Need help fast - Unbalanced Preamp to Balanced Amp

Thanks guys. I didn't mean to highjack the discussion, as I'm not the originator of the thread, but I do have an interest in the topic.

For those who might be interested, here's my actual situation. . .

I'm waiting on delivery of a pair of Maggie 1.7's. I thought I would use my very good sounding Rowland Concerto Class "D" integrated with internal phono. However a friend let me try a DIY SS Class "A" 45w amp that is based on a Nelson Pass design. As good as the Rowland sounded, I fell in love with vocals thru the DIY amp. Hard to beat Class "A" for midrange purity.

With this in mind, I thought I might purchase either a Pass XA30.5 or XA 60.5 mono's to go with the Maggies. My room is small (12'x 14'x 10') so I shouldn't require a huge amount of power for them to play at satisfying levels for my usual jazz, blues, female vocal listening. A stereo XA30.5 might be all I need. The heat wouldn't be as big a factor vs the mono's either.

I'm currently using a custom Placette RVC (unbalanced) that has 2 inputs. My Squeezebox is connected to one input via a PS Audio PerfectWave dac, and I'd like to use the phono of the Rowland thru the 2nd input. Unfortunately the Rowland only has balanced connectors for it's output, thus the need for conversion.

Not knowing whether I'll keep the Rowland or not, I'd rather not spend a lot of $$ on cabling at this point. Almarg & Kal have provided several alternatives for me to explore. I thank them and everyone else who has provided suggestions. Hopefully I will discover something that works and is relatively inexpensive.

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.

If this is an entry level NAD preamp, and depending on the quality of the amp, it might be better to just sell the C162 and replace it with a balanced preamp.

KenL,

I'm not sure whether or not the Pass XA series amps would be good matches for a passive volume control, because of the XA's relatively low input impedance (20K unbalanced, 30K balanced). Although I note that Stereophile's measurements indicate that those impedances are "constant across the audio band," which will help matters. You should probably raise that question in a separate thread if you decide to pursue that possibility.

As you may be aware, the XA30.5 is considerably more powerful than its rating suggests, and JA's measurement in the Stereophile review was 195 watts into 4 ohms (although the amp leaves Class A operation at considerably less than that).

The Maggie 1.7's sensitivity of 86db/1m/2.83V corresponds to 83db at 1 meter for 1 watt input into its 4 ohm nominal impedance. 195W will result in about 106db at 1 meter. Given that for planar speakers volume levels fall off relatively slowly with distance, that strikes me as reasonable in relation to your small room dimensions and music preferences, assuming that the spec is accurate.

As far as interfacing the balanced output of the Rowland to the unbalanced input of the RVC is concerned, my suggestion would be to have a custom cable made up which connects xlr pin 2 to the rca center pin (assuming the Rowland conforms to the USA convention of pin 2 non-inverted and pin 3 inverted); xlr pin 1 to the rca ground sleeve; and leaves xlr pin 3 open. If you were to use a standard adapter, that would most likely ground pin 3, which would mean that the Rowland's output signal on that pin would be shorted to ground. Most equipment can tolerate that, but some cannot. Check with Rowland on that point if you would prefer to use an adapter.

If you want to consider a transformer instead of an adapter cable, my instinct with gear of that caliber would be to go with a Jensen. Although for the price at which B&H is currently offering the ART DTI, there is essentially no downside to experimenting with it.

Regards,
-- Al

Ralph - Thanks so much for your detailed response. I have a good friend who can make a cable as you describe. He tried once before, but apparently had it grounded differently, thus the hum.

Almarg - The Placette is a temporary thing. I will eventually go with an active preamp. Thanks again for your advice.