Caution: This page is for tinkerers, tweakers and gearheads. These are some more technical things that will hopefully be useful around the studio. This is for those of you who want to get out your soldering iron and start optimizing your gear.
Hums and Buzzes:
Sometimes it's tough to keep your tangle of wires and cables from picking up hums, buzzes and the local oldies station. There's several ways to combat these problems. First, you should run your wiring neatly with high quality patch cables. Try to seperate your audio wires from power cords, surge protectors, etc. If your audio and power MUST be near each other, then try to let them cross only at 90* angles. When power wires run parallel to signal wires, they can induce 60Hz hum into the signal, crossing at 90* greatly reduces this effect.
1. Be sure you are using good shielded cable between your mixer and your power amp or powered speakers.
2. Be sure ALL of your equipment is plugged into the same AC power circuit. If using more than one wall jack, be sure they are on the same circuit in the breaker panel, and that they are properly grounded. Likewise, makesure that your audio gear is the ONLY thing on that circuit.
3. Unplug all inputs to the mixer, leaving only the output to the speakers. If it sounds OK, you can proceed to the next step; If not, unplug the speakers and listen to the mixer with headphones. If it still hums, it's broken.
4. Start plugging in your other equipment starting with balanced gear, then unbalanced gear. Eventually, something will start buzzing, and I'll bet it'll be an unbalanced in or out.
When you find a piece of hummng gear, there's a few things you can do. The best option, in my opinion, is to buy an audio isolation transformer. These come various configurations with combinations of RCA, 1/4" TS, 1/4" TRS and XLR connectors. There should be a model available for any need. Use a good quality model to make sure you don't degrade the signal. It's also possible to buy a bare transformer and some jacks and make your own. If you're trying to rid hum in unbalanced gear, it may be wise to buy an unbalanced to balanced transformer.
You can also try ground lifting the equipment. This involves disconnecting the shield of the audio wire at one end. This is also called a "telescoping shield". I recommend connecting the shield on the mixer end, and floating it at the other end. You may also have to physically isolate the metal case from other components around it.
DO NOT by any circumstances try to do a ground lift by removing the ground leg on the 3 prong power cord, or by using a 2 to 3 prong adapter. The ground wire is essential to make sure that the equipment is safe. If there is ever a failure in the gear, and the ground is not connected, you can be electrocuted by touching the case.
There's also times that your gear will pick up radio stations, CBs and other annoying types of interference. Here's a few things to try when having RF problems:
Everyone hates having 10 wallwarts on the back of their rack. They take up too much space, and often block the outlet on either side of them. For very little money, and a trip to radio shack, you can build a box that will power several effects boxes, distortion pedals, etc. This is a project for people with some decent electronics skills, but if you're good at making simple circuits, then this should be easy.
One recommendation I make is to only offer ONE voltage on your power supply. 9 VDC is the most common voltage that I find in my gear, and is a prime candidate for this project. I don't recomend have polarity switches on the box, because this causes all sorts of voltage potentials between your gear if you're not careful. It CAN be done, but I think it's too risky for me to explain clearly over the net.
This circuit is designed around the 7809 voltage regulator IC. It's a very cheap and easy way to make a simple power supply. I personally like to use a seperate regulator for each output to provide a little isolation between the components. I find that some devices with a LFO (low frequency ocsillator) like phasers and flangers will dirty up the power and add "swooshing" sounds to your other equipment... using seperate regulators really helps this.
To build this project, you'll all of the obvious electronics tools, and the following supplies:
Power Isolation Transformers:
There's no doubt that a power isolation transformer really helps to eliminate interference from the mains power. To build one, get yourself an isolation transformer that has dual primaries & dual secondaries, the kind that's usually used to step up from 117 to 240 or down from 240 to 117. Wire both the primary & secondary in series as if you were using it as a 240-240 (1:1).
Now, ground the center tap of the secondary, (that's the connection you made to join the two secondary windings). Do not ground the primary. You now have a 1:1 transformer that has a single-ended input & a balanced output. It is of course expecting to see 240 volts on its front end and wants to give you the same on the other side. It will think you're a wimp for only providing 117 but will be happy to oblige with 117 on the output .
A nice bonus to this type of hookup is that since our transformer friend did indeed brace himself for 240 volts but only got roughly half of that, he will tend to exhibit much less hum to boot. Now you just need to add an appropriate fuse and 3 prong power cord, and put it in a case.
Wiring your own cables:
There's no big trick to building your own cables. Soldering is pretty easy, and just takes a little practice. The first thing to do is to select an appropriate type of wire. An XLR or 1/4" TRS cable will need 2 conductors plus a shield, and a 1/4" TS cable will need 1 conductor plus shield.
You must also choose the right type of shield for your application. Foil shields provide better hum and noise rejection, but they are only good in a fixed installation (something that won't be moved often - behind the rack, or in a wall). Braided shields are more flexible, and can be used for mic, instrument, and patch cables. Get the highest percentage of shield coverage as possible.
I always use heat shrink tubing and high quality connectors. Neutrik and Switchcraft are fine choices. I put heat shrink tubes over each terminal in the connector... this prevents the possibility of them ever becoming shorted if the cable is strained hard.
To build a cable, you chop off the appropriate sized piece of wire, and strip both ends. Generally, you'll strip the outside jacket back about 3/4" to 1". Twist the shield wires together neatly, and tin them with the soldering iron. Strip back about 1/4" to 1/2" of the inner conductor. Twist it neatly, and tin it. Repeat for each inner wire.
