Need help fast - Unbalanced Preamp to Balanced Amp

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.

Most line transformers, including the ISOMAX are designed to load at 600 ohms or thereabouts.

If you don't load the transformer it can ring. The best place to load it is at the end of the interconnect that it is driving, if that is an XLR you can run that cable 50 feet if you want to with no degradation. I've never in 40 years heard of a requirement to keep XLR interconnects short unless its for cosmetic purposes.

Hi Ralph,

See the following datasheet, and the note near the bottom of page 1 of the user manual for the PI2XX (the unit shown has balanced inputs and balanced outputs, but a similar model can be ordered with unbalanced inputs):

Datasheet: http://www.jensen-transformers.com/datashts/pi2xx.pdf

User Manual: http://www.jensen-transformers.com/datashts/pi2xx_usr_man.pdf

Note that the recommended load impedance range is 10K to infinity. And that the output impedance of the transformer at 1kHz, with a 600 ohm source impedance connected, is very high at 4.65K, of which 1.9K is dc resistance.

Given the very high output impedance, it would seem to make sense that cable capacitance should be kept very low, to avoid high frequency rolloff. That would apply whether the output is balanced or unbalanced.

Your comments appear to be applicable to output transformers, such as this one, which as explained in this application note will not perform as well in terms of cmrr as will an input transformer.

Best regards,
-- Al

Hi Al, I would not use that transformer for this application, because it does appear to leave you susceptible to the cable due to the high impedances involved.

Instead if you are running a single-ended preamp to a power amp, I would use something that steps down to 600 ohms, and then see if I had enough output at the amp end- if not I would step it back up again.

Of course, we could be talking at cross purposes, the main reason I use balanced lines is to get rid of the need for expensive and often short cables!

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.