My Yamaha rcvr has inputs aplenty, but it does NOT have a "phono" or turntable input. Since I only have the turntable, the built-in tuner, and a DAT deck coming into the receiver, I need some adjustable preamp or volume control to interpose between the turntable R & L outputs and one pair of the receiver inputs. They are: V-aux, VCR, D-TV/Cbl, DVD, MD/CD-R, (tuner), and CD. The DAT deck works fine with "CD", and I can get turntable audio through "V-aux" and "VCR", but It comes through much too loud. I realize that all I need to do is adjust the volume, but is there not a commercially-available filter or preamp that I can use to "normalize" the turntable output, or a circuit I can build myself?

 

Do a search for RCA Attenuator/volume control.

 

You need a phono preamp. This type of amp is designed specifically for use with a turntable. The phono preamp boosts lower frequencies (see RIAA curve) to compensate for the way in which records are produced.

 

If the signal from the turntable comes thru "TOO LOud" then you do not need any more amplification.
It is the Phono Cartridge that is the single item responsible for converting the tiny wiggles from the stylus into electrical signals.
So backing up a bit, there are two common kinds of technology-pickups used for playing records. Those types are magnetic and crystal transducer types. The magnetic pickups produce a very small signal and always require a high gain, low noise, frequency compensated pre-amplifier. It has been claimed that they sound better and certainly they cost a lot more.
The other category is "crystal" pickups, which is a broad realm of products. Those pickups use the piezoelectric properties of a material that generates a voltage when the small wiggle of the stylus imparts a bit of strain on the material. Those pickups produce a much greater output voltage. Some of them deliver several volts, enough to drive the control grid of a small power output tube in a cheap record player. So probably your turntable has a crystal pickup. There is a chance that it also includes an internal amplifier, but that is rare.
So what you will require in that case is a frequency compensated attenuator. Frequency compensated because the recording process includes different gains for different frequencies. The reason is complicated but the compensation required is vastly documented, so I will not go into those (important) details here.