Vacuum Tube Stereo Microphone Preamp

(C) 2010 G. Forrest Cook

Introduction

This microphone preamplifier is designed to interface a dynamic microphone directly to a computer's sound card input. It can also be used as a mic preamp for mixing boards and tape recorders. The circuit is configured like a mixing board input strip so that each microphone has its own bass and treble cut/boost control. With a larger power supply, this preamp could easily be doubled to provide four channels.

The project was inspired by your author's attempts to record live audio using various small solid-state mixing boards with built-in mic preamps. The preamps all produced high levels of hiss and a quieter mic preamp was desired. The hiss problem is multiplied when making a multi-track audio recording since each new track adds to the combined hiss level.

A number of different tricks were used in this project in order to keep the noise level low and the 60 cycle hum to a minimum. These include powering the tube filaments with DC, isolating the power supply ground, having good filtering on the B+ line and isolating the microphone input with a transformer. The result is a circuit with excellent performance.

Warning

This project uses potentially dangerous high voltages including 160 VAC and 220 VDC. The project should only be attempted by someone who has experience working with high voltage circuitry. The power cord should always be unplugged when working on the preamp. The circuitry is designed to discharge the capacitors when power is removed, but it's always a good idea to short out the electrolytic capacitors before working on the preamp.

Connections

Power Input - grounded 120 VAC
Microphone Inputs - Low Impedance Dynamic Mics
Line Outputs - 50K, 5V P-P (approx)

Controls

Power On/Off
Bass (2 channels)
Treble (2 channels)
Volume (2 channels)

Theory

The microphone signal is sent via a balanced and shielded mic cable to a female XLR jack and microphone transformer in the preamp. The mic transformer serves a dual purpose, it interfaces the Low Z mic signal to the High Z vacuum tube control grid and it converts the balanced / ungrounded microphone input signal to the unbalalanced grid signal. The mic transformer was salvaged from a 1970s vintage solid-state Shure mono mic preamp, other medium to high impedance transformers should work here.

The 6AU6 tube is wired as a high gain class-A pentode amplifier. The cathode circuit has a 100uF bypass capacitor in parallel with a 100nF capacitor to provide a wide signal bandwidth. The cathode bias is set by the 2.7K resistor. The screen grid (pin 6) is pulled high via a 220K resistor and is AC bypassed with a 100nF capacitor.

The output of the 6AU6 pentode stage is DC bypassed with a 50nF capacitor and sent to a Baxandall tone stack. The output of the tone stack is fed to one 12AU7 triode section wired as a class A amplifier. The 12AU7 stage acts as a buffer stage for the tone stack and provides some drive for the volume control and output.

The power supply was built in a separate enclosure with a four foot power line, this keeps the power transformers from introducing hum into the preamp circuitry. The high voltage supply uses a 160VAC transformer to supply AC to a bridge rectifier and pi-network ripple filter. The filter capacitors are fairly high value (100uF), lower values produced audible hum from the preamp. A 1K/22 uF RC filter is mounted in the preamp box to further filter the high voltage DC and remove any noise picked up in the remote power supply cable.

The filament supply produces a floating 12.7VDC source. The 15VAC is sent to a bridge rectifier and 2200uF filter capacitor to provide DC for the 7812 voltage regulator IC. The 7812 output voltage is raised 0.7V via the 1N4002 diode in the ground reference lead, it produces 12.7V for the tube filaments. Resistance from the remote power supply wire drops the voltage to a perfect 12.6V. The 12AU7 tube's filament is driven directly from the 12.6V supply, the two 6AU6 (6.3V) filaments are wired in series and driven by the 12.6V supply. The only ground connection for the filament circuit comes from the center tap between the two 6AU6 tubes, this provided the lowest hum.

The three tubes consume 450 mA of current so the 7812 regulator should be able to power two of these circuits (4 channels) if it is properly heat sinked. The 2200uF capacitor on the input side of the 7812 would need to be doubled when making a higher current filament supply.

Construction

The power supply components were mounted on a 4"x4" electrical utility box, a standard romex style clamp was used to secure the AC input and DC output cables. One of the transformers and the choke were mounted on the outside of the power supply box. Note that the AC ground was only connected to the power supply box frame and the DC ground was only connected to the preamp box frame. The preamp box gets its external ground connection through the computer or mixing board that it connects to. This configuration will eliminate a hum-producing ground loop.

The preamp components were mounted in a pair of 4"x4" electrical utility boxes, the two boxes were bolted together and holes were punched out between the boxes for the internal wiring. An aluminum plate was fashioned to fit inside of each box, the plates were screwed behind the three knockout holes in each box and were drilled out for the tone and volume controls. Knockout holes were removed from the tops of the boxes to alow mounting the three tube sockets. Knockout holes were also removed for the two XLR jacks.

Solid box cover plates were used for the bottom of the boxes and stick-on rubber feat were attached to the plates. A number of terminal strips were added inside of the boxes for connecting the various resistors and capacitors. The wiring is somewhat crowded, especially in the box with the two tubes. Beginners would be advised to use a larger box.

Parts

Most of the parts used in this project came from my junk box. The transformers may be difficult to find, transformers with similar parameters may be substitued. If you can't find a 15V transformer for the filament circuit, a more common 16V unit can be used. The microphone input transformer was scavenged from a 1960s vintage Shure solid state microphone preamp, a single-ended tube output transformer or even a small filament transformer would probably work as a substitute.

The Strat Monger lists a number of suppliers for vacuum tube parts. eBay is a good place to find tubes and other parts. A number of different 6AU6 tubes were tried until a pair with low microphonics was found. There was a significant difference between tubes in a batch of around ten tubes. Luckily, 6AU6 tubes were mass-produced in the 1950s and 1960s and they are still available for low prices.

It should be possible to use common 12AU6 pentodes instead of 6AU6 pentodes, the two 12AU6 filaments should be wired in parallel with the 12.7V supply and the filament supply ground should be moved to the center tap (pin 9) of the 12AU7 filament.

Use

Connect your microphones to the preamp using good quality shielded mic cables. Connect the output signals to a computer line-in jack or the line input of a mixing board. Audio should be monitored on the computer or mixing board if possible. Set the preamp's tone controls for the best sound and adjust the output levels so that the signal does not clip.

A comparison was performed between this preamp and a Behringer Eurorack UB1202 mixer using Shure SM57 mics. One microphone was sent to one of the UB1202 mic inputs and the other was sent through the tube preamp and into one of the UB1202 line inputs. Audio was monitored from the UB1202 headphone circuit. The levels were set for equal headphone volume between the channels. The difference between the two channels was not subtle at all. The hiss coming from the Behringer's preamp was quite noticeable and the tube preamp's hiss was almost undetectable. A lot of audio detail that could be heard on the tube channel was completely obscured by the noise in the Behringer's mic preamp stage.

One weakness with this circuit comes from tube microphonics. The 6AU6 is normally used as an RF tube, not an audio tube. The high gain of the 6AU6 makes it produce a lot of microphonics. Tapping on the side of the tubes produces a fair amount of sound and the preamp should never be placed close to a loud sound source such as a guitar amplifier. In environments where loud sound is unavoidable, such as in a room with a drummer, it would advisable to locate the preamp box inside of a padded case for sound shielding.


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