someone please explain direct coupled amps

It is easy: almost all amps made today are direct-coupled with means that the output transistors are directly connected to the load (speakers). This is in contrast to a (solid-state) transformer-coupled amplifier in which a transformer buffers the load so that transistors are NOT directly connected to speakers anymore, but rather to a constant-impedance transformer.

Transformer-coupling has advantages: It is much easier on the output stage, thereby inceasing amplifier life span, and allows optimization of the output stage for certain loads for better linearity (lower distortion) among other things. The disadvantage is that it is very hard to wind a transformer for low losses and large bandwidth - both of which are needed to prevent altering the sound.

Despite the advantages, only McIntosh Laboratory uses transformer-coupled outputs (a patented design they have used for a long time) in their solid-state (and tube, of course) amps. The transformers use a special bifilar winding technique that doesn't easily lend itself to mass production but they figured out how to do it well. Seems like I knew of another company at some time that did it too but I can't remember now. Others may know.

Anyway, I can give you lots more technical details because I researched this topic for a while in another electronics application - just email me. Take care - Arthur

Just for the record it has a somewhat broader meaning in tube amps. It does not refer to the connection between the output stage and the speakers. (Conventional tube amps all use transformers)

With tube amps it refers to how the stages (separate gain circuits) of a tube amp are connected (coupled). Usually a resistor and capacitor are used. You can use an inductor or a transformer too. The cap or transformer blocks the dc plate voltage of stage 1 from getting to the next stage (at the grid). In a directly coupled amp the plate of stage one is connected to the grid of stage 2 with no cap or transformer in the path, so dc is allowed to pass. There is nothing between the plate (of stage one) and grid (of stage 2) so the connection is "direct."

This makes it a bit more complicated as the grid of stage 2 has a lot of dc on it. The 2nd stage requires considerable higher voltage. On the good side you do not get the phase distortion, time delay or frequency loss that is introduced by a cap. Some think it sounds best.

Clueless got it right...we are talking about interstage coupling capacitors being eliminated. A Direct coupled amp has response down to DC: which means that if you put a constant 1 volt on the input you will get a constant 25 volts (or so) at the output.

The idea of a using a direct coupled amp for music is not entirely logical, since the music only goes down to 20 Hz or so. Coupling capacitors large enough to pass 10 Hz or so should suffice. However, there is a certain elegance in following the input signal precisely, even down to dc, and some amps made this way sound good, although that might be for different reasons.

A dc coupled audio amp is really a controllable dc power supply, such as might be used in an electonics lab. In fact, some years before dc coupled audio amps were marketed one manufacturer of power supplies was demonstrating the high slew rate of his product by playing music through it.

Thanks for the info.. That makes sense now : )

So its pretty safe to assume the most SS amps are going to be direct coupled?

Thanks Again
Bob

Clueless and Eldartford got carried away on tube amps but in response to your question, yes indeed, nearly all SS amps are direct coupled (except McIntosh, as I said).