Well, let me begin my describing my equipment. I have a basic turntable with a lower-end Shure cartridge and stylus (as soon as I can afford it, I intend to upgrade this item). This phono plugs into a Denon receiver-amplifier with excellent sound reproduction. The output of the amplifier plugs into an older home-made computer with a 500Mhz AMD processor and 256 megs of RAM. This computer sits beside the stereo equipment, and is dedicated to the purpose of recording sound. The computer is equipped with a basic 16 byte Creative Soundblaster soundcard. This computer is running Windows XP. Okay, that's it--very unimpressive, but perhaps not too far different from what you will be using to record. I must note that although this is the computer I use to record, it is NOT the computer I use to process the recording (denoising, declicking, etc.). It will do these jobs, but it is very slow. After I have made a good digital recording, I will send it to my fast, powerful laptop over my wireless network to do the final processing.
The first VERY IMPORTANT step is to make sure that everything is grounded together. You simply run wires connecting metal surfaces of each and every piece of equipment you are using--computer-monitor-phono-amplifier-preamplifier (if any)-and whatever other electronic components may be nearby. If you don't do this, you are going to get annoying hums. It's okay to attach wires to case screws or mounting screws, just as long as the connection is solid and is going to bare metal.
After this, the first challenge, which really shouldn't be much of a challenge, is to run a cable from your amplifier to your computer soundcard. The easiest way to do this is to have your computer take the place of your cassette deck: the "record" outputs on the amp go to the line-in on the soundcard. The "playback" inputs on the amp hook up to the "line out" or "speaker" plug in on the soundcard. Now your soundcard will probably have 2 inputs and 1 or 2 outputs. The soundcard input that you will want to connect your amplifier to is the "line in" connection. You do not want to hook it up to the "microphone input." Now this soundcard input will most likely have a little 1/8'' standard stereo plug socket (like the headphone socket on your Walkman), and so, you will need a commonly available cable with a 1/8 standard stereo plug-in on one end, and standard RCA stereo connectors on the other end:
You will need a second cable like this to connect your soundcard output to the cassette-deck playback input on your amplifier (or any other available input).
Now, it is possible that you will find that the volume coming from your amplifier's cassette-deck "record" jacks will turn out to be too loud for your soundcard. In other words, it is possible that the volume is so high coming into the soundcard, that you cannot adjust it downward far enough with your soundcard controls to compensate. I will cover this in more detail in the "Recording" section, but for now I want to suggest an alternate hookup. You can run a cable directly from the headphone jack on your amplifier to the "line-in" jack on your soundcard. You can then adjust the input volume with either the standard volume control on your amp, or with the separate headphone volume control (if your amp has such a feature). This gives you much more control over the input volume, rather than having to depend on the fixed volume output of the tape deck record jack. This is what I had to do with my Denon amplifier-Soundblaster hookup.
And, as another illustration of the way things have a tendency to work out all wrong, let me recount the sad story of my amplifier's pre-amp. Well, as you know, your common integrated home stereo amplifier has a preamp built in. It is the job of the pre-amp to amplify the very small signals coming from your phono cartridge to a sufficient level so that the main amplifier can deal with them. When you hook up your turntable to the "phono" input of your amplifier, you are actually hooking it up to the preamp circuitry in your receiver. That is why you cannot hook up your phono directly to the CD input, or the tape deck input, and achieve a usable signal. The meager phono output must first be "pre-amplified." For the same reason, you cannot run the output of your phono directly into the "line-in" jack of your computer's soundcard.
Well, I noticed that in some of my recordings I was getting suspicious drop-offs in volume in one of my channels from time to time. At first I suspected an intermittent failing in my phono cartridge, but I then found that the dropoffs always occurred in the same channel, no matter which way the phono cables connected to the amplifier were attached. I gradually became convinced that the failure was occurring in my pre-amp. Further investigation and probing and wiggling and fiddling around indicated the problem was a cold-solder joint connecting the input jack of one channel to the main amplifier's circuitboard. And now I discovered the true joys of trying to disassemble an amplifier that was never meant to be operated upon by human hands. I was an electronics technician in the Marine Corps, and so had a modicum of knowledge about circuitry and soldering. I spent a ridiculous amount of time trying to disassemble that thing. I even drilled a hole in the bottom of the amplifier to try to get at the defective solder joint--but all to no avail. Before I knew I had been defeated and concluded I had to "cut-and-run," stalking off in search of a six-pack, I had pretty much used up my Marine Corps vocabulary.
I did not want to throw out the amplifier, because it sounded really good, and I really did not have the money to go out and buy another decent amp. Then one day while browsing through E-Bay, I discovered a little Radio Shack phono preamp for sale. I bid on it and got it for $5.00. Now this item is only the size of a couple of packs of cigarettes, and it has only one input and one output: Phono In and Signal Out. There is no volume control to adjust the output. At first I tried plugging the output from this little amp directly into my soundcard, but it turned out that the output volume was once again too high, and I could not adjust it down. I then ran the output of the preamp to the CD input jack on my amplifier, and this worked just fine. I should note that there are many specialized preamps available with separate volume controls, and they range in price from cheap to expensive. Some of these feature special circuitry that undoes the complex equalization that recording engineers use when making records. The output of these can be fed directly into your soundcard, completely bypassing the main amplifier.
I guess the moral of this rather pathetic and protracted story is that you do what you've got to do and keep fiddling around until you find a hookup combination that works. As always, having the money to get around your problems really can help.