Balanced Lines and Attenuators

Balanced Lines - Behringer Mixer Xenyx 502

Balanced Lines and H-Padsbalanced lines

Anyone doing audio work is going to come across balanced lines in long cable runs for either input or output and having a degree in radio technology is the best way to start but in all probability you’ll still at some time or other end up with a noisy network which can be a horrendous task to eradicate.

There are four things to remember when running any cable over a considerable distance:

a) is wireless an option? In most cases the user or console may be static but not so with a lead singer, vocalist, your local pastor or person setting up a large audio system. From my experience for wired as much as possible. We will show you how. Decent wireless is extremely expensive to get the same quality as wired.

b) with wired systems always try to get the signal as high as possible before the feed – this may use an active system which implies a high input impedance to low input impedance at +dBU or 0.775V RMS in audio engineering speak or use transformer coupled devices. Musos prefer transformer coupled devices because they believe it adds colour to the signal. This is true but not always the right kind. Good quality audio transformers are expensive but if you are a DIYer this may be a better alternative than active. (keeping costs down).

Balanced Lines - Behringer Mixer Xenyx 502
Behringer Xenyx 502

c) audio works best when coupling low impedance to high input impedance from output to input, audio amplifiers to the designed load and never lower. In the input stages there is a certain give or take (audible but not always damaging) but not so in the power amplifier where unnecessary stressing will cause damage.

d) low voltage, high input impedance signal runs over three meters or ten feet the engineer will always ensure that the cable run is balanced. What is balanced?

e) and there is fifth point as well! Balanced lines are usually taken from XLR and TRS plugs and sockets. Always ensure that these sockets are indeed balanced because as luck would have it they can be used in a stereophonic setup as well. Consumer audio will have a stereo 6.3mm socket perhaps – the tip will be channel one, the ring, channel two, sleeve ground. These are not balanced.

XLR - usually used in balanced lines
XLR – usually used in balanced line systems

The cable consists of a shielded twisted pair with the negative (-) and plus (hot) running at an equal but opposite polarity ensuring that any noise, e.g. electrical interference, is cancelled out at the output end. Very low impedances are not as prone to electrical noise pickup as high impedance runs from instruments and microphones such as used at a live event. Engineers try to run this at the lowest possible impedance with the highest drive possible, without distortion.

In radio work it is essential that all stages inside the receiver or transmitter are as closely coupled and matched to each other as possible. In audio we feed high low input impedance into high input impedance, in other words as long as the preceding stage is at a lower impedance than the following stage you should be alright. Electrical engineers know this as line transmission – cable matching can be critical, which obviously the includes the impedances as used in a telephone network.

balanced lines
Balanced Lines

The dBu, dBV and dBm are terms heard regularly in the audio field although the dBm is a measurement of power. +4dBu and -10dBv are the most commonly seen figures, one applied to professional audio and the other, consumer audio. dB is all about references. I cannot just say this car is better than that car – I need to quote figures. Nowadays fuel economy is more important than horse and torque. Sometimes 🙂 . When we talk electronics, an engineer won’t just say that this voltage is 30V because it has no meaning. A voltage is in fact a potential difference which must imply more than one point, so a better way to say this would be, “the potential difference between point A and point B is 30V”. Even better, “the potential difference between point A and B, which has a resistance of 30 Ohms is 30 Volts.” Now I also know that 30V across a 30 Ohm resistor equals one Ampere. When we state dB we are making a reference to something and quite simply, dBu is a professional standard which implies that 0 dBu = 0.775V R.M.S. (Route Mean Square)  and 0dBV = 1V R.M.S. Note that there is no reference to the circuit load resistance (load impedance) that this potential difference (alternating current) is measured across. A measurement that does take this into account is the dBv which is 0.775V R.M.S. across 600 Ohms. This is an old term used in ‘telephone circuitry‘. In essence:

  • 0dBu and 0dBV are referenced to a non-terminated load.
  • 0dBv is across a terminated load (600 Ohms).

