Mesa Rectifier Presence Controls

Posted: November 16, 2015 in Mesa Boogie Dual Rectifier, Modification
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This is in regards to the Presence circuits for a 3 Channel Dual/Triple Rectifier and the Roadster/Road King.  I do not have information on the Reborn or miniature versions, though I suspect they kept it pretty much the same.

Mesa came up with a clever Presence circuit for the 2001 3 Channel Rectos, which carried over to the Road King and Roadster on their relative channels.  For the sake of flexibility, the Presence pot is flanked by relays to swap the function between modes.  The modes are used to determine power amp response due to negative feedback and the amount of distortion it is capable of producing.  From most to least: Modern, Vintage, and then Raw.

Using Modern, the negative feedback is disconnected and treble frequencies are being attenuated by an RC filter connected between the the Presence pot and the wiper of the Treble pot.  Using Vintage, negative feedback is being fed from the transformer to the PI.  The amount of feedback is controlled by the Presence pot, which also sets the frequency cutoff; it has dual functions as a resistor divider and a filter.   Raw is a special case and is discussed on its own below.  I will be using Channel 2 for the examples, but I will contrast the circuit differences with channel 3.

Modern
Modern mode presence control circuit, Ch 2.

Modern mode presence control circuit, Ch 2.

When Modern is selected, the RC filter creates a roll-off which moves about from an axis at 1.6 kHz.  The 25 k pot controls the depth of the cut.  Since it is connected to the treble wiper, it affects all the sound being passed to the channel volume.

Channel 3 has the same overall form with different values.  Its axis swings from 2.24 kHz and the pot is 100 k, which gives a wider, longer response.

I’ve read dismissive comments about this attenuator on message boards.  Considering the lack of negative feedback, a traditional presence control is impossible for Modern.  Having the ability to tailor the similar frequency bands, with the same pot, when switching between modes, is awesome.

Vintage
Presence circuit in Vintage Mode, Ch 2.

Presence circuit in Vintage Mode, Ch 2.

When Vintage is selected, the relays place a ground point where the cap and pot meet in Modern mode; lug 1 floats (which acts the same as if it was tied to the wiper).  The wiper is moved to the negative feedback loop and the entire pot is placed in parallel with the tail resistor of the phase inverter (PI).  This forces the pot and resistor to interact and reduce to equal value.  The pot is free to change value based on the position of the wiper, but all the values are reduced from their relative positions due to the parallel resistance.  The resistor is 4.7 k and the pot is 25 k.  This makes the maximum value of the pot and resistor 3.96 k when Vintage mode is used.  The feedback resistor and the tail resistor form a voltage divider for AC signals, while the pot is a variable filter.

When the pot is fully counter-clockwise, it is removed from the circuit and the wiper cap is connected to ground.  The 4.7 k tail resistor jumps to its stated value and is now in parallel with the cap.  This creates a couple of different changes, by creating partial bypassing to the resistor and providing a path to ground for the majority of the negative feedback.

The above is why turning the Presence back a tiny bit from fully clockwise creates such a dramatic change to the sound.

Another bit of filtering is regarding the remnants of the Modern/Raw Presence circuit which is still working during Vintage.  The RC filter on the Treble wiper is shunting frequencies above 1.6 kHz to ground at all times.  On Channel 3, the frequency is 2.24 kHz.  These filters partially account for the difference in brightness between the channels.

Contrast

The difference in the way the Presence works between modes is that Vintage is filtering highs in the power amp and Modern is filtering them in the preamp.  Negative feedback causes the power amp to reduce distortion and gain across the entire spectrum.  The Vintage Presence control enables a person to allow some frequencies to pass more easily.  The Modern mode Presence control allows a person to select the maximum frequency to pass along to the power section, which has no feedback and, therefore, no reductions.

