Thanks a lot Folks,
Even though what you have written is extremely lucid, there is only so much I have understood which I set out below:
1. Music frequency starts from about 20 hz so 0 hz is not much point. Does this also include sounds that we feel rather than hear? May be some of the body in music comes from the sub-20 hz frequencies
2. The "sonic advantages" if any, of DC coupled may have something to do with the absence of coupling capacitors which cause frequency related 'phase shifts' which get blocked in conventional amps. (I think I know what 'out of phase' sounds like, not sure if that is relevant here)
3. It is useful to enquire how much global feedback is provided by a DC coupled amp. The sound characteristics of the amp is connected with this.
4.One should make sure that a design has coupling capaciiors at the output to protect speakers
I did not quite get the point about reduced gain below 10 hz, is this is important from the point of operation or design? I am afraid "Servo circuit to maintain offset close to zero" was Greek to me.
Muchas Gracias
I wrote a long explanation but it got lost ! So here is a quick short one.
Most music lies above about 30 Hz . The disco type bass , kick drum etc. lies around 50 to 100 Hz ! Lower than 20 Hz is usually only from an organ or LFE effects in movies like thunder , explosions, .....running Dinosaurs etc !
The sonic advantage is due to eliminating the input capacitor. But this is not due to the phase shift it introduces with frequency. It due to the behaviour of the capacitor , which is highly debated. Like polypropylene sounds better than polyester etc. Some agree and others do not. But they sometimes do sound different to me.
Feedback can be dc or ac coupled. However dc coupling only affects the bandwidth of the amp. Response goes down to dc rather than rolling off at say 10 Hz or so, if it is dc coupled!
Capacitors at the output of an amp is not good and not used in most modern amps. It used to be done in old generation amps or amps that have a single supply . This means that it will have dc at the output ( about 1/2 supply Voltage ) and so you need a capacitor to block the dc. These are large electrolytic capacitors and are not as good as small film caps used typically at the input.So if you want to block dc , do it at the input.
The amp should have a fast acting dc sensing circuit at the output which will operate a protection circuit , often a relay , to disconnect the speaker if a large dc voltage is found at the speaker terminal.
DC servos are only to correct a small dc offset at the output that all amps have. That works only in the milli Volt range.
When you use a capacitor at the input,the frequencies below the roll off frequency ( say 20 Hz) will get reduced ( lower gain !) . It ( the signal level) keeps dropping as the frequency falls tending to zero at dc ( blocking it !).
