TRL Dude or Joule 150 MKII for Major Pre Upgrade

Tim (Mitch2), Ralph's comment that pin 3 should be grounded when adapting to an XLR input using a simple RCA-to-XLR adapter is of course correct. I would add that the 23.5K nominal input impedance of your Jensen PI-RX transformers is specified based on a 20K load being connected across its secondary. See the datasheet. With a 100K load I would expect its input impedance to be, as a rough approximation, more like 100K!

A separate question would be how well the transformer will perform when loaded with 100K and driven by the Dude's output impedance (I don't know what that is). I note that the datasheet indicates a maximum allowable load of infinity ohms, but a maximum allowable source impedance of 2K. A conversation with Jensen would seem to be in order on that question.

An ideal transformer (which of course is something that does not exist) having a turns ratio of one-to-one will present an input impedance identical to the load impedance that is connected across its secondary (output) winding.

Best regards,
-- Al

As noted I use the Dude with my balanced amps (100K input) without any issue too. The adapters are wired just that way.

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.

Tim, sounds like the single-ended preamp has a ground loop problem when working into the balanced amp via an adapter, the loop being broken, as might be expected, when the transformer is used.

An adapter-based approach that I suspect would provide better results, although probably not as good as the transformer-based approach, would be to connect RCA male to XLR male adapters at the preamp outputs, and run XLR cables from there to the amp. That would essentially be equivalent to the arrangement shown in Figure 2.1 of this Jensen paper on interfacing balanced and unbalanced equipment.

With respect to using the transformer, I wouldn't necessarily conclude that you have to stay below 2K output impedance for the preamp, given that your amp has an input impedance of 100K. It seems conceivable to me that the 2K figure in the datasheet is based on the assumption of a 20K load, just as the input impedance spec is based on that assumption.

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.

My guess is that the person you were speaking with was not Mr. Whitlock :-)

Maybe another phone call is in order.

Yes, that would seem clear.

On the other hand, you could buy one of Ralph's preamps, and the issues would all be moot :-)

(I have no affiliation).

Best regards,
-- Al