The Heart of a Quality Tube Mic Preamp is a Good Quality Power Supply.
The old adage “You get what you pay for” is as true as it’s ever been.This includes microphone preamps. The vacuum tubes used in high quality tube preamps require a hefty power supply to give the tubes the high voltages they need for optimal performance. In the Edwards LE-10 series preamps, we use high quality tubes, top of the line audio and power transformers, and many other high end components, to come up with a World Class preamp.
At the heart of any quality tube gear is a good quality power supply. When designing the LE-10 series preamps we looked for a power supply that sits on the floor with one cord going to a wall outlet and the other going into the preamp. This type of supply is called a “brick”. Another type of power supply has a transformer that plugs directly into the wall with a cord leading to the device it is powering. This type of supply is known as a “wall wart”.
Since many studios use expensive Power Conditioners, we have chosen to use the brick form of supply, which takes up the space of only one outlet on a power conditioner. Having an external supply in the form of a “brick”, gives us protection from electrical surges before they even get into the preamp. In addition, the brick that we use is UL Approved (Underwriters Lab) which satisfies an important manufacturing requirement. This brick has an output of 24 Volts AC. That voltage is way too low to operate our tubes in their best operating ranges. Not only that, but the 24 volts is AC voltage and the tubes require DC voltage.
When designing our power supply, we consulted the RCA Tube Manual, the “go to” book for tube design. We discovered that the 12AX7, our main tube, operates best with plate voltages between 250 and 300 Volts DC. The plate voltage is the highest voltage in the circuit. If plate voltages are too low or too high, everything about the sound of the preamp suffers. But how do you get 250 or more volts DC out of a 24 Volt AC brick?
Since we are using an AC Voltage brick, we are able to use a transformer to step up the 24 volts AC to a higher value of 120 Volts AC. From there, we use a common circuit which raises the voltage and at the same time changes it from 120 Volts AC to 240 Volts DC. This circuit is called a “Voltage Doubler”. But, as they say on the infomercials, “There’s more”. The voltage doubler gives us a voltage that is more than double. The preamp components then draw current, creating a resistive load which drops the voltages to where they should be for optimal performance.
You may ask “How does all of this apply to a mortal like myself?” The answer is simple. If you are thinking about acquiring a microphone preamp, check the specifications very carefully. However, even specifications can be deceiving. The best thing to do is to hear one. In our case you can easily find our demos of music recorded through an Edwards LE-10 Microphone Preamp.
Many tube preamps do not use adequate power supplies. The result is poor frequency response, high distortion, low headroom and other issues. On the other hand, a well designed preamp with a very strong power supply will give you lots of gain, good frequency response, low distortion, plenty of headroom and a great sound.
The Edwards Audio Research Microphone Preamps have been described by reviewers and customers alike as “World Class”. It takes a lot of good engineering to come up with this kind of a product.
At Edwards Audio Research, we are always excited about our products, and would love to share more info with you. Simply click on the Contact Tab for info on how to reach us. And don’t forget to check out our Demos!
Mic Level and Line Level Audio as related to Mics and Mic Preamps
It seems like everywhere you go there are more audio devices. Wherever there is sound being amplified, there is a microphone picking up that sound. That mic could be on the your football coach’s “bull horn”. Another microphone could be 20 or 30 feet above a Symphony Orchestra as they record a Beethoven Symphony for future generations to enjoy.
Speaking of recording, the recording studios have a wide variety of mics. We all have cell phones. Although we are not conscience of the hows and whys of this miracle, it too has a built-in microphone. The microphone list goes on and on and includes: walkie talkie sets, hearing aids, electronic tuners, telephones, 2 way radio communication devices, and much more. There are microphones used in Houses of worship, police cars, radio and TV broadcast facilities and even on our laptop computers. There are microphones on my grandson’s favorite truck, the fire engine. Needless to say, microphones are seemingly everywhere.
All of the microphones we just mentioned have a preamp associated with them. As you might guess, Preamp is short for pre-amplification. You might say it’s an amplifier before the amplifier. If you think about how the sound gets from the singer on stage to the loudspeaker, the preamp has a vital role.
To explain this, we will assume, for our purposes, that there are three volume levels of audio. The lowest level is the output of a microphone, known as Mic Level. Mic level signals are amplified to the next level known, in the audio industry, as Line Level. Line level is not enough level to adequately drive loudspeakers, but will usually drive headphones or earbuds. Audio mixers require Line Level signals for a mix to happen. A Speaker Level signal is needed to complete the audio journey from the microphone on stage to the speaker to your ears.
All of the gear that microphones plug into, use microphone preamps. In the case of a recording or PA system console, a broadcast mixing board, and all the other gear we mentioned earlier, microphone preamps are built-in. Then, Why do I need a preamp? Perhaps the question should be Why do I want a Microphone preamp?
In high-end audio equipment, there are some very fine built-in microphone preamps. But if you want a world class microphone preamp, you look for a preamp that is not just a part of a console or mixer but a preamp specifically engineered to deal with those unique problems associated with microphone level audio signals. Some of these problems include electronic noise, RF (radio frequency) interference, 60 hz hum and other issues.
When you use an Edwards LE-10 Mic Preamp, your mic plugs into the LE-10 mic input and the output of the LE-10 plugs into the console or mixer’s line input. When this is done, the built-in preamp is replaced by the LE-10 and engineers agree that the sound of the Edwards LE-10 is superior. That’s why a growing number of audio engineers are using the Edwards LE-10 Vacuum Tube Microphone Preamps.
Insights on the Design of a High End Tube Mic Preamp
The first stage of amplification is very critical because microphones produce a low audio output. In that stage of amplification we use a 12AX7 tube. This tube is commonly used in the electronics industry because of its reliability, high quality and availability. What many people do not know about this tube, is that the 12AX7 is actually two tubes in one casing.
Tubes have filaments inside them, much like the filament that lights up an incandescent light bulb in your favorite lamp. The filaments can run on either AC or DC voltages. Our designer, Bill Bruins, chose to run the filaments of both the 12AX7 and the EF86 on DC voltages. Since AC voltages operate at 60hz, it is possible to add hum, which is also at 60hz, to an audio circuit by using AC voltages, and the associated wiring, to run the filaments. That problem is eliminated by operating the filaments with a regulated DC power supply.
The filament voltages for our tubes is 6.3 volts (either AC or DC). After much research and thought we have elected to run our filament voltages about four tenths of a volt lower, causing the vacuum tube preamp to operate relatively cool and giving longer life to the tubes. All of this without compromise to frequency response, signal to noise ratio, and all the other audio specifications.
The EF86 tube is legendary for its fine frequency response and quiet operation. This tube is used, along with a Jensen transformer, to create a balanced 600 ohm output feeding into a Neutrik 3 pin XLR connector.
Tubes are noted for their warmth and musical sound. In every phase of design we have built circuitry that makes the tubes operate at their optimal values. Our tubes are user changeable. And because of the negative feedback circuitry designed into our mic preamp, the LE-10 works extremely well with a wide variety of tubes, ranging from moderately priced tubes to expensive ones.
You can find and read about Vacuum Tube Microphone Preamps all over the internet. But, hearing the sound of an Edwards LE-10 series preamp will quickly convince you that THIS Microphone preamp should be the next item in YOUR studio…………… The Mic Pre that will make your music come to life!