take a set of these adaptors apart (they seem glued together) I could simply run a jumper between pins 1 and 3, and then try it again with my current single-ended preamp. If I cannot get the adaptors apart, I have the guts of some XLR connectors around here and it would be easy enough to wire up a set and try them.

With a 100K load I would expect its input impedance to be, as a rough approximation, more like 100K!

Almarg, funny you should point out, I previously discussed this very issue with someone at Jensen and I remember being told that very same thing. However, when deciding on whether to purchase this single-ended Dude, I called them again this past week and was told the input impedance would be a maximum of 25K ohms, as shown on the data sheet. Maybe another phone call is in order.

The preamp I am using has an output impedance of only 12 ohms, so it works just fine with the Jensens. I just like to try new stuff sometimes and the balanced only thing limits me somewhat, at least in this case where the Dude has a higher than typical output impedance. If I can get the adaptors to work without noise, that might be the ticket.

Ok, I checked out my generic rca/xlr adaptors both using a multi-meter and also by taking one apart. These have a jumper from pin 1 to pin 3. Hooked up my single-ended preamp using these adaptors and have the same moderately loud buzz. Run through the Jensen transformers the same preamp/amp combo is dead quiet. Also using my MUSE balanced preamp, or the balanced active outputs from my MUSE Erato II player, the amp is dead quiet without the transformers. Therefore, choices are to keep using the transformers and preamps that work well with those (e.g., preamps having output impedance preferrably below about 1K ohms, and no greater than 2K ohms); find a balanced preamp I like better than my preamp/transformer combo; or buy new amps with single-ended inputs.

Grannyring, I have an extra transformer since I went from a two-channel transformer box (both L and R channels in one box) to mono transformers (two boxes) when I moved my monoblocks further apart (although they are now closer together again). The balanced cables out of the transformers should be no more than 1M long. The ones I use are about 2 feet long. To use the one-box version, your monoblocs must be reasonably close together.

I had offered to let another member try the transformers. Since he is not having the buzz issue and I do not think he needs/wants to try the transformers right now, maybe you would like to try them and see if they help. This would be interesting information for me too, because then I would learn whether a single-ended Dude might work for me.

My plan earlier this week was to try the single-ended (SE) Dude that was up for sale on this site and if I liked it to go ahead and order a balanced Dude while continuing to use the SE Dude. I planned to use the SE version for the 3 months it takes TRL to make these and then sell it after I received my new balanced Dude. I declined to buy the SE Dude after my research indicated the 5K ohms output impedance may not work with the transformers. However, Almarg is correct, I do not believe I spoke with Bill Whitlock when I called Jensen last week.

Almarg, I know Ralph's preamps are fully balanced and would work properly with my set-up. While I respect Ralph and his company immensely, I just didn't care for the sound of the Atma-Sphere preamp that I owned some years ago. It was a tricked out MP-3 with teflon, regulated power, and Caddocks. I am aware of an Atma-Sphere preamp for sale now that is called an MLS-1 Linestage, which is apparently a one-box precurser to the MP-1. I know it is old because it has the yellow board they used early on. This pre was apparently at Atma-Sphere recently where it received; new filter caps, HV regulated power supplies, fast rectifiers, the latest 3.1 version volume attenuator, main power transformer, and state of the art copper CuTF V-Cap coupling caps, as well as a full cleaning of all controls and input/output connections. The price is reasonable but I have no idea where something like that would fall between the MP-3 and MP-1. It would have to offer something above the MP-3 I owned for me to consider trying it.

Back to the OP, the Joule will likely be easier to find here used (if that is your intent) than the Dude, simply because more have been made over a longer period of time. There is an LA 150 MkII for sale here now. I owned that preamp and it would be a huge upgrade over the Rotel. You would also have the option to upgrade it to the Signature Edition [LA-150 MkII SE] version in the future for $1,750. In addition to a very nice appearance, the Joule offers a couple of neat features that can have practical applications such as selectable output impedance of 400 and 1200 ohms, and selectable gain from 0 to 11 dB. The 6350 tubes used are no longer made but Joule has thousands for when you need more, and they are also available in other places...here is something I found about those tubes:

6350: This tube was mainly manufactured for IBM and other computers in the late 1950s to mid 1960s. Sylvania and GE blackplates are considered the best of all with this tube. It has a mu or gain factor of 18 so is very close to the 12AU7. These were made to tight specs and carefully quality checked, since replacement in a computer of the day was difficult and costly, and the tubes were expected to operate 24-7. These make impressive upgrades to the standard 12AU7!

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.