Our switchable Output Stage - and why Impedance matching to amplifier inputs is important
It's not something commonly talked about, but line level impedance matching is something we need to be aware of as sometimes it can cause a loss of performance.
It's this reason that I find people sometimes have had a bad experience when trying to integrate valve output sources with certain transistor amplifiers.
The issue is impedance matching, if we try to feed a source with a high output impedance into an amplifier with a low impedance then the source cannot properly 'drive' the amplifier's input. The sonic result is normally a dull, undynamic sound, often quite quiet and sometimes distorted and lacking in bass.
To ascertain if this is an issue we really need to know the input:
If you look at the specifications for the line level impedance on your amplifier (i.e. the place you'd plug an external phono stage, DAC, CD player or similar item into) then it should have a figure expressed in 'k Ohms' (thousand Ohms).
A figure of 47k Ohms is quite common but far from guaranteed. This value is considered an easy load, and this or anything higher (100k Ohms is about as high as you tend to see) can be driven by pretty much any source.
Figures below 47k Ohm, become increasingly difficult to drive, and certain transistor amplifiers, Naim being a prime example are 18k Ohms, and older Exposure amplifiers as low as 10k Ohms.
Some TVC pre amplifiers (transformer volume controls) can also exhibit a low impedance load, this is difficult to put into a set value of 'k Ohms' as it can vary depending upon frequency, volume setting and the connected power amplifiers impedance.
Solid state sources tend to have a very low output impedance, so will shrug off a low impedance load without problems, but a lot of valve sources will not.
The reason for this is many valve sources use what's called a capacitor coupled Anode follower output stage, it's a very simple, elegant and good sounding design that produces lots of gain but it has the side effect of a relatively high output impedance. Therefore these generally need to be connected to a 47k Ohm load or greater.
There are two solutions to this problem:
-A transformer coupled Anode follower circuit, when designed correctly, should solve this issue as the transformers do the impedance matching for us, but this isn't very common as you require high quality output transformers and they're never inexpensive!
-A Cathode follower output stage, effectively it's a buffer, it doesn't add any voltage gain but due to its local feedback it offers a very low output impedance.
What we have done in our designs (possibly uniquely) is to include a switchable output stage. You can use it in Anode follower mode for use into amplifiers with an impedance over 47k Ohms and when a lower impedance is expected you can switch in the additional Cathode follower stage - giving you the best of both worlds and the easy ability to see what sounds best in your system.