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I was messing around with LT-SPICE and came up with a simple emulation of a prototypical preamplifier circuit. The top circuit is a basic 12AX7 gain stage consisting of an input circuit and a Marshall-type cathode follower, such as is often used to drive a tone stack, running off of 300V, simulated using Duncan Munro's 12AX7 model. The bottom circuit is a opamp circuit running off +12 and -12, using the official TL072 SPICE model. They look very different, yet produce nearly identical output in silico:
Red lines indicate similar breaks between subcircuits in both examples. The tube circuit uses a single 12AX7 for the input gain stage, while the opamp circuit uses 2 TL072 stages as a single stage cannot provide sufficient gain with a full 20KHz frequency response. The green box shows the subcircuit which provides an approximation of grid current draw.
Here's the output! Red trace indicates the input signal. Green trace is the simulated tube circuit, and the blue trace is the simulated opamp circuit. There are obvious similarities here - grid bias shift produces asymmetrical clipping and dynamics, the output is almost identical and I bet it'd sound pretty much the same.
So a simulated tube amp, and a simulation of a simulation of a tube amp
The CF tube stage is actually, surprisingly, pretty much a symmetrical square wave clipper. Pretty much all the delicious even-order harmonics come in because of bias shifting! At least in my simulations. Probably in real life, as well! The TL072 circuit with 2 Zener diodes in the feedback loop gives pretty much the same output for the same input voltage, except that it's level shifted somewhat and is scaled down by a factor of about 25 in voltage. Because the TL072 can't swing 300 volts! The funny green-box circuit provides an approximation of grid current draw here, scaled up by a factor of 10. This requires a 0.22 uF coupling capacitor instead of a 0.022 uF cap, to give identical bias-shifting effects, but allows the use of resistances 10x smaller in this circuit to avoid what might otherwise become serious noise problems. The 3.9K resistor is a simulation of plate resistance which the actual tube stage has, as opposed to the near-zero output impedance of an opamp. It's also bad bad bad to run an opamp into a weird load without series resistance, I've found they tend to oscillate if I do that.
If anyone wanted to build the opamp circuit, I'd recommend bypass caps on the ICs, an input protection network for that first noninverting stage, all that usual stuff. This wouldn't be a high-gain thing by any means, but put a tone stack circuit after it and you might have quite a 'tubey' sounding preamp! And a higher-gain tube circuit could totally be simulated this way, I think, as well.
So . . . interesting? I think so.
Red lines indicate similar breaks between subcircuits in both examples. The tube circuit uses a single 12AX7 for the input gain stage, while the opamp circuit uses 2 TL072 stages as a single stage cannot provide sufficient gain with a full 20KHz frequency response. The green box shows the subcircuit which provides an approximation of grid current draw.
Here's the output! Red trace indicates the input signal. Green trace is the simulated tube circuit, and the blue trace is the simulated opamp circuit. There are obvious similarities here - grid bias shift produces asymmetrical clipping and dynamics, the output is almost identical and I bet it'd sound pretty much the same.
So a simulated tube amp, and a simulation of a simulation of a tube amp
The CF tube stage is actually, surprisingly, pretty much a symmetrical square wave clipper. Pretty much all the delicious even-order harmonics come in because of bias shifting! At least in my simulations. Probably in real life, as well! The TL072 circuit with 2 Zener diodes in the feedback loop gives pretty much the same output for the same input voltage, except that it's level shifted somewhat and is scaled down by a factor of about 25 in voltage. Because the TL072 can't swing 300 volts! The funny green-box circuit provides an approximation of grid current draw here, scaled up by a factor of 10. This requires a 0.22 uF coupling capacitor instead of a 0.022 uF cap, to give identical bias-shifting effects, but allows the use of resistances 10x smaller in this circuit to avoid what might otherwise become serious noise problems. The 3.9K resistor is a simulation of plate resistance which the actual tube stage has, as opposed to the near-zero output impedance of an opamp. It's also bad bad bad to run an opamp into a weird load without series resistance, I've found they tend to oscillate if I do that.
If anyone wanted to build the opamp circuit, I'd recommend bypass caps on the ICs, an input protection network for that first noninverting stage, all that usual stuff. This wouldn't be a high-gain thing by any means, but put a tone stack circuit after it and you might have quite a 'tubey' sounding preamp! And a higher-gain tube circuit could totally be simulated this way, I think, as well.
So . . . interesting? I think so.
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