External DAC -general questions

I've used digital to analog converters off and on for many years mostly in attempts to make resulting sound more "analog like". The only application that I was able to tell a difference, though, was when I installed one between an old Sony jukebox and a Sphinx integrated amp. In other words, I think I really added DAC's because it just seemed like it should improve the signal.

My questions may be based on unfounded assumptions but here's where I'm confused; I assume that in all of audio, for there to be amplified sound, some form of conversion to analog is necessary and would be built in to the equipment. My recent amps and preamps boast of good built in DAC's, tube DAC's at that, and I still have external converters between them and various sources. So here's the question; if an analog converted signal is being delivered to an internal circuit designed to convert from digital to analog already, how does that work. I know it does and maybe I don't need to understand it to use it, but I would like to have some understanding of it.
You don't put an analog signal into a DAC. You take the direct digital out from the single-wire digital output and plug that into the digital input of the DAC. The signal is only converted to analog once - before the amplifier. But it wouldn't work if you did it any other way, so if you're getting sound, you must be doing it right. I think.
So if using a quality converter ahead of the preamp it sounds like it defaults to using the already converted signal. This being the case, assuming the outboard DAC is better than the preamp's converter, leaving it installed is probably the best choice.
I don't understand what you're saying. An analog signal will
have a pair of RCA or XLR outputs. In order to use the DAC in
your preamp, you will still have to run a single-wire digital
signal from the source into the single-input digital input of
the preamp. At least in every piece of gear I've ever seen.
You are not using the DAC in your preamp if you're running a
stereo pair of wires into one of the analog stereo inputs in the
preamp. I don't think.
Thanks, Chayro. Where was my brain; of course, the signal coming from the DAC, wouldn't be seen as digital by the preamp. My preamp does have multiple ways of receiving digital signals and I use them for a couple of sources. These, then, would be using the internal converter. I apologize to all for this confusing post.