Audio Amp
This is an audio amplifier project, with emphasis on audio quality.
One things that seperates this amplifier from many others is that it uses an active crossover filter, before the power amplification stage, in order to get better performance in the crossover band. A typical amplifier and speaker system uses passive RLC links for the crossover between different frequencies, giving an uneven magnitude and phase response. By using an active filter like the one below the magnitude of the power from the high-frequency output and the low-frequency output can be kept constant, rather than having a dip at the crossover frequency as with a typical passive LC filter, and the phase difference can be compoensated by placing the speakers at well chosen positions (compare figure 8a with 10 in the LM833 appnote below). This also removes the need for large, bulky power capacitors and inductors.
The amplifier uses a standard transformer – rectifier – capacitor unregulated power that outputs +-22V, and with a +-15V linear regulation board for small-signal opamps.
The active crossover filter is based on the one found in national’s LM833 appnote on page 6 and 7, with some additional pre and post processing (see eagle schematics below) using an OPA2134 instead of the LM833. Though it is not specified in the datasheet the phase delay between the high and low outputs is fixed (most easily tested in a SPICE simulation) (though the phase delay for a given output varies with the frequency as with any analog filter).
The power amplification stage for the midrange and tweeter is based on national’s LM1876, where I’ve essentially done a straight rip-off of the schematic in figure 4 on page 6 (auxiliary amplifier application circuit, again see eagle scematics below). The base stage is based on national’s LM4780, which drives the base in a push/pull configuration. The schematic for this board is incredibly badly done and I haven’t uploaded it, however it’s pretty much identical to the circuit on page 5. There are a total of two stereo crossover boards, one that first seperates the base signal from the rest, and then one afterwards that seperates mid from tweet.
I’ve found the audio quality to be very good from this amplifier, with a very clean sound in a pair of good monitors. I have unfortunately not been able to do any real objective tests, however a more audio-interested friend of mine said that the audio quality is in the range of amplifiers that cost 5000 to 10000 SEK, or rougly 1000 to 2000 USD. I myself am completely happy with the audio quality from the amplifier and can’t find much to complain about.
Audio amplifier schematic/board (midrange/tweet)
Images are of version without base amplifier.
