Why so few balanced phono inputs?

thank-you all for the great response.
Sean

Hi Ralph, I always have this question in mind and you are probably one of the best persons to answer.

I noticed that some supposedly full balanced phonos, even when offering both XLR and RCA input jacks, actually have single-ended circuit for the first input stage (the XLR jacks are really single-ended connection), and then convert the signal to balanced mode.

I always wonder why the designers are doing this when they have full balanced circuit and have XLR input jacks already. Is the main reason, as you have mentioned, just dealers don't like to change the cable when they are doing auditions, or there are some intrinsic difficulties in making a balanced phono input stage, such as noise or needing very tight match of parts etc.?

The thing you have to be careful about with this statement is the cable used. If a shield is involved and it is tied to pin 3 while pin 2 has the inside connection, its not going to work regardless of the impedance to ground.

Ralph, if you would note my earlier post on 5/08 I already mentioned that a coaxial cable would make for a very poor balanced line. I was trying to make the point that just because some piece of equipment has an XLR jack on its panel, that doesn't necessarily mean it's truly balanced. The definition of a balanced output or input is equal impedance from the two signal lines to ground. As an example, with some OP amp based balanced to single-ended conversion circuits the impedances do not exactly match and are off by a few ohms. Doesn't sound like much but it can seriously degrade the CMRR.

^^ got it John!

Thekong, you hit the nail on the head- dealers don't like to change cables- removing a tone arm cable from its connector under the 'table can be a bit of a pain. Anytime you see an XLR connection but the circuit is single-ended, its likely that the connector is just there for show, unless it is connected to an input transformer, in which case the connection might be entirely legit, as transformers can have excellent CMRR numbers.

Building a balanced phono stage itself is another matter. There are a lot of ways to do it, some overly complex (feeding into that notion that all balanced circuits have to have twice as many components, which is otherwise not true). You do have to have matched parts and I think the one that stymies many designers is how to do the EQ. That can be quite tricky if you don't know what you are doing!! But overall one of the nice benefits is lower noise per given stage of gain, which might mean that you don't have to have as much gain to make it work (we have two stages in our phono section for example)...

You do have to have matched parts and I think the one that stymies many designers is how to do the EQ. That can be quite tricky if you don't know what you are doing!!

Ralph, thanks for the reply! When I discussed the issue with some local DIYers, they did mention having difficulities in doing the EQ in balanced mode !