What makes a good tube preamp?
There's quite a few tube preamps available from the cheapest Chinese import to the exotic and expensive, so what makes a tube preamp good?
The most important feature is low output impedance ie the ability to drive loads. A power amplifiers load (input impedance) will vary from typically 5000 ohms (solid state) to 100,000 ohms(tube) and its generally recognised that a ratio of 1:10 is suitable, ie the preamp will have an output impedance 1/10 of the input impedance.
So if a tube preamp is going to sucessfully drive a solid state power amp with an input impedance of 5000 ohms it should have an output impedance of around 500 ohms.
This is just about worst case- 5000 ohms input is quite low, but not uncommon.
What happens if a preamp's output impedance (Z) is too high?
The preamp will not be able to transfer its frequency response in a linear fashion, it will be inaccurate and "sloppy' sounding.
This can happen when an unsuitable high resistance tube is used in a circuit that does not do any "shifting" from high to low output Z.
Tubes like the 12AX7, 6SL7 are often used in simple, minimal preamps but their high resistances require circuits such as the cathode follower to be used , which lower the output Z.
Unfortunately there's a price to pay , as 100% feedback is employed in this circuit, which does introduce a bit of haziness and veiling.
Without the cathode feedback, the high resistance tubes sound slow, warm and so "romantic" they will put you to sleep pronto.
Another way to lower Z is to use a "transformer" a step down device which can drop the ratio by a chosen amount, for example a 4P1L triode tube has a resistance of 1000 ohms, if we use a 4:1 ratio step-down we will end up with a Z of 250 ohms, which is low for a tube preamp, and lower than you will ever need!
Transformers vary tremendously in quality , and they must be carefully matched to each tube and application. Transformers don't work well with high resistance tubes and even with well matched tubes, its quite an art to build a really competent and high performance transformer.
I use the 6SN7 tube in my preamps , with a special circuit and Lundahl or Magnequest output transformers the preamps have an output Z of 1300 ohms , nice and low .
So output Z is important for a preamp in order to control a power amp with accurate frequency response and drive.
Tomorrow I'll talk about something else that's important.
