Amplifiers

Compared to the damage done by mastering amplifiers may be a minor worry but there are things to look for when choosing one. The most important aspect of an amplifier is it's design, that can then be well implemented with decent quality components.

Design of an amplifier needs to some first
Quality of implementation comes second

Like much HiFi however often a great deal of money is spent on implementing poor designs, when it would cost exactly the same to build a truly great amplifier. Some component choices do influence design however, such as the choice between tubes/valves and solid state.

Terms

Term Description

THD Total harmonic distortion, a popular way of representing distortion.
It's also a poor way because THD treats every harmonic as equal but the ear does not.

OPT Output transformer. Used to isolate and impedance match a speaker to an amplifier - usually a tube one.

NFB Negative Feedback. A mechanism of feeding back an inverted signal from the output back to the input.
Used to control gain, reduce noise and measured distortion and flatten frequency responses.
NFB also lowers the output impedance which is very useful.

Local NFB Feedback around a single stage, intrinsic to some devices like triodes. Good for sound.

Global NFB (GNFB): Feedback around multiple amplifier stages. Bad for sound (see later).

PSU Power Supply Unit.

Current A measure of the rate-of-flow of electrons from one place to another along a conductive path.
An analogy is counting cars going past a point per minute, a slow multilane road can be matched only by a fast single lane road, imagine then a small single lane road (thin wire) heating up with the traffic as opposed to a (thick wire) multilane road just drifting by.

Voltage A measure of the potential for current to flow if a path is offered.

AC Alternating current - like that created by music and from the wall socket.

DC Direct current - like that created by a battery or PSU.

Resistance The DC ratio of voltage to current through a device or speaker.

Impedance The resistance at AC frequencies of a device or speaker.
A low impedance means lots of current, a high impedance generally means less current.
You can think of voltage as the requirement and current as the enforcer that does the actual work.

SET Single Ended Triode, a term for a popular tube amp design that uses no global feedback at all that uses a nice big (often expensive) triode as the main power device. Often they have a low output power but tend to be the best sounding topology.

Super triode A new term denoting a multi-tube solution using NFB around just the tubes to replace a single expensive triode.
This allows a better performing amplifier for much lower cost.

SuperSET A new term denoting a SET amplifier made with a super triode. This gives all the benefits of both SET designs but greater power and control for a lower cost.

Basic design issues

There are some good hybrid designs - for instance tube front ends with solid state followers that have pros and cons but are not looked at here.

Tube or solid state?
Class A, AB, B or D?
Feedback schemes..

Tubes/valves vs transistors

Tubes are old fashioned by the most linear device is still a triode, in particular directly heated cathode ones, but even the simple dual triodes like the 12AX7 are still very good. In addition to their linearity they run at high voltage so the actual section of the transfer curve used can be rather small, adding to the linearity.
BJTs have a rather non linear response but make very good followers.
MOSFETs are more linear than BJTs and are available with great power.
FETs are usually too small for outputs but are closest to tubes in linearity and behavior.

As the most linear devices are tubes we should therefore choose those, unless we are using class D which is a switching design.

Conclusion

Use tubes for amplification.

Amplifier class

Class A. This is the best sounding class, despite being slightly asymmetrical in concept. The reason is that it uses a constant power so the power supply does not intrude on the sound.
Class B. Class B is popular because it's cheap to make. To work properly it has to switch between the +ve and -ve transistors at exactly 0mA which is a problem because:
- That's the most common place for an amplifier so it affects the sound the most
- Devices tend to be the most non-linear when they switch off.
Class AB is class B with a little bias current run through the transistors to make them a bit more linear at the critical crossing region. Unforunately this results in a situation where both devices are switched on around the central point but one switches off as the voltage leaves that region. This causes the impedance to half every time that happens, which means in anything but a small signal the impedance is continually changing with the waveform.
Class C is a partial conduction not suitable for audio.
Class D is best done by very fast chips and as such is not so DIYable, although TPA3116 modules are very cheap and can be upgraded to excellent sound.

Conclusion

Use class A, but class D has some interesting properties.

Feedback

NFB improves frequency response, noise, impedance and the gain is controllable but as with everything it has a cost. The cost is that NFB only really works as advertised in a linear system, any non linearity gets multiplied which has the effect than non linearities merely spread upwards in frequency and spread themselves over the bandwidth. It's a type of Conservation of Non-linear Distortion effect.
This effect still helps the THD figures look good.

So NFB's pros and cons are:

Pros: Better drive (lower output impedance), less noise, less gain but a preset, controllable gain.
Cons: Multiplication of harmonic distortion, instability (i.e slow, phase shifting amplifiers can become oscillators).
The most insidous and often unnotoiced con is that the obvious distortion is replaced by a subtler one. For this reason NFB is really only best applied to amplifiers which are already linear, which is a problem for BJT based amps.

Feedback in tube amplifiers

GNFB is usually used in tube amplifiers in moderate amounts, despite the long known fact that feedback works best if there is lots of it. Often a mild amount is worse than none at all - just enough to multiply but not enough to do any good. Individual stage feedback is wlways present anyway, a triode has an instrinsic local feedback, all followers do and anything with a (un bypassed) cathode resistor does to. Local feedback works around each device itself and so it very 'fast', and therefore always the best feedback. Additionally the harmonics it multiplies are simpler, not the sum of the preceeding 4 stages but just of the device itself.

Tube amplifiers tend to get away with GNFB because they are already pretty linear.

Global feedback is bad, especially in phase compromised amplifiers
Local feedback is good and unavoidable anyway

This leads us to reconsider the practice of including the OPT (output transistor) in the GNFB loop. It's easy to dp - especially in push-pull amplifiers - but it's a bad idea because:

The OPT is a band limited phase lagging component that has no business in a feedback loop
The phase lag reduces the 'phase margin' meaning that instability becomes a bigger issue causing overshoot, ringing and in extreme cases turning your amplifier into an oscillator. I.e. this phase lag stops the proper application of decent amounts of feedback.
The band limiting nature causes the feedback to be reduced at the bass and treble. Higher quality OPTs can alleviate this but the effect still occurs so even if we manage a decent amount of feedback (before noticeable instability) the feedback where we need it most - at the frequency extremes - will be far lower. Remember feedback only works at high levels.

These problems (and the solution) is something Thorsten Loesch has pointed out and has been proven in a design we did that incorporated the idea.

The solution is to follow the SET design but instead of using a triode - to create a 'super-triode' out of two or more tubes with tight feedback around them. This 'super-triode' will then have a very linear response and - more importantly - a very low output impedance. This low impedance then drives the OPT correctly and allows it to work as it should.

Conclusion

We need to use local feedback and avoid feedback over output transformers - even with the more linear tubes.
The best amplifier will therefore be:

Tube based
Class A (Single ended)
SET based design but with a composite 'super-triode' made with multiple cheap tubes.

Using these principles an extremely well sounding amplifier can be built for the cost of a pretty cheap mediocre one. The sound will be light, sweet, realistic with good bass and sweet treble and the music will come alive, freed of the constraints of a tired feedback loop trying to keep fundamentally nonlinear components together. It's just physics/electronics/theory but put together it does make a difference.