Need help fast - Unbalanced Preamp to Balanced Amp

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

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.

Hi Mitch -- Assuming you are referring to the LAMM LL2 Deluxe, you've most probably seen JA's measurements, which indicate an output impedance rising to 3.3K at 20Hz. A phone call to Jensen sounds like it would be in order, to ask them what effects that would have.

Best regards,
-- Al