PART 1: PREAMPLIFIER, STANDARD CONFIGURATION
© 2004 Aleksander Niemand/ANACON Technology
Disclaimer: NO voodoo-magic here, just solid electronic design based on over 25 years' experience to guarantee top performance, quality, functionality and reliability.
Note: A PDF file with block diagram of the preamplifier is available in the DOWNLOAD section. It will be easier to follow the text if you open the PDF in a separate window or print it out.
Input amplifier has 2 gain stages built around a 12AX7/ECC83S dual triode using a proprietary Variable Impedance Matching Adjustable Gain circuit topology (VIMAG) developed by ANACON Technology. Some of the classic gain-structure shaping techniques are used in the TUBEWONDER circuit due to their simplicity and effectiveness. One such technique is changing cathode decoupling capacitors value in the 1st input stage for gain boost and bright/normal function. This is what the Gain switch does and thats where any likeness to existing preamp circuits ends. The real innovation is in careful implementation of VIMAG circuit.
VIMAG circuit gives full control over distortion generation in the preamp by varying two key parameters:
- Precisely controlled variable impedance matching between the two gain stages gives control over harmonic content in the distortion spectrum. This can be changed from even to odd harmonics with a flip of the Harmonics switch.
- Continuous gain adjustment over a 10db range in the 2nd gain stage varies the amount of distortion in the preamps output signal from clean/soft onset of clipping to hard clipping. This is done with the DRIVE control, which is active in both positions of the Harmonics switch.
Additional benefits of the VIMAG circuit:
- Dynamic sensitivity. Since total gain and gain structure are actively variable, the preamps dynamic response is very sensitive to the input signal level i.e. the guitars volume control and your playing style.
- Full tone preservation. Because there is no tone stealing attenuator between the gain stages (as is common practice in standard multistage preamps) your guitars full tone is preserved and available for the EQUALIZER section to work on.
To further expand available range of gain structures and matching to different pickups the input has a sensitivity switch that attenuates guitar signal by 6db. At the same time input impedance changes from 1Mohm to 120kohm thus presenting different loading on the pickup. There are in total 4 controls in the input stage to give you a wide range of gain structures and distortion characters.
Additionally a 5th control can be added as a custom option, this is a Bass extender switch that expands low frequency bandwidth from standard 110Hz down to 60Hz.
Preamps output is fed via LEVEL potentiometer to the EQUALIZER through an INTERSTAGE BUFFER that uses one half of the 2nd 12AX7 tube. The buffer has two separate low impedance outputs, one feeding the EQUALIZER and the other feeding REVERB tank drive amplifier. The buffers low output impedance ensures signal integrity and minimizes unwanted interaction between input gain/distortion controls and equalizers boost/cut controls.
The EQUALIZER section is built with three 12AX7 triodes, one for each of the 3 frequency bands Bass, Middle & Treble. The circuit topology and principle of operation is similar to that used in professional parametric equalizers but realized using tubes only.
There are several benefits to active equalization compared to passive tone control circuits:
- No signal level loss with boost/cut controls in center (-0+) position (passive tone controls attenuate the signal up to 20db when set to flat frequency response).
- Drastically reduced interaction between each bands boost/cut controls allows precise and repeatable setting
- Boost/cut characteristics of each band can be tailored easily and independently of each other.
Bass & Treble equalizer sections are switchable between peak/dip and shelving characteristic while Middle peak/dip equalizer can be switched between three center frequencies.
Output from the equalizer is mixed with reverb return signal in an active mixer amplifier that also provides another 6db gain. This brings the signal to a level sufficient to drive the output amplifier to its full power and beyond to produce power tube compression. Mixer output is fed directly to the MASTER VOLUME potentiometer.
The mixer is a virtual-ground inverting amplifier and thus prevents reverb signal from loading the equalizer circuit and vice versa, this makesREVERB control a true mixer:
it changes proportions between DRY and WET signals while keeping the sum of both signals constant for smooth panning between DRY only and WET only without significant change in volume.
The virtual-ground mixer circuit can be easily expanded to accept additional signals from effects sidechain or external preamplifier. (This feature is optional)
High-cut filters are usually implemented as a simple RC-network (a capacitor in series with a potentiometer) connected across the anodes of phase inverter tube in the power amplifier. High-cut filters are often called Brilliance, the difference is the potentiometer wired in reverse so that maximal cut effect is with the potentiometer in its CCW position (counterclockwise).
This filter implementation, although efficient, presents a heavy load on the phase inverter and seriously limits its ability to drive the output tubes to full power. In some amplifiers this loading effect is used for Master Volume control by connecting only a potentiometer between outputs of the phase inverter.
We chose to realize High-cut function as an active filter in the Mixer amplifier preceding the phase inverter. The result is a smooth acting High-cut filter that limits high frequency content of the signal reaching the phase inverter but does not affect its drive capability.This means that output tubes can always be driven to full power and beyond for power tube compression/distortion.
The High-cut filter can be completely switched off or set to two cutoff frequencies: 600Hz or 1200Hz for Soft or Hard filter action. The potentiometer sets the amount of attenuation of frequencies above the cutoff point.
REVERB DWELL MIXER & TANK DRIVER
The reverb drive circuit consists of two sections:
- Summing mixer amplifier that combines the dry signal from the INTERSTAGE BUFFER and a portion of recovered delayed (WET) signal via the DWELL control potentiometer.
- Reverb Tank driver, this a standard SE class A driver using a paralleled ECC82 tube and a transformer
Feeding a portion of WET signal back to the reverb tank produces an effect of increasing the delay time.
REVERB RECOVERY & TONE BALANCE
Wet signal from reverb tank is amplified by ½ of an ECC83 tube. Amplified signal is then fed to an active TONE BALANCE circuit that uses the other triode in the ECC83 tube. Part of amplified wet signal is fed back to the DWELL MIXER via dwell control potentiometer (see above).
The TONE BALANCE circuit has flat frequency response when the control potentiometer is in the center position (-0+). Turning the potentiometer to the left boosts bass and attenuates the treble while turning it to the right cuts bass and boosts treble in the wet signal. Reverb TONE BALANCE operates on delayed signal only while the main EQUALIZER acts only on the dry signal. This allows for even more flexible control of the total amplifier tone.
PART 2: POWER AMPLIFIER will follow shortly /AN Dec 5, 2004