I can’t believe there’s no coupling cap/s somewhere between input and output somewhere.
But then it’s a shame if the rest of the system is not true direct coupled also?
The problem with direct coupling from input to output is that the circuit goes lower in frequency (to DC) while the power supply (which is supposed to go to DC) in fact has a very low frequency time constant, probably several seconds. If the circuit can go lower than the power supply its possible to modulate the power supply which in turn will cause intermodulation distortion.
So its useful to limit the circuit’s ability to go all the way to DC. This also prevents it from amplifying DC should any be applied to the input of the circuit.
So we have direct coupled inputs and outputs, but coupling caps in between. The advantage here is that since the capacitors are buffered inside the circuit (IOW we’re able to control the impedances which they drive) we are able to keep their values relatively low. This helps with transparency; the larger value a coupling capacitor is, the more coloration it can impose (caused by inductance, ESR and dielectric constant).
When designing a high performance audio circuit, bandwidth is important to prevent phase shift in the audio passband. Generally speaking the circuit has to go to 1/10th the lowest frequency to pass and 10x the highest frequency to pass. So you need 2Hz to 200KHz if you really want zero phase shift. This is challenging if you have an output transformer or coupling cap; in the case of the former getting it to cover that much bandwidth is a real trick! In the case of the latter there is no way to know to what sort of amplifier the preamp will be connected so the coupling cap is often larger on that account, and as a result isn’t going to go all the way to 2Hz in some cases (probably with a solid state amp being driven).
Now in the case of vacuum tubes while it is *possible* to direct couple from input to output, its really impractical and would likely be a reliability issue. So you aren’t going to find a vacuum tube preamp that is direct coupled from input to output. You might find a solid state preamp that is, but even then there are practical issues (most of which I’ve outlined) which make such a thing unwise.
Because coupling caps and coupling transformers are thus a necessary evil there are quite a few boutique products of both types around. FWIW a good line driving transformer is more expensive than a good coupling cap so that is a good portion of the reason that output transformers in preamps are uncommon. But a secondary reason is its also more difficult to get bandwidth and unless one has been quite careful in the design and application of the transformer, higher distortion too.
