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The so-called 'Trimode' system used by most car amps seems mysterious, as there is no speaker switching, and commonly only a simple stereo/ Mono switch. Even without the switch being operated, a speaker connected between the +ve (Red) terminal of one amp (typically the left channel) and the -ve (black) terminal of the other will still work with a typical stereo signal.
There is nothing magical about this, and the basic concept is used in almost every car amp known. What follows is a simplification, but describes the process and how it works.
A schematic of a trimode amp is shown below, in Figure 1. As you can see, there are two completely normal power amplifiers, and a simple inverter circuit is used in front of the Right channel amp. The 'trick' is that the output terminals for this inverted channel are reversed, so the +ve Red terminal is actually connected to ground (chassis), and the -ve Black terminal connects to the amp's output. For any normal stereo signal, this maintains the correct polarity of the system, and the speakers will not be out of phase.
Figure 1 - Trimode Amplifier Schematic
The opamp is usually nothing special, and virtually any standard opamp (such as a TL071) can be used. If another opamp is used in the front end, then the device may be a dual opamp - I have deliberately not added pin numbers, since you will need to work out for yourself what you need here.
Since this is not intended as a design as such, if you want to use this method you will have to work out the details for yourself. Please do not send me e-mails asking for a complete design! There is enough detail here for anyone who is capable of wiring a simple circuit, and who understands the basics of amplifiers in general terms.
When connected in BTL, the same speaker 'sees' 20V at one terminal, and an inverted 20V signal on the other - a total of 40V RMS (I shall leave the proof of this to the reader) Using the same formula, 40²/ 4 = 400W - four times the power. But ... each amp now sees only half the load impedance (think of an imaginary centre tap in the voice coil, connected to ground). The amplifier must be stable into 2 ohms, or this method will not work. Of course, you can use an 8 ohm speaker and still get 200W if the amplifier cannot drive 2 ohms safely.
If you have ±12V available (from the main switchmode power supply for example), then you can use the Project 87B dual balanced line driver/ transmitter circuit to obtain a defined input impedance and the non-inverting and inverting outputs.
The method described here is in almost universal use for car amplifiers, is simple and works well. Strangely enough, it is rarely used in (home) hi-fi amplifiers, possibly because it may be considered 'unacceptable' by audiophiles (for whatever reason).
As stated above, please don't e-mail me for further information on the construction of high power car amps. There is a project article for a high powered switchmode converter, and that can be used with almost all amplifiers shown on these pages - however, none of the amps I have presented will drive 2 ohms safely, and they are not recommended for car audio use. Most car amps are very rugged, but are typically of 'sub-hifi' performance, often having relatively high levels of crossover distortion (especially audible at low impedance and low volumes).
A power amplifier for car audio use may be offered at some later time, but at the present there is nothing available on these pages that I consider suitable. Several people have enquired about using LM3886 power op-amps, and although these will work, they really need to be connected in parallel. There is an excellent Application Note (it was AN-1192, but is now SNAA021B) on how best to do this, originally by National Semiconductor but now TI. See snaa021b (the document is a PDF file - approx 802k in size). There is also a simplified method of achieving the same result as shown above, but bear in mind that input impedance will be low (and there are possible stability concerns as well).
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