Professionals have it easy, or so you might think, whereas we private studio owners perched on the sharp edge of reality are always trying to find ways to do the impossible with the minimum of equipment. The purpose of this brief article is to explain a few practical 'workarounds' based around a typical domestic hi-fi.
Vinyl Challenge: I recently had a phone call from a reader who had a small line-level mixing desk, but needed to use some material from a record deck. You can't just plug a record deck directly into a line-level mixer, not just because the levels and impedances are all wrong, but also because records need to be processed via an RIAA preamp. Without getting too technical, records are not recorded 'flat' but instead use a form of pre-emphasis which boosts high frequencies. When played back through a filter with the opposite characteristics, the original sound is restored and the level of background noise is also reduced -- very much like tape noise reduction. All amplifiers with record deck inputs include RIAA preamps which perform this very task, but the only mixers with RIAA preamps built in are those designed for DJ or broadcast use.
Of course I was ready with a solution: "Simply plug the recording output of your hi-fi system into the mixer," I said. "That'll give you a line-level feed from your record deck, complete with RIAA correction -- easy." That would have been it, except that he'd got one of those integrated Midi (no -- not that kind of MIDI) systems where everything is in one box with no obvious connections to the outside world. There was no record out or anything remotely like it -- but there was a headphone socket. As it turns out, most headphone outputs will match passably well to a mixer's line input, so the solution was to buy a 'stereo-to-two-monos' jack adaptor and record directly from the headphone socket. The quality ultimately depends on how good the headphone amplifier is, but most are good enough for this purpose. In this case the problem was solved and the reader was happy.
Mic Preamp: Another way in which your hi-fi can come in handy is to provide a mic input -- if you have a cassette deck with a couple of mic input jacks and not one of the aforementioned Midi systems designed with a total absence of useful orifices. Surprisingly, even relatively cheap cassette decks can have quite good mic amps, so if you're working with a line-level keyboard mixer and want to feed in a mic, just put the cassette deck into record/pause mode and feed its output into your mixer. The mic level may then be adjusted using the record level control on the cassette deck.
Aside from providing a convenient way to get a mic input into a line mixer, this trick also helps get around a lack of insert points if you want to compress or otherwise process a signal as you record it. The mic goes into the cassette deck, the output of the cassette deck feeds the compressor's input, and the output of the compressor feeds the mixer's line input.
Headphone amp: Finally, if you have an old hi-fi amp lying around (car boot sales are a good source of these), you can use it as the basis for an inexpensive headphone monitoring system. Figure 1 shows the wiring arrangement, which involves feeding the speaker outlets to a row of stereo headphone jacks, via 330 ohm resistors to limit the power. Without the resistors, you run the risk of overdriving the headphones to the point where your eardrums meet in the middle of your head! It's also a good idea to include a 33 ohm resistor directly across the speaker outlets as shown, just in case the amplifier in question isn't too happy about running without a speaker load. The 33 ohm resistor should be a 10 Watt, wire-wound type, whereas the remaining resistors can be either 1W metal film types or low wattage wire-wounds.
The system described will work with both high and low impedance headphones, though the headphone mix will be in mono only. With amplifiers that permit the left and right speaker negative terminals to be linked, it's easy enough to create a stereo headphone feed, but as not all amps will operate this way, I've played safe and stuck to mono. The amplifier volume control will regulate the overall level of the headphone mix, while the balance control sets the relative level of the two headphone groups. The amplifier's Aux, Tuner or CD input could be fed from the foldback output of a mixer to provide a cue mix while recording. If you find some of your phones unacceptably louder than others in the same group, you could double (or more) the value of the 33 ohms resistors feeding that particular socket to reduce the power.