Quite often I get questions about running balanced connections and mixing RCA and XLR connections. Since I had finally started an FAQ over at the new website I though I would add a few notes over there. I am pasting them into this thread so any questions can be posted here.
From the FAQ:
Can the TAP System Run Fully Balanced?
I use a fully balanced Slagleformer based pre-amp here myself and it works wonderfully. The modular nature of the TAP system makes it easy to setup a fully balanced system. Simply include 2 additional Slagleformer (or Hybrid Resistor) modules and you now have a fully balanced attenuator. This is much like going from 2 decks to 4 decks on a stepped attenuator. Also the input selector pcb's are easy to setup for balanced operation. If you are building an AVC-1 Slagleformer based pre-amp I do have some back plates here that can include 2 balanced inputs and 2 balanced outputs - please send an e-mail or call to discuss options that fit your exact requirements.
Can The TAP System Mix RCA and XLR Connections?
Yes - but I like to be sure that the system is optimized and that you are not wasting $ on the extra parts needed if the system does not really benefit from it. There are several ways that OEM and DIY pre-amp builders include balanced connections in their products. All these options are ok as long as the end user understands just what is implemented. So much of the time I fear that when this is not well understood the result could be wasteful - spending $$$ to make use of balanced connections when the system as a whole does not benefit. Note that I have nothing against balanced connections - as mentioned above I run my system that way here myself.
Here are two of the more common ways to MIX RCA and XLR connections along with a few notes on each:
1- Install XLR and RCA connectors but run only single ended (RCA) circuits inside
If the internal circuits of the pre-amp are by design single ended but users want to connect up gear that might only have XLR connections this is a handy way to do it - BUT it does not make any use at all of the XLR source(s) or amp. In a pre-amp wired like this just the PIN2 (hot) and PIN1 (gnd) side of the XLR connection is used and the PIN3 side is not used (no pin3 connection at the XLR input / grounding pin3 at the XLR output). The plus to this is that it is really cheap to implement. Also it does save the user from inserting RCA/XLR converters into the signal path. The troubling downside with this is that if the user does not understand what is actually being connected internally they might spend extra $$$ specifically for an XLR source, XLR pre-amp, or XLR amp not knowing that one component in the chain is simply throwing away 1/2 of the balanced signal. It likely will sound great but the extra $ spent to go balanced connections when the source OR pre-amp OR amp run this way are completely wasted..... All 3 need to be implemented internally with fully balanced circuits to get the benefits.
2- Install both XLR and RCA connectors and use fully balanced circuits inside
This works well and is what I do here in my listening room. When using 4 Slagleformer modules the XLR inputs and XLR output signal path is fully balanced in every way. An example of this kind of build can be found by clicking here:http://www.audiocircle.com/index.php?topic=118082.0
When an RCA input is selected the input switching routes the hot(+) from the RCA jack to the pin2 side of the internal circuits and then usually grounds the pin3 side of the internal circuits. This is not specific to the TAP system - many balanced pre-amps run just this way. This setup is perfect when your amp likes to run balanced AND you have a source or two that are best run balanced as well but also need to mix in some RCA sources. Note that if you use the RCA output in this setup you make no use at all of the extra modules. Note also that not all amps and sources with XLR jacks installed are best used in that mode. This is especially true when it comes to amps. Some amps might include a balanced connector but really only use the pin2 side of that connection (the internal circuits are single ended) thus making no use of the balanced circuits in the pre-amp or the source(s). Worse yet (because you actually loose out sonically) some amps not using balanced circuits throughout include extra 'stuff' in the signal path to make the conversion from balanced to single ended - meaning a direct connection to the RCA jack would almost certainly be a better way to go. Here are some guidelines to use when checking if this configuration fits for your system:
- If BOTH your amp AND your sources are best used balanced but you also need a few rca inputs then this is a good way to go
- If all of your better sources are RCA output then do not use this construction - $ are wasted since you make no use of the extra modules
- If you intend on using the RCA output then do not use this construction - $ wasted and you are not making use of the sources XLR output
- If there is a good chance you will want to run balanced later then consider making provisions for it in the enclosure and then adding the
modules later on.