Need help fast - Unbalanced Preamp to Balanced Amp

If you use an adapter, sometimes you can wind up with a hum because the adapter is not grounding the unused input.

With a balanced amp you have a non-inverted input and an inverted input that is part of the XLR connection. If the RCA to XLR adapter is only using the non-inverted input, often there will be no connection on the inverted input. This can leave you with some hum, and often less gain.

To correct this, open up the XLR side of the adapter and have someone (if you can't solder) connect pin 1 of the XLR (ground) to the unused connection (often pin 3, which is the inverted input).

This will get rid of the hum and the amp will have normal gain.

It would be better to have a cable that does the conversion rather than an adapter, as there are less connections.

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. Then run the balanced cable from the transformer to the amplifier- this cable can be a lot longer than you are used to with single-ended, and you will not have to sink as much cash into it for it to sound right.

The transformer will have some artifact; although the Jensens are very good I find they reduce bass impact and liveliness, so if your cable can be short from the preamp to amp I would go with the adapter rather than the transformer.

With regard to Al's comment immediately above, you could set up the cable so that only pin 2 of the XLR at the preamp end is used. That way the inverting out would not be grounded and possibly damaged.

FWIW in the case of our preamps they are completely tolerant of pin 3 being grounded to pin 1.

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 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!

Mitch2, my advice if you look for a preamp is look for one that is fully differential and balanced internally, not just at the inputs or outputs. If it is built right, the length or quality of the interconnect will be of no consequence.

If your amplifier does not accept balanced inputs, it won't help to run a balanced cable that then uses an adapter to convert to single ended.

FWIW though your amplifier can easily be modified without changing any of the circuit design to accept a balanced input. We have been doing this mod for a while- its very simple and there are no transformers involved.