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.

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.