This page deals with pre-amplifiers and power amplifiers. The preamplifier discussion also includes those for phono correction, using RIAA and other curves.
In a normal hifi set up, there are usually many sources (record deck, radio tuner, cd player, etc.) which need to be interfaced to the power amplifier, which is connected to the loudspeakers. This interface accommodates the voltage outputs (music signal) from the sources, and feeds it to the power amplifiers. If the source voltages are high enough (up to 1-2 volts), then the preamplifier is just a line stage, and will have an attenuator ('volume control'), sometimes tone controls and other switching facilities, and usually a line stage amplifier, although some pre-amplifiers are passive, meaning they do not contain an amplifier, just controls.
The phono preamplifier:
The correction curve is usually implemented in a number of ways, either in a feedback loop, or as a passive circuit between two (or more) active units, or a combination of the two. The active components can be thermionic valves or transistors, either discrete or in an integrated circuit (op amp). The easiest implementation is a two stage discrete JFET transistor circuit, but the simpler the circuit, in general, the more complicated the power supply. Second easiest uses op amps, and the most difficult to implement are valves, if only because of the need for high voltages and extra heater supplies. Except for this, the discrete transistor (JFET) and valve circuits can be virtually identical, except for component values. Passive circuits (using just resistors and capacitors between the active elements) are best used in JFET/valve circuits, and feedback configurations best suited to op amp circuits. Each has their advantages and disadvantages, supporters and detractors.
The power amplifier:
I like simple power amplifiers, the more complicated ones sound, well, more complicated. Valve (tube) based amplifiers are usually simpler in design than their solid state counterparts, and this may be partly responsible for their liking for hifi duty. However, I'm sure transistor amplifiers can be made which are simple, but have simpler power supplies and easier to implement than their valve counterpart. The power amplifier design I like is a straight forward FET output amplifier, about 60 watts into 8 ohms. Does that sound low powered? Well, the circuit components are readily available and very cheap; it would be cost effective to use many smaller amplifiers, one to feed each drive unit of a loudspeaker, which may have up to 5 drive units, per side. But this amplifier has one special feature.
The output of most modern transistor amplifiers have many passive devices strapped across their output, lowering damping factors, and generally lowering fidelity. Feedback is used to straighten out the transfer function. These devices, (resistors, inductors and capacitors, maybe even a fuse) are necessary for stability and safety. The design I am considering has two sets of output FET's, one set with the associated stability devices, feedback loop and a dummy 8 ohm load, and the other just connected to the speakers, probably via a fuse.
For a couple of pseudo-active loudspeaker projects, I have decided to use a 'chip' amplifier, that is a complete (or nearly complete) amplifier on a single silicon chip, its the TDA7294, a 50 watt (into 4, 6 or 8 ohms) with a modest power supply. Data sheet is here: here
As mentioned the relevant pages on loudspeakers, I'm using these amplifiers in a pseudo-active configuration, that is, the power amplifier (just one) is mounted inside of the enclosure, with the heat sink outside, of course! Power is from an external supply, which will power the other loudspeaker, as well.
Here is a picture of the amplifiers (from the supplier) :
The input capacitor (the green one) will be changed for a non-electrolytic one, perhaps a polypropylene one. Note that there is enough space on the pcb to change the input capacitor, as the quality of this component can be important.
Here is the circuit diagram of the complete amplifier:
and the block diagram of the internals (just the chip amp):
Note the ubiquitous bipolar input long tail pair, but unusual gain stage, and the output MOSFETs, all built on one chip, so thermal problems should not occur!
The chip amps came today (14th Dec), already built, (I thought they were not assembled! saves me a job!). They are smaller than I imagined, but that's not a bad thing. I managed to source transformers and heat sinks from ebay, and the rest of the power supply components from Rapid Electronics. Now I need to decide on enclosures for the power supplies, and connectors and leads. Here they are:
to be continued......