TRL Dude or Joule 150 MKII for Major Pre Upgrade

Mitch2, as much as I think you should be buying my preamp :) instead of the Dude, I am sure the Dude can work with your amplifier. The problem is that the adapters you were using were not wired right.

The problem is that pin 3 of the XLR was left floating, allowing the amp to buzz of hum. Cardas adapters are often wired this way. Its easy to fix- you just connect pin 3 to pin 1 and you're all set.

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.