"The 'Shroom"

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The 'Shroom is an innovative balanced microphone preamplifier,
designed to add that special tube "warmth"
for professional and semi-professional use.

Characteristics:

• Overall voltage gain: approx. 1000 (60 dBv)
• 30 dBv attenuator switch at output, if less gain required.
• Balanced input, balanced output.
• Output may optionally be operated single-ended, using either phase.
• Input impedance (balanced) = 1200 ohms.
• Output impedance (balanced) = approx. 10k (60 dB), or 1k (30 dB)
• Frequency response:
  (balanced 10k output load, 60 dBv setting)
  • High-frequency 3 dB corner: 37.5 kHz.
  • Low-frequency 3 dB corner: 22 Hz
• Output Headroom: approx. 32 dB (ref. 1V RMS)
• Unique direct-coupled input. No transformers or capacitors between mic and first stage.
• Self-derived phantom power for condenser microphones. Also usable with dynamic mics.
• Low AC power requirement (less than 20 watts).
• Number of tubes: 3 per preamplifier section

  Qty:	Type:	Description:	Function
  2	717A or 6AK5	Low-noise Pentode	Direct-coupled input stage
  1	12AT7A/ECC81	Dual triode	Second stage RC-coupled amplifier

• Inputs: Lo-Z XLR-3
• Outputs: Hi-Z (60 dBv) or Lo-Z (30 dBv) XLR-3.
• Filament hum-balance pots to minimise hum.
• Internal DC balance potentiometers
• 8-ohm or 4-ohm operation
• Valves only in signal path. Semiconductor diodes used only for power supplies.

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Worst-Case Distortion


This is a spectrogram of a 440-Hz. sine wave at the output of "The Shroom" at a signal level just below the clipping point (approximately 44 volts RMS). This represents about +36 dB over 0 dB (reference: 1V RMS). As a point of comparison, this would represent a sound pressure level of +118 dB into a "typical" microphone with a sensitivity comparable to a Shure SM58. The sample was taken with the gain switch set to the 30 dB position, feeding a 10k balanced load, with a 600-ohm source impedance at the input.

Note that even harmonics are considerably suppressed (compared to the odd harmonics), due to the balanced nature of the preamplifier. A slightly greater reduction would be possible (if deemed desirable) by gain-matching between the two push-pull halves.

Note also the "cyclical" nature of the harmonic spectrum. This can be considered the amplifier's "voice-print" and, in a sense, defines its acoustic colour. At lower levels, the voice-print becomes less and less pronounced, becoming virtually "transparent" at low SPL levels. This graduated characteristic gives it the unique tube "colour,", or timbral shift with changing dynamics, with crispy clean performance at quiet portions of a song, and becoming richer and more penetrating at higher sound levels.

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Progress to date:

(May 1, 2002)
Here is a photo of the first prototype, consisting of two identical channels on a single small chassis. Note the mushroom-like appearance of the input tubes, Type 717A's, which were originally designed for low-noise microwave amplifiers in radar receivers. Functionally these are quite similar to the later miniature type 6AK5, which was similarly extensively used in low-noise high-frequency preamplifier applications. The difference in appearance between the two channels is due to different manufacturers (Raytheon on the left side, Tung-Sol on the right side). Performance-wise, the difference between the two was found to be insignificant.

One of the two transformers supplied filament voltage for the second-stage tubes, the classic 12AT7/ECC81, which sport a high-gain combined with the capability for higher output drive than the more usual 12AX7 types. The other transformer steps this filament voltage back up, for the higher voltages required by the tubes' plate circuits.


'Shroom Prototype 1, Top View

(Under the hood)
Here's what it looks like under the chassis. Two additional small transformers supply the filament voltage for the input tubes. Using separate transformers allows per-channel "hum-bucking" via the two small balance pots accessible from the top.

Note the direct connection from the input connectors (front) to the first-stage tube sockets. There are no transformers or capacitors in the signal path between microphone and first stage. Furthermore, the tubes themselves supply the necessary 24-volt phantom power source required by condenser microphones, with no resistor-capacitor networks to any external voltage source.

Note the comparative small physical size of coupling capacitors. This is due to the relatively low voltage B+ supplies used in this design, which is one of the factors allowing the "graduated voice-print" characteristic of this device. In spite of this lower-than-usual B+ supply, the unit still sports a whopping +36 dBv of headroom (referenced to 1 volt RMS). The attenuator (switches on the rear apron) is provided to accomodate other preamplifiers, matching devices or effects units requiring a lower input drive.


'Shroom prototype 1, underside

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Contact: You may contact me regarding this project via email. I'd be happy to hear from you, and any bug reports or suggestions for improvement are always welcome. To find out what other kinds of trouble I get into, check out my sites listed below:

Back to Tube Projects Main Page My main Dogstar Music site Fred's ZX81 retro computer page My Theremin Exploits Original music on IUMA.com ... and on Soundclick. Nice bio.