Need help fast - Unbalanced Preamp to Balanced Amp

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.

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.

Thanks Al, I had not noticed the impedance information in the literature. I would be interested in a more thorough reporting of the output impedance of the transformers at different frequencies resulting from a range of input impedances. The output impedance of my preamp at 1k ohms is low, at about 250 ohms, but rises at lower frequencies due to limited coupling cap size. Although my amps fortunately have a relatively high input impedance of 100k ohms, at lower frequencies approaching 20 hz, the output impedance of my preamp may exceed the 2k ohm "maximum" posted on Jensen's website. I suspect I may be getting some LF roll-off resulting from this. This condition could explain why my balanced preamp performs better in the bass than my single-ended preamp running through the transformers - although there are also other possible reasons for this such as the balanced preamp having higher gain and being SS, while the single-ended preamp is tubed (although it is noted for it's robust bass). I need to find a fully balanced (not pseudo-balanced) tubed preamp that I really like, but have not yet done so.

Al, your suggestion to call Jensen resulted in some good information. I spoke to their designer, Bill Whitlock, who informed me the specifications shown on the website are referenced to a “typically worst-case scenario” where they considered a load (amp impedance) of only 10K ohms. The good news for me is that when the input impedance of the amp is higher, the load seen at the input of the transformer is also higher. Therefore, when using the transformers with amps having an input impedance higher than 10K ohms, the recommended 2K ohm preamp output impedance becomes less of an issue. Bill said, since the input impedance of my amps is 100K ohms, there would be no sonic penalty with regards to bass response resulting from my single-ended tubed preamp's higher output impedance into lower frequencies (3.3K ohms at 20Hz).

Ralph, I agree entirely that a fully differential and balanced preamp would be the best solution for my amps. However, by only looking for preamps that are fully differential and balanced, the field of available contenders has been significantly reduced. I do currently have a pretty good sounding differential balanced solid state preamp I am using, but the field for balanced tubed preamps is much smaller. Therefore, since I like the sound of my single-ended tubed preamp, and can apparently use it with the Jensen transformers without a sonic penalty, I believe (at least for now) that using the transformers is a good solution for me, and possibly also for the OP (depending on his preamp/amp impedances).