Dual Channel Tube Microphone Preamp
|Objective: I wanted to build a
tube-based microphone preamplifier from one of the classic designs of
the 1940's - 1960's. While searching the web for suitable
designs, I came across the schematic
and user manual for the Pultec
MB-1 Microphone/Booster Amplifier, hosted by Rafael Texiera.
|Dual channel version: By using more
compact components, I built two channels of the circuit into slightly
than Pultec needed for one channel. I did not change the circuit
in any significant way, but I did have to use different transformers.
Front panel : each channel has an impedance selector (50, 200, 500 ohms) and a 10 step gain selector (28 dB - 48 dB). Knobs and switches are mil-spec beauties from the surplus store. The 2U rack enclosure is the Sescom 2RU7. It is shipped completely disassembled, and consists of 2 x 0.125" end panels, 4 extruded rails, and 4 x 0.062" skin panels, all anodized aluminum. It is held together by eight screws through the end panels into the ends of the rails. The skin panels slip into slots in the rails. The box becomes acceptably stiff when it's all put together. It's not the most rugged box available, but it'll survive in the studio, or mounted in an SKB (or similar) portable rack box with a DAT machine for location recording.
Rear panel: balanced outputs on Switchcraft 1/4" TRS phone jacks, inputs on Neutrik XLR's.
Top view: Two identical preamp modules and power supply fit comfortably in 2U x 7" deep chassis. I probably have enough space left over to add a phantom power supply module, which I intend to prototype in the next few months. The turret boards and tube/transformer mount are attached to threaded aluminum standoffs, that are screwed into a nut captured by the chassis rail extrusions. The Sescom site shows assembly details. The tube and transformer mount is 6.7" x 1.5" x 0.125" Alumi-Trim anodized aluminum cut from a 4' strip bought at the home center. The 1" hole near the bottom allows mounting an octal socket for a plate-to-line output transformer such as the Altec 15095 15K:600 ohm.
Top view of one preamp channel, after adding output transformers: I originally omitted output transformers, because I did not need balanced output. However, the signal output voltage at the cathode follower's coupling cap was too high (>100V P-P) to feed into any unbalanced line input without major attenuation. NewYorkDave over at GroupDIY turned me onto a line of low cost impedance matching transformers made by Edcor (needs MSIE browser) in Carlsbad, NM. While their standard WSM 10K:600 model would have worked fine, Dave told me they would custom-wind a 15K:600 ohm version for the same price and delivery as the stock item. One week after calling Edcor, 4 freshly varnished 15K:600 plate-to-line transformers arrived by UPS for less than US$10 apiece delivered! Specified response is 20 Hz - 30 kHz +/- 0.5 dB. The WSM's are unshielded, open frame units, but I haven't had any issues with noise pickup, or feedback into other stages of the circuit. The spec'd power level is 500 mW, which is actually higher than the 400 mW spec Triad HS-50's Pultec used in the original. I mounted the transformers sideways over the octal socket hole, attached with a screw through one of the mounting ears which I bent at a right angle. With the transformers, the preamp puts out over 22V P-P into 600 ohms (unbalanced) at clipping.
Turret board after buss lacing, before mounting components. It's 6.7" x 2.125" x 0.125" G10-FR4 (Garolite) from McMaster-Carr. I worked out the layout myself, generally following the style I picked up from Doug Hoffman, who supplied the turret terminals.
Side view of one preamp channel. On the left is the UTC A-11 mic-to-grid input transformer. The shock-mount tube sockets contain a GE-labeled 12AX7 (Made in Gt Britain), and a Sylvania Gold Brand 5963 (industrial 12AU7). The Wima MKS 4 3.3 uF film capacitor couples the output of the cathode follower to the Edcor output transformer. The cap is hot-glued to a block of Delrin so it doesn't flop around mounted on top of the turret terminals. My 15 year old daughter Karen plaited the 6 strands of #24 teflon over silver plated copper wire that connects the balanced input to the impedance selector and input transformer. All half watt resistors are 1% RN60D metal film. I used a 5W 18K cathode resistor for the cathode follower instead of the 10W specified in the original schematic, because it only needs to dissipate <1.5 W.
Power transformer is a Triad R-4A, which produces 555 VCT at 40 mA, and 6.3 VCT at 2A with 120 VAC input. The rectifier, first two filter stages, and the filament reference voltage divider on the power supply board are shared by both channels. The 20 mA current needed for two channels still leaves a 100% margin for this transformer, but would drop the voltage a bit lower than the prototype. I used UF4007 silicon diodes instead of the 6X4 tube rectifier of the original and prototype to avoid its 22V voltage drop, along with its plate and filament dissipation. The power transformer and rectifier/filter board are mounted on standoffs threaded into the 1/8" aluminum end panel / rack ear. The transformer is far enough away from the preamp modules so radiated hum isn't an issue.
|Performance: As of April 2005, I
only know that the unit is quiet, the channels are well matched, the
frequency response is within 1dB from 20 Hz to beyond 20 kHz, and
it sounds fine
Shure Beta 58 and AKG D112 microphones I have handy. I'll do more
serious testing at Arpad's studio in a few weeks. We'll do some A/B
comparisons with his Soundcraft and Ampex gear and assortment of
and modern ribbon, dynamic, and condenser microphones. So far, I
am very excited about the the audio performance and visual appearance
of this project. May 15, 2005: Arpad and I
auditioned the Pultec with his Beyer M500 ribbon mic, a pair of
Nakamichi CM-100 condensers, and a pair of AKG D112 bass instrument
mic's. We recorded some electric guitar at relatively low volume,
with the paired mics side-by-side. There was no obvious
difference in tone between the Pultec and his Ampex MX-10 preamp, but
the Pultec was at least 10 dB quieter. There was plenty of gain
on tap for the low output ribbon mic. After another test round to
understand how it behaves on quiet acoustic and loud electric sources,
Arpad should be ready to start using it for session work.
Planned enhancements: Switchable pad and phase on the output. Engraved front panel. Switchable 48V regulated phantom power supply.
|Additional information: The as-built
schematic, turret board
layouts, and assembly sketch are here.
I intend to
publish a bill of materials as time permits. Of course, I'll
questions about this, or my other projects. I hang out regularly
with the other guitar amp builders at AX84
and Ampage, and the pro audio
crazies at GroupDIY.