Hello!
No problem with the questions - all good ones!
The way a tranformer handles impedances is one of it's bigest advantages. In many ways it behaves electrically more like an active pre-amp with an easy load for the source and a low output impedance driving your amp. I'll take your questions one at a time:
>>What is the exact relationship between source and amp impedances?
The output impedance of the pre-amp is a measure of how well it will drive your amp's input impedance. A lower output impedance means more current drive to your amp.
The output impedance is a bit complex to calculate exactly but the rule of thumb is that the source impedance divided by the square of the turns ratio at that volume setting gives the output impedance - or at least thats the basic relationship.
The turns ratio is the number of turns on the primary of the transformer divided by the number of turns on the secondary (at that volume setting). As you turn down the level the turns ratio quickly becomes high the square of that number becomes very high. This means that as the level is turned down the output impedance gets lower. The fact it is related to the SQUARE of the turns ratio means it goes down very fast so after a few steps from the top volume the tx102 starts to display a quite low output impedance.
Hope I did not get too techie! There is a chart at S&B's info page showing output impedance at each step:
http://www.stevens-billington.co.uk/page102.htmNote that this is with an unusually high source impedance of 1K ohms. Most all sources will display output impedances significantly lower than those shown in the chart.
> I understand that lower source impedances are preferred, and the >lower the better.
Although lower is better beyond a certain point it becomes academic so I don't recomend searching out sources with very low output impedances specifically. The table at S&B's site linked above shows input impedance of the tx102 and you can see it is quite high and an easy load for most any source. Other factors between sources will have more influences on the sound quality of your system than the sources output impedance. A general rule is to keep it below about 1K and almost all commercially availiable sources are lower than this - most much lower. I use an Audio Note Dac and have many customers with Audio Note dacs or players. These have output impedances on the high side but present no problems for the tx102.
> I understand there is a minimum desired amp impedance of around >10K Ohm, but is there a maximum? Is higher better?
Just like the output impedance of the source this 'higher amp impedance is better' is a general rule but don't pick an amp just for that spec. Other factors will dwarf the affect of input impedance. I have gone lower than 10K on my test gear with fine results and I have some customers running under 10K (and right around 10K) with no problems. Anything from 10K up or so is fine. There is no maximum. Typical amp input impedance values are from about 20K to 100K.
The source impedance enters into this as well. A typical source may not have any problems driving a low impedance amp with the tx102 in between but a really high source impedance AND a very low input impedance amp may not be a good combo. These kind of mixes are really hard to put together and almost all gear works out just fine.
Hope this helps!
Thanks!
John