For reference I will include that 0dBm = 1 milliWatt dissipated across a 600 Ohm load caused by the 0.775 V R.M.S. signal across it.

  • Power = I^2 * R . I = V/R.
  • I = 0.775V / 600 = 0.0013A or 13mA.
  • P = 0.0013A^2 / 600 = 0.001W or 1mW.

I like the 0dBv because it is all self explanatory. But it is not to be. Consumers use 0dBV and professionals use 0dBV. In fact the standard is based on +4dBu and -10dBV. You want more information? Sengpielaudio has the answer. In South Africa ‘piel’ is a dirty word. Jokes aside, for the budding audio adventurer the website is very informative.

Another extremely informative website is that of Rane (you should definitely know them if you are planning the professional route, some of the most incredible equipment comes from them). Here you can get all the gen on the expressions used to make you sound like an expert. And all the gen that you should look out for that tells you something is dodgy. Audio Specifications, by Rane.

Balanced Lines from Unbalanced Sources

A question I have often come across in the forums whilst browsing for a simple padding circuit is “How to make an RCA socket or signal” balanced. I covered this last because from my own experience you won’t find RCA sockets on a professional mixer except for CD or Tape input. What happens if you control the mixer by remote and the tape deck is situated 30 meters away from the mixer. Not really something you would want to do but who cares, an interesting question in any event. The first thing I would do is read up on this very interesting article, once again by ‘no-nonsense, tell it is as it is” Rod Elliott. Here he explains how to build a Direct Injection circuit which allows for a non-balanced input to be output balanced through the use of two opamps (TL072). I have built this circuit and it works exceptionally well. Be cautious of phantom powering and ground lift as he has stipulated. For the more ambitious there are certain ways to circumvent the issues arising but it may be advisable to consult a professional electronics person for this task. Phantom power is used to supply +48V DC to condenser type microphones. In many cases condenser mics offer superior performance to dynamic microphones.

Balanced Lines and Attenuation

So now you have built your DI Box but are infuriated to find that the signal output s too high for the input, which invariably is going to be a microphone input. In the good days as a consumer you would add a double ganged pot with the wiper arm connected to the input of the amplifier (just like in a pre-amp). Alas, pro-audio has it’s advantages but one of them is not ease of change.  I will refer you again to a link:  http://www.electronics-tutorials.ws/attenuators/attenuator.html. Once you have an idea on what an attenuator is all about then you can move on to the calculator at http://www.nu9n.com/tpad-calculator.html – for your reference a T-Pad and H-Pad are similar. Two T-Pads make up an H-Pad and this is used in balanced line networks.

A problem I had recently was to match a computer output (computers invariably have the most unsophisticated audio outputs known to man) to the Pro Art MPA digital pre-amplifier. It has a balanced input stage so I built a simple H-Pad device. If you want a picture and schematic please contact me via these pages.

The balanced line approach is possibly pretty much confusing to the newcomer but the reality is that if you know Ohm’s Law and how a Wheatstone Bridge works you will win the battle. Yes, the battle will be far from over until you have enough experience under the belt in tackling typical P.A. issues, a basic grasp of the fundamentals is very relevant. Doing is the most important aspect to learning, not just reading. Balanced lines are critical to the performance of any public address or live audio system.

 

PAIA Corporation

admin 2014-01-10 07:04:00
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Company/Business PAIA Corporation
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Core Business Manufacturer and Online Reseller of Electronic Kits
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About PAiA
Various PAiA Images PAiA is a leader in the development of kit products for the musician and electronic experimenter. The love of technique drives us to create for others, and to share what we have learned. Entertainment, encouragement and education are our goals. Our company both creates its own products, and works with innovative developers in the community to bring a broad spectrum of new products to the market. PAiA is a Texas corporation with offices in Edmond, Oklahoma and Austin, Texas. Our unique name is trademarked and owned by the John Stayton Simonton, Jr. Revocable Trust and used under license.