Each mode’s Presence control performs essentially the same function, but in different ways, with different responses.  The power amp set to Modern will have a greater emphasis in the mids, with less low bass and less brilliance.  Vintage will have a flatter power amp response and the more high and low frequencies will be amplified.  While it is an audible difference, the feel is affected most of all.  Modern doesn’t have much in the way of touch-sensitivity and stays tighter by comparison.

Raw

Raw is a hybrid between Modern and Vintage.  It has negative feedback, but the pot is not part of the circuit.  The 47 k feedback resistor and 100 n capacitor reduce distortion and gain for all frequencies above 34 Hz in the power amp.  It receives more feedback than Vintage, since the tail resistor is 4.7 k and provides a greater amount of feedback from the potential divider and has a flatter response.  The pot uses the Modern circuit to attenuate high frequencies.

The combination of filtering and a relatively larger amount of negative makes Raw the most sensitive to player dynamics.  If any power amp distortion is being produced, it will fade quickly when the Gain control is between off and Noon.  Going past Noon will provide a tighter, less saturated distortion that isn’t too unlike Vintage, but the feel and sustain are different.

Tone Matching

The manual says the difference in taper between channels makes it possible to match sounds between them by observing a 10:00 setting on Channel 3 as the same as the maximum setting on Channel 2.  This only applies to Modern and is still not entirely true.

  • Modern: The axis by which each channel controls their respective frequencies is different.  10:00 on Channel 3 will be close to maximum on Channel 2, but would be brighter, and can never be completely exact without modification.  The cap on the Channel 3 Volume control also shunts highs to ground, making matching more difficult.
  • Vintage: Each pot is reduced in value by being in parallel to 4.7 k.  The 100 k pot on Channel 3 creates a slightly higher overall value than the 25 k pot on Channel 2 (4.48 k and 3.96 k, respectively).  Therefore, the difference in value would mean rotating the Channel 3 pot slightly behind, or Channel 2 slightly ahead, to match them.  However…..
  • All: The differing RC filters between the treble wipers and ground makes Channel 3 brighter than Channel 2 at all times and, again, can not be exactly matched without modification.  To match settings as closely as possible, the Channel 3 Presence would need an even slightly greater reduction in regards to Channel 2 than would be calculated by the positions of the pots in relation to the value differences.

Rather than trying to match sounds, Channel 3 could be considered a “Bright Channel” and used appropriately.  If a person performs the Presence Mod to match channels on the Pre-Reborn Dual or Triple models, the treble-cut RC filter needs to be changed and the Channel 3 Volume capacitor needs to be removed to be an exact match.  The Road King/Roadster do not have the Volume cap.

A Few Last Words

Mesa states in the manual that Channel 3 of the Dual and Triple Rectifiers are tailored specifically for the Modern mode.  It seems to me that the combination of greater midrange emphasis of the Modern power amp voicing, the mid-boost in the Channel 3 tone stack when using Modern, and the enhanced brightness of Channel 3 from the custom voicing are meant for Channel 3 to be used for soloing.  The amp itself is attempting to create similar conditions in feel and distortion as using a pedal to boost the signal.

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Comments
  1. […] 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 system as a […]

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  2. Shemham says:

    I came back to this very informative post after trying the Modern mode RC filter with a SLO design (with negative feedback). It’s interesting how it shapes the sound. It doesn’t simply cut the highs. It also boosts lows and more than subtly. With 22k/3.3nf filter the sound is pretty dark and lows receive more space. With additional resistance such as 22k/3.3nf/68k the effect of filter reduces. I think this is actually one key difference between SLO and Rectifier series tonewise. The filter voices Rectos much darker and bottom heavy compared to SLO.

    It’s worth to note that the 2-channel Rectifier had to channel cloning feature which allowed using two presence controls – one for the RC filter and one for the NFB. This allowed much closer tone to SLO. This also makes me wonder whether the early Rectifier revisions had smaller cap in the RC filter. That would explain less lows and brighter tone.

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