Concurrent use of balanced and unbalanced outputs: Lamm L1

Is there a foreseeable issue with using a balanced and unbalanced output for a biamping? My specific situation is using a Lamm L1 preamp for "main" speakers along with two powered subwoofers which have line-level (both balanced and unbalanced) inputs only. My current hope is to use an RCA-to-RCA cable and an XLR-to-RCA cabling (or possibly a Purist Audio Design XLR/RCA adapter) from preamp to amps to avoid a gain mismatch. I have emailed Lamm early this week without response yet and will try again soon with another address. Thank you.   

I would definitely wait for a response, but my gut feeling is don't try it.
Maybe Ralph of Atma-sphere or Almarg will chime in...
Thanks, Bob. Ohlala, the answers to your questions depend on the specific designs that are involved.

First of all, though, I would keep in mind that most XLR-to-RCA adapters short the signal on XLR pin 3 to ground (XLR pin 1). That would be appropriate when adapting to an XLR input, and may be appropriate when adapting an XLR output that is transformer coupled. But in the case of **some** designs that have actively driven XLR outputs it may cause problems or conceivably even damage, eventually if not sooner. See this thread for example. I believe that Cardas can supply adapters on special order which leave pin 3 unconnected. I have no knowledge concerning the Purist adapters.

But in any event I would think it likely that you could accomplish proper gain matching just by using the gain control on the subs, without an adapter or adapter cable.

The other possible issues relate to sonics. I couldn’t find relevant information on the L1, as it is an older model, but to be able to be confident of good sonic results if RCA cables are connected to one destination (amp or sub) and XLR cables to the other the following needs to be determined:

1)Are the L1’s RCA and XLR outputs for each channel driven by separate output stages (i.e., are they independently "buffered"), or are they driven by the same output stage, as is the case in many designs which provide both kinds of outputs. In other words, is the center pin of the RCA connector for each channel simply connected directly to one of the two signal pins on the XLR connector for that channel?

2)What are the output impedances of the L1’s RCA and XLR outputs?

3)What are the input impedances of the subs and the power amps?

4)Approximately how long are the cables that connect the preamp to the power amps, and that connect the preamp to the subs?

If the answer to the first question is that the RCA and XLR outputs are driven by the same output stage, and the signal provided to the RCA connector is therefore the same signal that is provided to one of the two signal pins on the XLR connector, the answers to the other questions become particularly important. In that situation the loads presented to the preamp by the sub’s input impedance and also by the capacitance of the cables to the subs may have adverse sonic effects on the signals received by the power amps, especially if the output impedance of the preamp is high.

Aside from the issue of adapters (and perhaps some adapter cables) shorting pin 3 to ground, though, there’s no harm in trying any of this. And if your power amps can accept both XLR and RCA inputs, I would first determine which of those connections sounds best on the main speakers, before deciding which connection to use for the subs.

-- Al

Thank you both for your helpful responses. I really appreciate your time and effort. Al, your post allowed to me find answers without chasing my tail so much, and it may prevent me from damaging my preamp.

This topic is of little to no interest on audiogon, but I'll report what I found anyways. Yes, biamping using those outputs is done. They are not independently buffered. Thankfully the L2, which is very close to the same as L1, has been measured by Stereophile.  The unbalanced output impedance ranges from 124ohms to 3.35kOhms (at 20Hz); balanced 248 to 7.7k. Lamm does state that its "high current drive capability" with drive any "real world" load. The downside is that the preamp's phase switch only affects the unbalanced output. The other thing I thought interesting is that one can biamp Lamm ML2.# amps by using the one of balanced/unbalanced inputs as an output to another amp. 

Thank you, again.
You’re welcome, Ohlala.

However output impedance rises to 3.35K and 7.7K at 20 Hz are fairly extreme, and are definitely not consistent with "high current drive capability" that can handle any "real world load." Also, the line level input impedances of many and probably most powered subs are rather low, often between 5K and 20K. The lack of independent buffering for the two preamp outputs will put the input impedance of the power amp in parallel with that impedance, for the signal that is common between the RCA and XLR connectors, resulting in an even lower overall impedance as seen by the preamp.

The ML2.x amplifiers that you mentioned in connection with biamping have a considerably higher input impedance than most subs have, at 41K. Driving two of them with the same signal would result in a load impedance of 20.5K, which is marginally acceptable in relation to an output impedance of 3.35K. (Note John Atkinson’s comment in the measurement section of Stereophile’s review that "the Lamm L2 should not be used with power amplifiers having an input impedance of less than around 22k ohms if the low frequencies are not to sound lightweight"). Although the **difference** in the load impedances presented to the two signals on the XLR connector might in itself adversely affect sonics, since it would unbalance the balanced signal pair to some degree.

As I said earlier there’s no harm in trying such an arrangement. But unless the sub you choose and your power amps both have relatively high input impedances, such as 47K or more, the impedance issue could very well result in adverse effects on the sonics of both the sub and the main speakers. If so, you may want to consider putting some sort of buffer stage between the preamp and the subs, that would present a high input impedance to the preamp while driving the subs with a low output impedance.

Another member here faced a similar situation a few years ago with his ARC Ref electronics and a sub. A gentleman named Tom Tutay, of Transition Audio Design in Florida, custom made a suitable buffer stage for him at modest cost (hundreds of dollars, not thousands), which worked out very well. I don’t think Tom has a website, but his contact info can be found via a Google search.

Good luck. Regards,
-- Al

Again, thank you very much, Al.  The YG powered sub has a 10kohm input impedance, so going to avoid it.
If using a Lamm preamp, there appears to be no advantage to using the XLR outputs as the preamp is only single-ended.

If you want to use balanced connections from the preamp I would seek a different preamp.