Presence and Voicing of the Mesa Boogie Dual Rectifier 3 Channel Amplifier

Posted: February 7, 2015 in Mesa Boogie Dual Rectifier, Tube Amps
Tags: , , , , , , ,

Convention, hype, and a confused amp tech’s handwriting have mucked up the exact nature of the Presence control settings on the Dual Rectifier 3 Channel amp.  For the non-geeks, I will say what the real nature is and then dive into detail afterward.

For Raw Mode and Modern Mode in Channels 2 and 3, the presence control is a treble attenuation circuit.  Modern has no negative feedback.  Raw has all possible negative feedback; as if someone turned the Presence all the way down and then also added the treble attenuation control to further reduce the high frequencies.  Turning the Presence all the way up in Raw is about the same as turning it all the way down in Vintage.

Vintage is designed in a more conventional fashion.  The Presence control adjusts the amount of negative feedback going to the phase inverter.

The switching is all shown on the Voicing page of the schematic, as shown:

Voicing Circuit MBDR

When a dirt channel is selected, either X (CH3) or Y (CH2) becomes a ground.  The pole of the switch at that ground point is connected to a “Mode” pole and the relays on that path are closed.

Relays 8 and 9 are to switch the “Presence” pot between its functions.  Relay 7 is for the connection of the feedback at the P.I..  Relay 6 is to enable a 2M2 blocking resistor directly after the circuit block for V1.  Using just channel 3, I’ll illustrate the way it works:

  • Modern: Relays 9 and 7 close.  The Presence pot is an attenuator and the feedback is shorted.
  • Vintage: Open circuit.  All relays are open.  Presence pot controls feedback.
  • Raw: Relays 9 and 6 close.  The feedback is activated at the P.I., but disconnected from the Presence pot.

As previously stated, since the pot is disconnected from the feedback circuit in Raw mode, the feedback level is almost the same as turning the presence completely down in Vintage Mode, but has slightly more treble.  A resistive component would introduce a further reduction in high frequency response.  When this component (the pot) is removed from the feedback in Raw, those frequencies have a very small boost by comparison to Vintage with the pot fully counter-clockwise.

The Presence pot now functions as a treble attenuator; the same as Modern.  Additionally, the 2.2M resistor, with treble boost, is enabled as the sole grid resistor coupled to the output of V1.  This reduces the signal coming to the gain pot, except for the frequencies above the treble boost knee.

The combination of negative feedback, an attenuating “presence” control, and increased grid resistor will: reduce gain, reduce distortion, add stability, and, in combination, are like a band pass filter that reduces all the guitar signal frequencies.  Bass and mids are reduced in the preamp and the treble accented.  As previously stated, all frequencies are reduced by the feedback at the P.I., but the treble is most affected, since it is dominant.  The faux-presence control placed after the tone stack determines the amount of treble allowed into the power amp.

Due to value and placement of filter capacitors in the P.I. and power amp, the shifting between modes of feedback do not cause instability.  I feel the switching, voicing, and presence circuits are a very clever and inspiring use of different parts of the amp to get a multitude of responses for several types of guitar styles.

Mesa Rectifier Presence Controls is a closely related article about the way the Presence pot changes for each mode.

Here is a copy of the schematic for educational purposes: boogie_dualrectifier_3ch_solo_head

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Comments
  1. Alejandro says:

    this is great, i’ve been busting my brain trying to unveil the truth behind that circuit

    Like

  2. […] looked at the input and Clean, the Mode voicing and Gain control, V2 and V3, EQ and modes and Presence.  I am going to bring it all together and take a less detailed, wider, view of the […]

    Like

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