No doubt you’ve heard terms like class A, push pull, , SET, full complimentary, single ended etc. These terms all refer to how we use an amplification device rather than the amplification device itself (like a tube or transistor).
Many of us have also heard that tubes produce a warmer sound due to the fact they have even harmonics and transistors are harsher and colder producing only odd harmonics. While it’s perhaps true that even harmonics (a simple doubling of the frequency) may sound more welcome that odd harmonics (times three) in the broadest sense it isn’t actually the device that is responsible for the focus on harmonics it’s the way it is used.
Most tube preamplifiers are single ended amplifiers while most solid state designs are not. This has a lot to do with how each sound – not because it’s a tube or a transistor but how it is used. In a single ended design there is one amplifying device covering the entire musical signal while in a push pull or complimentary design there are two: one for one half of the signal and the other the remaining half. The simple fact that second order harmonics are louder than third order harmonics in most any device dominates the single ended approach while push pull and complimentary designs cancel these distortions by the very nature of their operation. What this means is that any device, tube or transistor, will display the same basic character with respect to odd or even harmonic distortions depending on how it is used. The fact that tubes more typically are used single ended than most transistors is the reason the audio “facts” have evolved about the warmth of tubes vs. transistors. Fact is, in practical applications it’s mostly true!
Designers of both tubes and transistors can make choices and tradeoffs to achieve their goals. For example, one of the tradeoffs of tubes is they have relatively high output impedance and are not easily capable of driving a loudspeaker directly – where the opposite is true for transistors. This is the reason most tube power amplifiers have large output transformers and transistor amplifiers do not. The transformer “fixes” the tube output issue but, of course, has many drawbacks sonically. Some designers have gone to great lengths to fix this problem by designing what’s called OTL or output transformerless amplifiers. While not ideal it does show that where there’s a will there’s a way.
The point I’d like to get across in today’s post is that much of what we relate to as “common knowledge” between the differences of tubes and transistors has as much to do with how they are implemented as the devices themsleves. Some of it is a Catch 22 – where the implementation differences are necessitated by the limitations and advantages of each device type – but much of it is simply a pure choice by the designer.
Paul McGowan / PS AUDIO