Low Voltage Tube Circuits

Some Low Voltage Tube Circuits

Doing research on the web it’s not surprising to find the many thousands of requests covering circuits which utilise the starved plate technique. As mentioned previously, I am not an advocate of this approach but it does work well – as long as you do follow the tried and practiced route, meaning using circuits which do what they are supposed to do and designed by engineers whom know tube technology.

PAIA Tube Circuits

In line with our website offering, we try to bring you really great weblinks of companies that supply kits, parts or accessories. The logo below this is of Paia Corporation, based in Texas and founded in 1959! This is just such a company.

Paia - Low Voltage Tube Circuits
Paia Corporation – Excellent Kits at very realistic prices for the musician and DIYer

If you really enjoy intuitive websites Paia is exceptional. Although we are by no means affiliate to this company they bring a big chunk of knowledge to the musician and DIYers corner – for the thousands of like minded individuals who enjoy building their own gear this gem of a website is one you must bookmark. From my own experience you will not be able to source cheaper and have the same result as purchasing the kit. There are also lots of forums where DIYers have purchased the kit and applied modifications either for customisation or to improve (kits cost money, manufacturers try to keep their and your costs down).

I recently built a copycat of the PAIA 9407K (see here) and besides for some critical comments from possibly more subjective testers found the pre-amp did what it was supposed to do. I have not tested the Behringer MIC-100, 200 or 2200 although I am trying to get my hands on a 2200. The Behringer mic preamplifiers no doubt are based on the same circuit design and funny enough many of the reviews on “Musician’s Friend” are most encouraging. Remember that these are musicians doing the testing – it’s not an audiophile review.

The circuit in the Paia uses low noise opamps which either drive or bypass the ECC83 tube which is used as a low voltage (starved plate) voltage amplifier. Supply rail is derived from a charge pump circuit which multiplies the 15V DC to about 45V DC. I don’t think there is anything possibly unique about this amplifier except running a tube which is not designed to run at 45V to get the desired outcome. For experimenters, there is a distinctive sound – this makes it unique. Fiddling with replacement tubes in this stage, called “rolling” is a true test as to the merits of tube sound as boasted by tube followers.

There is however two other circuits which deserves some mention – one from an old Elektor magazine where the tube drives a MOSFET class A (see here) – this circuit deserves mention because it serves as a stepping stone to the experimenter. It does use higher voltages and starts running into the domain which I like, using the tube where it starts acting as a tube and not necessarily as a low gain non-linear device. The Roger Gomez Headphone Amplifier is also a hybrid (see here) but is a high quality circuit using starved plate again. This circuit uses the ECC82 or 12AU7tube. The distinct differences between the AU7 and the AX7 lies in the gain. Many tube starved plate experimenters prefer the 12AU7 for this purpose and in my mind, depending on what you opt for (Paia or Gomez) bare in mind tube gain makes a big difference. Both are complete projects and both are simple to build, baring in mind that the Paia circuit does use many more components and a higher voltage.

Tube circuits – Oxy Morons use CMOS

If you are experimenting with these circuits there are some simple rules to follow – know your limitations. Never build circuits using high voltages if you are inexperienced. The low voltage starved plate circuits here are not exhaustive, there are many similar circuits on the web – always take care when building a circuit and ensure that you have either a current limiter in circuit or a fuse. Although you may use low voltages, static electricity will damage CMOS devices. Earth your hands, do NOT take the devices from it’s protective holder until you are going to use it. Although tubes are robust they will break if dropped and if the heater voltage is exceeded. Although the current draw of preamp tubes is in the order of a hundreds of micro or a few millamperes, going over their rated value can easily strip them of their magical powers.

Non Tube Circuits and knowing your own limitation

Talking about limitations. When I was about 13 a friend of mine plugged his 3V transistor radio into the 240V mains socket. I offered to repair it.  Now that is not knowing your own limitation.