dbx Disc

Ever since I saw it demonstrated in a Techmoan video, I've wanted to give dbx Disc a go.

It promises significant noise reduction and a high dynamic range from vinyl. But is it worth the investment?

dbx is essentially a compander - a compressor and expander. The (potentially) 100dB or so of dynamic range of the source being recorded will not fit within the 60dB or so dynamic range of a typical record (or cassette). The quiet sounds will be lost down at the noise floor of the disc or tape, caused by disc surface noise or tape hiss and peaks will distort, or saturate the tape.

dbx compresses that dynamic range down, with a ratio of 2:1, making the loud sounds quieter and the quiet sounds louder. The quiet parts of the music are now higher in level than the noise floor.

On playback the signal is expanded back by the same ratio and the (nearly) full dynamic range is restored. Also, on expansion as the quiet sounds which were increased in level previously are brought back down to their original low level, the noise of the disc or tape is also reduced in level, and a much quieter recording is the result.

Unfortunately discs and decoders are few and far between these days. I got lucky on eBay and found a model 122 unit for the bargain price of £25 delivered. The one dbx encoded disc I have cost me the same - hardly a budget way to listen to music. I think this is what led to its gradual demise back in the 80s.

The dbx 122 offers dbx noise reduction for tape sources and also can decode dbx encoded discs. The basic difference between the tape and disc modes is an additional 30Hz high pass filter in disc mode to avoid rumble or warp from affecting the decoding process. There was a dbx version I, which was intended for professional studios. dbx II was the consumer version.

Inside are two removable circuit boards which perform the compression or expansion for the left and right channels depending on whether you have Record or Play selected. The rest of the unit is fairly straightforward - a power supply providing +/-12v, and the various switches and level adjustments.

Unsurprisingly for the priced I paid for it, my unit wasn't working. It just produced clicks and pulses which caused the speakers woofers to flap about unnervingly. It seems the problem was with the connectors on the removable circuit boards. You can see in the photo above the hairline breaks in the solder joints around some of the pins. I reflowed the solder to all of the pins and it seemed to work fine after that.

There is what looks like a date on the boards - could that be 1974? That would be about right. I believe this unit, with its rectangular buttons instead of round, is an earlier version.

Connections are confusing at first, but essentially the unit is connected to the tape out/monitor loop on the amplifier and whatever source is selected on the amp can be run through the unit.

So just how good is it? Really good!

Granted I don't have a lot of material to try out, just one disc of the London Symphony Orchestra, but it definitely does have the kind of dynamic range you would normally expect from a CD.

And the noise reduction works brilliantly. The image above is from the dbx unit going straight into Audacity. On the top you can see the stereo signal from a gap between songs with the unit in Bypass. On the bottom, the unit with dbx turned on. Both of these are amplified by 20dB. The level matching (using the 'Play Level Match' control on the unit) may not be perfect, but you get the idea.

Even with the amp turned way up, the record is silent.

It's a brilliant system, with a major flaw - dbx encoded discs are rare, and command a high price.

The next thing for me to try is how it performs with tape recordings. Will it give Dolby S a run for its money?