Here is a new design that I have been working on over the last few years - a pedal designed to mimic the performance of a typical negative feedback tube amp power stage driving a Celestion quad box or similar.
The pedal has the negative feedback presence and depth (resonance) controls typical of Soldano, Marshall, Fender, Peavey etc.
Tube power amplifiers have a high output impedance compared to solid state class AB and digital class D amplifiers, which have very low output impedances. This means that the tube amplifier dynamic power output is considerably influenced by the changing frequency impedance characteristic of the speaker, resulting in higher output power delivered at low end speaker/cabinet resonance and at higher frequencies due to the continually rising speaker voice coil inductance above 400Hz. This is why tube power amplifiers always sound warmer in the bass frequencies and have more "sparkle and reverberance" in the high frequencies because the dynamic power output is increased in these regions. A solid state or class D amplifier is not affected by the changing frequency impedance response of the speaker because of it's very low output impedance - consequently these amplifiers always sound weaker in the bass frequencies and darker in the higher frequencies compared to the tube amplifier.
After simulating the response characteristics of a Soldano SLO output stage driving an approximate electrical equivalent of a celestion quad box, I have come up with this functionally similar 9v dc pedal which can be placed anywhere in your effects pedal chain, but preferably as the last pedal before a solid state amplifier or mixer, daw recording device etc.
The signal is fed via an input level (DRIVE) control (ideally set to 18% to maintain unity gain), to one input of an differential instrumentation amplifier. The voltage gain of this amplifier is the same as the 12AX7 differential input stage of the typical tube amplifier (12dB). After this stage the signal is fed to an op amp gain stage designed to simulate the speaker rising frequency response at high frequencies and a gyrator designed to introduce the "hump" at 100Hz typical of the speaker cabinet resonance. The power tubes have unity voltage gain, so there is no need to model them in this design. Next the signal is fed to the output level control, the top of which is fed (via two back to back clip limiting diodes to ground,to simulate the power amp tube saturation - these can be omitted if desired) to the DEPTH and PRESENCE controls and then to the remaining input of the differential input stage (this completes the feedback loop and sets the voltage gain to the same as the tube amplifier output stage it is modelled on; in this case the Soldano SLO).
The presence and depth controls work exactly as they do in a real tube output stage.
Ideally the input drive is set low and the output level is set to maximum to obtain the clearest possible signal output, however increasing the drive and decreasing the output level can cause the clipping diodes to conduct especially on maximum presence control settings.
No attempt has been made to model the frequency response characteristic of the speaker in this design, just the electrically equivalent interaction with the tube power amplifier output stage.
Here is the schematic, and some pictures of my prototype build (shocking construction I know! - I am working on a much neater pcb design
).The graphs show the response at: (top to bottom), minimum depth minimum presence; middle depth middle presence; maximum depth maximum presence; maximum depth minimum presence; minimum depth maximum presence.
Note: Do not forget the 10k resistor on the output - severe oscillation will occur if omitted!
I do hope you enjoy building it, and as usual, your comments and questions are always welcome.
Happy New Year
bajaman
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