Fine Tunes #16
Get thee hence to the nearest RadioShack. Ask for their $11.99 Mini Audio Amplifier/Speaker (part #277-1008), and don't forget a 9V battery. Then pick up part #42-2421---a short lead with a 1/8" mini-phone plug on one end and a pair of alligator clips on the other. Then get part #42-2444, another small cable with a mini-plug to a pair of RCAs. They'll set you back something around $2.50 each. Then, after making sure no one sees you coming out of RatShack, find a pair of RCA shorting plugs. If you can't score a few at your local high-end dealer or electronics supply house, make them up yourself: Separate a cheap interconnect pair, cut it in half, strip the wire off behind two of the RCA connectors, and simply tie hot and ground together. Shorting plugs are optional, by the way. If you can't find 'em and don't wanna do anything with your hands besides . . . then don't worry, be happy. If your system runs balanced you can still get organized, but it's a bit more work finding the parts and putting them together.
Back at home, fire up the system and leave the preamp on Mute, or run the volume down. Leaving the speaker cables in place, attach the alligator clips to the speaker binding posts, click the mini-phone plug into your Mini Amp, and turn up its volume. With 40dB of gain, you'll hear your system's noise floor. Do not play music or let any signal appear at the amps' outputs, or else: fried Mini Amp. And another thing---this little guy should never be put across a power supply! AudioPrism makes the Noise Sniffer for snuffing out the gremlins riding the power line, but the Mini Amp isn't meant to carry 120VAC.
Perhaps you'll hear hum, hiss, noise, or tube rush. Try switching front-end components with the source selector on your preamp and listen to the noise floor. It's always a good idea to mute or lower the volume when switching sources. Listening to your home-made noise sniffer with the amplifier's inputs shorted makes clear what the amplifier's contribution to the overall noise level is. It might be less than you think. Of course, be sure to shut off the amp before plugging in or out.
Next, switch to the mini-plug-to-RCA cable and insert the leads into your source components' outputs, both tubed and solid-state. The rest of your system can be turned off while you do this, with only the device under test powered up. You might be surprised at how your phono stage roars and spits, or at the character of a tubed preamp's noise floor. While typically lower in level, a solid-state noise floor bears exploration as well. Lightly tapping the chassis at different points will show up any resonant hot spots resulting in zinging tubes or other microphonic components. You can try gently tapping capacitors and resistors in both tubed and solid-state gear as well.
What to do if your solid-state gear is resonant or noisy? Try adding a little weight to the top cover, or mount the unit on inexpensive Vibrapods or other Sorbothane-like footers. There's always the bubble wrap/air-bladder route to inexpensive low-resonance tuning. And Marigo Audio Labs makes available several interesting vibration-control devices. They're little damping dots you press directly onto capacitors, resistors, and even ICs. (They even offer Window and Room Tuning Dots!) The Marigo devices aren't exactly cheap, but you can experiment with a little glob of Blu-Tack and see if it's worth further investigation.
The Mini Amp Method is also a cheap, easy way to explore the noise characteristics of a new tube set or replacement tube pair. Use a pencil with an eraser, or find the Dynaclear Tube Tapper you always knew would come in handy one day. (This is an audiophile pencil, believe it or not, with a T top and two eraser heads. Bongggg . . . paging David Lynch!) One gently taps a tube while listening to the Mini Amp. Now that's what I call a cheap tube tester!
In my experience, modern tubes are quite robust and nonmicrophonic, be they Russian, Chinese, or Slovak. Some desirable NOS (new old stock) Telefunkens and Bugle Boys can be noisy when tapped. That doesn't mean they won't sound perfectly wonderful. But with microphonic tubes of any pedigree, you can now experiment with different tube-damping accessories to see which one most quiets the racket. Damping microphonics doesn't guarantee improved sound, but I'd call it a good bet.
In general, a certain amount of hiss from thermionic circuitry is normal and acceptable. A roaring sound might indicate a gassy tube. Thunder---described in the manual for the George Kaye Small Signal Tube Checker as "a rumbling low-frequency avalanche"---generally indicates leakage or a short in the heater cathode. The tube might work, but not well. Tinkling bell-like sounds or whistles mean you've found a microphonic tube. Valves, as the British call tubes, can oscillate in the audible and supersonic ranges whether they're being bonged or not. And here's another little tube tidbit: Did you know that tubes are photosensitive? Listen for a change in the noise floor as you vary the amount of light falling on the component under test.
Another great setup trick from Tweaker Tiscareno is perfect for eliminating RFI-induced hum in high-gain phono stages. Simply loop the interconnects between turntable and preamp, which de-tunes the antenna effect. Then move the "antenna coil" around until you find a null point, et voilà---quiet vinyl replay.
Ah, life is good.