Non-Linear Analogue Emulation Amplifier

A culmination of the work I started in 1994, yielding a first publication of 12ax7 simulation plots in 1998.

Maillet-GlassAudio98 PDF

These plots show an accurate rendition of your typical 12ax7 bias transfer curve, ie., simulated under the same conditions seen in Fender, Marshall and other vintage tube preamp circuits.

The transfer response corresponding to the usual Cathode resistor value of 1.5k ohms is taken from my tube amp book “Inside Fender and Marshall Tube Amps” and highlighted below:

IFMTA-book-pdf

This Common-Cathode Triode gain stage transfer curve is the benchmark I chose for defining tube-like behavior. To be clear, we are talking about large-signal response in the DC-transfer sense, ie extracted adiabatically and applying only to DC-coupled circuits.

The goal is to come up with circuitry that will produce the desired electronic transfer function, so far I have found two ways of doing it. In my opinion any suitable solution needs to meet two conditions: (i) the circuit has to be 100% DC-coupled internally (ie., no caps or inductors used to achieve wave-bending), and (ii) the circuit form needs to have an implementation that is practical in the designing of a greater (amplifier) circuit. It can’t just be a novelty circuit.

Ten years ago almost I came up with the following solution, one using an n-channel jFET device:

transferCurvature-TubeSimulation

From this idea, I built a couple of stand-alone “non-linear boosters” shortly afterwards and had some great success // but then put the idea aside to pursue other research … since then, I’ve learned quite a few things about vintage class-A EQ circuits (thanks to conversations with Roly Roper yrs ago), and recently found a suitable combination of gain/EQ/non-linearity circuit blocks to make this all work properly and logically

so here it is … my sonic high performer:

a Tube/Triode Emulating preamp // with tone controls not commonly seen in modern (SS) times …

featuring 6 physical inductors with a cut-boost BASS control, cut-boost sweep-able MIDS, and the usual TREBLE cut, GAIN and MASTER … controls that have an uncanny range to them, and in particular with the ability to adjust the degree of wave-bending independently of output (the Master circuit is linear)

to meet my design criteria any amplifier design at the very least should be able to produce a classic mid-scoop (Fender guitar amp) Frequency Response – as shown above here … but this design does more, in fact mid-scoop or boost – and continuously variable from 700hz to about 6khz …

this following simulation plot shows the range of adjust-ability for the semi-parametric MID filter section:

in addition, the amplifier has to produce dynamic Triode-Transfer emulation /… the amplitude-dependent wave-bending part

A compact 60watt TUBE-EMULATION amplifier can be built by boxing up a class-D power module with the TUBE-EMU-lator preamp … this wide-range EQ and tube emulating wave-shaper combination preamp makes for a surprising experience when driven into a good power amp and guitar speaker … the latest class-D modules running on TPA3118 IC’s sound very good

dumbfounded, buddy looked at it … said it sounded like a Marshall

… he meant an old Marshall



2 comments on “Non-Linear Analogue Emulation Amplifier
  1. Brienne R. says:

    Hi JC,

    Just curious how I can build or procure one of these emu-preamps?

    Many thanks,
    Brie

  2. jcm says:

    HI BRIE, THANKS FOR ASKING …

    AT THE MOMENT THE THING IS A PROTOTYPE AND I’M TRYING IT OUT AS MUCH AS I CAN (MAINLY AT REHEARSALS) … I THINK I WAS HAVING ISSUES WITH THE 65W LAPTOP SWITCHED-MODE POWER SUPPLY … CONVERSELY I COULD POSSIBLY BE HAVING ISSUES THE 60W CLASS-D MODULE I GOT OFF EBAY, MAYBE THE AUTOMATIC SHUTOFF KICKS IN WHEN I WAS DRIVING THE THING HARD // THO HARD TO TELL IF IT’S THE LAPTOP SWITCH-MODE PSU I’M USING OR THE CLASS-D MODULE … I’M NOW USING BIG BATTERIES TO SEE IF I COULD USE IT AS A SERIOUS BUSKING AMP – AND SO FAR SO GOOD … I DON’T THINK IT HAS ANYTHING TO DO WITH THE PREAMP SECTION AS SUCH, BUT IT JUST COULD BE THAT THE NON-LINEAR OUTPUT IS JUST TOO MUCH FOR THE DOWN STREAM SWITCH-MODE COMBO I HAVE FOLLOWING IT … I NEED TO TRY IT OUT WITH MY ACOUSTIC 361 POWER AMP CLONE AT SOME POINT

    ALSO, I’M NOT SURE IF I’M ALSO HAVING ISSUES WITH THE MID-RANGE PART OF THE SOUND // IT’S A LITTLE TOO MIDRANGEY SOUNDING AT TIMES … BUT THAT COULD BE DUE TO THE OPEN BACK SPEAKER I’VE BEEN USING WHEN PLAYING IT ON GUITAR (1X12 CELESTION 70/80 MINI CAB AND VOX DA20 SPEAKERS – THE LATER BEING OK AS FAR AS MID-RANGE GOES) … I HAVE YET TO TRY IT ON A MARSHALL 4X12 AND OTHER GOOD CABS // OTHERWISE THE DYNAMIC GRIT I’M GETTING IS VERY PALATABLE AND TUBE-LIKE …

    AS WELL, IT TURNED OUT TO BE “VERY” GOOD AS A BASS AMPLIFIER GOING INTO A PROPER BASS CAB … SOMEBODY AT THE STORE I TRIED IT AT WANTED ONE RIGHT AWAY UPON HEARING IT … SO, IT’S SHOWING PROMISE // JUST GOTTA FIGURE OUT WHAT TO FOLLOW IT WITH TO GET BEST RESULTS … JUST TO COMPARE, “QUILTER” FOR EXAMPLE HAD TO DESIGN A SPECIAL SWITCH-MODE PSU TO GET HIS DESIGNS WORKING RIGHT … AS FAR AS I CAN TELL, SOME CARE IS NEEDED HERE – MIGHT (LIKELY) BE WE CAN’T JUST PLUG THIS PREAMP INTO ANYTHING AND EXPECT MIRACLES … PLUS, I’M NEW AT THE SWITCHED-MODE STUFF // MIGHT HAVE TO BE A LINEAR POWER SUPPLY, AND QUASI-COMPLIMENTARY (OLD-SCHOOL) SS POWER TO GET OPTIMUM RESULTS

    I NEED TO BUILD A STANDALONE VERSION OF THIS SO I CAN TRY THE PREAMP ALONE INTO OTHER POWER AMPS TO SEE HOW THINGS GO, THOUGH AT LEAST ANOTHER PERSON WHO UNDERSTANDS MY IDEA BUILT WHAT HE DUBS A “TUBE AMP KILLER” … I JUST ORDERED ANOTHER BATCH OF INDUCTORS FROM DAVID GEREN AT CINEMAG AND WILL BE BUILDING A STAND-ALONE UNIT AT SOME POINT THIS WINTER … IF I THINK IT’S SAFE ENOUGH FOR SOMEONE ELSE TO TRY OUT I’LL CONTACT YOU AND SEE IF WE CAN GET ONE IN YOUR HANDS … LIKE I SAID, IT’S VERY MUCH A PROTOTYPE WITH ORIGINAL EQ FUNCTIONS AND DYNAMIC SHAPING – IF YOU’RE COOL WITH THAT MAYBE WE CAN MAKE IT HAPPEN // CHEERS FOR NOW

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