Need help fast - Unbalanced Preamp to Balanced Amp

You guys are great. Really appreciate learning a thing or two every time I have a question. Thanks guys!

Based on my use of transformers with my current gear, I will respond to a couple of Ralph's transformer comments,

If you go the transformer route as mentioned above, place the transformer as close as you can to the preamp can keep the cable to it short.

This applies if you are using output transformers. However, I had much better luck with input side transformers. Since my preamp to amp run is less than 3M, I use 2.5 to 3M single-ended IC's to two separate single channel Jensen input transformers then very short balanced IC's (less than 1M) from the transformers to my balanced Clayton M300 monoblock amps.

The transformer will have some artifact; although the Jensens are very good I find they reduce bass impact and liveliness,

The first part of this statement is true, but with the Jensens it is my experience this is inaudible, or at least unnoticable. So far, I prefer using my favorite single-ended preamp with the transformers to using the balanced preamp I have tried without transformers. In other words, I have found the sonic differences in preamps to be much greater than the sonic penalty resulting from losses that may occur when using the transformers. Some highly regarded manufacturers use transformers from companies like Jensen and Lundahl on the output side of preamps and and/or the input side of power amps. The literature provided by Jensen shows pretty much flat response throughout the audible frequency band. Source impedance (the output impedance of your preamp) must be below 2K ohms and the load impedance (input impedance of your power amp) must be above 10K ohms according to the literature, and they recommend keeping the transformer to amp cables as short as possible. I have found this to be an excellent solution to the problem of driving balanced amps using a single-ended preamp. I have not tried the transformers to drive balanced preamps from single-ended sources, although it seems that should also work if the output and input impedances are within the limits provided. One last thing, the Jensen transformers are relatively inexpensive, being just over $200 for the stereo model and around $125 for the single channel model (which comes in a slightly more robust case for pro applications). You have to ask them for the single-ended model, but it is perfect when using monoblocks that are more than a couple of feet away from each other. Check out the link to Jensen posted above by Al.

Thank you Mitch.

What a useful information! I also have single ended preamp (Joule-Electra LA-300ME) and Spectron balanced monoblocks.

I did not understand your comment exactly:

"single-ended IC's to two separate single channel Jensen input transformers then very short balanced IC's (less than 1M) from the transformers to my balance "

Left and Right preamp RCA outputs can have only one IC each. If you need for each one of them two separate single channel input transformer then, as I understand it, you need to use "Y" connector at the end of each of your single ended SE interconnect to connect to two separete single channel transformer inputs

is this what you have in mind of each of your single channel transformer has one input for RCA and one output for XLR?

Thank you

is this what you have in mind of each of your single channel transformer has one input for RCA and one output for XLR?

Dob, yes there would only be one rca cable for each channel, connected between the preamp output and the input side of the transformer. And of course one xlr cable for each channel, connected between the output side of the transformer and the power amp input.

As Mitch indicated, for an "input transformer" the xlr cable should be very short, less than 2 feet iirc.

Jensen's site indicates that before ordering it is usually best to speak with them directly, for help with model selection and other guidance.

Best regards,
-- Al

Dob, I could have been clearer. Al nailed it above.

Most of the pictures on the Jensen site show their ISOMAX transformers in a stereo box (both channels into and out of one box). After asking them, they set me up with mono transformers (only one channel input and output per box). Jensen most commonly sells these single channel transformers to their pro clients and the boxes are more heavy duty, but the transformers are identical to the stereo model. The single channel boxes were better for my monoblocks since the amps sit about 7 feet away from each other. If I had used the stereo box, the XLR cables from the box to the amps would have been longer than I wanted them to be. Al's comment on length is perfect, mine are about 2 feet long, and Jensen recommends as short as possible and no longer than about 1M.