Now you slip the barrel of the connector over the wire, and put heat shrink tubing on each wire (just slip it on, don't heat it yet). If you're making XLR's, make sure you put on the strain relief before soldering, too. On 1/4" connectors, I usually solder the tip and sleeve connectors first, and the shield last. The layout of XLR's make them easy to solder in any order. When you solder connectors, you'll need a relatively big iron to heat the terminals sufficiently. When you're done soldering, use a heat gun to shrink the tubing over your connetions, then close the barrel. When you've finished, use an ohmmeter to test them for open circuits, miswiring, and shorts.
And, just so you don't get your wires mixed up, here's what each conductor is for both types of balanced connections. You can also make your own XLR to 1/4" TRS wires to connect your gear.
Making Inline Pads:
Many microphones (especially large diapragm condensors) have a pretty hot output, especially when used on a loud source like a kick drum. Expensive mics will often have a pad to attenuate the output if it's too hot for your mic preamp. It's pretty easy to build you own pads of varying attenuation levels.
The design is called an " H" pad, and is used to attenuate balanced signals. there are 4 resistors of identical value (R1) and one which has a different value (R2). The four identical resistors should be 1% tolerance (or you can just match them with a good digital multimeter). The values you choose depends upon the amount of attenuation you need, and the impedance required. If you're making a low impedence connector, use a 600ohm resistor for R2; For high impedence, you can use 470K.
Neutrik makes male XLR to female XLR "adapters" that are essentially a 3" long metal tube with each gender of XLR on it. You may use these as a housing for your attenuators, and perhaps put a stripe of colored electrical tape around the barrel to identify the attenuation amount.
One of the factors that makes budget gear cheaper is the selection of components in the circuitry. The manufacturer's selection of opamps is a spot that can commonly be improved on by a crafty do it yourselfer.
While many opamps are fine, I often see lesser quality opamps like the TL071 and TL072 used in audio gear. For a significant improvement, The Burr Brown OPA132 and 2132 are drop in replacements for the TL071 and TL072, respectively. They have less distortion, much better output driving capabilities, lower noise and lower DC offset. The only drawback is their cost... if you buy in moderate quantities, they cost around $1.50 and $2.50 each. One other thing to worry about: they do draw more supply current than the TL07x. If you're changing a lot of chips in one piece of gear (like a console) you may have to beef up the power supply.
In the never ending quest to improve the sound of your studio, you can consider capacitor upgrades in your audio gear. A lot of less expensive gear manufacturers put lots of small electrolytic capacitors in the signal path. They're usually to stop a DC bias from passing between different portions of the circuit.
I recommend changing these capacitors to Panasonic HFS or HFQ series capacitors. I would also bypass any larger output capacitors with .01uf poly film caps, this will clean up your high end considerably. Mouser carriers the XICON brand, you'll have to get the 400 volt caps but they'll work fine.
Choosing an Oscilliscope:
Tektronix scopes are readily available at test equipment brokers (expensive) and ham swap meets (cheap) and are generally good investments. They are much more stable and reliable than scopes intended for the consumer repair field such as B+K, Leader, etc. A good model for audio is the Tek 465B or 475. Both are portable scopes with very good performance and they are rugged. Prices range from $900 from brokers to $200 at swap meets. If the CRT is in good condition a swap meet special is a good bet.
If it needs calibration, it can be done with a DVM and known calibration source and the excellent service/operators manual. The 465 series are easy to fix and keep in good calibration. Complete cal only takes a few hours the first time. You'll find that a 465 in good condition will keep within spec for several years before needing any adjustments. In any event a scope is not a shop's primary time (frequency/period) or amplitude reference so minor calibration deviations won't have any effect on your measurements. The best analog scope only have 2% amplitude and 5% time accuracy. If a particular measurement requires high precision other instruments need to be used in conjunction with your scope. Manuals are available from several sources including Manuals Plus here on the net.
Unless you need some of the plug ins available for the 7000 series lab scopes stay with the portables. The CRTs last longer and are more stable. The 7000 series, however has some great plugins such as the 7A13 differential amp or the band limited high gain preamps. They also offer on-screen range display. If your budget is bigger, the newer 2445 or 2464 are great scopes.
For general purpose work you may find a low cost "T" series is just what you need. For example a T932 can be found at swap meets for <$50 and offers most of what any shop would need including calibrated delayed dual sweep. H-P also made some good scopes but are not held in as high regard as Tektronix for general purpose scopes.
Some of the portable scopes from both companies had built in DMMs but most were pretty crude compared to your typical bench DMM. One model, the Tektronix 2236 had a very useful digital display that is super for analog work. It's frequency counter function would display the frequency of any triggered signal by computing the period between trigger point crossings which meant low freq.. signals were displayed with very accuracy instantly. Normally a conventional frequency counter counts a slow signal good accuracy and resolution requires very long gate periods (>10 sec). The 2236 is not as cheap on the used market even though it's new price was only $3,600. Expect to pay $1,000 for a good 2236. For about $650 you might be able to get a good 2246, another good utilityscope that has screen readout.
For new scopes look to the TDS 300 or 400 series which both offer very good performance for the money ($3,300-$8,400). A popular scope for bench work is the fairly new TDS 200 series digital scopes that cost $900 for a 60 mhz and $1,800 for a 100mhz version. While offering a lot of features few people working with audio find using a DSO is as comfortable as an analog scope like the 465.
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